The term air pollution signifies the presence of substances generated by human activities in concentrations that interfere with human health, safety or comfort; or injurious to vegetation, animals and other environmental media resulting in chemicals entering the food chain or being present in drinking-water and thereby constituting additional source of human exposure. The air pollutants can influence the structure and function of ecosystems; can affect plants, animals, and soil, thereby affecting the quality of life.
Sources of Air Pollution
The main sources of air pollution are:
- AUTOMOBILES: Motor vehicles emit hydrocarbons, carbon monoxide, lead, nitrogen oxides and particulate matter. In addition, diesel engines, when misused or badly adjusted are capable of emitting black smoke and malodorous fumes.
- INDUSTRIES: Combustion of fuel to generate heat and power produces smoke, heat, sulphur dioxide, nitrogen oxides and fly ash. Many industries discharge carbon monoxide, carbon dioxide, ozone, hydrogen sulphide and sulphur dioxide
- DOMESTIC SOURCES: Domestic combustion of coal, wood or oil result in emergence of smoke, dust, sulphur dioxide and nitrogen oxides.
- TOBACCO SMOKE: The most direct and Important source of air pollution affecting the health of many people is tobacco smoke. Even those who do not smoke may inhale the smoke produced by others called passive smoking.
- MISCELLANEOUS: These comprise of burning refuse, incinerators, pesticide spraying, wind borne dust, fungi, molds, bacteria and nuclear energy programmes. All these contribute to air pollution.
Although the Earth’s atmosphere extends to several layers above the surface, man is directly concerned with only 8-10 km of the atmosphere.
The level of atmospheric pollution depends upon
- Topography-Pollutants concentrate in the breathing zone if the topography is dominated by tall buildings.
- Air movement-Wind helps in the dispersal and dilution of pollutants.
- Climate-Vertical diffusion of pollutants depends on the temperature gradient. Rapid cooling of lower layers of air reduces vertical motion and traps the pollutants and water vapours resulting in smog. The temperature inversion or rapid cooling of inner layers of air is a threat to human health.
More than 100 pollutants have been identified. The important ones are carbon monoxide, carbon dioxide, hydrogen sulphide, sulphur dioxide, hydrocarbons, ozone, lead and cadmium. Contaminants differ greatly from place to place depending upon the specific complex of contaminant source. The combination of smoke and fog is called 'smog’.
- Carbon monoxide: a product of incomplete combustion of carbon containing materials. Concentrations in urban areas depend on weather and traffic density.
- Sulphur dioxide: results from the combustion of sulphur containing fossil fuel, the smelting of sulphur containing ores and industrial processes, domestic fires.
- Lead: The combustion of alkyl lead additives in motor fuels accounts for lead emissions. The degree of pollution depends on vehicle density and the lead content of petrol. Children upto 6 years are at risk for lead exposure and adverse health effects. Since blood brain barrier is not yet fully developed in young children they have a lower threshold for hematological and neurological effects of lead than adults. Pregnant women and her fetus are next affected category to lead exposure, since the placenta is not an effective biological barrier.
- Carbon dioxide: is not an air pollutant, as it is a natural constituent of the air and does not take part in any significant reaction with other substances in the air. However, its global concentration is rising above the natural level to affect global warming and climate change.
- Hydrocarbons: Man-made sources of hydrocarbons include incineration, combustion of coal, wood, processing and use of petroleum hydrocarbons participate in the chemical reactions causing photochemical smog.
- Cadmium: emission is due to steel industry, waste incineration, volcanic action and zinc production. Since tobacco contains cadmium, smoking contributes to the uptake of cadmium. Cigarettes may contain from 05 to 3 pg cadmium per gram.
