Air Pollution – Definition, Causes, Effects and Control

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Air Pollutants – Environmental and Health Impacts of Air Pollution

When due to human activities or some natural processes the amount of solid wastes or concentration of gases other than oxygen, increases in the air more than the normal percentage of different gases, the air is said to be polluted and this phenomenon is regarded as Air pollution.

Air pollution means the presence in the atmosphere, or injection into it, of substances that are not present naturally or, present naturally but are in much lesser concentrations, and that may harm living organisms directly (Allaby, 1995).

Standard or Normal Components of Air

• Nitrogen: 78.11%
• Oxygen: 20.6%
• Carbon dioxide: 0.03% – 0.04%
• Water vapour, Helium, Neon, Argon, Ozone, Methane, Hydrogen: 0.96% – 0.97%

The components may vary in industrial areas and metropolitan cities.

Sources of Air Pollution

Various gaseous emissions are responsible for air pollution which are injected continually into the atmosphere from different sources. The probable sources are given below:
1. Emissions from Industrial Chimney:
There are many industries which act as the major sources of air pollution. Of those petroleum refineries, cement factories, stone crushers, food processing industries, chemical fertilizer factories, etc. emit gases through the chimneys of the factories. Among these, petroleum refineries are the major sources of SO₂ and NO₂. Mathura-based petroleum refinery has been accused of aggravating the pollution-related decay of the Taj Mahal in Agra as well as the monuments of Fatehpur Sikri. Similarly, dusts released from cement factories cause health hazards. Stone crushers and hot-mix plants also create a menace, the SPM (Suspended Particulate Matter) is five times more than the safety limits. Smokes from fertilizer and food factories emit various poisonous gases. Acid vapour is emitted continuously from chemical factories.

2. Thermal Power Stations:
Both standard and superthermal plants are present in our country and coal is used as fuel in these plants. The fly ash, SO₂, and other gases and hydrocarbons are regularly released in the air and these make the air polluted and unhealthy.

3. Automobile Exhaust:
Toxic exhaust of automobiles is a source of considerable air pollution (60%). It contains CO, SO₂, NO₂, CO₂, and other toxic substances.

4. Agriculture Practices:
Such types of air pollution are, not in massive quantities but bear significance, due to the harmful substances they release. This is agriculture, which is responsible for pollution through pesticides, insecticides, herbicides, and dust from agricultural practices.

5. Domestic Combustion:
In rural areas, firewood and coal are used as fuels. Combustion of these fuels produces CO_x [i.e., CO, CO₂), NOx (all oxides of nitrogen), SO_x (SO₂, SO₃), and hydrocarbons which, though in a lesser amount, still pollute the air.

6. Radioactive Elements:
Radioactive elements like ²³⁵Uranium, ²²⁶Radium, etc., are now being used in research and medical science. After their use, the residues are not always disposed of with care. All these radioactive elements pollute the environment.

Components and Sources of Air Pollution:

Components of Air Pollution	Sources
1. Oxides of Carbon – CO, CO2	1. Automobile exhaust, uses of fossil fuel, emission from industry.
2. Oxides of Nitrogen – NO, NO2	2. Petroleum refineries, combustion of biomass, uses of chemical fertilizers.
3. Oxides of Sulphur – SO2, SO3	3. Burning of fossil fuel, industrialization.
4. Methane and other hydrocarbons.	4. Forest fire, fossil fuel, and farming.
5. CFCs (Chlorofluro Carbons)	5. Air conditioner, Plastic, colour, foam, etc.
6. O3, PAN, PBN [Peroxy Benzoyl Nitrate]	6. Thermo chemical reaction.
7. SPM	7. Coal burning, smoke emission from cars, buses, etc.

Classification of Air Pollutants

Air pollutants may be classified into 2 broad categories.

A. Natural Contaminants

• Plants: Decomposed plant bodies emit methane (CH₄), hydrogen sulphide (H₂S), and carbon monoxide (CO), and other than that plant spores also create air pollution.
• Aerosol: Minute molecules of solid and liquid substances e.g., dust, smoke, mist, fog, and fuels.
• Soil: Dry soil dust with bacteria, viruses, fungus.
• Ocean: Different types of oceanic salts.
• Volcanoes: Sulphur compounds, fly ash, SO₂, SO₃, fumes, and other gaseous substances.
• Cosmic substances: Cosmic rays, meteor, aerolith, comet dust, etc.

