Role of plants in controlling air pollutants-Legislation & air quality standards

 

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Role of plants in controlling air pollutants-Legislation & air quality standards

Plants are world's natural pollutant sink. Trees are known to fix and metabolize carbon-dioxide and carbon monooxide both photosynthetically and non-photosynthetically. In recent years, the utility of trees in controlling the pollution problems has been recognized particularly air and noise pollutions. Growing ornamental plants result in relief of certain gaseous pollutions and other plants have got purifying qualities such as bamboo, palms, lily etc. Plants weaken sound is now well known established fact. Plants with fleshy and thick leaves deaden the sound such plant could be Euphorbia neriifolia, Ficus elastica, F. integnoflia, Citrus sp. etc. Similarly plants with branches that move and vibrate to absorb and mask sound like Pipal, Mango, Conifers, Bamboo etc.

The effects of urban trees on air quality

Urban vegetation can directly and indirectly affect local and regional air quality by altering the urban atmospheric environment. The four main ways that urban trees affect air quality area: Temperature reduction and other microclimatic effects Removal of air pollutants Emission of volatile organic compounds and tree maintenance emissions

Energy effects on buildings

Temperature Reduction: Tree transpiration and tree canopies affect air temperature, radiation absorption and heat storage, wind speed, relative humidity, turbulence, surface albedo, surface roughness and consequently the evolution of the mixing-layer height. These changes in local meteorology can alter pollution concentrations in urban areas

b . Although trees usually contribute to cooler summer air temperatures, their presence can increase air temperatures in some instances

c . In areas with scattered tree canopies, radiation can reach and heat ground surfaces; at the same time, the canopy may reduce atmospheric mixing such that cooler air is prevented from reaching the area. In this case, tree shade and transpiration may not compensate for the increased air temperatures due to reduced mixingd . Maximum mid-day air temperature reductions due to trees are in the range of 0.04oC to 0.2oC per percent canopy cover increasee . Below individual and small groups of trees over grass, mid-day air temperatures at 1.5 m above ground are 0.7oC to 1.3oC cooler than in an open areaf . Reduced air temperature due to trees can improve air quality because the emission of many pollutants and/or ozone-forming chemicals are temperature dependent. Decreased air temperature can also reduce ozone formation. Removal of Air Pollutants: Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfacesg . Trees also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most particles that are intercepted are retained on the plant surface. The intercepted particle often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fallg . Consequently, vegetation is only a temporary retention site for many atmospheric particles. In 1994, trees in New York City removed an estimated 1,821 metric tons of air pollution at an estimated value to society of $9.5 million. Air pollution removal by urban forests in New York was greater than in Atlanta (1,196 t; $6.5 million) and Baltimore (499 t; $2.7 million), but pollution removal per m2 of canopy cover was fairly similar among these cities (New York: 13.7 g/m2 /yr; Baltimore: 12.2 g/m2 /yr; Atlanta: 10.6 g/m2 /yr)h . These standardized pollution removal rates differ among cities according to the amount of air pollution, length of in-leaf season, precipitation, and other meteorological variables. Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 8 cm in diameter (0.02 kg/yr)k . Air quality improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter, 0.45% for ozone, 0.43% for sulfur dioxide,

0.30% for nitrogen dioxide, and 0.002% for carbon monoxide. Air quality improves with increased percent tree cover and decreased mixing-layer heights. In urban areas with 100% tree cover (i.e., contiguous forest stands), short-term improvements in air quality (one hour) from pollution removal by trees were as high as 15% for ozone, 14% for sulfur dioxide, 13% for particulate matter, 8% for nitrogen dioxide, and 0.05% for carbon monoxideh . Emission of Volatile Organic Compounds (VOCs): Emissions of volatile organic compounds by trees can contribute to the formation of ozone and carbon monoxide. However, in atmospheres with low nitrogen oxide concentrations (e.g., some rural environments), VOCs may actually remove ozonei,j . Because VOC emissions are temperature dependent and trees generally lower air temperatures, increased tree cover can lower overall VOC emissions and, consequently, ozone levels in urban areasl . VOC emission rates also vary by species. Nine genera that have the highest standardized isoprene emission ratem,n, and therefore the greatest relative effect among genera on increasing ozone, are: beefwood (Casuarina spp.), Eucalyptus spp., sweetgum (Liquidambar spp.), black gum (Nyssa spp.), sycamore (Platanus spp.), poplar (Populus spp.), oak (Quercus spp.), black locust (Robinia spp.), and willow (Salix spp.). However, due to the high degree of uncertainty in atmospheric modeling, results are currently inconclusive as to whether these genera will contribute to an overall net formation of ozone in cities (i.e., ozone formation from VOC emissions are greater than ozone removal). Some common genera in Brooklyn, NY, with the greatest relative effect on lowering ozone were mulberry (Morus spp.), cherry (Prunus spp.), linden (Tilia spp.) and honey locust (Gleditsia sp.)n . Because urban trees often receive relatively large inputs of energy, primarily from fossil fuels, to maintain vegetation structure, the emissions from these maintenance activities need to be considered in determining the ultimate net effect of urban forests on air quality. Various types of equipment are used to plant, maintain, and remove vegetation in cities. These equipment include various vehicles for transport or maintenance, chain saws, back hoes, leaf blowers, chippers, and shredders. The use and combustion of fossil fuels to power this equipment leads to the emission of carbon dioxide (approximately 0.7 kg/l of gasoline, including manufacturing emissionso ) and other chemicals such as VOCs, carbon monoxide, nitrogen and sulfur oxides, and particulate matterp . Trees in parking lots can also affect evaporative emissions from vehicles, particularly through tree shade. Increasing parking lot tree cover from 8% to 50% could reduce Sacramento County, CA, light duty vehicle VOC evaporative emission rates by 2% and nitrogen oxide start emissions by less than 1%q . Energy Effects on Buildings: Trees reduce building energy use by lowering temperatures and shading buildings during the summer, and blocking winds in winterr . However, they also can increase energy use by shading buildings in winter, and may increase or decrease energy use by blocking summer breezes. Thus, proper tree placement near buildings is critical to achieve maximum building energy conservation benefits. When building energy use is lowered, pollutant emissions from power plants are also lowered. While lower pollutant emissions generally improve air quality, lower nitrogen oxide emissions, particularly ground-level emissions, may lead to a local increase in ozone concentrations under certain conditions due to nitrogen oxide scavenging of ozones . The cumulative and interactive effects of trees on meteorology, pollution removal, and VOC and power plant emissions determine the overall impact of trees on air pollution.

Combined Effects: Changes in urban microclimate can affect pollution emission and formation, particularly the formation of ozone. A model simulation of a 20 percent loss in the Atlanta area forest due to urbanization led to a 14 percent increase in ozone concentrations for a modeled dayl . Although there were fewer trees to emit VOCs, an increase in Atlanta’s air temperatures due to the urban heat island, which occurred concomitantly with tree loss, increased VOC emissions from the remaining trees and anthropogenic sources, and altered ozone chemistry such that concentrations of ozone increased. A model simulation of California’s South Coast Air Basin suggests that the air quality impacts of increased urban tree cover may be locally positive or negative with respect to ozone. The net basinwide effect of increased urban vegetation is a decrease in ozone concentrations if the additional trees are low VOC emitterst . Modeling the effects of increased urban tree cover on ozone concentrations from Washington, DC to central Massachusetts reveals that urban trees generally reduce ozone concentrations in cities, but tend to slightly increase average ozone concentrations in the overall modeling domain. Interactions of the effects of trees on the physical and chemical environment demonstrate that trees can cause changes in pollution removal rates and meteorology, particularly air temperatures, wind fields, and mixing-layer heights, which, in turn, affect ozone concentrations. Changes in urban tree species composition had no detectable effect on ozone concentrationsu . Modeling of the New York City metropolitan area also reveal that increasing tree cover 10% within urban areas reduced maximum ozone levels by about 4 ppb, which was about 37% of the amount needed for attainmentv . Urban Forest Management: Urban forest management strategies to help improve air quality includew : · Increase the number of healthy trees (increases pollution removal). · Sustain existing tree cover (maintains pollution removal levels). · Maximize use of low VOC emitting trees (reduces ozone and carbon monoxide formation). · Sustain large, healthy trees (large trees have greatest per tree effects). · Use long-lived trees (reduces long-term pollutant emissions from planting and removal). · Use low maintenance trees (reduces pollutants emissions from maintenance activities). · Reduce fossil fuel use in maintaining vegetation (reduces pollutant emissions). · Plant trees in energy conserving locations (reduces pollutant emissions from power plants). · Plant trees to shade parked cars (reduces vehicular VOC emissions). · Supply ample water to vegetation (enhances pollution removal and temperature reduction). · Plant trees in polluted areas or heavily populated areas (maximizes tree air quality benefits). · Avoid pollutant sensitive species (increases tree health). · Utilize evergreen trees for particulate matter reduction (year-round removal of particles).

Phylloremediation of Air Pollutants: Exploiting the Potential of Plant Leaves and Leaf-Associated Microbes

Air pollution is air contaminated by anthropogenic or naturally occurring substances in high concentrations for a prolonged time, resulting in adverse effects on human comfort and health as well as on ecosystems. Major air pollutants include particulate matters (PMs), ground-level ozone (O₃), sulfur dioxide (SO₂), nitrogen dioxides (NO₂), and volatile organic compounds (VOCs). During the last three decades, air has become increasingly polluted in countries like China and India due to rapid economic growth accompanied by increased energy consumption. Various policies, regulations, and technologies have been brought together for remediation of air pollution, but the air still remains polluted. In this review, we direct attention to bioremediation of air pollutants by exploiting the potentials of plant leaves and leaf-associated microbes. The aerial surfaces of plants, particularly leaves, are estimated to sum up to 4 × 10⁸ km² on the earth and are also home for up to 10²⁶ bacterial cells. Plant leaves are able to adsorb or absorb air pollutants, and habituated microbes on leaf surface and in leaves (endophytes) are reported to be able to biodegrade or transform pollutants into less or nontoxic molecules, but their potentials for air remediation has been largely unexplored. With advances in omics technologies, molecular mechanisms underlying plant leaves and leaf associated microbes in reduction of air pollutants will be deeply examined, which will provide theoretical bases for developing leaf-based remediation technologies or phylloremediation for mitigating pollutants in the air.

Introduction

Air pollution is referred to as the presence of harmful or poisonous substances in the earth's atmosphere, which cause adverse effects on human health and on the ecosystem. Major air pollutants include particulate matters (PMs), nitrogen oxides (NO₂), sulfur dioxide (SO₂), ground-level ozone (O₃), and volatile organic compounds (VOCs)). Effects include  respiratory illness, cardiovascular disease to bladder and lung cancer

The world has experienced unprecedented urban growth during the last three decades. Urban population is expected to increase at 2.3% per year in developing countries from 2000 to 2030. Urbanization is often associated with rapid economic growth. For example, China's urbanization grew from 17.92% in 1978 to 52.57% in 2012, and China's gross domestic products (GDPs) increased from 454.6 billion Chinese Yuan in 1980 to 51,894.2 billion Yuan in 2012. The increased economic growth has been accompanied with elevated energy consumption. China's energy consumption, primarily fossil fuels like coal, increased from 602.75 million tons in 1980 to 3,617.32 million tons in 2012.

