Air pollution encompasses a diverse array of natural and anthropogenic emissions, including gaseous constituents, volatile chemicals, aerosols (particulate) and their atmospheric reaction products. Ambient air acts as an atmospheric sink, where all emissions are released . Many such emissions are in such small quantities that they get immediately dissipated and absorbed but continuous release of these pollutants build up in the air and pose hazards to human health. Despite pollution control effects, the air quality has been threatened to alarming levels in several cities throughout the world. The World Health Organisation (WHO) estimates that as many as 1.4 billion urban residents in the world breathe air exceeding the WHO air quality guidelines (World Resource Institute, 2000). The health consequences of exposure to dirty air are considerable. On a global basis, estimates of mortality due to outdoor air pollution is estimated to be around 2.0 to 5.7 lacs, representing about 0.4 to 1.1 percent of total annual deaths. The adverse effects of air pollution are more pronounced in the developing countries. On the one hand, the developing countries are grappling with the environmental problems associated with inadequate economic developments, the hallmarks of which are malnutrition, poor sanitation and lack of basic human needs. On the other hand , in their pursuit for rapid economic development , the developing countries are confronted with a newer set of environmental problems due to increasing air pollution on account of industrialization, urbanization and motorization.

Severe air pollution episodes during last century have shown that breathing dirty air can be dangerous and at times deadly. The 1948 'Killer Fog' in small town of Donora, Pennsylvania that killed 50 and the particularly virulent 'London Fog' of 1952, in which some 4000 died, were associated with widespread use of dirty polluting fuels. Since then, many countries have adopted ambient air quality standards to safeguard the public against the most common and damaging pollutants, which include sulphur dioxide, suspended particulate matter, ground level ozone, nitrogen dioxide, carbon monoxide and lead, which are directly or indirectly released by the combustion of fossil fuels. Although substantial investments in pollution control in some countries have lowered the levels of these pollutants in many cities, poor air quality is still a major concern throughout the industrialized world.

1.1 Indoor Air Pollution

There are four principal sources of pollutants in indoor air viz. combustion, building material, the ground under the building and biological agents. As dangerous as polluted outdoor air can be to health, indoor air pollutants can pose even a greater health risk. Indoor air pollution is a concern where energy efficiency improvements sometimes makes the house relatively air tight thereby reducing ventilation and raising indoor pollutant levels. Indoor air pollution is usually associated with occupational situation particularly through combustion of biomass fuels. The greatest threat of indoor pollution exists where the people continue to rely on traditional fuels for cooking and heating. Burning such fuels produces large amounts of smoke and other air pollutants in the confined space of home, a perfect recipe for high exposures. Liquid and gaseous fuels such as kerosene and bottled gas although not completely pollution free is many times less polluting than unprocessed solid fuels (Fig 2). In these circumstances, exposure to pollutants is often far higher indoors than outdoors. The health problems due to indoor air pollutants are more widespread than those caused by outdoor air pollutants for the following reasons.

• The exposed persons are in close proximity to the source of indoor air pollutants.

• A recent report of WHO asserts 'the rule of 1000' which states that a pollutant released indoor is one thousand times more likely to reach the lungs than a pollutant released outdoors.

• Women and children, particularly those in rural sector spend more time indoors than outdoors.

• In rural areas, indoor air pollution is responsible for much greater mortality than ambient air pollution.

Epidemiological studies have linked exposure to indoor air pollution from dirty fuels with at least four major categories of illness. These are:

• acute respiratory infections (ARI) in children

• chronic obstructive pulmonary disease (COPD) such as asthma and bronchitis;

• lung cancer and

• pregnancy related problems. Of these, ARI appears to have the greatest health impact in terms of the number of people affected .

