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Major air pollutants can be divided into two categories, namely: gaseous pollutants and particulate pollutants.


1. Carbon Monoxide (CO)

Carbon monoxide is a colourless, poisonous, lethal gas, which is one of the most serious air pollutant.


(a) Natural activities. Major amount of CO in the atmosphere comes from natural sources as given below:

  • From forest fires and volcanic activities.
  • From combustion of methane (which is produced by the bacterial decay of organic matter) into CO.
  • From synthesis and decay of chlorophyll (a green pigment of plants).


(b) Human activities.

  • Incomplete combustion of carbonaceous matter in automobile engine and also in defective furnaces. This produces major chunk of CO in air. For example,
    complete combustion of octane (C8H18) requires 12.5 mole of oxygen per mole of octane

C8H18 + 12.5 O2 → 8CO2 + 9H2O

(Complete combustion)

However, if we have only 10 mol of O3 then we shall get 3 mole of carbondioxide and 5 mol of CO.

C8H18 + 10 O2 → 3CO2 + 5CO + 9H2O

Octane    (Incomplete combustion )

  • Incomplete combustion of fossil fuels, agricultural and slash matter.
  • Reaction of carbon dioxide and carbon at high temperatures in industrial processes

CO2 + C → CO

  • Dissociation of carbon dioxide at high temperatures

2CO2 → 2CO + O2


Toxic effects of CO

Toxic effect of CO arises from its capacity to bind itself to the haemoglobin present ill red blood corpuscles (RBC) of the blood more strongly (about 200 times) than oxygen. As a result of this amount of haemoglobin available in the blood for the transport of oxygen to the body cells decreases. Thus CO cause obstruction in the normal functioning of blood which further produces disorders in normal metabolism due to less O2 level. The presence of CO in blood can cause mental retardation, respiration problems, muscular weakness, dizziness and even death depending on its concentration in blood.

The maximum permissible concentration of CO in the atmosphere is 40 ppm for an exposure of 6-8 hours. The higher concentration of CO or longer exposure start causing adverse effect. At concentrations greater than 100 ppm difficulty in breathing starts which produces headache or
dizziness. Concentrations greater than 750 ppm causes acute oxygen-starvation which is referred to an asphixiation or anoxia which leads to coma or even death.


Sinks of CO

Though human activities are releasing CO in atmosphere and it is expected to double its concentration in surrounding atmosphere every 5 years. However, the actual increase in CO in atmosphere is less because of some natural sinks.

(i) Soil is the major sink for CO. Some microorganisms in soil remove CO from the atmosphere.

(ii) Some of the species in atmosphere such as hydroxyl (HO) and perhydroxyl (HOO) radicals, atomic oxygen and ozone help in the oxidation of CO to CO2 and consequently, remove it from the atmosphere.


2. Carbon Dioxide (CO2)

Carbon dioxide (CO2) is a natural constituent of the atmosphere and is vital to all forms of plant life

Normally, it forms about 0.03% by volume of atmosphere.


It is released mainly into the atmosphere by the combustion of fossil fuels (coal, oil etc.) in factories and also at homes. Carbon dioxide is also produced by biological decay of plants. It is also given out by plants and animals in the process of respiration.


Ocean is a main sink for carbon dioxide. It contains most of the dissolved carbon dioxide in form of carbonates, bicarbonates and organic compounds. It is also removed from the atmosphere by photosynthesis process of plants.

In a balanced ecological system, the CO2 is released into the atmosphere and is regularly removed by green plants for photosynthesis which in turn produce oxygen, thus
maintaining the cycle.

Forests on the earth account for about 70% of all photosynthesis occurring at any time. However, because of deforestation, there is an increasing build up of CO2 in the atmosphere.       .

Carbon dioxide causes mild narcotic effects, stimulation of respiratory centre and leads to asphixiation. The increasing concentration of carbon dioxide also. change climatic conditions especially by raising the global temperature. The phenomenon is known as greenhouse effect,


3. Oxides of Sulphur (SO2)

The two common oxides of sulphur SO2 and SO3 are represented by the formula SOx. These are probably the most harmful of the gaseous pollutants.


(a) Natural activities. SO2 the common oxide of sulphur is released into the atmosphere through volcanic eruptions. It accounts for about 67% of the total SO2 present in the atmosphere. The remaining 33% comes from human activity.

(b) Human activities. The human activity which release SO2 into the atmosphere are as follows:

  • Combustion of sulphur-bearing fuels such as coal and oil in thermal plants.
  • Roasting and smelting of sulphide ores such as CuFeS2, PbS, Cu2S, ZnS, etc.
  • A part of SO2 undergoes photolytic and catalytic oxidation in the atmosphere to form SO3 (SO2 + O2 → SO3). Thus, SO2 present in the atmosphere is generally contaminated with SO3. The O3 so formed gets converted into H2SO4 in the presence of moisture. This acid comes down from the atmosphere in the form of sulphuric acid rain.