 The increased combustion of fossil fuels with relatively low combustion efficiency along with weak emission control measures have resulted in drastic increases in air pollutants, such as PMs, SO₂, NO₂, O₃, and VOCs. Per unit of GDPs in 2006, China emitted 6–33 times more pollutants than the United States (US). As a result, air quality has become a major focus of environmental policy in China. India experiences similar situations as China. Urbanization coupled with rapid economic development in India increased energy consumption and also air pollution in some megacities. For example, PM₁₀ in Delhi was almost 10 times of the maximum PM₁₀ limit at 198 μg m⁻³ in 2011

The World Health Organization (WHO) air quality guidelines stated that the mean limits for annual exposure to PM_2.5 (particle diameters at 2.5 μm or less) and PM₁₀ (particle diameter at 10 μm or less) are 10 μg m⁻³ and 25 μg m⁻³, respectively; and the limits for 24-h exposure are 25 μg m⁻³ and 50 μg m⁻³, respectively. The limit for 8-h exposure to O₃ is 100 μg m⁻³. Annual mean for NO₂ is 40 μg m⁻³ or 200 μg m⁻³ for 1 h, and 24-h exposure to SO₂ is 20 μg m⁻³ or 500 μg m⁻³ for 10 min.

PMs have become the most pressing environmental problems in China and India. For example, during the first quarter of 2013, China experienced extremely severe and persistent haze pollution that directly affected about 1.3 million km² and about 800 million people. Of which daily average concentrations of PM_2.5 measured at 74 major cities exceeded the Chinese pollution standard of 75 μg m⁻³, which is approximately twice that of the US EPA (United States Environmental Protection Agency) standard of 35 μg m⁻³, for 69% of days in January, with a record-breaking daily concentration of 772 μg m⁻³.

Recent studies from the International Agency for Research on Cancer showed that there were 223,000 deaths in 2010 due to air pollution-resultant lung cancer worldwide, and air pollution has become the most widespread environmental carcinogen. The WHO reported that around 7 million people died of air pollution exposure directly or indirectly in 2012. This data was more than double previous estimates and confirmed that air pollution has become a substantial burden to human health and is the world's largest single environmental health risk. Additionally, air pollution also harms animals, plants, and ecological resources including water and soils.

Measures for Reducing Air Pollution

To reduce air pollution, the first step is to eliminate or reduce anthropogenic-caused emissions. The second step is to remediate existing pollutants. Different strategies, policies, and models for air pollution abatement have been proposed or implemented. For example, the Chinese government has imposed restrictions on major pollution sources including vehicles, power plants, transport, and industry sectors and promulgated the “Atmospheric Pollution Prevention and Control Action Plan” in September 2013, which was intended to reduce PM_2.5 by 25% by 2017 relative to 2012 levels. Science-based technologies have been developed for control of air pollutants, such as diesel particulate filters and activated carbon filtering as adsorbent for xylene and NO₂ . Catalytic oxidization and chemisorption methods have been used for indoor formaldehyde removal. Photocatalysis as one of the most promising technologies has been used for eliminating VOCs.

Air pollutants can also be mitigated through biological means, commonly referred to as biological remediation or bioremediation. It is the use of organisms to assimilate, degrade or transform hazardous substances into less toxic or non toxic ones. Plants have been used for remediation of pollutants from air, soils, and water, which has been termed as phytoremediation. Microbes such as bacteria and fungi are also capable of biodegrading or biotransforming pollutants into non toxic and less toxic substances, which is known as microbial biodegradation. Microbes as heterotrophs occur nearly everywhere, including plant roots and shoots. Both roots and shoots have been reported to be able to remediate air pollutants, but little credit has been given to microbe activity.

Plant shoots or the above-ground organs of plants colonized by a variety of bacteria, yeasts, and fungi are known as phyllosphere. However, most scientific work on phyllosphere microbiology has been focused on leaves. This review is intended to explore the potential of plant leaves and leaf-associated microbes in bioremediation of air pollutants, or simply known as phylloremediation. Phylloremediation was first coined by Sandhu et al. , who demonstrated that surface-sterilized leaves took up phenol, and leaves with habiated microbes or a inoculated bacterium were able to biodegrade signficantly more phenol than leaves alone. Previous reports also documented that both plant leaves and leaf-associated microbes mitiagted air pollutants, such as azalea leaves and the leaf-associated Pseudomonas putida in reducing VOCs, leaves of yellow lupine plants along with endophytic Burkholderia cepacia for toluene reduction, and poplar leaves and the leaf-associated Methylobacterium sp. decreased xenobiotic compounds. Phyllo originated from Greek word of phullon, meaning leaf. Thus, phylloremediation should be defined as a natural process of bioremediation of air pollutants through leaves and leaf-associated microbes, not the microbes alone.

Plant Leaves and Phyllosphere

Leaves are the primary photosynthetic organs with distinctive upper surface (adaxial) and lower surface (abaxial). The upper surface has a layer (<0.1–10 μm) of waxy cover called cuticle. Wax contents and compositions frequently differ among plant species. The primary function of cuticle is to prevent evaporation of water from leaf surfaces, and it is also the first barrier for the penetration of xenobiotics. The leaf surface is filled with trichomes, which are epidermal outgrowths in various forms. Trichomes play roles in mechanical defense because of their physical properties and also in biochemical defense due to the secretion of secondary metabolites. Epidermis cells are directly underneath the cuticle layer in which stomata often occur. Xylem and phloem are situated within the veins of leaves as the plant vascular system, which are connected from root tips to leaf edges. There is a layer of compactly arranged cells around the vein called bundle sheath regulating substance circle around the xylem and the phloem. Xylem transports water and nutrients from roots to shoots, and phloem transports assimilated products from source and sink tissues. Under the epidermis, there are mesophyll cells in two layers: column-like palisade cells and loosely packed spongy cells. The air spaces among the spongy cells promote gas exchange, and photosynthesis takes place in chloroplasts packed in the mesophyll cells. The underside of leaves also has a layer of epidermal cells where most stomata are located. There are two guard cells surround the stomata, and stomatal pore opening and closure is regulated by changes in the turgor pressure of the guard cells. Stomata regulate the flow of gases in and out of leaves and also able to adsorb or absorb other chemicals.

Leaves also play pivotal roles in supporting phyllosphere microbes. The phyllosphere is estimated to have area up to 4 × 10⁸ km² on the earth and is the home for up to 10²⁶ bacterial cells. Phyllosphere bacterial communities are generally dominated by Proteobacteria, such as Methylobacterium and Sphingomonas. Beijerinckia, Azotobacter, Klebsiella, and Cyanobacteria like Nostoc, Scytonema, and Stigonema also reside in the phyllosphere. Population of γ-Proteobacteria such as Pseudomonas could be high as well. Dominant fungi in the phyllosphere include Ascomycota, of which the most common genera are Aureobasidium, Cladosporium, and Taphrina. Basidiomycetous yeasts belonging to the genera Cryptoccoccus and Sporobolomyces are also abundant in phyllosphere. The microbes can be epiphytic by living on the surface of plant organs and/or endophytic occurring within plant tissues without causing apparent disease.

Plant species significantly influence the composition of a phyllosphere community. In a study of 56 different tree species, reported that different species harbor distinct microbial communities in phyllosphere. This principle was also confirmed for trees in temperate and tropical climates and for Mediterranean perennials. Using high-throughput sequencing technology, 

Fungal communities on leaves were dominated by the phyla Ascomycota, which accounted for 79% of all sequences, followed by Basidiomycota (11%) and Chytridiomycota (5%). More than half of the variation in fungal community composition could be explained by plant species differences. Leaf chemistry and morphology as well as plant growth status and mortality were closely related to fungal community structure. These results may suggest that different tree species host different fungal communities. Additionally, microbial compositions within plant species may differ due to geographic locations. The differences could be caused by climatic variation or due to the limited dispersal of the colonizing taxa. Furthermore, phyllosphere microbial community may differ between urban and non-urban locations and also differ by seasons.

Roles of Leaves and Phyllosphere Microbes in Air Remediation

The close association between plant species and specific microbial communities in the phyllosphere suggests their adaptation and coevolutionary relationships. Recent studies show that leaf bacterial diversity mediates plant diversity and ecosystem function relationships. We hypothesize that a long-lasting exposure of leaves and leaf-associated microbes to air pollutants could result in plants or microbes individually or coordinately developing mechansims for adapting to the polluted substances. Such mechanisms may include leaf adsorption or absorption and pollutant assimilation as well as microbial biodegradation, transformation or metabolic assimilation of the substances. The coordination between leaves and micriobes could be synergistic or antagonistic. Information regarding phyllospere microbes in remediation of PMs, SO₂, NO₂, and O₃ is scarce

Remediation of PMs

PMs have become the most dangerous pollutants in some countries. Chemical species of PMs, derived from the available data over China included SO2−4SO42-, NO−3NO3-, NH+4NH4+, organic carbon, and elemental carbon, which were in a range of 2.2–60.9, 0.1–35.6, 0.1–29.8, 1.5–102.3, 0.2–37.0 μg cm⁻³ in PM_2.5, and 1.6–104.6, 0.5–46.6, 0.2–31.0, 1.7–98.7, and 0.3–26.8 μg cm⁻³ in PM₁₀, respectively. PM_2.5 is the major component of PM₁₀, accounting for 65%. PMs are also composed of microorganisms. In a study of PMs in Jeddah, Saudi Arabia, the average concentrations of PM₁₀ and PM_2.5 were 159.9 and 60 μg cm⁻³, respectively and the concentrations of O₃, SO₂, and NO₂ averaged 35.73, 38.1, and 52.5 μg cm⁻³, respectively. Microbial loads were higher in PM₁₀ than PM_2.5. Aspergillus fumigatus and Aspergillus niger were the common fungal species associated with PMs. Microbes were also found in PMs in Austria, including fungi from genera Aspergillus, Cladosporium, and Penicillium and aerobic mesophilic bacteria. Using metagenomic methods, identified 1,315 distinct bacterial and archaeal species from 14 PM samples collected from Beijing, China. The most abundant phyla were Actinobacteria, Proteobacteria, Chloroflexi, Firmicutes, Bacteroidetes, and Euryarchaeota. Among them, an unclassified bacterium in the nitrogen fixing, filamentous bacteria genus Frankia was the most abundant, and the most abundant classified bacterial species appeared to be Geodermatophilus obscures. The abundance of airborne bacteria was reported to be in a range from 10⁴ to 10⁶ cells m⁻³ depending on environmental conditions, and materials of biological origin might account for up to 25% of the atmospheric aerosol.

