Pollution can be defined as the introduction
by humans, deliberately or inadvertently, of
substances or energy (heat, radiation, noise)
into the environment—resulting in a
deleterious effect. Whereas, contamination is
the presence of elevated concentrations of
substances in the environment, food, etc.,
which may not necessarily be harmful or a
Pollution can be ‘primary’, having effect immediately
on release to the environment ; or ‘secondary’, the
product of interaction after release with moisture,
other pollutants, sunlight, etc.
Pollution may be local, regional, transboundary, or
global. The effects may be direct, indirect or
cumulative, felt intermittently or constantly, immediate
or after a delay ; affecting the atmosphere, water
bodies, oceans, ground water, soil or be restricted to
certain organisms, produce or localities.
The effects of pollution may be short term or longer
term ; pose a hazard or a nuisance; be toxic or nontoxic ; take the form of a chemical, biological,
radiation,heat, light, noise, dust, or odour problem.
• Air pollution is basically the presence of
foreign substances in air in excessive
concentration which adversely affects the well
being of the individual or causes damage to
• According to World Health Organisation, air
pollution is defined as, “substances put into
air by the activity of mankind into
concentration sufficient to cause harmful
effect to his health, vegetables, property or to
interfere with the enjoyment of his property.”
• Indian Standards Institute define air pollution
as, “Air pollution is the presence in ambient
atmosphere of substances, generally resulting
from the activity of man, in sufficient
concentration, present for a sufficient time
and under circumstances which interfere
significantly with the comfort, health or
welfare of persons or with the full use or
enjoyment of property.”
Historical Overview
(i) London Smog.-1952,
(ii) Meuse Valley, Belgium. Dec. 1, 1930
(iii) Donora, Pennsylvania (USA). October 1948
(iv) Pittsburgh. Prior to 1948 the nickname of
Pittsburgh was “Smokey City”, and it seemed to
be appropriate as a black pall of smoke and soot
often turned day into night, and blackened the
brightest buildings in a few months.
(v) Los Angeles, California. In early 1950’s due
to large volume of traffic on Los Angeles
streets, photochemical smog is formed by the
interaction of HCs and oxidants (like NOx, CO,
O3) in the presence of sunlight to form toxic
PAN and ozone, causing eye irritation, visibility
reduction and damage to crops and rubber
(vi) Bhopal Gas Tragedy December, 1984
Sources of Air Pollutants
Classification of Air Pollutants
1. According to Origin
• Primary air pollutants are those which are
emitted directly to the atmosphere and found
there in the form in which they are emitted. For
example, particulates, carbon monoxide (CO),
oxides of sulphur (SOx),oxides of nitrogen (NOx),
hydrocarbons (HCs), radioactive compounds,
particles of metal, pollen, bacteria, etc. The five
main primary air pollutants (viz. particulates, CO,
SOx, NOx and HCs) contribute more than 90% of
global air pollution.
•Secondary air pollutants are those which are
produced in the air by the interaction among two
or more primary air pollutants, or by reaction
with normal atmospheric constituents, with or
without photoactivation. For example, ozone
(O3), peroxyacetyl nitrate (PAN), formaldehyde,
formation of acid mists, smog (coal induced and
photochemical smog),etc.
2. According to Chemical Composition
On the basis of chemical composition, air
pollutants can be divided as—organic and inorganic
air pollutants.
• Organic compounds contain carbon and
hydrogen, and many also contain certain
elements such as oxygen, nitrogen, sulphur and
phosphorus. Examples of organic air pollutants
are hydrocarbons, aldehydes, ketones, carboxylic
acids, organic sulphur compounds, etc.
• Inorganic air pollutants include compounds, such
as CO, CO2 , SOx, NOx, O3, etc.
3.According to State of Matter
On this basis, air pollutants are classified as—
particulate and gaseous air pollutants.
• Particulate air pollutants include finely divided
solids and liquids dispersed in gaseous media.
Dust, smoke, fly ash, flumes, etc., are examples of
solid particulates ; while mist, spray, fog, etc., are
liquid particulate air pollutants.
• Gaseous air pollutants are organic gases like
benzene, methane, butane, aldehydes, ketones,
etc., as well as inorganic gases like CO2, SOx, CO,
NH3, H2S, NOx, etc.
