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Air Pollution: Air Pollutants, Classification of Air Pollutants

Global Environmental Issues and Environmental Degradation

The important global environmental issues are:

  • Pollution
  • Biodiversity Loss
  • Desertification
  • Depletion of Ozone Layer (explained in Geography > Climatology > Polar Vortex > Ozone Depletion)
  • Acid Rain
  • Oil Spills
  • Dumping of Hazardous Wastes
  • Climate change due to greenhouse effect and global warming.
  • Environmental degradation is making the environment unfit or less suitable for the survival of different life forms thereby causing immense ecological damage.
  • Population explosion, urbanisation and the associated increase in human needs and comforts have resulted in rapid industrialisation.
  • Rapid industrialization in turn has led to overexploitation of natural resources.
  • The consequences of such exploitation are evident in the form of soil erosion, desertification, loss of biodiversity, pollution of land, air and water bodies.

Major Causes of Environmental Degradation

Air pollution
  • Pollution: Gaseous pollutants: Oxides of carbon, nitrogen and sulphur.
  • Particulate matter: Fine metal dust, fly ash, soot, cotton dust and radioactive substances.
  • Burning of plastics: Emits polychlorinated biphenyls (PCBs).
  • Industrial catastrophes: Accidental release of some poisonous gases like phosgene (COCl2) and methyl isocyanate (as it happened in Bhopal in 1985) were fatal.
  • Secondary air pollutants formed from complex reactions between primary pollutants, such as smog and acid rain.
  • Global warming: Increasing use of fossil fuels is a leading cause of increasing levels of CO2 and other greenhouse gases in the atmosphere.
Water pollution
  • Industrial and domestic effluents: Pathogens, heavy metals in water bodies and soil.
  • Hazardous waste/Toxic substances: Use of a various type of chemicals today have serious health implications. The incidence of cancer, genetic mutations and damage to nervous, immune and hormonal systems.
Soil degradation
  • Deforestation, overgrazing, intensive cultivation, over irrigation etc. results in the loss of topsoil and fertility of the land. Prolonged degradation of land leads to desertification.
Damage to ecology and biodiversity
  • Habitat fragmentation and destruction: Industrialization and associated infrastructure development have led to the destruction of habitats of many species.
  • Invasive populations: Introduction of new alien species or non-native species reduces the populations’ growth of native species.
  • Overexploitation and introduction of a new or genetically modified species reduce the productivity of natural ecosystems.

Consequences of Environmental Degradation

  • Increased sensitivity to diseases: Cultivated species of plants, fishes and other domesticated animals have become increasingly sensitive to pest and diseases.
  • Genetic resistance: An increased use of insecticides, pesticides and antibiotics has speeded up directional natural selection and caused genetic resistance in pathogens.
  • Effect on nutrient recycling: Use of fertilisers in agricultural fields interferes with the natural biogeochemical cycles.
  • Loss of biodiversity: Habitat destruction and habitat fragmentation are threatening the survival of many endangered species.
  • Climate change/global warming: Air pollution is making the planet inhospitable due to events like global warming, acid rain, ocean acidification etc.
  • Melting of ice caps and glaciers/Sea level change: Global warming is posing a serious threat to Himalayan and polar ecosystems by melting snow at a faster rate. Rise in sea level in future is a serious threat to coastal landforms.
  • Change in weather patterns: The incidence of extreme weather events and change in weather patterns have become common due to environmental degradation.
  • Depletion of water resources: Water pollution has created a tremendous scarcity of quality water resources.
  • Desertification: Lot of arable lands is converted to wasteland due to erosion, salinity and loss of fertility.