 Ammonia oxidizing archaea (AOA), ammonia oxidizing bacteria (AOB), and complete ammonia oxidizers (Comammox) were identified in PM_2.5 collected from the Beijing-Tianjin-Heibei megalopolis, China. Of which Nitrosopumilus subcluster 5.2 was the most dominant AOA, Nitrosospira multiformis and Nitrosomonas aestuarii were the most dominant AOB, and the presence of Comammox was revealed by the occurrence of Candidatus Nitrospira inopinata.

The mean cell numbers of AOA, AOB, and Ca. N. inopinata were 2.82 × 10⁴, 4.65 × 10³, and 1.15 × 10³ cell m⁻³, respectively. The average maximum nitrification rate of PM_2.5 was 0.14 μg (NH4⁺-N) [m³ air h]⁻¹ . AOA might account for most of the ammonia oxidation, followed by Comammox, while AOB were responsible for a small part of ammonia oxidation. The assay of nitrification activity was performed in laboratory conditions. However, the nitrification potential of such bacteria in PMs after being deposited on leaf surfaces is unknown. We hypothesize that the nitrification process could be more active once such PM-containing bacteria settled on leaves. Further investigation on nitrification of PM-associated bacteria in the phyllosphere could provide insight into how the phyllosphere could potentially act as manufactories in the nitrification of ammonia.

The current literature regarding phylloremediation of PMs has been primarily focused on plant leaves. Plant canopy is a sink for PMs. This is due to the fact that leaves are in the air and they span more than 4 × 10⁸ km² on a global scale, which is about 78.4% of the total surface area of the earth; leaves thus physically act as a natural carrier for PMs. Leaves differ greatly in surface structure and metabolic secreted substances as well as microbial composition. The amount of surface waxes and compositions show different capacity to retain and embrace PMs.

Leaf physical characteristics such as leaf shape, hairs or trichomes, and stomata significantly affect PM accumulation. Needle leaves were reported to accumulate more PM_2.5 than broad leaves. The effectiveness was attributed to the higher capture efficiency and higher Stoke's numbers of needles compared to those of broad leaves. Additionally, small individual leaf area and abundant wax layer also contribute to the effectiveness. Leaf trichomes have been shown to increase PM_2.5 accumulation. The trichome density was positively correlated with amount of PM_2.5 accumulated on leaves, and plant species with abundant hairs, such as Catalpa speciosa, Broussonetia papyrifera, and Ulmus pumila were able to retain more PM_2.5 than those with fewer hairs. The adaxial surface of leaves accumulated more PMs than the abaxial leaf surface, which is probably due to the fact that the abaxial surface in general has few trichomes and less rough surface. Stomata may play some roles in accumulation of PMs. The length of stomata ranges from 10 to 80 μm and densities varies from 5 to 1,000 mm⁻² depending on plant species and environmental conditions.

Stomatal pore areas range from 46 to 125 μm² , thus stomata could retain or adsorb either PM_2.5 or PM₁₀. A study of PM deposition on leaves of five evergreen species in Beijing, China showed that PM diameter up to 2 μm was in the stomatal cavity. 

The effects of PMs on 12 common roadside plant species and found that stomatal sizes were reduced due to air dust deposition, but plant growth was not affected, suggesting the potential of plants in adsorbing air pollutants.

Growing evidence has suggested that plant leaves are able to capture PMs and act as biofilters. On average, the upper leaf surface of 11 plant species intercepted 1,531 particles per mm⁻² . Needles of Pinus sylvestris accumulated 18,000 mineral particles per mm² . Upper leaves of Hedera helix captured about 17,000 particles per mm² . Trees removed 1,261 tons of air pollutants in Beijing, of which 772 tons were PM₁₀. In New Zealand, urban trees removed 1,320 tons of particular matter annually due to the existence of woodlands in Auckland.

Trees within cities removed fine particles from the atmosphere and consequently improved air quality and human health. Tree effects on PM_2.5 concentrations and human health are modeled for 10 U.S. cities. The total amount of PM_2.5 removed by trees varied from 4.7 tons in Syracuse to 64.5 tons in Atlanta in the U.S annually. All the reported removal of PMs is attributed to plant leaves. It is unknown at this time if phyllosphere microbes could break down the PMs on leaves and if mineral elements released from the broken PMs could become plant nutrients. Considering the fact that the microbes can biodegrade a wide range of substances including petroleum, we hypothesize that some microbes should be able to break down PM. Future research in this regard will be conducted, and identified microbes could be used for PM reduction.

Remediation of SO₂

Sulfur dioxide (SO₂) was among the first air pollutants identified to harm human health and ecosystems. The combustion of fossil fuels has substantially increased SO₂ in the air. China has contributed to about one-fourth of global SO₂ emission since 1990. The emission of SO₂ from Guangdong province totaled 1,177 Gg in 2007, of which 97% was emitted by power plants and industries. SO₂ can be oxidized photochemically or catalytically to sulfur trioxide (SO₃) and sulfate (SO2−4SO42-) in the air. With the presence of water, SO₃ is converted rapidly to sulfuric acid (H₂SO₄), which is commonly known as acid rain. While in sulfur assimilation, SO2−4SO42- is reduced to organic sulfhydryl groups (R-SH) by sulfate-reducing bacteria, fungi, and plants. Sulfur oxidizing bacteria such as Beggiatoa and Paracoccus are able to oxidize reduced sulfur compounds like H₂S to inorganic sulfur, and thiosulfate to form sulfuric acid. Sulfate reducing bacteria like Archaeoglobus and Desulfotomaculum can convert sulfur compounds to hydrogen sulfide (H₂S). Oxidation of H₂S produces elemental sulfur (S°), which is completed by the photosynthetic green and purple sulfur bacteria and some chemolithothrophs. Further oxidation of elemental sulfur produces sulfate. Sulfate is assimilated through the sulfate activation pathway, which is consisted of three reactions: the synthesis of adenosine 5′-phosphorylation of (APS), the hydrolysis of GTp, and the 3′-phosphorylation of APS to produce 3′-phosphoadenosine 5′-phosphosulfate (PAPS).

 In Mycobacterium tuberculosis, the entire sulfate activation pathway is organized into a single complex. Additionally, sulfate reducing bacteria have been shown to use hydrocarbons in pure cultures, which can be used for bioremediation of benzene, toluene, ethylbenzene, and xylene in contaminated soils. Such bacteria may also colonize leaf surfaces and could be used for remediation of air pollutants.

Plant leaves absorb SO₂ via stomata. At apoplastic pH, it is hydrated and oxidized successively to sulfite and sulfate, both of which can inhibit photosynthesis and energy metabolism if they accumulate to a high concentration. Such inhibition can cause SO₂ toxicity. Symptoms include interveinal chlorosis and necrosis in broad-leaved species, and chlorotic spots and brown tips in pine conifers. Until the 1970s, SO₂ was considered to be a key contributor of acid rain causing forest dieback. Interestingly, when the Clean Air Acts came into action in the 1980s, the reduction in atmosphere SO₂ resulted in sulfur (S) deficiency in crops, particularly Brassica species. The S deficiency was responsible for the increased incidence of disease caused by Pyrenopeziza brassicae. The explanation is that plants could become injured in a SO₂ concentration range from 131 to 1,310 μg m⁻³; plants, however, can rapidly assimilate SO₂ and H₂S into reduced sulfur pools such as cysteine and sulfates.

 A recent transcriptome analysis of Arabidopsis responses to SO₂ showed that plant adaptation to SO₂ evokes a comprehensive reprogramming of metabolic pathways including NO and reactive oxygen species (ROS) signaling molecules, and also plant defense response pathways. The importance of this study revealed that plant responses to SO₂ stress is at the transcription level with initial activation of cross tolerance and followed by sulfur assimilation pathways. Cysteine metabolism in particular is associated with the network of plant stress responses, thus improving plant growth in soils where sulfur supply is limited. It has been shown that an atmospheric level of 79 ng m⁻³ SO₂ could contribute to 10–40% of leaf sulfur assimilation. Elevated SO₂ concentrations around natural CO₂ springs have been documented to enhance accumulation of sulfur metabolites and proteins in surrounding vegetation. Therefore, plants can be selected for growing in SO₂ polluted environments. In 2000, about 42.62 Mg of SO₂ was removed from the atmosphere by urban trees in Guangzhou, China. Additionally, S metabolism can be genetically engineered for improving plant resistance to SO₂. Transgenic tobacco plants overexpressing cysteine synthase or serine acetyltransferase gene were highly tolerant to SO₂ and sulfite.

Remediation of NO_x

There are several oxides of nitrogen (N) in the atmosphere: nitrogen dioxide (NO₂), nitric oxide (NO), nitrous oxide (N₂O), nitrogen trioxide (N₂O₃), and nitrogen trioxide (N₂O₅). Among them, the USEPA regulates NO₂ only because it is the most prevalent form of NO_x generated anthropogenically. NO₂ also participates in the formation of ozone (O₃) and NO. NO_x emissions in China increased rapidly from 11.0 Mt in 1995 to 26.1 Mt in 2010. Power plants, industry, and transportation were major sources of NO_x emissions, accounting for 28.4, 34.0, and 25.4% of the total NO_x emissions in 2010, respectively. The total NO_x emissions in China are projected to increase 36% based on the 2010 value by 2030.

A group of bacteria like Azotobacter and Rhizobium and fungi such as mycorrhizas are capable of fixing atmospheric N. Cyanobacteria are able of using a variety of inorganic and organic sources of combined N, like nitrate, nitrite, ammonium, urea or some amino acids. These microbes are often associated with plant roots. Nitrifying bacteria including species from the genera Nitrosomonas, Nitrosococcus, Nitrobacter, and Nitrococcus oxidize ammonia to hydroxylamine, and nitrite oxidoreductase oxidizes nitrite to nitrate. Nitrifying bacteria thrive in soils, lakes, rivers, and streams with high inputs and outputs of sewage, wastewater and freshwater because of high ammonia content. Phyllosphere diazotrophic bacteria, like Beijerinckia, Azotobacter, and Klebsiella and also Cyanobacteria, such as Nostoc, Scytonema, and Stigonema can use atmospheric dinitrogen (N₂) as a source of nitrogen. N₂ is fixed by the nitrogenase enzyme encoded by nif genes, and the gene nifH has been widely used for analysis of their community structure. The abundance of N₂-fixing bacteria was also reported to improve drought tolerance, suggesting their adaptability to plants grown in different environmental conditions.

Plants absorb gaseous NO₂ more rapidly than NO because NO₂ reacts rapidly with water while NO is almost insoluble. The uptake of NO₂ per unit leaf area was reported to be nearly three times that of NO when the two gases occurred in the same concentration. As a result, NO₂ has been considered to be more toxic than NO. Visible symptoms resulting from NO₂exposure are relatively large, irregular brown or black spots. However, phytotoxicity of NO₂ is rare and much less than SO₂ and O₃. This is due to the fact that NO_x are plant nutrients. When NO and NO₂ are absorbed and dissolved in the extracellular solution of leaves, they form nitrate (NO₃) and NO₂ in equal amounts and proton (H⁺). NO₃ is then utilized by plants in the same way as it is absorbed from roots and used as a nitrogen source for synthesizing amino acids and proteins. Foliar absorption of NO₂ varies widely depending on plant species.