Effects of Air Pollution on
Human Health
•The air we breathe has not only life sustaining
properties, but also life damaging properties. An
average man breathes 22,000 times a day
and takes in 16 kg of air each day. The impurities
in the inhaled air can affect human health in a
number of ways, depending upon the nature
and concentration of the pollutants, duration of
exposure, and age group of the receptor.
The various health effects are :
(i) Eye irritation can be caused by many air
pollutants such as Nox,O3, PAN, smog,
particulates, etc.
(ii) Nose and throat irritation can be caused by SO2,
NOx, insecticides,pesticides, etc.
(iii) Gaseous pollutants like H2S, SO2, NO2 and
hydrocarbons can cause odour nuisance even at
low concentrations.
(iv) Irritation of the respiratory tract can be caused
by SOx, NOx, O3,CO, etc.
(v) Increase in mortality and morbidity rate.
(vi) A variety of particulates, particularly pollens,
can initiate asthmatic attacks.
(vii) High concentrations of SO2, NO2, SPM (suspended
particulate matter) and photochemical smog can aggravate
chronic pulmonary diseases like bronchitis and asthma.
(viii) Carbon monoxide, which is two hundred times more
reactive than oxygen, combines with haemoglobin in the
blood and consequently increases stress on those suffering
from cardiovascular and pulmonary diseases. Similarly, nitric
oxide (NO) can react with haemoglobin and reduce the
oxygen carrying capacity of the blood.
(ix) Hydrogen fluoride can cause fluorosis and mottling of teeth.
(x) Air pollutants such as polycyclic organic compounds, aliphatic
hydrocarbons, etc. can cause cancer.
(xi) Dust particles can cause dust specific respiratory
diseases, such as, silicosis (associated with silica
dust), asbestosis (associated with asbestos dust),
(xii) Heavy metals, like lead (emitted from vehicles),
may enter the body through the lungs and can
cause poisoning. Its high concentration can
damage liver and kidney, and can cause
abnormality in fertility and pregnancy, and
mental development of children gets affected.
(xiii) Exposure to radioactive isotopes like Iodine
131, Phosphorus 32, Cobalt 60, Radium 226, etc.
can cause anaemia (iron deficiency), leukaemia
(RBC deficiency), cancer, and genetic defects.
Effects of Air Pollution on Animals
The process by which the animals get poisoned is
entirely different
from that by which human beings exposed to air
pollutants are poisoned.
In case of animals, it is a two-step process :
(i) accumulation of air pollutants in the vegetation
and forage; and
(ii) subsequent poisoning of the animals, when they
eat the
contaminated vegetation/forage.
The pollutants mainly responsible for most livestock
damage are :
Fluorine : Of all the farm animals, cattle and sheep are
the most susceptible to fluorine toxicity. Horses are
quite resistant, while poultry are probably the most
resistant to fluorine of all the farm animals. Fluorine is
a cumulative poison under conditions of continuous
exposure to subacute doses. Its effects are lack of
appetite, rapid loss of weight,lameness, periodic
diarrhoea, muscular weakness, wearing of teeth, and
Lead : Chronic lead poisoning has been observed
frequently in animals that have been grazing near
smelters and lead mines. It causes paralysis and
difficulty in breathing. In case of acute lead
poisoning, the onset is sudden and the course is
relatively short. There is complete loss of appetite,
paralysis, and diarrhoea.
Arsenic : In acute cases, it can cause severe
salivation, thirst,vomiting, irregular pulse and
respiration, abnormal body temperature, and death
in few hours. Chronic arsenic poisoning causes
cough,diarrhoea, anaemia, abortion, paralysis, and
Economic Effects of Air Pollution
Air pollution damage to property/material is a very important
economic aspect of pollution, and it covers a wide range :
(i) Corrosion : Air pollution damages materials chiefly by corrosion
of metals. The prime air pollutant responsible for metallic
corrosion is SO2. In the presence of oxygen and moisture, it is
converted to sulphuric acid. Deposition of this acid on metal
parts of building roofs, railway tracks, overhead wires, metal on
bridges, and other structures cause enormous loss due to
(ii) Damage to building materials : The acid deposition reacts with
lime stone, marble, and other building materials to cause
deterioration and disfigured the building materials.