Air Pollution

  • Pollution may be defined as the addition of undesirable material into the environment as a result of human activities. The agents which cause environmental pollution are called pollutants.
  • A pollutant may be defined as a physical, chemical or biological substance released into the environment which is directly or indirectly harmful to humans and other living organisms.
  • Pollution may be of the following types: Air pollution, Noise pollution, Water pollution, Soil pollution, Thermal pollution and Radiation pollution.
  • In order to control environmental pollution, the Government of India has passed the Environment (Protection) Act, 1986 (Bhopal disaster) to protect and improve the quality of our air, water and soil.
  • Air pollution may be defined as the presence of any solid, liquid or gaseous substance including noise and radioactive radiation in the atmosphere in such concentration that may be directly and/or indirectly injurious to humans or other living organisms, property or interferes with the normal environmental processes.
  • An ever-increasing use of fossil fuels in power plants, industries, transportation, mining, construction of buildings, stone quarries had led to air pollution.
  • Fossil fuels contain small amounts of nitrogen and sulphur.
  • Burning of fossil fuels like coal (thermal power plants) and petroleum release different oxides of nitrogen and sulphur into the atmosphere.
  • These gases react with the water vapour present in the atmosphere to form sulphuric acid and nitric acid. The acids drop down with rain, making the rain acidic. This is called acid rain.
  • Acid rain corrodes the marble monuments like Taj Mahal. This phenomenon is called as Marble cancer.
  • Other kinds of pollutants are chlorofluorocarbons (CFCs) which are used in refrigerators, air conditioners and as pressurising agents in aerosol sprays. CFCs damage the ozone layer of the atmosphere.
  • The combustion of fossil fuels also increases the number of suspended particles in the air. These suspended particles could be unburnt carbon particles or substances called hydrocarbons.
  • Presence of high levels of all these pollutants causes visibility to be lowered, especially in cold weather when water also condenses out of the air. This is known as smog and is a visible indication of air pollution.

Smog is already explained in detail in Geography > Climatology > Forms of Condensation > Smog

Ozone Depletion is already explained in detail in Geography > Climatology > Polar Vortex > Ozone Depletion

Classification of Pollutants

According to the form in which they persist after release into the environment.

  • Primary pollutants: These are persistent in the form in which they are added to the environment, e.g. DDT, plastic, CO, CO2, oxides of nitrogen and sulphur, etc.
  • Secondary Pollutants: These are formed by interaction among the primary pollutants. For example, peroxyacetyl nitrate (PAN) is formed by the interaction of nitrogen oxides and hydrocarbons.

According to their existence in nature

  • Quantitative Pollutants: These occur in nature and become pollutant when their concentration reaches beyond a threshold level. E.g. carbon dioxide, nitrogen oxide.
  • Qualitative Pollutants: These do not occur in nature and are human-made. E.g. fungicides, herbicides, DDT etc.

Particulate pollutants

  • Particulate pollutants are matter suspended in air such as dust and soot.
  • Major source of SPM (suspended particulate matter) are industries, vehicles, power plants, construction activities, oil refinery, railway yard, market place, industries, etc.
  • Their size ranges from 0.001 to 500 micrometres (µm) in diameter.
  • Particles less than 10 µm float and move freely with the air current.
  • Particles which are more than 10 µm in diameter settle down.
  • Particles less than 0.02 µm form persistent aerosols.
  • According to the Central Pollution Control Board (CPCB), particulate size 2.5 µm or less in diameter (PM 2.5) are responsible for causing the greatest harm to human health.
  • These fine particulates can be inhaled deep into the lungs and can cause breathing and respiratory symptoms, irritation, inflammations and pneumoconiosis (disease of the lungs caused due to inhalation of dust. It is characterised by inflammation, coughing, and fibrosis – excess deposition of fibrous tissue).

Fly ash

  • Fly ash is ejected mostly by thermal power plants as by-products of coal burning operations.
  • Fly ash pollutes air and water and may cause heavy metal pollution in water bodies.
  • Fly ash affects crops and vegetation as a result of its direct deposition on leaf surfaces.
  • Fly ash particles are oxide rich and consist of silica, alumina, oxides of iron, calcium, and magnesium and toxic heavy metals like lead, arsenic, cobalt, and copper.
  • Major oxides are present are aluminium silicate (in large amounts), silicon dioxide (SiO2) and calcium oxide (CaO).
  • Cement can be replaced by fly ash up to 35%, thus reducing the cost of construction, making roads, etc.
  • Fly ash bricks are light in weight and offer high strength and durability.
  • Fly ash is a better fill material for road embankments and in concrete roads.
  • Fly ash can be used in the reclamation of wastelands.
  • Abandoned mines can be filled up with fly ash.
  • Fly ash can increase crop yield when added to the soil. But if it gets deposited on the leaf, it will reduce photosynthesis.
  • It also enhances the water holding capacity of the land.
Policy measures of MoEF
  • The Ministry of Environment and Forests has made it mandatory to use Fly Ash-based products in all construction projects, road embankment works, and low lying landfilling works within 100 km radius of Thermal Power Station and mine filling activities within 50 km radius of Thermal Power Station.
Q. With reference to ‘fly ash’ produced by the power plants using the coal as fuel, which of the following statements is/are correct?
  1. Fly ash can be used in the production of bricks for building construction
  2. Fly ash can be used as a replacement for some of the Portland cement contents of concrete
  3. Fly ash is made up of silicon dioxide and calcium oxide only and does not contain any toxic elements.