The most efficient woody plants for absorbtion of NO₂  included Eucalyptus viminalis, Populus nigra, Magnolia kobu, and Robinia pseudoacacia, and the most herbaceous plants include Erechtites hieracifolia, Crassocephalum crepidioides, and Nicotiana tabacum.

Nitrogen dioxide could be a plant signal molecule that improves plant growth. About one-third of NO₂-derived N absorbed by leaves was converted into a previously unknown Kjeldahl-unrecoverable organic nitrogen, which comprise a novel heterocyclic Δ2 1,2,3 thiadiazoline derivative and nitroso- and nitro-organic compounds. These results indicate that NO₂ is not only known as a pollutant or a supplemental source of N, but also acts as an airborne reactive nitrogen species signal. This is in agreement with the reports that endogenously produced NO_x such as NO act as a vital plant signal.

To further analyze atmospheric NOx effects on plants,  fertilizer and concomitantly reduce NO₂ concentrations. The authors found that application of 282 μg m⁻³ NO₂, equivalent to the heavily polluted urban air, to plants for 10 weeks almost doubled the biomass, total leaf area, the contents of carbon (C), N, S, phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) as well as free amino acid contents and crude proteins. The mass spectrometric analysis of the ¹⁵N/¹⁴N ratio showed that N derived from NO₂comprised less than 3% of total plant N, meaning that the contribution of NO₂-N to total N was relatively low. These results imply that NO₂ could be a multifunctional signal to stimulate plant growth, nutrient uptake, and metabolism.

Remediation of O₃

Anthropogenic O₃ is primarily generated from the reaction of atmospheric O₂ with ground-state O (3P) radicals that result from the photolytic dissociation of ambient NO₂. Thus, the presence of NO and NO₂ in the lower atmosphere is closely linked with ground-level of O₃. In China, O₃ levels increased at a rate of 2.2 μg m⁻³ per year from 2001 to 2006. Average O₃ concentrations in Beijing varied from 45 to 96.2 μg m⁻³ depending on locations. In Shanghai, 1-h average concentration of O₃ was 54.2 μg m⁻³. O₃ level increased during spring, reached the peak in late spring and early summer, and then decreased in autumn and finally dropped in winter. The highest monthly average O₃ concentration (82.2 μg m⁻³) in June was 2.7 times greater than the lowest level (30.4 μg m⁻³) recorded in December.

Ozone is considered an effective antimicrobial agent against some bacteria and fungi. There have been no reports on microbial-mediated O₃ reduction. However, in a study of O₃ effects on phyllosphere fungal populations, 

A chronic exposure of mature Valencia orange trees (Citrus sinensis) to O₃ or SO₂ for 4 years decreased populations of phyllosphere fungi. In a same experiment conducted by the authors, a short-term fumigation of O₃ to giant sequoia (Sequoiadendron giganteum) and California black oak (Quercus kelloggii) did not significantly affect the numbers of phyllospere fungi. Plant absorption of O₃ is mainly through stomata, O₃ is easily dissolved in water and reacts with apoplastic structures and plasma membranes to form reactive oxygen species (ROS), such as O−2O2-, H₂O₂, and OH radical. The O₃ or ROS can disturb cell membrane integrity and attack sulfhydryl (SH) groups or ring amino acids of protein, thus causing phytotoxicity. Injury symptoms include white, yellow or brown flecks on the upper surface of leaves. The threshold concentrations that cause a 10% reduction in yield are 80μg m⁻³ for sensitive crops and 150 μg m⁻³ for the most resistant crops. Adaptation of plants to O₃ stress has resulted in plants developing mechanisms against O₃ toxicity. First, O₃ can be removed from the air by chemical reactions with reactive compounds emitted by vegetation, particularly monoterpenes. Second, semi-volatile organic compounds, such as different diterpenoids exuded by trichomes on leaves are an efficient O₃ sink. Tobacco leaves can secret diterpenoid cis-abienol, which acts as a powerful chemical protection shield against stomatal O₃ uptake by depleting O₃ at the leaf surface. As a result, O₃ flux through the open stomata is strongly reduced. As to O₃ absorbed by leaves, an oxidative burst occurs as the initial reaction to O₃, followed by activation of several signaling cascade and plant antioxidant systems including ascorbate-glutathione cycle and antioxidant enzymes to alleviate the oxidative burden resulting from O₃ exposure.

Remediation of VOCs

VOCs are organic chemicals that have a low boiling point and a high vapor pressure at room temperature causing large numbers of molecules to evaporate into the surrounding air. VOCs are numerous and ubiquitous including naturally occurring and anthropogenic chemical compounds. VOCs participate in atmospheric photochemical reactions contributing to O₃ formation and also play a role in formation of secondary organic aerosols, which are found in PMs. The strong odor emitted by many plants consists of green leaf volatiles, a subset of VOCs called biogenic VOCs, which emit exclusively from plant leaves, the stomata in particular. Major species of biogenic VOCs include isoprene, terpenes, and alkanes.

Anthropogenic VOCs include large groups of organic chemicals, such as formaldehyde, polycyclic aromatic hydrocarbons (PAHs), and BTX (benzenes, toluene, and xylenes). The most significant sources of formaldehyde are engineered wood products made of adhesives that contain urea-formaldehyde (UF) resins. BTX come from painting and coating materials used for interior decoration and refurbishment. Motor-vehicle exhausts, tobacco smoke, and heating also contribute to the presence of VOCs. A great concern over VOCs has been indoor air quality. Indoor formaldehyde in recently renovated homes ranged from 0.14 to 0.61 mg m⁻³, and benzene, toluene, and xylenes were 124.0, 258.9, and 189.7 μg m⁻³, respectively. The formaldehyde concentration is 65–100% higher than indoor air quality standards of China. Formaldehyde and BTX as main indoor VOCs contribute to the so-called “sick building syndrome”. This review regarding VOCs is thus emphasized on indoor air quality.

As early as in the 1970s, NASA (U.S. National Aeronautics and Space Administration) conducted research on the use of foliage plants for remediation of air quality in space shuttles. Foliage plants are those with attractive foliage and/or flowers that are able to survive and grow indoors. Results showed that foliage plants removed nearly 87% of air pollutants from sealed chambers within 24 h. For example, each plant of peace lily (Spathiphyllum spp. ‘Mauna Loa’) removed 16 mg of formaldehyde, 27 mg of trichloroethylen, and 41 mg of benzene from sealed chambers after a 24-h exposure to the respective chemical. Generally, plants absorb gaseous pollutants via leaf stomata. Some of the VOCs are recognized as xenobiotics by plants, and they are detoxified through xenobiotic metabolism, involving oxidoreductase or hydrolases, bioconjugation with sugars, amino acids, organic acids, or peptides, and then removed from the cytoplasm for deposition in vacuoles. In addition to plant leaves, rhizosphere microbes also contribute to reduction of VOCs under interior environments. Using a dynamic chamber technique, formaldehyde removal by potted foliage plants and found that formaldehyde removal was attributed not only to the formaldehyde dehydrogenase activities of plant leaves but also to the absorption and metabolism by microorganisms in the rhizosphere. Such bacteria have been isolated from soils, water, and different tissues of plants in polluted environments. Many pure cultures of bacteria, including various strains of P. putida, have been evaluated for biodegradation of air pollutants. Some fungi strains are also able to use volatile aromatic hydrocarbons as sole source of carbon and catalyze degradation reactions we mainly discuss phylloremediation of formaldehyde, benzene, toluene, and xylene as well as phenols and PAHS.

Formaldehyde

Formaldehyde is a colorless, flammable gas or liquid that has pungent and suffocating odor. It poses a significant danger to human health due to its high reactivity with proteins and DNA, thus formaldehyde is known to be a human carcinogen. Plants can directly absorb formaldehyde and transform it to organic acids, sugars or CO₂ and H₂O.

Exposed shoots of Chlorophytum comosum to 8.5 mg m⁻³ gaseous [¹⁴C]-formaldehyde over 24 h and found that about 88% of the recovered radioactivity was associated with plant metabolites as ¹⁴C, which had been incorporated into organic acids, amino acids, free sugars, lipids, and cell wall components. Formaldehyde responsive genes were identified from golden pothos (Epipremnum aureum). Glutathione (GSH)-dependent formaldehyde dehydrogenase (FADH) and formate dehydrogenase (FDH) can detoxify formaldehyde to formate and further to carbon dioxide. A wide range of foliage plants have been documented to be able to remove formaldehyde. 

86 species of foliage plants were exposed individually to 2 μl L⁻¹formaldehyde in sealed chambers and found that formaldehyde removed per cm² leaf area in 5 h ranged from 0.1 to 6.64 mg m⁻³, depending on plant species. The most efficient species in removal of formaldehyde include Osmunda japonica, Selaginella tamariscina, Davallia mariesii, and Polypodium formosanum. Surprisingly, these efficient plants belong to pteridophytes, commonly known as ferns and fern allies. Why this group of plants is more efficient than the other foliage plants in formaldehyde removal deserves further investigation.

Formaldehyde can also be assimilated as a carbon source by bacteria. Such assimilation occurs in Methylobacterium extorquens through the reactions of the serine cycle, in Bacillus methanolicus through the RuMP cycle, and in Pichia pastoris through the xylulose monophosphate cycle. Some fungi also assimilate formaldehyde. 

a fungal strain (Aspergillus sydowii HUA) was   isolated which was able to grow in the presence of formaldehyde up to 2,400 mg l⁻¹ and the specific activity of formaldehyde dehydrogenase and formate dehydrogenase were as high as 5.02 and 1.06 U mg⁻¹, respectively, suggesting that this fungal isolate could have great potential for removing formaldehyde. Some of the bacteria and fungi used to colonize roots can also colonize leaves and could be used for phylloremediation of formaldehyde in the air.

BTX

BTX refers to benzene, toluene, and three xylene isomers [ortho– (or o–), meta– (or m–), and para– (or p–)], which are major components of gasoline. Due to their low water solubility and acute toxicity and genotoxicity, BTX components have been classified as priority pollutants by the USEPA. Plants leaves can absorb BTX mainly through stomata, which are converted to phenol or pyrocatechol, and subsequently to muconic acid and fumaric acid. Foliage plants, such as Dracaena deremensis and Spathiphyllum spp. have been documented to remove BTX indoors.

73 plant species were  fumigated with 478.5 μg m⁻³ benzene gas and found that 23 of the 73 species showed inability to reduce fumigated benzene, the rest varied in benzene reduction, ranging from 0.1 to 80%. The most efficient plant species were Crassula portulacea, Hydrangea macrophylla, and Cymbidium ‘Golden Elf’. Foliage plants that are effective in removal of toluene include H. helix, Philodendron spp., Schefflera elegantisima, and Sansevieria spp. The wax of Sansevieria trifasciata and S. hyacinthoides is rich in hexadecanoic acid, which could pay an important role in absorption of toluene.

plant absorption of xylene was also evaluated. The tested 15 plant species were able to remove xylene with removal efficiency ranging from 59.1 to 88.2%, of which Zamioculcas zamiifolia was the most efficient species.

