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Air Pollution


Table of Content

Air pollution is defined as the addition of undesirable material into the atmosphere either due to natural phenomena or due to human activities. Air pollution is the atmosphere condition in which the presence of certain contaminants produces harmful effects on man and his environment. These substances include:

(i) Gases such as oxides of sulphur, CO, oxide of N2 ,hydrocarbons, ozone and other oxidents

(ii)  Particulate matter such as dust, smoke, fumes etc.

(iii) Radioactive material & many others.

The chemical substances causing air pollution are known as air pollutants.

More than 90% of the air pollution is caused by oxides of S, 

oxides of C, oxides of N, volatile organic compounds and particulate matter.

  • Major Sources of Air Pollution

The air pollution is caused by both human activities and natural phenomena.

The natural sources of air pollution include volcanic eruptions, forest fires, pollen grain dispersal, evaporation of volatile organic compounds, microbial decomposition of organic matter, wind erosion of soil and nuclear radioactivity.

Human activity like burning of fuels for energy is the major source for air pollution. Some other human activities responsible for air pollution are.Major Air Pollutants

  1. Combustion of gasoline in the automobile

  2. Deforestation: This increases the amount of carbon dioxide gas in air and the amount of oxygen in air gets decreased.

  3. Fast Industrialization: The smoke of which is produced by industries contains gases like CO, CO2,H2S, NO & NO2, These industries are responsible for 20% of total air pollution.

  4. Agricultural activities: The pesticides are added in the soil in order to kill the harmful pests. While doing so, a large amount of pesticides escape into environment by mixing with air.

  5. Wars: The nuclear weapons are used in war which emits the harmful radiations.

  • Particulate Pollutants

  1. Soot: produced by incomplete combustion of carbonaceous fossils fuels such as coal, fuel oil, natural gas, wood etc in insufficient supply of oxygen.

  2. Metal particles: These are released by various metal finishing operation. The micro particles of toxic metal & SO2 gas present in the polluted atmosphere get absorbed on the particles rendering them highly toxic.

  3. Metal oxides: They are generated by combustion of fuels containing metallic compounds.

  4. Lead salts: Their source is lead tetraethyl (Pb(C2H5)4) which is added to gasoline to improve its antiknock property. In order to avoid deposition of PbO suitable amounts of C2H4Cl2 & C2H4Br2 are added to gasoline along with Pb(C2H5)4.

  5. Fly ash: It originates from the combustion of high ash fossil. It contains partially burnt particles of the fuels.

  6. Asbestos dust: It originates from industrial units manufacturing asbestos sheets, gaskets ropes etc. Asbestos flowing & asbestos insulations also contribute towards asbestos dust in the atmosphere.

  7. Solid Hydrocarbons: These are emitted from petroleum refineries & comprise of paraffins, olefins & aromatics.

  8. Dust Particulates: Originate from natural, domestic, industrial or agricultural sources. These are thrown into atmosphere by volcanic eruptions, blowing of dust by wind, mining operations etc.

  9. Acid mist: Sulphuric acid mist is produced when SO3 present in the atmosphere comes in contact with moisture. Nitric acid mist is produced when oxides of nitrogen, viz, NO & NO2, undergo the series of reactions in the atmosphere.

 Harmful effects of particulates

  1. Effect on human beings: Affect the human respiratory system & cause several respiratory illnesses.The particles with small size are more harmful in this context. The particulates in fact, become the carriers of the toxic substances from the atmosphere to the human & cause big health hazards.

  2. Effect on visibility: Particulates in the atmosphere cause scattering & absorption of sunlight & reduce the visibility.

  3. Effect on Materials : The adverse effect of particulates on materials include corrosion of metals (when the atmosphere is humid), erosion & soiling of building, sculptures & painted surfaces & soiling of clothes & draperies.

  • Gaseous Pollutants





Carbon monoxide (CO)

(a) Incomplete combustion of carbonaceous matter, automobile engines & also in defective furnaces, Incomplete combustion of fossil fuels, agricultural , slash matter and other carbon 

2C + O2 → 2CO


(b) Dissociation of carbon dioxide.  

2CO2   \rightleftharpoons  2CO + O2 

(c) Reaction of carbon dioxide with carbon containing compounds at high temperature.

CO2 + C  \rightleftharpoons  2CO 

Hydroxyl & perhydroxyl  radicals, atomic oxygen & ozone help in the oxidation of atmospheric CO into  CO2 .


Soil is major sink for CO. Some of the microorganism present in soil remove carbon monoxide from air.


Carbon monoxide is poisonous  as it combines with hemoglobin of red blood cells about 300 times faster than O2, thus forming carboxyl hemoglobin.


This decreases the transport of oxygen to the body organs & cells.


Carbon dioxide (CO2­)

It is released mainly into the atmosphere by the combustion of fossil fuels (coal, oil etc) in factories & also at homes. CO2 is also produced by biological decay of plants

Ocean is a main sink for CO2.



Green plants for photo synthesis.

CO2 causes narcotic effect, stimulation of respiratory center & leads to asphyxiation. The increasing concentration of CO2 also changes climatic conditions especially by raising the general temperature.


Oxides of sulphur


Volcanic eruptions (natural activity) & also through combustion of sulphur bearing fuels such as coal & oil (human activity).

This pollutant is also produced during roasting & smelting of sulphide ores (human activity)


A part of SO2 undergoes photolytic & catalytic oxidation to form SO3. The SO3, so formed gets converted to H2SO4 in the presence of moisture. This acid comes down from the atmosphere in the form of sulphuric acid rain.



It causes cough, shortness of breath & spasm of larynx, acute irritation to the membrane of gas resulting tears & reduces hearing ability.

SO2 irritates the respiratory system of animals & human, produces leaf injuries (called necrotic bloating) to board leaved plants & gases. It also causes deterioration of fabric (cotton, rayon) paper & leather.


Oxides of

Nitrogen (NOx)

NO is produced in small amounts by microbiological processes in soil. However significant amount of NO & NO2 are emitted in to the atmosphere by natural activity.


Many natural processes acts as sink for oxides of nitrogen. These oxides are inherently unstable & decompose to N2 & O2 after some time. Therefore, the concentration of nitrogen oxides in the atmosphere tends to remains low.


(i)   NO is biologically less active & less toxic than NO2. Like CO it binds hemoglobin & decreases oxygen transport efficiency of blood.

(ii)   Inhaling of nitrogen oxides by human results in pulmonary odema & hemorrhage.

(iii)  The oxides of nitrogen cause damage to plants. Exposure of plants to NOcauses leaf spotting & break down of plant tissues.

(iv)  The sunlight reacts with NO2 to produce highly active oxygen atoms.


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