Select the correct answer using the code given below

  1. 1 and 2
  2. 2 only
  3. 1 and 3
  4. 3 only
  • Fly ash does contain heavy toxic elements like arsenic, cobalt, lead etc.

Answer: a) 1 and 2


  • It is present in petrol, diesel, lead batteries, paints, hair dye products, etc.
  • It can cause nervous system damage and digestive problems and, in some cases, cause cancer.
  • Lead affects children in particular.
  • Tetraethyl lead (TEL) is used as an anti-knock agent in petrol for a smooth and easy running of vehicles.
  • The lead particles coming out from the exhaust pipes of vehicles is mixed with air.
  • It produces injurious effects on kidney and liver and interferes with the development of red blood cells.
  • Lead mixed with water and food can create cumulative poisoning.
  • It has long term effects on children as it lowers intelligence.

Metallic Oxides

  • Oxides of iron, aluminium, manganese, magnesium, zinc and other metals have an adverse effect due to deposition of dust on plants during mining operations and metallurgical processes.
  • They create physiological, biochemical and developmental disorders in plants and also contribute towards reproductive failure in plants.

Nanoparticles (NPs)

  • Nanoparticles are particles with dimensions comparable to 1/10of a meter (1 divided by 100 crores).
  • Major natural processes that release NPs in the atmosphere are forest fires, volcanic eruptions, weathering, dust storms from desert etc.
  • Naturally occurring NPs are quite heterogeneous in size and can be transported over thousands of kilometres and remain suspended in the air for several days.
  • Nanotechnology has a global socioeconomic value, with applications ranging from electronics to biomedical uses (delivering drugs to target sites).
  • Man-made NPs are unknowingly or purposely released in the environment during various industrial and mechanical processes.
Effects of Nanoparticles on the environment
  • After releasing in the environment, NPs will accumulate in various environmental matrices such as air, water, soil and sediments including wastewater sludge.
  • NPs in the environment influences dust cloud formation, environmental hydroxyl radical concentration, ozone depletion, or stratospheric temperature change.
Effect of NPs on dust cloud formation
  • NPs in environment coagulate and form dust cloud.
  • Dust cloud formation decreases sunlight intensity.
Asian brown clouds impact on Himalayan glaciers
  • Asian brown clouds carry large amounts of soot and black carbon (NPs) and deposit them on the Himalayan glaciers.
  • This could lead to higher absorption of the sun’s heat (reduced albedo) and potentially contributing to the increased melting of glaciers.
NPs and ozone depletion
  • NPs can result in increased production of reactive oxygen species (ROS), including free radicals like Cl.
  • Radicals like Cl– destroy ozone. (Explained in Geography > Climatology > Polar Vortex > Ozone Depletion)

In chemistry, a radical (a free radical) is an atom, molecule, or ion that has unpaired valence electrons.

Effect of NPs on stratospheric temperature
  • NPs in the troposphere interact with molecular hydrogen accidentally released from hydrogen fuel cells and other sources.
  • Molecular hydrogen along with NPs moves up to the stratosphere, resulting in the abundance of water vapour in the stratosphere.
  • This will cause stratospheric cooling due to the formation of Stratospheric clouds (mostly ice crystals).
  • Stratospheric clouds destroy ozone. (Geography > Climatology > Polar Vortex > Ozone Depletion)
Q. There is some concern regarding the nanoparticles of some chemical elements that are used by the industry in the manufacture of various products. Why?
  1. They can accumulate in the environment and contaminate water and soil.
  2. They can enter the food chains.
  3. They can trigger the production of free radicals.

Select the correct answer using the code given below.