Bacteria including some strains of Rhodococcus rhodochrous, Alcaligenes xylosoxidans, and P. putida and also fungal cultures of Cladophialophora sp. are able to degrade BTX. Many Pseudomonas species are leaf colonists and some are plant pathogens. BTX are actual growth substrates for a number of organisms, such as P. putida. In a study of bioremediation of airborne toluene, it was found that the time required for 95% reduction of the initial toluene concentration of 339 mg m⁻³ was 75 h by Azalea indica plants along. Such reduction by the plants inoculated with P. putida TVA8 under the identical conditions was only 27 h. Subsequent additions of toluene further increased the removal efficiency of plants inoculated with the bacterial strain, but the toluene-removal rate was comparably low in plants without inoculation. Hence, inoculation of the leaf surface with P. putidaTVA8 was considered to be essential for rapid removal of toluene. These results clearly demonstrated the importance of both plant leaves and leaf-associated microbes in phylloremediation of indoor air pollutants. The genetics and biochemistry of strains F1 and mt-2 of P. putida have been intensively studied. Such information could be important for exploring these strains for effective removal of air pollutants.

Air Borne Phenols and Polycyclic Aromatic Hydrocarbon (PAHs)

Air borne phenols are a class of chemical compounds containing a hydroxyl group bonded directly to an aromatic hydrocarbon group, whereas PAHs are hydrocarbon comprising only carbon and hydrogen with multiple aromatic rings. Phenol and PAHs are major air pollutants in urban areas, and some PAHs have been considered carcinogenic. It has been reported that Bacillus cereus can degrade phenol via meta-cleavage pathway. Pseudomonas sp. CF600 can mineralize phenol on bean and maize leaves by dmp catabolic pathway.

Leaves were collected from trees growing in an area that was known to have high concentrations of VOCs. Unsterilized and surface-sterilized leaves were then exposed to radiolabeled phenol in closed chambers for 24 h and the amount of phenol degradation was compared. The phenol degradation by the non-sterilized leaves was significantly greater than the degradation by the sterilized leaves, indicating that degradation of VOCs was enhanced by the presence of the phyllosphere communities. This work indicates that plant leaves can accumulate phenols, which may be subsequently available for bacteria in the phyllosphere for degradation.

Plant leaves can absorb atmospheric PAHs. A study on deciduous forest in Southern Ontario, Canada, confirmed that amounts of phenanthrene, anthracene, and pyrene were reduced within and above the forest canopy during bud break in early spring. Plant species differ in removal of PAHs, the differences could be attributed to specific morphological and chemical constitutions of plants as well as leaf-associated microbes. Phyllosphere bacteria on 10 ornamental plant species were studied based on their diversity and activity toward the removal of PAHs. The phyllosphere hosted diverse bacterial species including Acinetobacter, Pseudomonas, Pseudoxanthomonas, Mycobacterium, and unculturable ones, of which PAH degrading bacteria accounted for about 1–10% of the total heterotrophic phyllosphere populations depending on plant species. The analysis of bacterial community structures using PCR and denaturing gradient gel electrophoresis showed that each plant species had distinct band patterns, suggesting that the bacterial communities are closely associated with leaf morphology and chemical characteristics of ornamental plant species. Furthermore, branches of fresh leaves of selected plant species were evaluated in sealed chambers for removal of a mixture of PAHs (acenaphthene, acenaphthylene, fluorene, and phenanthrene). Bacteria on unsterilized leaves of all tested plants showed an enhanced removal of phenanthrene. Bacteria on leaves of Wrightia religiosa in particular were able to reduce all the tested PAHs. Therefore, phyllosphere bacteria on ornamental plants may play an important role in natural attenuation of airborne PAHs and plant species differ in supporting microbes in PAH removal.

Development of Phylloremediation Technologies

This review has documented that plant leaves and leaf-associated microbes individually can reduce air pollution and the combination of the two generally exhibits enhanced remediation of air pollutants. Since air pollution never before has become such an urgent problem in countries like China and India, now is the time to seriously consider all options for reducing the pollutants. Phylloremediation is a natural and environmentally friendly way of bioremediation of air contaminants.

Developing phylloremediation technologies includes (1) selection and evaluation of appropriate plant species and microorganisms that are tolerant to pollution and able to remove one or more air pollutants; (2) testing and analysis of the compatibility of plant leaf surfaces with isolated microbes for synergetic interactions in reduction of pollutants in laboratories, in simulated indoor environments, and in outdoor settings; (3) analysis of experimental data and development of phylloremediation technologies; and (4) implementation of the technologies for remediation of air in both indoor and outdoor environments.

Plant Selection

Plants should be selected from four categories: (1) trees, (2) shrubs or small tress, and (3) ground cover plants for use in outdoor environments as well as (4) foliage plants for indoor environments. Trees are referred to as perennial plants with elongated stems or trunks, supporting branches and leaves. Shrubs (or small trees) are those small to medium-sized woody plants that grow under some degree of shaded conditions. Ground covers are any plants that can grow over an area of ground and they can grow below the shrub layer including turfgrass and other woody and herbaceous selections. Foliage plants are those which can grow and survive indoors for interior decoration.

Plant species not only differ greatly in adsorption, absorption, and assimilation of air pollutants but also vary significantly in pollution tolerance. Air pollution tolerance index has been used for evaluation of plants specie in response of pollutants. Information generated by the index is useful, but the index may require revision for better reflecting the ability of plants in tolerance of air pollutants. An initial large-scale evaluation of plants from the four categories should be conducted for identifying candidate species that are able to tolerate PMs, O₂, SO₂, NO_x, and VOCs individually or collectively and can also substantially retain or assimilate these pollutants. Plants should also tolerate abiotic stresses, such as drought, heat, and cold, and biotic stresses like plant pathogens. Leaves of plants should be able to support one or more selected microbes. Trees should have a relatively fast growth rate. Needle-leaved plants should be particularly considered. As mentioned before, needles are rich in waxes for capturing PMs, and they are also used as as passive bio-samplers to determine polybrominated diphenyl ethers. Broad-leaved plants should have more hairs or trichomes and more stomata with a large canopy. Leaf water and nutritional contents, leaf cuticular wax composition, hairs or trichomes, and surface physical characteristics should be suitable for microbial colonization. Shrubs and ground cover plants should have similar leaf physical and chemical properties but be able to tolerate slight shade. For foliage plants, they should substantially tolerate shade and can survive and grow under indoor low-light conditions.

Plant species possessing the aforementioned traits should be selected from particular regions where plants survive and thrive under heavily polluted environments. The rationale is that plants that are able to grow in the polluted environments may develop mechanisms for adaptation to the stressful conditions. Thus, some regions of China and India could be ideal locations for initial selection of plant species. Plants have been documented to tolerate multiple stresses, which include induced cross tolerances and the ability of particular variants to resist multiple distinct stresses. Reactive oxygen species are key molecular signals produced in response to multiple stresses, which are aimed at the maintenance of cellular equilibrium. Glutathione-S-transferase (GST) genes play an important role in the maintenance of ROS equilibrium. Salicylic acid, jasmonic acid, and ROS interplay in the transcriptional control of multiple stresses. Additionally, omics technologies should be used for identifying molecular mechanisms in regulation of plant responses to multiple stresses. Such information, particularly transcriptional factors, key regulatory genes or enzymes should be incorporated into the plant selection processes.

Genetic engineering is an option for improving plants to remediate air pollutants.

Cysteine synthase is a key enzyme to utilize H₂S and SO₂ as a sulfur source to synthesize cysteine. Overexpression of cysteine synthase in rice was shown to enhance sulfur assimilation upon exposure to a high level of H₂S. Nitrite reductase catalyzes the six-electron reduction of nitrite to ammonium. Transgenic Arabidopsis plants bearing chimeric spinach NiR gene enhanced nitrite reductase activity and NO₂ assimilation. Cytochrome P450 2E1 has strong and specific capacity of decomposing organic pollutants in animal bodies. Transgenic tobacco plants overexpressing CYP2E1 gene showed increased ability to detoxify broad classes of pollutants such as chlorinated solvents and aromatic hydrocarbons. Unlike tobacco, poplar (Populus tremula × Populus alba) plants are a fast-growing tree species with large canopies. Poplar plants overexpressing a mammal CYP2E1 exhibited increased metabolism and enhanced removal of organic pollutants from hydroponic solution and the air. Some genes from microbes can also be used for engineering transgenic plants for phylloremediation. The ribulose monophosphate (RuMP) pathway is one of the formaldehyde-fixation pathways found in microorganisms. The key enzymes of this pathway are 3-hexulose-6-phosphate synthase (HPS), which fixes formaldehyde to D-ribulose 5-phosphate (Ru5P) to produce D-arabino-3-hexulose 6-phosphate (Hu6P) and 6-phospho-3-hexuloisomerase (PHI), and then converts Hu6P to fructose 6-phosphate (F6P). Co-expression of HPS and PHI in tobacco plants resulted in 20% reduction of formaldehyde compared to the control plants. In another study, a chlorocatechol 1,2-dioxygenase gene (tfdC) derived from the bacteria Plesiomonas was introduced into Arabidopsis thaliana. Transgenic plants showed enhanced tolerances to catechol, an aromatic ring. Transgenic plants were also able to remove a large amount of catechol from their media and highly efficient in convertion of catechol to cis, cis-muconic acid, suggesting that degradative genes derived from microbes can be used to produce transgenic plants for bioremediation of aromatic pollutants in the environment.

Selected plants should be evaluated in controlled environmental chambers to measure their capacity for tolerance and also assimilation of air pollutants. Seedlings could be exposed to particular pollutants or a mixture of pollutants in different concentrations and durations. Plant responses to the exposures could quickly evaluated based on stomatal conductance, net photosynthetic rate, the maximum quantum efficiency of photosystem II using the new LI-COR6800. Their morphological appearance, i.e., leaf greenness, leaf size, and plant height and canopy dimension compared to control treatments should be evaluated. The ability of plants to remove pollutants should be tested using GC-MS. For evaluation of plant responses to PM, in addition to the mentioned plant characteristics, leaf morphology, particularly leaf surface characters should be examined under microscopes and stomatal size and density recorded. If needed, isotopic labeling techniques could be used to track the fate of particular compounds. The evaluation results once analyzed and compared, plants that tolerate stresses and are able to adsorb or absorb or assimilate pollutants could be identified from each type of plants for subsequent compatiablity tests with selected microbes.

Microbe Selection

Cultivable bacteria only account for a small fraction of the total diversity in the phyllosphere, which has greatly hampered the use of some valuable microbes. New approaches, such as the use of improved culture and advanced devices (i-Chip), co-culture with other bacteria, recreating the environment in the laboratory, and combining these approaches with microcultivation should be employed to convert more uncultivable bacteria into cultured isolates in the laboratory.

