Monday, November 4, 2013

Air Pollution - causes, effects and control measures

Air pollution - Air pollution may be defined as the presence of one or more contaminants like dust, mist, smoke and colour in the atmosphere that are injurious human beings, plants and animals.
  1. Rapid industrialization
  2. Fast urbanization
  3. Rapid growth in population
  4. Growth of vehicles on the roads and
  5. Activities of human beings have disturbed the natural balance of the atmosphere.
The composition of Air is given below:
Nitrogen                                     78%
Oxygen                                      21%
Argon                                        less than 1%
Carbondioxide                           0.037%
Water vapour                            Remaining
Ozone, Helium and ammonia     Trace amount

Sources of Air pollution
Sources of  air pollution are of two types.
Natural sources and Artificial sources

Natural sources of pollution are those that are caused due to natural phenomena. Ex: Volcanic eruptions, Forest fires, Biological decay, Pollen grains, Marshes, Radioactive materials.

Artificial sources are those which are created by man. Ex: Thermal power plants, Vehicular emissions, Fossil fuel burning, agricultural activities etc.

Classification of Air Pollutants
Depending on the form of pollutants present in the environment, they are classified as:
  1. Primary pollutants and
  2. Secondary pollutants
Primary pollutants are those that are directly emitted in the atmosphere in the harmful form
Ex: CO, NO, CO2, SO2 etc.

Secondary pollutants are those that are formed by reacting with other components or some basic component of the atmosphere to form new pollutants.
Ex: Oxides of Nitrogen (NO2 or NO3) react with moisture in the atmosphere to give Nitric acid

Indoor air pollutants are primary air pollutants. The most important indoor air pollutant is Radon gas.
Sources of indoor air pollutants are:
  1.  Radon gas is emitted from building materials like bricks, concrete, tiles, etc that are derived from soil containing radium
  2. Radon is also found  in natural gas and ground water and is emitted while being used.
  3. Burning fuel in the kitchen and cigarette smoke release pollutants like CO, SO2, HCHO (Formaldehyde) and BAP (Benzo-(A) pyrene).
Carbonmonoxide: It is a colourless, odourless gas that is poisonous animals. It is formed by incomplete combustion of carbon containing fuels.
Source of carbonmonoxide is cigarette smoking and incomplete combustion of fossil fuels (more than 77% comes from motor vehicle exhaust)
Health effects include reduced ability of red blood cells to carry oxygen to body cells and tissues. This leads to headache and anemia. At high levels it causes coma, irreversible brain damage and death.

Nitrogen Dioxide: It is a reddish-brown irritating gas that causes photochemical smog. In the atmosphere, it gets converted into nitric acid (HNO3). It is caused by burning fossil fuels in industries and power plants.
Health effects include lung irritation and damage. Environmental effects involve acid deposition leading to damage of trees, lakes, soil and ancient monuments. NO2 can damage fabrics.

Sulphur Dioxide: It is a colourless and irritating gas that is formed by combustion of sulphur containing fossil fuels such as coal and oil. In the atmosphere it is converted into Sulphuric acid which is a major component of acid deposition.
Health effects involve breathing problems for healthy people.
Environmental effects involve reduced visibility and acid deposition on trees, lakes, soils and monuments leading to their deterioration and adverse effect on aquatic life.

Suspended Particulate Matter (SPM): Includes a variety of particles and droplets (aerosols) that can be suspended in atmosphere for short  to long periods.
Human sources for SPM include burning coal in power and industrial units, burning diesel and other fuels in vehicles, agriculture, unpaved roads, construction, etc.
Health effects include nose and throat irritation, ling damage, bronchitis, asthama, reproductive problems and cancer.
Environmental Effects include reduced visibility and acid deposition. Acid deposition may lead to damaged trees, soils and aquatic life in lakes.

Ozone is a highly reactive gas with an unpleasant odour occurring in the stratosphere where it protects mankind fro the harmful ultra-violet rays from the Sun. However on earth, it is a pollutant.
It occurs on earth due to reaction between Volatile Organic Compounds (VOCs) and Nitrogen Oxides. It moderates the climate

Photochemical smog is a browinsh smoke that frequently forms on clear, sunny days over large cities with significant amounts of automobile traffic.It is mainly due to chemical reactions among nitrogen oxides and hydrocarbons in the presence of sunlight.
Health effects include breathing problems, cough, eye, nose and throat irritation, heart diseases, reduced resistance to colds and pneumonia.
Environmental effects involve damage to plants and trees. Additionally, Smog reduces visibility.

Lead is a solid and highly toxic metal. Its compounds are emitted into the atmosphere as particulate matter.
Human Sources: Paint, Smelters (metal refineries), lead manufacture, storage batteries, leaded petrol, etc
Health effects: Lead accumulates in the body and brain leading to nervous system damage and mental retardation (especially in children), digestive and other health problems. Lead containing chemicals are known to cause cancer in test animals.
Environmental Effects: It can harm wildlife.

Hydrocarbons Lower haydrocarbons accumulate due to decay of vegetable matter.
Human effects: They are carcinogenic

Chromium: It is a solid toxic metal emitted into the atmosphere as particulate matter.
Human sources: Paint, Smelters,  Chromium manufacture, Chromium plating.
Health Effects: Perforation of nasal septum, chrome holes, etc.

The atmosphere has several built-in self cleaning processes such as dispersion, gravitational settling, flocculation, absorption, rain-washout, etc to cleanse the atmosphere. However, control of contaminants at their source level is a desirable and effective method through preventive or control technologies.
Source control: Some measures that can be adopted in this direction are:
  1. Using unleaded petrol
  2. Using fuels with low sulphur and ash content
  3. Encouraging people to use public transport, walk or use a cycle as opposed to private vehicles
  4. Ensure that houses, schools,  restaurants and playgrounds are not located on busy streets
  5. Plant trees along busy streets as they remove particulates, carbon dioxide and absorb noise
  6. Industries and waste disposal sites should be situated outsdide the city preferably on the downwind of the city.
  7. Catalytic converters should be used to help control emissions of carbon monoxide and hydrocarbons
Control measures in industrial centers
  1. Emission rates should be restricted to permissible levels by each and every industry
  2. Incorporation of air pollution control equipment in design of plant layout must be made mandatory
  3. Continuous monitoring of the atmosphere for pollutants should be carried out to know the emission levels.
Air  pollution can be reduced by adopting the following approaches.
  1. Ensuring sufficient supply of oxygen to the combustion chamber and adequate temperature so that the combustion is complete thereby eliminating much of the smoke consisting of partly burnt ashes and dust.
  2. To use mechanical devices such as scrubbers, cyclones, bag houses and electro-static precipitators in manufacturing processes. The equipment used to remove particulates from the exhaust gases of electric power and industrial plants are shown below. All methods retain hazardous materials that must be disposed safely. Wet scrubber can additionally reduce sulphur dioxide emissions.
  3. The air pollutants collected must be carefully disposed. The factory fumes are dealt with chemical treatment.

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