- Hydrogen sulphide: due to human activities, coke production, viscose rayon production, waste-water treatment plants, wood pulp production, sulphur extraction process, oil refining and in tanning industry hydrogen sulphide is generated. Effects of hydrogen sulphide are-
- unpleasant odour at low concentration
- conjunctival irritation
- diffuse neurological and mental symptoms at concentrations of less than 30 µg/m3
- Ozone: Ozone is one of the strongest oxidizing agents: There are no significant anthropogenic emissions of ozone into the atmosphere. Existing ozone has been formed by chemical reactions that occur in the air. In the troposphere, the ozone producing and ozone-scavenging processes involve absorption of solar radiation by nitrogen dioxide. The maximum ozone concentration that can be reached in polluted atmosphere appears to depend not only on the absolute concentrations of volatile organic compounds and nitrogen oxides but also on their ratio. At intermediate ratio (4:1-10:1), conditions are favourable for the formation of appreciable concentration of ozone. Seasonal variations in ozone concentrations occur and are caused mainly by changes in meteorological processes. Quarterly mean ozone concentrations are highest during the second and third quarters of the year.
Indoor Air Pollution
Indoor air pollution is one of the four most critical global environmental problems, probably exposes more people worldwide to important air pollutants than does pollution in outdoor rural population in developing countries particularly, women and children fall a prey to indoor air pollution.
It contributes to
- acute respiratory infections in young children, mainly pneumonia
- chronic lung disease and cancer in adults, exposed to indoor coal smoke
- adverse pregnancy outcomes (such as stillbirths).
Table 1 shows the sources of indoor air pollutants.
Tobacco smoke, Stove, Aerosol sprays
Stove, gas heaters
Gas cookers, cigarettes
Particle board, carpet adhesives, insulation
Monitoring of Air Pollution
The best indicators of air pollution are sulphur dioxide, smoke or soiling index, grit and dust, coefficient of haze and suspended particles. Air pollution index is an arbitrary index which takes into account one or more pollutants as a measure of the severity of pollution.
Central Pollution Control Board (CPCB) sponsored National Air Quality Monitoring Programme, facilitates evaluation of long-term air quality trends for health-related criteria pollutants such as inhalable dust, sulphur dioxide, nitrogen dioxide, lead, hydrogen sulphide and ammonia. The trend analysis showed that Suspended Particulate Matter (SPM) exceeds the CPCB standards in major Indian cities throughout the year. Chromium, copper, nickel, arsenic, lead, iron, zinc sulphate, nitrate, chloride, fluoride, ammonia, sodium and potassium are secondary pollutants analyzed for the assessment of dry deposition of air pollutants. Wet deposition of air pollution has been evaluated by analyzing rain water. The first rain has maximum concentration of pollutants with low pH, higher sulphate and nitrate contents. Thus, leads to acid rain, the phenomenon lasts for a short period in urban atmsphere of Indian cities.
Effects of Air pollution
Air quality has generally improved during the past two decades in developed countries, but has deteriorated in many developing countries. The reasons being
- Rise in industrial activity,
- increasing power generation,
- Congestion of streets with poorly maintained motor vehicles that use leaded fuel. Air pollution can affect in two ways:
- Health aspects:
- Immediate – borne by the respiratory system resulting state is acute bronchitis
- Delayed – intense pollution results in immediate death by suffocation. Chronic bronchitis, lung cancer, bronchial asthma, emphysema and respiratory allergies are the delayed effects of air pollution. Lead poisoning though affects many systems in the body, is particularly dangerous to children with developing brain and nervous system. There was a reported association between increased levels of lead and impaired neuropsychologic development as measured by loss of IQ poor school performance and behavioural difficulties.
It is difficult to quantify the effects of air pollution as the amount of exposure to polluted air and influence of related variables like smoking, nutrition, occupation and climate are not measurable. Air pollution damages the human respiratory and cardio respiratory system. Most vulnerable population includes elderly, children, smokers and those with chronic respiratory difficulties. Sudden increase in pollution is reported to augment the mortality and morbidity.
- Social and economic aspects: Includes
- destruction of plant and animal life
- corrosion of metals
- damage to buildings
- cost of cleaning, maintenance, repair and esthetic nuisance.
- reduces visibility in towns
- soil and damage clothing