 Similar to plant selection, initial microbial selection could be carried out in areas where plants have been contaminated by air pollutants. In coordination with plant selection, microbes could be isolated from leaves of plants identified in plant selection. This is because the pollutants may exert selective pressures to phyllosphere microbial diversity. For example, bacterial communities hosted by Platanus × acerifolia leaves from different locations of Milan (Italy) were analyzed by high throughput sequencing. The results showed that biodiversity of bacterial communities decreased but hydrocarbon-degrading populations increased along the growing season, which suggest that air contaminants might play an important role in the selection of phyllospheric populations in urban areas.

A particular attention should be given to endophytic microbes. There are about 300,000 plant species on the earth; each plant could host one or more endophytes. Endophytes are resided inside plant tissues and generally have no harmful effects on plants. Endophytic bacteria that colonize leaves could be particularly desirable as they could not be washed away by precipitation. Recent advances in endophyte-assisted remediation have been reviewed. Endophytic B. cereus ZQN5 isolated from natural Zamioculcas zamiifolia leaves enhanced ethylbenzene removal rate on sterile Z. zamiifolia . Microbes could also be isolated from the rhizosphere of plants contaminated by air pollutants as more endophytism occurs in roots. Some of leaf endophytes could be initially established in roots and subsequently transported to shoots. 

reported that some microbes isolated from roots can also colonize leaf surfaces. An endophytic strain of B. cereus ERBP from roots of Clitoria ternatea was able to colonize the leaf surface of Z. zamifolia. During a 20-d fumigation with formaldehyde, the inoculation of ERBP did not interfere with the natural shoot endophytic community of Z. zamiifolia. ERBP inoculated Z. zamiifolia exhibited a significantly higher formaldehyde removal efficiency when compared to the non-inoculated plants.

Microbes, once identified and cultured, could be engineered to improve phylloremediation capacity. A pTOM toluene-degradation plasmid from B. cepacia G4 was introduced into Bacillus cepacia L.S.2.4, a natural endophyte from yellow lupine (Lupinus arboreus). After the engineered bacteria were inoculated into aseptic lupine seedlings, the recombinant endophytics degraded 50–70% more toluene and provided much more protection against the phytotoxic effects of toluene than that obtained from soil bacteria. Horizontal genes can transfer among plant-associated endophytic bacteria in plants. Poplar was inoculated with the yellow lupine endophyte B. cepacia VM1468, which contains the pTOM-Bu61 plasmid coding for constitutively expressed toluene degradation. Inoculated plant growth was enhanced in the presence of toluene, and the amount of toluene release via evapotranspiration was also reduced. Although no inoculated strains were detected in the endophytic community, there was horizontal gene transfer of pTOM-Bu61 to different members of the endogenous endophytic community. The TCE-degrading strain P. putidaW619-TCE also can be engineered via horizontal gene transfer in poplar plants.

Efforts on microbe selection should also be placed on the identification of microbes that could remediate PM, SO₂, NO₂, and O₃. As mentioned above, a group of microbes can assimilate SO₂ and NO₂, further research should explore those microbes for effective assimilation of the two pollutants. Thus far, it appears that no information is available regarding microbial remediation of PM and O₃, which may not be the case in the nature. Extensive research should be conducted to determine if nature has offered microbes that can break down PMs and can also biodegrade or biotransform O₃.

Selected microbes could be domesticated by growing them in different cultures varying in pH, carbon source, temperature, and O₂ to identify appropriate culture media and conditions for maximizing their growth. Morphological characterization and internal transcribed spacer rDNA analysis should be conducted to determine their phylogenetic relationships with other microbes. Their ability to biodegrade particular or a group of air pollutants should be evaluated in the laboratory. Microbial characteristics including their utilization of organic compounds, decomposition rate of pollutants, adaptability, competition, and growth rate should be recorded and analyzed. Competitive strains that show promise in bioremediation should be identified. A series of bacterial and filamentous fungal genomes have been sequenced recently. More than hundreds of bacterial and fungal transcriptomic and proteomic datasets are available. With the advent of increasingly sophisticated bioinformatics and genetic manipulation tools, mechanisms underlying the biodegradation or transformation of pollutants by the isolated microbes could be elucidated. This information, in turn, will significantly improve our understanding of the microbes and provide us with molecular bases for manipulation of the microbes for enhancing phylloremediation.

Evaluation of the Compatibility between Plant Leaves and Microbes

Plants selected from the four categories should be inoculated with selected microbes to determine the compatibility of each selected microbe with each selected plant species. The test could begin first in laboratory settings using entire leaves in designated chambers or utilizing young seedlings in relative large growth chambers to evaluate if inoculated microbes could grow on leaf surfaces and if the specific inoculation affects plant growth. Compatible combinations would be exposed to pollutants at different concentrations and durations to determine the potential for pollutant reduction. A microbe that is compatible with one plant species may not be compatible with another. For example, B. cereus ERBP isolated from roots of C. ternatea was compatible with the leaf surface of Z. zamifolia but not with the leaf surface of Euphorbia milii. ERBP-colonized Z. zamifolia grew well and showed high efficiency in removal of formaldehyde, but ERBP-colonized E. milii were less effective in removal formaldehyde and the plants exhibited stress symptom. Laboratory evaluation will generate a large number of plant-microbe combinations that are specifically effective in removal of a particular pollutant or a particular group of pollutants. Bacteria would be propagated using bioreactors and corresponding plants would be propagated through either cuttings or tissue culture. The plants would be transplanted into greenhouses or specific regions with air pollution for testing the effectiveness of the combinations in real-world situations.

Plants and microbe combinations that pass the real-world test will be investigated using the next-generation sequencing (NGS) technologies (metagenomics, metatranscriptomics, metaproteomics, and metabolomics) and the rapid evolution of SIP (Stable isotope probing) for identifying molecular mechanisms underlying microbial and plant interactions in facilitation of phylloremediation. The compatibility evaluation and molecular analysis would ultimately result in the development of protocols for culturing microbes and producing corresponding plants. Some protocols will be catered to trees, others used for shrubs or small trees. Some would be effective for improving groundcover plants, and some will be used for indoor foliage plants.

If the test is to be conducted in a large scale, satellite image acquisition and analysis should be used. The analysis of the data will finally validate the protocols, i.e., particular plants can be inoculated with a specific group of microbes for use in remediation of a particular pollutant or a mixture of pollutants.

Implementation of Phylloremediation Technologies

The protocols will be implemented for phylloremediation. We propose three types of plantscape: (1) manufactory plantscape, (2) urban plantscape, and (3) interior plantscape. The plantscape for manufactories and cities should have three levels of greening: the sky with trees, the ground with groundcover plants, and shrubs in between. Additionally, climber plants can be used to build green walls and small trees and shrubs as well as groundcovers can be used to build green roofs. For interior plantscape, each room should have a minimum of one potted foliage plant. Foliage plants can also be used to install green walls in interior environments for enhance remediation of indoor air pollutants.

The implementation of phylloremediation technologies should also take landscape design concepts into consideration, resulting greenbelts, green parks, green walls that fulfill roles not only for air remediation but also for recreation. Depending on the occurrence of pollutants and the scale and degree of the overall pollution, relevant protocols to the particular situations would be implemented. The remediation efficiency could be monitored over time using specific models in connection with satellite imagine data to determine how much of individual pollutants have been removed.

Conclusion

Air pollution is real, and it is adversely affecting human comfort and health and jeopardizing the ecosystem. The causes are multidimensional including increased population, urbanization, and industrialization accompanied with increased energy consumption and economic growth along with weak regulation, deforestation, and climate change. A recent article published suggested that circulation changes including the weakening of the East Asia winter monsoon induced by global greenhouse gas emission contribute to the increased frequency and persistence of the haze weather conditions in Beijing, China. This claim could be true. The fact is that air pollutants released anthropogenically has caused the global warming. Our attention nevertheless should focus on how to control the emissions and how to remediate the pollutants. Although rhizosphere (roots and root associated microbes) contributes greatly to remediation of air pollutants, in this review, we specifically discuss phylloremediation. The role of plant leaves and leaf-associated microbes in remediation of air pollutants has not been well explored.

Using the Urban Forest Effects Model,  the influence of the urban forest on air quality was studied in Beijing, China and found that the 2.4 million trees in the central part of Beijing removed 1,261.4 tons of pollutants from the air in 2002, of which 720 tons were PM.

We believe that phylloremediation is an environmentally friendly, cost effective way of remediation of air pollutants. The key component of this technology lies in plants. It is plants that can adsorb or absorb pollutants and plants that support microbes in biodegradation or biotransformation of pollutants. To develop phylloremediation technologies, some basic questions should be addressed: (1) Anatomical, physiological, biochemical and molecular mechanisms underlying plant responses to each pollutant should be investigated. Previous research has documented plant responses to pollutants such as NO_x, SO₂, O₃, and VOCs, but the research was largely intended to identify how plants were injured. We need to exploit why many plants are tolerant to the pollutants, what are the underling mechanisms, and how can we manipulate the mechanisms for increased tolerance and for use in phylloremediation. There is little information regarding plant responses to PM. Do plants simply adsorb PM? What are the fates of stomatal absorbed PM? (2) Phyllosphere microbes are still largely a mystery and many are not culturable. Methods for collection, identification, and cultivation should be developed. Some microbes isolated from the rhizosphere can also be used for leaf colonization. Mechanisms for biodegradation and transformation of pollutants have been mentioned in this review. However, we still do not know if there are microbes that can remediate PM and O₃. An important question that should be immediately addressed is the roles of microbes within the PM.

Do the microbes become active once settled on leaves?

Do they have the ability to break down the PM?

With the advances of omics, these questions will be answered, and new strains with high efficiency in breaking down pollutants are expected to be isolated and utilized.

A large scale and intensive test for the compatibility among identified plants and identified microbes should be carried out. Specific plant-microbe groups or combinations that can effectively reduce one or more pollutants should be identified, tested, and confirmed in real-world situations and corresponding protocols for using each combination should developed.

(4) New methods for analyzing dynamic changes of air pollutants in the atmosphere should be developed and standardized for monitoring the effectiveness of the phyllosphere technologies.

(5) Research and development of phyllosphere technologies is a multidisciplinary project requiring collaboration among researchers with different academic backgrounds at regional, national, and international levels. Nature has offered healthy alternatives for remediation of air pollution; we should collaborate with nature as a partner to restore nature's identity.

 

 Air (Prevention and Control of Pollution) Act

With development and industrialization, environmental preservation has become a serious concern. The rising power, India, has been facing the environmental pollution due to rapid development and lack of proper implementation of environmental pollution control standards. Environment is directly related with article 21 of Constitution of India which deals with right to life of individual. The two main laws that regulate air pollution in India:  The Air (Prevention and Control of Pollution) Act, 1981 (Air Act) and Environment (Protection) Act, 1986 (EPA). This article is primarily concerned with critical study of provisions under these two acts.