B. Man-made Contaminants

• Gaseous substances: Combustion products of coal, petroleum, and fossil fuel, emissions from industries, automobile exhausts, e.g., CO, CO₂, SO₂, SO₃, PAN, O₃, etc.
• Solid wastes: Minute dust, SPM from different factories, industries, etc.
• Radioactive substances: Atomic power stations, radioactive fuel, atomic explosion, atomic war, etc.

Types of Air Pollutants

The air pollutants released from different sources may be Primary Air Pollutants or Secondary Air Pollutants.

A. Primary Air Pollutants and their Effects

The main primary air pollutants are:

1. Suspended Particulate Matter (SPM):
Except for pure water, any particle having dimensions ranging from 0.001 pm to 100 pm is called a suspended particle. It may include dust of various types, soot, fly ash, smoke, or even natural substances like fur, hair, spores, volcanic smoke, rock dust, ashes from forest fires, etc. Smoke contains a suspension of carbon and other particles given off due to the burning of organic matter. Some of their effects are

• The smoke and dust particles deposit on plants and clog stomata and also enter our respiratory tract and produce allergies, and asthma.
• Emissions from iron mills, flour mills, mines, and cement, cause emphysema, pneumoconiosis, etc.
• Sometimes pollen spores cause severe allergies in humans.
• Tolerable SPM standards in industrial areas are 500 µg/m³ but have increased to 1200 µg/m³ and that of the domestic area is 100 µg/m³ which has also increased to 800 µg/m³ causing severe effects on humans.

2. Carbon Monoxide (CO):
Incomplete combustion of organic matter and metallurgical activities are the main producers of carbon monoxide. Automobiles release CO at a high rate. In closed rooms or congested areas, if coal or any fuel is burned, CO is produced which is extremely harmful as it may diffuse into the blood and react with haemoglobin to form carboxy haemoglobin. CO can convert 7.5% of haemoglobin into carboxyhemoglobin (COHb) within 8 hours at 50 ppm. COHb hinders oxygen transport causing headaches, dizziness, cardiovascular malfunction, etc.

3. Carbon dioxide:
It is a major greenhouse gas. Increasing concentration of CO₂ is posing a great problem of global warming.

4. H2S:
The main source of H₂S gas is the treatment plants of sulphur ores, refineries, etc. It can affect the plants by causing chlorosis and defoliation. In humans, it can cause eye irritation, throat irritation, and nausea.

5. Sulphur Oxides:
The main form of oxides of sulphur is SO₂. It is produced during the smelting of metallic ores and the burning of fossil fuels. SO₂ when present in excess amounts in the air combines with rain water and forms an acid H₂SO₃, causing acid rain (discussed later).

Some harmful effects are:

• It can cause chlorosis and necrosis in plants even at concentrations as low as 0.032 ppm.
• Lichens like Parmelia, and Usnea die in the presence of SO₂, so, are very good indicators of SO₂ pollution.
• Human beings are affected at concentrations above 1 ppm. It can cause eye irritation and respiratory disorders like bronchitis, and emphysema. SO₂ is proven also to be a potent mutagen.
• It causes a high death rate in fish.
• It also causes the deterioration of buildings through discolouration and corrosion of paints and sculptures.
• The wonder of the world Taj Mahal, made up of white marble, is facing the threat of corrosion and yellowing due to SO₂ emission from the nearby refinery at Mathura.
• SO₂ can even corrode metals like iron and zinc, thus posing a threat to metallic structures like bridges.

6. Nitrogen Oxides (N₂O, NO, NO₂, N₂O₄, N₂O₅):
Several sources are there that emit nitrogen oxides viz, electric storms, high energy radiations, solar flares, combustion in industries, automobiles, incinerators, and nitrogen fertilizer plants. Effects include:

• chief constituents forming peroxyacetyl nitrates or PAN.
• photochemical smog.
• corroding of metals in the presence of moisture.
• discolouration of textiles.
• cause necrosis, lesions, defoliation, and finally death in plants.
• lung edema, blood congestion, and dilation of arteries in humans. Also has some mutagenic properties.