(iii) Damage to paints and protective covering : Pollutants like SO2, O3,
H2S, and aerosols damage protective coating and paints of the
(iv) Damage of textile dyes and textile fibres : The fading of textile dyes
and deterioration of natural and synthetic textile fibres is caused
by SOx, NOx, and O3.
(v) Rubber Cracking : Rubber cracking of tyres and various forms of
electrical insulation is caused by ozone and PAN.
(vi) Deterioration of leather and paper : Sulphur dioxide causes leather
to lose much of its strength and ultimately disintegrate; which
has posed a serious problem of storage of leather bound books in
libraries. The impurities in paper absorb SO2 and convert it into
H2SO4 in the presence of moisture, which makes the paper
extremely brittle and decreases its folding resistance.
(vii) Effect on glasses and ceramics : Although
glasses and ceramics are especially resistant to
the chemical action of air pollutants, but
long exposure for years showed a change in
their surface appearance.
Air Pollutants
1. Particulates
Aerosols or particulates are dispersed solid or liquid particles of
microscopic size in gaseous media. The term aerosol is applicable
as long as the particle is suspended in air. Once the particle
settled on a liquid or solid surface, the term no longer applies.
However, particulate matter is a nuisance both as an aerosol
and as settled or deposited matter.
Particulates can either be natural or anthropogenic. Natural
particulates include pollen, viruses, bacteria, fungi, spores,
protozoa, plant fibres, volcanic dust, etc. while anthropogenic
particulates include smoke, fly ash, root particles, acid
droplets, inorganic dusts, metallic oxides and salts, metallic
fumes, etc.
It is the physical characteristics of the particulates which are
quite significant in air pollution. The physical characteristics
include size, settling properties, mode of formation and
optical qualities. Size is one of the most important physical
properties of particulates. The surface area of the particulates
presents good sites for adsorption/absorption of various
organic and inorganic matter. The sizes of the particles, which
are of major interest in air pollution, range from 0.01 to 100
μm in diameter.
Particulates can generally be classified as suspended or
Suspended particulates vary in size from less than 1.0 μm
to approximately 20 μm. They can remain suspended in
the atmosphere for long periods of time. The settleable
particulates (or dustfall) are larger and heavier
particulates. They are generally greater than 10 μm in
size and settle out close to their sources. However, the
settling rate of particualtes depends upon the size and
density of the particulates as well as turbulence of air.
• According to their mode of formation, the
aerosols are classified as — dust, smoke, fly
ash, fumes, mist and spray ;
• Dust is made up of solid particles larger than
those found in the colloids, and are capable of
temporary suspension in air or other gases.
• Dust is produced by the crushing, grinding or
blasting of organic and inorganic materials
such as coal, stones, cement, wood, etc. It
may range in sizes from 1 to 1,000 μm.
• Smoke is made up of fine solid particles resulting
from the incomplete cumbustion of organic
particles such as coal, wood, etc. The dia. of
smoke particles vary from 0.5 to 1.0 μm.
• Fly ash consists of finely divided, non-combustible
particles contained in flue gases arising from
combustion of coal. Like dust, it has particles that
range in size from 12 to 1,000 μm. Like smoke, it
results from burning of coal; and like fumes, it
consists of inorganic metallic or mineral
Fumes are fine solid particles formed by the condensation of
solid materials, such as molten metals. For example, metallic
oxides of zinc, lead, etc. The particles sizes range from 0.03 to
0.3 μm.
Mist consists of liquid particles or droplets formed by the
condensation of a vapour, or by the dispersion of a liquid (as
happens in foaming or spraying), or by a chemical reaction
(such as the formation of sulphuric acid mists). Mists are
usually less than 10 μm in dia. If the mist concentration is high
enough to reduce visibility, then it is called a fog.
Spray consists of liquid particles formed by the atomization of
parent liquids such as pesticides and herbicides. Its size ranges
from 10 to 1,000 μm.
• Effects
• Particulate’s effects on human health are
immense. The fine particles (< 3 μm in size)
are the worst as they can penetrate deep into
the lungs through the respiratory tract. Once
they are lodged into the lungs, they can cause
severe breathing trouble by physical blockage
and irritation of the lung capillaries.
2. Carbon Dioxide (CO2)
It is a colourless, tasteless, odourless and noncombustible gas that provides the carbonation
in soft drinks and beverages. It has been part of
the earth’s atmosphere as long as the earth has
had an atmosphere. At present the carbon
dioxide concentration in the atmosphere is
approximately 350 ppm.