Introduction

The term “clean air” means air which is clean, unpolluted and neither harmful for humans nor harmful to the surroundings where we live. To maintain this, various countries in the world have their own laws. A state cannot ignore environment and only concentrate on economic growth. India is having worst environment among 132 countries according to Environment Performance Index. There are several factors degrading air quality of India. Today, it has been a challenge to maintain clean air in most developed countries like China etc. Smog has been common in big cities. In article 48A of the Directive Principles of Constitution of India it has declared that the state endeavors to protect and improve the environment and to safeguard the forest and wildlife of the country whereas article 51A imposes similar fundamental duty to citizens as well. Hence, it is necessary to have good laws and mechanisms.

Brief Summary of Legislations Related to Clean Air

Air (Prevention and Control of Pollution) Act, 1981

The objective of the Air Act 1981 is to preserve the quality of air and control of air pollution.Chapter 3 of this act deals with powers and functions of boards.There are two boards namely Central Board and State Boards. Some of their important functions are to improve the quality of air and to prevent, control or abate air pollution in the country, to advise the Government on any matter concerning the improvement of the quality of air and the prevention, control or abatement of air pollution, to plan and executed a program for the prevention, control or abatement of air pollution, to collect, compile and publish technical and statistical data relating to air pollution and the measures devised for its effective prevention, control or abatement and prepare manuals, codes or guides relating to prevention, control or abatement of air pollution, to lay down standards for the quality of air, to inspect, at all reasonable times, any control equipment, industrial plant or manufacturing process and to give, by order, such directions to such persons as it may consider necessary to take steps for the prevention, control or abatement of air pollution, to inspect air pollution control areas at such intervals as it may think necessary, assess the quality of air therein and take steps for the prevention, control or abatement of air pollution in such areas.The Central Board and State Board work in collaboration of each other. The Central works throughout the nation whereas State Boards work within its state.Likewise, chapter four states about the prevention and control of air pollution. State Government after consultation with State Board can declare any area or areas within the State as air pollution control area or areas for the purposes of this Act, can alter any air pollution control area whether by way of extension or reduction, can declare a new air pollution control area in which may be merged one or more existing air pollution control areas or any part or parts thereof.

Environment Protection Act, 1986

The Environment Protection Act came in 1986. Prior to this act, there was Department of Environment which was established in 1980 in India. In 1985, it converted into Ministry of Environment and Forests. Similarly, The Air (Prevention and Control of Pollution) Act came before this act in 1981. The objective of this act is to take appropriate steps for the protection and improvement of environment and prevention of hazards to human beings, other living creatures, plants and properties.This act has defined “environment pollution” as the presence of any environmental pollutant in the environment and “environment pollutant” as any solid, liquid or gaseous substance present in such concentration as may be, or tend to be injurious to environment. Similarly, chapter two deals with general power of Central government. Central Government shall have power to take all such steps it thinks necessary for the preserving and improving the quality of the environment and preventing and controlling environmental pollution, to prohibit and restrict on the handling of hazardous substance in different areas, to prohibit and restrict on the location of industries and the carrying on of the process and operations in different areas, to carry out and sponsor investigations and research relating to problems of environmental pollution, to safeguard for the prevention of accidents which may cause environmental pollution and for providing for re-medical measures for such accidents etc.

Besides, third chapter talks about the ways of prevention, control and abatement environmental control. It prohibits any person to carry on any industry operation or process shall discharge or emit or permit to be discharged or emitted any environmental pollutant in excess of such standards as may be prescribed and to handle or cause to be handled any hazardous substance expert in accordance with such procedure and after complying with such safeguards may be prescribed. Whoever fails to comply or contravenes will be punished with five years imprisonment or with fine which may extend to one lakh rupees, or both, and in the case of failure or if contravention continues, with additional fine which may extend to five thousand rupees for every day during which such failure or contravention continues after the conviction for the first such failure or contravention. Finally, if it continues more than a year from the date of conviction shall be punishable with imprisonment for the term which may extend to seven years.

Again, if a company commits any offense under this act, every person such as director, manager secretary or another officer of the company who at the time offence was committed, was directly in charge of and was responsible to the company for the conduct of the business of the company, as well as company shall be deemed to guilty of the offence and shall be liable to be proceeded against and punished accordingly.

 

Brief Summary of Some Landmark Cases

Bhopal Disaster Case

On December 3, 1984, the worst industrial accident in history occurred. Around 40 tons of Methyl Isocyanate (MIC) gas mixed with other poisonous gasses from a chemical plant which is owned and operated by Union Carbide (India) Limited. At least 3,800 people were killed and several were injured in this incident. This incident caused victims throats and eyes to burn, induced nausea because the gases remained low to the ground. Those who were exposed to such toxic gas gave birth to physically and mentally disabled baby even after 30 years.

The Union Carbide Corporation paid a sum of U.S. Dollars 470 millions for full settlement of all claims, rights and liabilities related to and arising out of the Bhopal Gas disaster to the Union of India. The principle of absolute liability was used by the Supreme Court made the Union Carbide Corporation pay compensation.It is relatively small in comparison to the offence which has long term effect in the human existence of that place. Even after this disaster, there has been rapid industrialization in India. While some affirmative changes in policy of government and conducts of a few industries have taken place, there still remain major threats to the environment from rapid and poorly regulated industrial growth. Due to widespread environmental degradation, adverse effect in human health consequences continues to happen all over India.

 

MC Mehta ( Taj Trapezium Matter) V. Union of India

Huge numbers of industries were around Taj Mahal. The main responsible factors for polluting the ambient air around Taj Mahal are industrial/refinery emissions, brick-kilns, vehicular traffic and generator-sets. The petition states that the color of marble has converted from whitish to yellowish and blackish. On 30th of December 1996 and the bench consisted of Justice Kuldip Singh and Justice Faizan Uddin gave the final verdict in this case. Taj Mahal, which is one of the world heritage sites as declared by UNESCO, has been source of revenue to the country because it has capacity to attract tourist throughout the world. The court was of the view that The Taj Mahal is a masterpiece and has international reputation. It is also an important source of revenue to the country because of the huge tourist attraction it commanded. So, there won’t be compromise regarding its beauty. The industries were supposed to relocate far from Taj Trapezium.

Principles laid down in this case are-

Sustainable development– Development of industry is essential for economy but at the same time environment has to be protected. Hence, the object behind this litigation is to stop the pollution.

Precautionary principle– the pollution created as an outcome of development so the state must anticipate, prevent and attack the harm caused to the environment.

Polluter pays principle– the court interpreted the principle in order to mean that the absolute liability to harm the environment is not only to compensate the victims of pollution but also for restoring the cost of environmental degradation.

Government Officials Announces Delhi Air Pollution an Emergency in India

The Indian Government has announced emergency situation and temporarily shut down construction sites, schools and a coal-fired power station due to severe levels of toxic air pollutant in Delhi.

A Delhi-based NGO “The Centre for Science and Environment” has said the Indian capital had seen the worst air quality in 17 years.

Similarly, Delhi Government has told farmers not to burn agricultural wastages. Depending on the last digit of their registration numbers The Delhi government is preparing to reintroduce a temporary scheme to only allow cars to drive on odd or even days.The patient suffering from respiratory diseases have increased in the hospitals in the city. According to World Health Organization’s report of 2012 out of 100,000, 159 died with respiratory disease which shows India has the highest rate in the world.

Analysis and Conclusion

The concept of sustainable development came up with challenging the concept of rapid development. For example, if anyone cuts a tree then he/she has to plant two or more trees. The notion of sustainable development rose up with the idea of preservation of environment. The development should be done in such a way that it will last for a long time and the future generation won’t get into problems. But the situation is contrary in the case of India. The pace of development is very fast. But it is failing to maintain clean air. There are acts, case laws, regulatory bodies and so on but still the situation of air is getting worse and worse. Lots of people are dying due to respiratory diseases and lung cancer. Especially in the city, where there is large population and where people from different rural parts of India come to seek facilities, are highly polluted. The life expectancy of people in India might go below then it is today. There are legislations like The Environment Protection Act 1986 and The Air Prevention and Control Act 1981 which have mentioned about preventive measures, regulatory board, punishment and compensation and the precedent established in the Bhopal Disaster Case and MC Meheta V Union of India, air pollution hasn’t been reduced but has increased which has been proved by air pollution faced by capital city Delhi recently.  It has already been three decades of these above laws which have come into existence but there is no improvement seen in the air and environment as a whole. From this point, it is clear that either there is problem in law itself or in the part of implementation. And the problem is in both laws and implementation. Laws overwhelmingly give discretion to make plans, investigate and research to the boards. It specifically does not address issues like removal of old vehicles, plantation of trees side by the road, dust management, stoppage of burning wastages etc. Therefore, the officials are silent and passive. They do not conduct any research, make plan and investigate to the issues. If the laws were clear regarding the respective issues then they would have been compelled to take action against such activities.

Similarly, if the officials had made planned to establish industrial area far from human residence, world heritage sites and cities then there won’t be problem. Due to lack of plan, at first the industry pollutes the environment, latter the case is filed and the court deliver verdict to relocate the industries in the case like MC Meheta V Union of India. Here seems problem in the implementation part.

Under the authority of the Air (Prevention and Control of Pollution) Act of 1981, India’s Central Pollution Control Board sets national ambient air quality standards and is responsible for both testing air quality and assisting governments in planning to meet such standards. State Pollution Control Boards are permitted to set stricter standards than those in effect nationally.

 

 

 

 

 

HISTORY

Interest in air quality management policies began in India during the 1970s. After the 1972 Stockholm Conference on the Human Environment, it became clear that the nation was in need of a uniform environmental law. As a result, the Air (Prevention and Control of Pollution) Act was passed by Parliament in 1981. With the goal of providing for the prevention, control, and abatement of air pollution, the first ambient air quality standards were adopted in 1982 by the Central Pollution Control Board (CPCB) and revised in 1994 and again in 2009.

Agencies responsible for air quality standard creation and monitoring include CPCB and several State Pollution Control Boards (SPCBs). All of these entities fall under the control of the Ministry of Environment and Forest (MoEF). The CPCB, working together with the SPCBs, provides technical advice to MoEF in order to fulfill the objectives outlined in the Air Act of 1981.

The Air Act mandates the CPCB and SPCBs to:

1.   Establish national ambient air quality standards for criteria pollutants,

2.   Assist government in planning future environmental prevention and control strategies,

3.   Carry out research to better understand environmental issues,

4.   Undertake nationwide air sampling to ascertain the ambient air quality in India and identification of the problem areas,

5.   Conduct air quality inspections in areas of concern.1

SPCBs can set more stringent standards than the existing national standards in their respective states. Such a process is similar to the local divisions used within the US EPA.

Another legal document, the Environment (Protection) Act of 1986 , does not specifically mention fuels or emission standards, but does authorize central and state governments to regulate activities that can harm the environment. Current ambient air quality standards are based on the authority of the Environment Act, 1986.