7. Fluorides:
Refineries are the main sources of emission of fluorides. Fluorides can cause chlorosis and necrosis in plants and finally abscission. In animals, it causes abnormal calcification of bones and teeth, weak bones, and gastrointestinal and neuromuscular disorders.

8. Volatile Organic Carbons (VOC):
These are produced naturally like marsh gas or by man-made products like paints, emulsions, etc. Chemically they contain polynuclear aromatic hydrocarbons (PAH) and formaldehyde causing severe lung disorders, respiratory congestion, and even cancer.

9. Chlorofluorocarbons:
They are used as coolants in refrigerators, propellants, air conditioners, etc. It is released by jet airplanes as aerosols while flying at great heights. It is the main cause of ozone depletion. By reacting with nitrogen oxides it causes holes in the ozone layer of our atmosphere. UV rays penetrate through this hole and cause harm to humans, animals, and even plants.

10. Other Pollutants:
Mercury through the burning of coal, phosgene through pesticide and dye manufacturing industries, methyl isocyanate from pesticide manufacturing plants and lead through the burning of commercial petroleum are added to the atmosphere causing huge hazards to nature and its beings.

11. Pollens and Microbes:
These are natural attributes of the atmosphere. However excess amounts of these cause severe disorders. Microbes can cause several diseases in both plants and animals while pollen causes severe allergic responses in humans, e.g., hay fever. Pollens of Amaranthus spinosus, Cyanodon dactylon, Ricinus communis etc, causes allergies.

The effects of air pollutants on human health are described below:

Air Pollutants	Their Effects
1. Carbon monoxide	Reduces oxygen-carrying capacity of hemoglobin, results in nausea, headache, drowsiness, breathing trouble, etc.
2. Sulphur dioxide	Causes irritation of the eyes, asthma, inflammation of the trachea, bronchitis, lung cancer, etc.
3. Nitrogen dioxide	Bronchitis, pneumonia, inflammation of the lungs, etc.
4. Hydrogen sulphide	Headache, nausea, diarrhea, irritation of eyes, pneumonia, etc.
5. Hydrogen cyanide	Headache, reduced eyesight, neuronal cell death, etc.
6. Ammonia gas	Irritation of eyes, inflammation of the trachea.
7. Chlorine	The whole respiratory system is affected.
8. Aldehyde	Inflammation of the trachea.
9. Chlorofluorocarbon (CFC)	It destroys the ozone layer, thus exposing living organisms to UV rays resulting in skin cancer.
10. Arsenic	Oxygen oxygen-carrying capacity of blood decreases.
11. Hydrogen Fluoride	Inflammation of various tracts of an organ.
12. Ozone	The effect of UV rays increases and skin cancer occurs.
13. Ashes, Smoke	Emphysema, irritation of the eyes, cancer, etc.
14. Carbon dichloride (Phosgene)	Coughing, inflammation of the trachea, lung oedema.

B. Secondary Pollutants and their Effects

These are pollutants that are produced due to photochemical reactions between nitrogen oxides and unsaturated hydrocarbons.

1. Ozone:
It is an extremely strong oxidant and has the ability to destroy the chlorenchyma and produce necrosis in plants. It can damage textiles and cause severe corrosion of marble statues and buildings. In humans, it can cause hemorrhages and eye irritation.

Effects on Plants:

Air Pollutants	Effects on Plants
1. Sulphur dioxide (SO2)	Chlorosis, necrosis of cells and tissues, destruction of lichens and mosses, reduction in growth and yield.
2. Fluoride and PAN	Damage to leafy vegetables like lettuce and spinach causes fluorosis, leaf chlorosis, necrosis of leaf margin and tips, and abscission of leaves, PAN destroys spongy parenchyma of young fruit’s leaf.
3. Oxides of Nitrogen and Fluorides	Reduces crop yield, destroys leaf tissue, and causes chlorosis and necrosis, lesions, defoliation, and death of the plant.
4. Hydrocarbons	Premature yellowing and falling of leaves, flower buds, decolouration, senescence.
5. Hydrogen Sulfide (H2S)	Functional damage to plants, falling of leaves.
6. Sodium Chloride	Falling of leaves, damage to plant tissue.
7. Ammonia	Retarded growth of stem and root, rust colouration on leaves, retarded seed germination.
8. Ozone	Decolouration and black spots on leaves, retarded growth.