3. Carbon Monoxide (CO)
It is a colourless, tasteless and odourless gas. It is slightly lighter
than air (0.965 times as heavy as air) and is insoluble in water. It is
chemically inert under normal conditions and has an estimated
atmospheric life of about two and a half months. It is a poisonous gas
and is generally classified as an asphyxiant. The atmospheric
background of CO is 0.1 ppm. It is produced by
(i) incomplete burning of the carbon in fossil fuels
2C + O2 ⎯→ 2CO
(ii) reaction between carbon dioxide and carbon containing materials
at very high temperatures in industrial processes, such as in electric
and blast furnaces
CO2 + C ⎯→ 2CO
(iii) and by dissociation of carbon dioxide at higher temperatures
CO2 CO + O
4. Hydrogen Sulphide (H2S)
Hydrogen sulphide is well known for its rotten egg smell. It is
produced by the reduction of sulphur. It is a very strong smelling
substance and is gaseous at room temperature having a boiling point
at – 60°C. It is slightly soluble in water at atmospheric conditions, but
groundwater may contain considerably higher concentrations under
pressure commonly found in deep aquifers. It imparts an unpleasant
taste and odour to water even in small concentrations. It is not toxic in
low concentrations that normally exist in the atmosphere (0.0002
ppm), but is toxic in high concentrations. The threshold limit value
(TLV) of hydrogen sulphide is 10 ppm.
• Effects
Exposure to hydrogen sulphide for short
periods can result in fatigue. But high
concentrations of H2S due to accidental
release, often cause fatalities. This occurs in
the production and processing of sour gas and
oil, which contain hydrogen sulphide. There
are many incidences of leakages of H2S from
natural gas processing plants killing hundreds
of people.
5. Oxides of Sulphur (SOx )
The oxides of sulphur (SOx) are probably the
most widespread and most intensely studied of
all the anthropogenic air pollutants. They
include six different gaseous compounds, viz.,
sulphur monoxide (SO), sulphur dioxide (SO2),
sulphur trioxide (SO3), sulphur tetroxide (SO4),
sulphur sesquioxide (S2O3) and sulphur
heptoxide (S2O7). In air pollution, SO2 in
particular and SO3 are the two most important
oxides of sulphur.
6. Ozone (O3)
Ozone is a bluish gas with a pungent odour. It can be created by
passing a high voltage through dry atmospheric air between two
stationary electrodes. It is unstable and breaks down to normal oxygen
and nascent oxygen (which is a powerful oxidizing agent).
Natural ozone mainly occurs in the stratosphere (between 16 to
40 km), where it serves a vital biological role in absorbing high energy
photons of ultraviolet radiation from sun and thus, protects the plants,
animals and human beings from the hazardous effects of UV-B
radiations*. Natural ozone is also present in troposphere, where it has a
background concentration of about 0.02 ppm.
Ozone is a life saviour, if present in stratosphere;
but a pollutant, if present in troposphere.
Ozone is a smelly and poisonous (at higher
concentrations) gas.Ozone, which is a major
component of photochemical smog along with
PAN, has an irritant action in the respiratory
track reaching much deeper into lungs than
oxides of sulphur. It can cause coughing,
shortness of breadth, air-way constriction,
headache, chest tightness, altered red blood
cells and eye, nose and throat irritation.
7. Oxides of Nitrogen (NOx)
Of the seven oxides of nitrogen, viz., nitrous
oxide (N2O), nitric oxide (NO), nitrogen dioxide
(NO2), nitrogen trioxide (NO3), nitrogen
sesquioxide (N2O3), nitrogen tetraoxide (N2O4)
and nitrogen pentaoxide
(N2O5), that exist in ambient air, only two oxides
of nitrogen (NO and NO2) are primarily involved
in air pollution.
8. Hydrocarbons (HCs)
Hydrocarbons are those organic compounds
which contain only carbon and hydrogen. Like
CO, they represent unburned and wasted
fuel. Most of the major chemicals in gasoline
and other petroleum products are
hydrocarbons, which are divided into two
categories— aliphatic and aromatic.
Aliphatic hydrocarbon group contains alkanes, alkenes and alkynes.
The alkanes are saturated hydrocarbons (i.e., methane) and are fairly
inert,and generally not active in atmospheric photochemical reactions.