Toxic effects of SOx

  • Both SO2 and SO3 are strongly irritating to respiratory tract of humans and animals. It has been reported that even at lower concentration, (= 5 ppm), SO2 causes cough, shortness of breath and spasm of larynx. (voice box). It also causes acute irritation to the membrane of eyes resulting tears and redness. SO3 is more harmful even at a concentration of 1 ppm.
  • SO2 also produces leaf injuries (called necrotic blotching) to broad leaved plants and grasses.
  • SO2 also produces corrosive effect on building materials such as lime stone, marble, roof slate etc., and metals such as iron, steel and aluminium. It also
    causes deterioration of fabrics (cotton, rayon) paper and leather.


Sinks of SOx

The oxides of sulphur are converted into H2SO4 which combine with lime stone minerals. In this way SOx are considerably removed from the atmosphere. Thus; marble structures, lime stone and geological sources of ammonia act As sink for atmospheric Sox.


4. Oxides of Nitrogen (NOx)

Nitrogen and oxygen fotms five oxides. However, three of these five oxides, namely; nitrogen dioxide (NO2), nitric oxide (NO) and nitrous oxide (N20) are present in the atmosphere in significant amounts. Of these three, only two oxides (NO and NO2) are atmospheric pollutants.


(a)   Natural activities. Significant amounts of NOx, mainly as NO is released into the atmosphere by natural bacterial action

  • Small amount of NO2 is produced in soil by microbiological processes.
  • Lightening discharge also results in the formation of  NO and NO2 from N2 and O2 present in the atmosphere.

(b)   Human activities. It involves the production of oxides of nitrogen from N2 and O2 present in the atmosphere during the combustion of carbonaceous matter such as coal, oil and natural gas at high temperature (exceeding 1200°C) in industries.


N2 + 02  2NO ;

2NO + O  2NO2


  • These oxides are also produced during combustion of gasoline or diesel in automobiles.
  • Chemical industries involving manufactures of H2SO4, HNO3 produce oxides of nitrogen as by-products which are released into atmosphere.


It may be noted that natural activity releases about 5 × 103 tonnes of NOx into air mainly as NO. On the other hand, human activity release about 5 × 107 tonnes of NOx every year all over the world. The concentration of NOx is about 10-100 times greater in urban areas (because of
industrialisation and automobile pollution) as compared to rural areas.


Toxic effects of oxides of nitrogen

The main harmful effects of oxides of nitrogen are:

  • NO is biologically less active and less toxic than NO2. Like CO, it binds to haemoglobin and decreases oxygen transport efficiency of blood.
  • Inhaling of the nitrogen oxides by humans results in
    pulmonary odema and haemorrhage.
  • The oxides of nitrogen cause damage to plants. Exposure of plants to NOx causes leaf spotting and breakdown of plant tissues. Excessive concentration (10 ppm) of NO causes decrease in the rate of photosynthesis.
  • The sunlight reacts with nitrogen dioxide to produce highly active oxygen atoms.


NO2   →   NO + O


The active oxygen attacks traces of hydrocarbons in the air and produces irritants which constitute photochemical smog, which is a big health hazard.


  • Acute exposure to NO2 can also cause damage to human health. The smaller concentration of NO2 leads to inflammation of lung tissues. NO2 has irritating
    effects on mucous membrane and higher doses of NO2 may cause bronchitis and respiratory problems. Excessive exposure can even cause death.
  • The nitrogen oxides also cause fading of dyes used in


Sinks of NOx

Many natural processes acts as sink for oxides of nitrogen. These oxides are inherently unstable and decompose to N2 and O2 after some time. Therefore, the concentration of nitrogen oxides in the atmosphere tends to remain low. Some natural processes and photochemical reactions also take care of NOx in the atmosphere and serve as sinks. The end product of these reactions is HNO3. The probable mechanism for the change of NOx to nitric acid are:

 (i) 2NO + O2 → 2NO2

3NO2 + H2O → 2HNO3 + NO

4NO2 + 2H2O + O2 → 4HNO3

(ii)  NO2 + O3 → NO3 + O2

NO3 + NO2 → N2O5

N2O5 + H2O → 2HNO3


5. Hydrogen Sulphide

The main natural sources of H2S are volcanic activity and biological decay of protein matter in stagnant water and swamps. A large number of industries such as petroleum refining, manufacture of paper, manufacture of sulphur dyes, tanning, etc., also produce considerable amount of H2S.


Toxic effect of H2S

 H2S gas is extremely toxic to humans. In higher atmospheric concentrations, it can be fatal. The reaction of H2S with essential proteins is primarily responsible for its toxic effects.

6. Hydrocarbons

These are the chemical compounds of carbon and hydrogen only. The hydrocarbons involved in air pollution are those which are volatile or gases under ordinary conditions. The hydrocarbons in air by themselves alone cause no harmful effects. However, they undergo chemical reactions
in the presence of sunlight and nitrogen oxides forming photochemical oxidants.