2. Peroxyacyl Nitrate (PAN):
They are formed when hydrocarbons react with nitrogen oxides present in the atmosphere. It can cause silvering, bronzing, and necrosis of leaves of plants by damaging chloroplasts, inhibiting the electron transport system and cellular metabolic enzymes.
NO + HC + O₂ → NO₂ + PAN

3. Smog:
These secondary pollutants accumulate and form an opaque dark fog condensed with water vapours, dust, smoke, and gases called smog. Some famous smog that occurred all over the world are the Los Angeles smog (1946), the London smog (1952) New York smog (1970). It is a type of air pollution, originally named after the mixture of smoke and fog in the air. Smog is a problem in many cities and is harmful to human health. Smoke from cars and industries, ground-level ozone, sulfur dioxide, nitrogen dioxide, carbon monoxide, unsaturated hydrocarbon, etc. mixed with air formed smog. It proved harmful for people with heart and lung diseases like emphysema, bronchitis, and asthma. It can cause eye and nose irritation also. Here are two types of smog are

(a) Classic smog (London):
It results from a large amount of coal burning in an area and is caused by gases like sulfur dioxide (SO2), hydrogen sulfide (H2S), smoke, and dust particles. It is dark brown thus opaque and has a reducing environment. Such fog is formed when water vapor condenses along with these harmful & gases. However secondary pollutants are not present. It occurred in London in December 1952 where it affected 50% of the population and killed 4000 people.
2SO₂ + O₂ → 2SO₃
SO₃ + H₂O → H₂SO₄

(b) Photochemical Smog (Los Angeles):
It contains secondary pollutants or photochemical pollutants like O₃, PAN, NO₂, H₂O₂, etc. It does not contain smoke. It is grey or yellowish-brown opaque smog having an oxidizing environment. Photochemical smog is a serious type of oxidizing pollutant and its main ingredients are O₂, NO_x, NO, NO₂, N₂O₃, N₂O₅, O, organic peroxides (R-O-O-R), Pb₂N, PAN (Peroxyacetyl nitrate), etc. These are formed by the photochemical reaction between NO_x, O₂, and hydrocarbons. It occurred in Los Angeles in 1940. Photochemical smog is formed at high temperatures. In intense solar radiations photochemical smog forms brown air while in case of low radiations of the sun, smog forms grey air.

If the O₃ of the atmosphere becomes more than 0.115 ppm then photochemical smog is formed. Some sulfates and nitrates are formed in photochemical smog. By the oxidation of H₂SO₄, such sulphates are formed. Similarly, by the oxidation of N₂O₅, NO₂ nitrates are formed. The presence of nitrates and HNO₃ in the smog irritates the eyes and throat and creates breathing trouble. The smog also causes great damage to buildings, monuments, plants, and animals.
NO₂ → NO + O
O + O₂ → O₃
O₃ + NO → NO₂ + O₂

Control of Air Pollution

Different kinds of air pollution can be controlled by modern technology. Emissions from factories and power plants can be made free from gaseous pollutants by three methods:

• Combustion Techniques: Only oxidizable pollutants can be removed by this method. Emissions are burnt at very high temperatures. This process is applied in the petrochemical and paint industries.
• Absorption Techniques: Here, scrubbers with packing materials are used to absorb gaseous pollutants. A fine spray of water is applied that dissolves NH₃, SO₂, etc. Sometimes a bed of lime is also employed to absorb SO₂.
• Adsorption Techniques: Activated charcoal, a chief adsorption material, is employed in this technique. It can adsorb toxic vapors, gases, and other harmful matters.