The alkenes, generally called olefins, are unsaturated and highly
reactive in atmosphere. The alkenes (such as ethylene), in the
presence of sunlight,react with nitrogen dioxide at high concentrations
to form secondary pollutants such as PAN (peroxyacetyl nitrate) and
ozone. The alkynes,though highly reactive, are relatively rare and thus
not of major concern in air pollution.
Aromatic hydrocarbons are biochemically and biologically active, and
some are potentially carcinogenic. They are derived from or related to
benzene. Though aromatics do not display the reactivity characteristics
of unsaturated aliphatic hydrocarbons, but the polynuclear group of
aromatic hydrocarbons are carcinogenic.
• Effects
• Hydrocarbons are generally not toxic at concentrations
normally found in the atmosphere, but they are major
pollutants because of their role in the formation of
photochemical smog.
• Experimental tests on humans and animals with
aliphatichydrocarbon concentrations of 500 ppm
produce no harmful effects. But polynuclear group of
aromatic hydrocarbons from automative exhaust
emissions are carcinogenic in nature.
• Ethylene, produced in automobile exhaust, is one of
the very few hydrocarbons that can cause plant
damage even at low concentrations. Tomato and
pepper plants, and orchids can be severely damaged if
they are exposed to ethylene (0.01 to 0.3 ppm) for
longer duration.
Air Pollution Control
Atmospheric Self-Cleansing Processes
The atmosphere, like a stream or river, has natural built-in
selfcleansing processes. Dispersion, gravitational settling,
flocculation, absorption, rain out, and adsorption are some of
the most significant natural removal mechanisms at work in the
Though not literally a removal mechanism, dispersion of
pollutants by wind decreases the concentrations of pollutants at
any place. Gravitational settling is one of the most important
natural mechanisms for removing particulates from the
atmosphere, especially particles larger than 20 μm in size. It also
plays an important role in several of the other natural
atmospheric cleansing processes
. In flocculation, larger particles act as receptor for smaller
ones to form a unit, and the process is repeated until a
small floc is formed, which is large and heavy enough to
settle under gravity. In the natural absorption process,
particulates or gaseous pollutants are collected in rain or
mist and then settle-out with moisture.
This phenomenon is known as washout or scavenging, it takes
place below cloud level. Rainout is another natural
cleansing process that occurs within the clouds, when submicron particulates serve as condensation nuclei around
which drops of water may form. This phenomenon has
resulted in increased rainfall and fog formation in urban
areas. Adsorption occurs primarily in the friction layer of
the atmosphere, i.e., the layer close to the earth’s surface,
such as soil, rocks, leaves or blades of grass, where they are
concentrated and retained.
Approaches to Air Pollution Control
(i) Dispersion of source locations. Air pollution can be controlled/
checked by dispersion of the sources of air pollutants, through
allocation of land, i.e., by proper planning and zoning of
industrial areas.
(ii) Dilution. By using tall stacks for industries or thermal plants, the
emissions or pollutants can be discharged at a sufficient height
from the ground, where the air movement, both horizontal and
vertical, is more and chances of downward movement of air (i.e.,
inversion conditions) are less. This will help in dispersion of
pollutants over a larger area in less time, and hence dilute the
concentrations of pollutants near the source.
(iii) Reduction at source by process changes. This can be achieved by:
(a) substitution of raw materials; e.g., the use of low-volatile coal in
place of high-volatile coal, eliminates smoke and soot.
(b) substitution of fuel ; e.g., desulphurization and de-ashing reduce
emissions of SO2, SPM (suspended particulate matter) and ash.
Similarly, natural gas can be used in place of coal, to minimise
(c) modification of the process; e.g., in case of disposal of combustible
refuse, sanitary landfill can be used instead of incinerators.
(d) modification of the process equipment, or repair and maintenance
of existing equipment helps in reducing atmospheric pollution.
(iv) Reduction at source by using control equipment. This is the most
effective method for reducing air pollution at source. Various
control devices are used for controlling different types of
pollutants emitted from different sources.
(1) Settling Chamber.
(2) Cyclone
(4) Electrostatic Precipitators (ESPs).
(5) Scrubbers or Wet Collectors.
Spray Tower
multiple fixed bed adsorber
Plate or tray towers
Packed towers

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