TECHNICAL STANDARDS

NATIONAL AMBIENT AIR QUALITY STANDARDS

National Ambient Air Quality Standards, as of 2009
Pollutant	Time Weighted Average	Concentration in Ambient Air
		Industrial, Residential, Rural and Other Area	Ecologically Sensitive Area (notified by Central Government)	Methods of Measurement
SO2, μg/m3	Annual*	50	20	·         Improved West and Gaeke ·         Ultraviolet fluorescence
	24 hours**	80	80
NO2, μg/m3	Annual*	40	30	·         Modified Jacob & Hochheiser (Na-Arsenite) ·         Chemiluminescence
	24 hours**	80	80
PM10, μg/m3	Annual*	60	60	·         Gravimetric ·         TOEM ·         Beta attenuation
	24 hours**	100	100
PM2.5, μg/m3	Annual*	40	40	·         Gravimetric ·         TOEM ·         Beta attenuation
	24 hours**	60	60
O3, μg/m3	8 hours**	100	100	·         UV photometric ·         Chemiluminescence ·         Chemical Method
	1 hour**	180	180
Lead (Pb), μg/m3	Annual*	0.50	0.50	·         AAS/ICP method after sampling on EMP 2000 or equivalent filter paper ·         ED-XRF using Teflon filter
	24 hours**	1	1
CO, mg/m3	8 hours**	2	2	·         Non Dispersive Infra Red (NDIR) spectrosopy
	1 hour**	4	4
Ammonia (NH3) μg/m3	Annual*	100	100	·         Chemiluminescence ·         Indophenol blue method
	24 hours**	400	400
Benzene	Annual*	5	5	·         Gas chromatography based on continuous analyzer ·         Adsorption and Desorption followed by GC analysis
Benzopyrene (BaP) – particulate phase only, ng/m3	Annual*	1	1	·         Solvent extraction followed by HPLC/GC analysis
Arsenic (As), ng/m3	Annual*	6	6	·         AAS/ICP method after sampling on EMP 2000 or equivalent filter paper
Nickel (Ni), ng/m3	Annual*	20	20	·         AAS/ICP method after sampling on EMP 2000 or equivalent filter paper
* Annual arithmetic mean of minimum 104 measurements in a year at a particular site taken twice a week 24 hourly at uniform intervals. ** 24 hourly or 8 hourly or 1 hourly monitored values, as applicable, shall be compiled with 98% of the time in a year. 2% of the time, theymay exceed the limits but not on two consecutive days of monitoring. Note – Whenever and wherever monitoring results on two consecutive days of monitoring exceed the limits specified above for the respective category, it shall be considered adequate reason to institute regular or continuous monitoring and further investigation.

 

MONITORING

The National Air Monitoring Programme (NAMP) is a nation-wide program headed by the Central Pollution Control Board whose purpose is to monitor levels of key air pollutants, report violations, and conduct research on pollution trends. NAMP monitors levels of SO₂, NO₂, Suspended Particulate Matter (SPM), and Respirable Suspended Particulate Matter (RSPM / PM₁₀) at 342 operating stations in 127 cities across India. The NAMP publishes a list of cities that violate air quality standards, which can be found here.

On 29 November 2011, MoEP announced the expansion of monitoring to include PM_2.5 at select locations in major cities.

 

India takes steps to curb air pollution

India’s air pollution problem needs to be tackled systematically, taking an all-of-government approach, to reduce the huge burden of associated ill-health. Patralekha Chatterjee reports.

Bulletin of the World Health Organization 2016;94:487-488. doi: http://dx.doi.org/10.2471/BLT.16.020716

Traffic congestion in Delhi, India.

Courtesy of Dinesh Mohan

Nine-year-old Neil suffers from asthma. When he is sick – with wheezing, breathing problems or sleeplessness – he misses many of his favourite activities.

“He’d like to be out playing more, doing the things children love,” says his mother, lawyer Leena Menghaney, who also has asthma. “Some months he misses as much as seven or eight days of school.”

The Menghaney family lives in the middle-class neighbourhood of Indirapuram in Ghaziabad, a city of 2.3 million inhabitants that flanks the Indian capital of Delhi.

Air pollution is a major risk factor for heart disease, stroke, chronic obstructive pulmonary disease (umbrella term for several progressive lung diseases including emphysema) and lung cancer, and increases the risks for acute respiratory infections and exacerbates asthma.

With the economy booming in many of India’s cities since the turn of this century the number of road vehicles and dusty construction sites have multiplied, and outdoor air pollution has become a major health hazard and a major killer.

This adds to the already large burden of ill-health caused by household air pollution from the use of solid fuels for cooking in the world’s second most populous country of some 1.3 billion people.

In India, an estimated 1.5 million people died from the effects of air pollution in 2012, according to WHO data. Globally, air pollution – both indoor and outdoor – caused nearly 7 million deaths, or 11.6% of deaths in 2012, making it the world’s largest single environmental health risk, according to World health statistics 2016.

About 98% of cities in low- and middle-income countries with more than 100 000 inhabitants do not meet norms set out in the World Health Organization’s (WHO) air quality guidelines, according to WHO’s global urban air quality database.

An increasing number of Indian cities are now measuring and reporting their air pollution levels to WHO and the number of such cities, globally, has nearly doubled to 3000 in 103 countries since 2014.

Reducing the deaths and ill-health from air pollution is one of the targets of sustainable development goal three and, last year at the United Nations climate change conference in Paris, governments recognized the need to curb emissions to reduce global warming.

The sources of India’s air pollution are many: indoor cook stoves, road traffic – including the ubiquitous auto-rickshaws that use a toxic mix of kerosene and diesel – industrial plants that burn fossil fuels and open burning of waste.

“We see the acute effects of air pollution, especially in young children and the elderly, and in people suffering from chronic obstructive pulmonary disease and heart disease,” says Dr Randeep Guleria, head of the Department of Pulmonology and Sleep Disorders at the All India Institute of Medical Sciences in Delhi.

Chronic obstructive pulmonary disease is a set of lung diseases that prevent normal breathing and can, eventually, be fatal.

“Exposure to high levels of pollutants affects lung capacity and predisposes children to respiratory problems in later life,” Guleria says, adding: “When the air pollution levels go up, the patients’ underlying disease worsens, and emergency visits to hospital and the need for medication go up dramatically too.”

Last year, the Steering Committee on Air Pollution and Health-Related Issues, set up by India’s federal Ministry of Health and Family Welfare, submitted a report to the federal government on the devastating effects of air pollution on people’s health in India.

It proposed measures that committee members argued would provide the largest reduction in exposure to air pollution and, as a result, improvements in people’s health.

For K Srinath Reddy, who co-chaired the committee, the report is important because it highlights the contribution of air pollution to the rise in cardiovascular diseases in his country.

“It was the first time that an official report in India examined air pollution as a health rather than an environmental issue,” says Reddy, who is also the president of the Public Health Foundation of India.

Since the sources of air pollution were so diverse, the committee proposed “a concerted and coordinated effort across the government” with the involvement of a dozen other ministries, including finance, agriculture, rural development, power and transport.

Proposals included switching to clean energy sources for cook stoves, public transport and industry, as well as measures to reduce road traffic by raising fuel taxes and parking fees, levying congestion charges, and creating vehicle-free zones and cycle paths.

“The tragedy is that there are perfectly feasible solutions to the air pollution problem, but these are surrounded by myths,” says Veerabhadran Ramanathan, professor of Atmospheric and Climate Sciences at the Scripps Institution of Oceanography at the University of California, San Diego.

For example the myth that tackling air pollution is expensive. “In California we found that if you clean up the air, each dollar invested in air pollution returned nearly US$ 30 to [the state of] California.

“There were huge health benefits along with a large increase in new jobs and thus in people’s well-being,” Ramanathan says.

In 2013, Ramanathan teamed up with the Air Resources Board, the clean air agency in California, and the Energy and Resources Institute, an Indian research agency, to initiate the India–California Air Pollution Mitigation Program.

Open burning in a street in Mumbai, India.

WHO /Diego Rodriguez

They compiled a report and issued 12 recommendations on how to reduce air pollution from the transportation sector in India. The findings called for a systematic approach across the country.

“You can’t tackle air pollution by just cleaning up locally. Delhi is a perfect example: switching to compressed natural gas vehicles helped temporarily in the 1990s, but Delhi still ranks among the world’s most polluted cities, just like Los Angeles in the 1960s,” Ramanathan says.

For Ramanathan, three systematic solutions are required for maximum impact. One, replace existing cook stoves with clean cook stoves, two, reduce pollution from diesel transport and, three, restrict open burning of biomass and fossil fuels.

Meanwhile, liquid petroleum gas and electricity, along with biogas and ethanol are some of the clean energy alternatives.

“India could cut its total air pollution by one third overnight by giving clean cooking stoves to all its villagers,” Ramanathan says. “When California wanted to pass its air pollution laws, there was tremendous resistance from industry.

“They said ‘it will destroy our economy, no one will come to California’, and the then president of the United States [of America] Lyndon B Johnson had to give California special permission to enact stricter laws than the rest of the country through congressional approval,” he says.

“When trucks from outside California came to California, they had to abide by California’s laws. Everything I see in Delhi today happened in California in the 1960s.

“That is why we – in the India–California Air Pollution Mitigation Program – looked at both the technical solutions, such as cleaning up your cars, and also structural solutions such as having proper regulatory bodies and proper monitoring,” Ramanathan says.

Perhaps India's capital city has the advantage of having many nongovernmental organizations (NGO) campaigning for better health, a vocal media which reports extensively on health problems caused by air pollution, and a supreme court that recently banned the registration of diesel vehicles in the capital.

More has been done in Delhi than elsewhere in India to tackle the problem. The auto-rickshaws run on compressed natural gas and, earlier this year, the state government piloted a congestion scheme to reduce the volume of traffic, in which vehicles with odd and even number plates could enter the city on alternate days.

Other recent measures in the capital include tighter vehicle emissions’ norms, higher penalties for burning rubbish and better control of road dust.

But while Delhi’s air quality has improved slightly, according to the WHO air quality database, air quality levels in smaller cities, such as Ghaziabad, where the Menghaney family live, have severely deteriorated in recent years, according to Indian NGO, the Centre for Science and Environment.

Public health advocates and clean air campaigners are keen to see action beyond Delhi.

Recently the Indian government took some steps in this direction committing to a 50% reduction in households using solid fuel for cooking and, last December, removing subsidies for polluting cooking gas to improve access to clean fuel for household cooking.

India recently included an additional target on reducing air pollution to the nine targets set out in WHO’s Global action plan for the prevention and control of NCDs 2013–2020 in its national NCDs strategy.

For Dr Kalpana Balakrishnan who heads the WHO Collaborating Centre for Occupational and Environmental Health at the Center for Advanced Research on Environmental Health in Chennai, such moves are thanks to a growing recognition of the double burden of outdoor and household air pollution for urban and rural populations.

“Recent efforts are an important first step in this direction,” says Balakrishnan.