The following steps have to be taken to control pollution at the source as well as after the release of pollutants into the atmosphere:
1. Prevention and Control of Vehicular Pollution:
For preventing and controlling air pollution created by vehicles, the following measures are to be adopted:

• Curbing the pollutant emission from vehicular exhaust by using various devices, such as positive crankcase, ventilation valve, and catalytic converter.
• Control of evaporation from the fuel tank and carburetter by several mechanical and chemical processes.
• Filters can be used to capture and recycle the hydrocarbons from the engine.
• Leaded petrol should be avoided, instead, unleaded petrol and low-sulfur diesel should be used.
• Periodic checking of vehicles for pollution control.
• Increased use of CNG can lower the amount of pollutants.

2. Prevention and Control of Industrial Pollution:
In different industries air pollution can be checked at 5 points of the control process:
(i) Removal of Particulate Matter:
It involves the use of the following equipment:

• Cyclone Collector: This is used for the centrifugation of waste gas-containing particles. Cyclone collectors can remove upto 70% of the particles.
• Electrostatic Precipitators (ESPs): To remove the suspended particles from the gas stream, the electrical forces are applied within the chamber in the precipitator. ESPs can remove 99% of the particulate pollutants from the chimney exhaust. ESPs work very efficiently in power plants, paper mills, carbon black plants, cement mills, etc. Sometimes bag filters or fabric filters are used for better results. Further, tall chimneys are used for vertical dispersion of air.

Limitation: It works on the gravitational setting method. The velocity of air between the plates is kept low to allow dust to settle down.

(ii) Removal of Gaseous Pollutants:
This uses the following three methods:

• Wet Systems: These are used in washing towers in which alkali fluid circulates continuously. This liquid reacts with SO₂ to produce a precipitate.
• Dry Systems: Here, the gaseous pollutants are allowed to react with an absorbent under a dry phase. Lime (CaO), and limestone (CaCO₃) are placed in the way of the flowing gas (SO₂, SO₃). This process is less expensive and effective.
• Wet-dry Systems: Here water in the absorbent reacts with the acid components. The absorbent Ca(OH)₂ slurry is spread into the hot gas stream in the form of small droplets. Calcium hydroxide reacts with SO₂ and the hot gases cause the water to evaporate simultaneously. The end product is a dry powder containing mostly fly ash and salts.

Air Quality Index:
Air quality is a measure of how clean or polluted the air is. Concentration is expressed in parts/mile (ppm) or parts per billion (ppb). The AQI level is based on the level of six atmospheric pollutants, i.e., SO₂, NO₂, CO, O₃, SPM (smaller than 2.5 µm), PM, (smaller than 10 µm).
AQI Category:

Case Study of Delhi

Delhi ranks highest in the number of vehicles in India. According to the record of 1990, the number of cars in Delhi was more than the total of West Bengal and Gujarat. Due to this, Delhi ranked fourth in the 41 most polluted cities of the world. Residents of Delhi were mostly suffering from burning eyes and respiratory problems. A public interest litigation (PIL) was filed in the Supreme Court. Supreme Court ordered the government to take measures against pollution by:

• Using CNG in public transport instead of petrol or CNG.
• Replacing old vehicles.
• Using unleaded petrol and diesel with reduced sulfur content.
• Making emission checks compulsory for all vehicles.
• Enforcing euro II norms.
• Fitting of catalytic converters.

Delhi first introduced the usage of CNG for autorickshaws in 2002.

• Its use is cheaper
• It burns completely
• It does not cause smoke
• It cannot be adultered like petrol and diesel.

Strict adoption of these measures by the Govt, decreased the level of pollution caused by gases like SO₂, NO_x, and CO, substantially.

Control of Air Pollution Through Law

In our country there have been several legislative measures both at the State and Central Government levels to prevent and control different types of air pollution:

• Bengal Smoke Nuisance Act, 1905;
• The Motor Vehicle Act, 1938;
• The Gujarat Smoke Nuisance Act, 1953;
• The Air Prevention and Control of Pollution Act, 1981;
• The Environment (Protection) Act, 1986;
• The Motor Vehicles Act, 1988 (This Act came into force from 01.07.1989).