  1. 1 and 2 only
  2. 3 only
  3. 1 and 3 only
  4. 1, 2 and 3

Answer: d) all

Major Gaseous Air Pollutants, Their Sources & Effects

Carbon monoxide (CO)

  • Carbon monoxide (CO) is a colourless, odourless, tasteless and highly toxic gas that is slightly less dense than air. It is short-lived (stay only a few months) in the atmosphere.
  • Carbon monoxide is produced from the exhaust of internal combustion engines and incomplete combustion of various other fuels. Iron smelting also produces carbon monoxide as a by-product.
  • It forms when there is not enough oxygen to produce carbon dioxide (CO2).
  • In the presence of oxygen, carbon monoxide burns with a blue flame, producing carbon dioxide.
  • Worldwide, the largest source of carbon monoxide is natural in origin, due to photochemical reactions in the troposphere.
  • Other natural sources of CO include volcanoes, forest fires, and other forms of combustion.
Health Effects
  • Carbon monoxide poisoning is the most common type of fatal air poisoning (poor ventilation and heat management in laptops and other electrons can cause the release of CO).
  • It is toxic to haemoglobin animals (including humans) when encountered in concentrations above about 35 ppm.
  • It combines with haemoglobin to produce carboxyhaemoglobin, which usurps the space in haemoglobin that normally carries oxygen.
Environmental Effects
  • Carbon monoxide (CO) is not considered a direct greenhouse gas (GHG).
  • In the atmosphere, it is spatially variable and short-lived, having a role in the formation of ground-level ozone (tropospheric ozone) and can elevate concentrations of methane (a strong GHG).
  • Carbon monoxide reacts with hydroxyl radical (-OH) to produce peroxy radical.
  • Peroxy radical reacts with nitrogen oxide (NO) to form nitrogen dioxide (NO2) and hydroxyl radical. 
  • NO2 gives O3 via photolysis (separation of molecules by the action of light).

Carbon dioxide (CO2)

  • Colourless and odourless gas. It is heavier than air.
  • Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids.
  • Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, in ice caps and glaciers and also in seawater.
Effects on Health
  • CO2 is an asphyxiant gas (asphyxia: a condition arising when the body is deprived of oxygen, causing unconsciousness or death.).
  • Concentrations of 7% may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, and unconsciousness.
Effects on Environment
  • Carbon dioxide is an important greenhouse gas.
  • Burning of carbon-based fuels since the industrial revolution has led to global warming.
  • It is also a major cause of ocean acidification because it dissolves in water to form carbonic acid.

Chlorofluorocarbons (CFCs)

  • Chlorofluorocarbons (CFCs) are used in refrigerators, air conditioners and aerosol sprays.
  • Since the late 1970s, the use of CFCs has been heavily regulated because of their destructive effects on the ozone layer. (Explained in Geography > Climatology > Polar Vortex > Ozone Depletion)
  • The Montreal Protocol on Substances that Deplete the Ozone Layer (a protocol to the Vienna Convention for the Protection of the Ozone Layer) is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances including CFCs which are responsible for ozone depletion.

Ozone (O3)

  • It occurs naturally in the stratosphere and absorbs harmful ultraviolet rays of the sun.
  • However, at the ground level, it is a pollutant (Greenhouse gas) with highly toxic effects.
  • Vehicles and industries are the major sources of ground-level ozone emissions.
  • Carbon monoxide, Nitrogen dioxide play a major role in converting O2 to O3.
  • Ozone makes our eyes itchy, and watery. It lowers our resistance to cold and pneumonia.

Nitrogen oxide (NOx)

  • NOx is a generic term for the various nitrogen oxides produced during combustion.
  • They are produced mainly in internal combustion engines and coal-burning power plants.
  • They are also produced naturally by lightning.

Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they produce various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler.

  • Agricultural fertilisation and the use of nitrogen-fixing plants also contribute to atmospheric NOx, by promoting nitrogen fixation by microorganisms.
  • NO and NO(contribute to global cooling) should not be confused with nitrous oxide (N2O – GHG), which is a greenhouse gas and has many uses as an oxidiser.
Effects on Health and Environment
  • They are believed to aggravate asthmatic conditions and create many respiratory health issues.
  • The reduction of NOx emissions is one of the most important technical challenges facing biodiesel.
  • NOx gases react to form smog and acid rain as well as being central to the formation of tropospheric ozone. (Explained in Geography > Climatology > Forms of Condensation > Smog)
  • When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog.
  • Mono-nitrogen oxides eventually form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain.
  • NO and NO2 emissions cause global cooling through the formation of -OH radicals that destroy methane molecules, countering the effect of greenhouse gases.

Sulphur dioxide (SO2)

  • It is a toxic gas with a pungent, irritating smell. It contributes to acid rain.
  • It is released naturally by volcanic activity. It is abundantly available in the atmosphere of Venus.
  • Sulphur dioxide is primarily produced for sulfuric acid manufacture.
  • Inhaling sulphur dioxide is associated with increased respiratory symptoms and premature death.
  • It also weakens the functioning of certain nerves.
  • It is also produced by
    • burning coal in thermal power plants and diesel fuels.
    • some industrial processes, such as the production of paper and smelting of metals.
    • reactions involving Hydrogen Sulphide (H2S) and oxygen.
    • The roasting of sulphide ores such as pyrite, sphalerite, and cinnabar (mercury sulphide).