The natural processes such as bacterial decomposition of organic matter, forest fires and vegetation are important sources of hydrocarbons in the environment. Gasoline fuelled
vehicles, industrial emissions account for 1/6th of all hydrocarbons in atmosphere.


In general, the term particulate refers to finely divided solid particles or liquid droplets suspended in air. The size of particulates varies from about 0.0002 µ to about 500 µ (1 µ = 10-4 cm), In the polluted air, the number of particulate particles is as high as 1011 per m3. In urban
areas the mass level of particulates varies from 60 µ g to 2 × 103 µ g per cubic metre.

Some Common Particulates and their Sources

1. Soot. The most common particulate injected into the atmosphere through human activity is soot which is produced by incomplete combustion of carbonaceous fossil fuels such as coal, fuel oil, natural gas, wood, etc., in insufficient supply of oxygen.

2. Metal Particles. These are released by various me finishing operations. The microparticles of toxic metals SO2 gas present in the polluted atmosphere get adsorbed the particles rendering them highly toxic.

3. Metal Oxides. Metal oxides are generated by combustion of fuels containing metallic compounds. For example, the particulate Fe3O4 is produced by the combustion of coal containing iron pyrites (FeS2).

3FeS2 + 8O 2 → Fe3O4 + 6SO2

4. Lead Salts. Lead used to be major air pollutant emitted by automobiles in their exhaust gases. Earlier lead tetraethyl [Pb(C2H5)4] was added to gasoline to improve its antiknock

property. Suitable amounts of C2H4Cl2 and C2H4Br2 are added to gasoline along with [Pb(C2H5)4] The volatile lead salts get eliminated through the exhaust. The complete reaction may be represented as follows:


Pb(C2H5)4 + 32O2 + C2H4Br2 + C2H4Cl2 → 20CO2 + 24H2O + PbCl2 + PbBr2

This problem has now been overcome by using unleaded gasoline in vehicles.

5. Fly Ash. It originates from the combustion of high-ash fossil fuels. It contains partially burnt particles of the fuels. This pollutant is released into the atmosphere from thermal

power plants, smelters and mining operations.

6. Asbestos Dust. It originates from industrial units manufacturing asbestos sheets, gaskets, ropes, etc. Asbestos flooring and asbestos insulations also contribute towards asbestos dust in the atmosphere. The-pollutant is also released into the atmosphere during the mining and processing


7. Solid Hydrocarbons. These are emitted from petroleum refineries and comprise of paraffins, ole fines and aromatics. The polynuclear aromatic hydrocarbons such as benzo (α-) pyrene, chrysene and dibenzanthracenes are carcinogens. These compounds are readily adsorbed on the

surface of soot particles present in the atmosphere and thus become serious health hazards.

8. Dust Particulates. 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. Depending upon sources, they may contain number of harmful pollutants such as Pb, Cd, Hg and hydrocarbons. These particles do not remain dispersed in air but gradually settle down on the surface because of gravitational forces.


9. Acid Mist. Sulphuric acid mist is produced when SO3 present in the atmosphere comes in contact with moisture. Similarly nitric acidmist is produced when oxides of nitrogen,
viz, NO and NO2, undergo the series of reactions in the atmosphere. For example,


2NO + O2 → 2NO2

2NO2 + H2O + O2 → 2HNO3

or 2NO2 + O3 + H2O → 2HNO3 + O2


Harmful Effects of Particulates

1.      Effect on Human Beings. The particulate pollutants seriously effect the human respiratory system and cause several respiratory diseases. The particles with small size are move harmful in this context. The larger particles are easily checked by the hairlike cilia in the nose while the smaller particles reach the lungs easily and get accumulated there. Exposure to particulates for long periods of times causes scarring or fibrosis of lung lining. This type of disease is well known in industrial settings and is termed as pneumoconiosis.


The particulates thus become the carriers of these toxic substances from the atmosphere to the humans and cause big health hazards. They cause bronchitis, bronchial asthma, emphysema and lung cancer and various allergic reactions.

 2.      Effects on Visibility. Particulates in the atmosphere cause scattering and absorption of sunlight and reduce the visibility. They also produce blurring effect on vision. The reduced visibility is dangerous practically for aircraft and motor vehicles drivers.

3.      Effects on Materials. The adverse effects of Particulates on materials include corrosion of metals (when the atmosphere is humid), erosion and soiling of buildings, sculptures and painted surfaces and soiling of clothes and 

  1. draperies. Particulate matter also damages the electronic Equipment through chemical or mechanical action on electrical contacts.

4.      Effects on Plants. Particulates get deposited on the

  1. leaves of the plants thereby blocking the stomata and retarding the transpiration of minerals from the soil. They also hinder the intake of CO2 from the air thereby causing obstruction in photosynthesis is process.


The various air pollutants and the sources from which they originate, are given in Table 52.1.