Volatile organic compounds (VOCs)

  • Volatile Organic Compounds (VOCs) are a large group of carbon-based chemicals that easily evaporate at room temperature.
  • For example, formaldehyde, which evaporates from paint, has a boiling point of only –19 °C.
  • Formaldehyde causes irritation to the eyes and nose and allergies.
  • The main indoor sources are perfumes, hair sprays, furniture polish, glues, air fresheners, moth repellents, wood preservatives, and other products.
  • Health effects: irritation of the eye, nose and throat, headaches, nausea and loss of coordination.
  • Long term health effects: suspected to damage the liver and other parts of the body.


  • Benzene is a natural constituent of crude oil and is one of the elementary petrochemicals.
  • Because benzene has a high octane number, it is an important component of gasoline (petrol).
  • Benzene increases the risk of cancer and other illnesses.
  • Benzene is a notorious cause of bone marrow failure.


  • Ethylene is widely used in the chemical industry.
  • Much of this production goes toward polyethylene, a widely used plastic containing polymer chains of ethylene units in various chain lengths.
  • Ethylene is also an important natural plant hormone, used in agriculture to force the ripening of fruits.
  • Ethylene is of low toxicity to humans and exposure to excess ethylene cause adverse health effects like headache, drowsiness, dizziness and unconsciousness.
  • Ethylene is not but ethylene oxide is a carcinogen (cancer-causing agent).

Biological pollutants

  • It includes pollen from plants, mite, and hair from pets, fungi, parasites, and some bacteria.
  • Most of them are allergens and can cause asthma, hay fever, and other allergic diseases.


  • Asbestos refers to a set of six naturally occurring silicate fibrous minerals –– chrysotile, crocidolite, amosite, anthophyllite, tremolite, and actinolite.
  • It is now known that prolonged inhalation of asbestos fibres can cause serious and fatal illnesses including lung cancer, mesothelioma, and asbestosis (a type of pneumoconiosis).


  • It is a gas that is emitted naturally by the soil.
  • Due to modern houses having poor ventilation, it is confined inside the house and causes lung cancers.

Noise Pollution

  • Noise by definition is “sound without value” or “any noise that is unwanted by the recipient”.
  • Noise level is measured in terms of decibels (dB). An increase of about 10 dB is approximately double the increase in loudness.
  • O. (World Health Organization) has prescribed optimum noise level as 45 dB by day and 35 dB by night. Anything above 80 dB is hazardous.
  • Noise pollution leads to irritation, increased blood pressure, loss of temper, mental depression and annoyance, a decrease in work efficiency, loss of hearing which may be first temporary but can become permanent in the noise stress continues.
  • Agreen belt of trees is an efficient noise absorber.
Ambient Noise Level Monitoring
  • Noise Pollution (Control and Regulation) Rules, 2000 define ambient noise levels for various areas.
  • The Government of India on Mar 2011 launched a Real-time Ambient Noise Monitoring Network.
  • Silence Zone is an area comprising not less than 100 metres around hospitals, educational institutions, courts, religious places or any other area declared as such by a competent authority.

Occupational Health Hazards

Black lung disease
  • In coal mining areas coal dust is the main air pollutant. The deposits of coal dust make miners lungs look black instead of a healthy pink and hence the name black lung disease.
  • Black lung disease is the common name for pneumoconiosis (CWP) or anthracosis, a lung disease of older workers in the coal industry, caused by inhalation over many years, of small amounts of coal dust.
  • The particles of fine coal dust accumulate in the lungs. Eventually, this build-up causes thickening and scarring making the lungs less efficient in supplying oxygen to the blood.

Chemicals and Biological Agents

  • Substances such as benzene, chromium, nitrosamines and asbestos may cause cancers of lung, bladder, skin, mesothelium, liver, etc.
  • Occupational asthma is caused due to exposure to organic dust, microorganisms, bacteria, fungi and moulds and several chemicals.
  • Silicosis first reported from Kolar gold mines in 1947 is a common disease among miners, pottery and ceramic industry workers.
  • Pneumoconiosis and byssinosis are common among mica and textile industry workers respectively.
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