Traditional biomass fuels amount for 80% of domestic energy consumption in our country. When these fuels are burnt in simple cook stoves during meal preparation, air inside homes get heavily polluted with smoke that contains large amounts of toxic pollutants such as carbon monoxide, oxides of nitrogen (NOx), sulphur dioxide (SO2), aldehydes, dioxins, polycyclic aromatic hydrocarbons and respirable particulate matter. The resulting human exposures exceed the permissible norms by a factor in multiples.

SOME KEY FINDINGS OF INDOOR AIR POLLUTION STUDIES (ESMAP,World Bank 2000)

• Exposure to biomass smoke increases the risk of acute respiratory infection (chest infection, coughs, colds and middle ear infections). Children in the Gambia Island found riding on their mother's back, during cooking over smoky stoves were more likely to develop Acute Respiratory Infection (ARI) then unexposed children.

• A study in Tanzania reported that the children below five years age died of ARI, were more likely to sleep in a room with an open cook stove than healthy children in the same age group.

• Studies in India and Nepal show that non-smoking women who have cooked on biomass stoves exhibit a higher prevalence of chronic lung disease (asthma and chronic bronchitis). The incidence of moderate and severe ailments among two year olds, increased as they spent greater hours near the fire.

• Exposure to high indoor smoke levels is associated with pregnancy related problems such as still births and low birth weights. One study in Western India found a 50% increase in stillbirths in women exposed to indoor smoke during pregnancy. Considerable amount of carbon monoxide has been detected in the blood stream of women cooking with biomass.

• Other than these four major categories of illness; indoor air pollution is associated with blindness and changes in the immune system. Eighteen percent of blindness may be attributed to the use of biomass fuels. Further, a 1995 study in Eastern India found the immune system of new borns to be depressed due to the presence of indoor air pollution.

Table 1. Annual Deaths Due To Air Pollution In Urban and Rural Areas

Region	Urban Outdoors	Urban Indoors	Rural Indoors	Total
Developed Countries	14 (0.5)	252 (8.4)	28 (0.9)	294 (9.8)
Developing Countries	186 (6.2)	644 (21.5)	1876 (62.5)	2706 (90.2)
Total	200 (6.7)	896 (29.9)	1904 (63.5)	3000 (100)

Source: Report of the Committee on Environment and Health (MoEF), May 2000

Source: Indoor Air Quality, ESMAP, World Bank, September 2000

Source: Indoor Air Quality, ESMAP, World Bank, 2000

Major Air Pollution Episodes

MEUSE VALLEY, BELGIUM (1930) A strong atmospheric inversion got settled over Meuse Valley on December 1, 1930 and remained until December 5. Effluents from several factories in the valley, chiefly oxides of sulphur, various inorganic acids, metallic oxides, and soot were then trapped in the stable atmosphere. Sixty three people (generally the old and infirm) died, and several hundred others deemed ill. Although many suspected sulphur oxides and hydrofluoric acid, the actual lethal substance could not be proved.

DONORA, PENNSYLVANIA, 1948 Donora, Pennsylvania is an industrial town on the banks of the Monongahela River about 30 miles south of the heart of Pittsburgh. The major industrial installations were steel and wire mill, a zinc smelter and a sulphuric acid plant. During a particularly calm and meteorologically stable period from October 27 to 31, 1948, air pollutants accumulated because of this many people were hospitalized and 20 died. Illness of several thousand persons was blamed on the episode, and over 130 separate lawsuits were filed. The causative agent of the deaths and illness could never be determined incontrovertibly but sulphur compounds were present in the air in abnormally high quantities.

LONDON SMOG, 1952 Historically, the longest record of intermittent air pollution problems belongs to the city of London, England. The notorious pea-soup fogs become especially offensive when mixed with coal smoke. The word smog (smoke and fog) was coined to describe this foul condition. In 1661, John Evelyn got published his well-known pamphlet, 'Fumifugium: or The Inconvenience of the Air and Smoke of London Dissipated'. His major recommendation had been the removal of all smoke-producing plants from London. But London did little about it until the famous London smog of December 1952, truly a major air pollution disaster. The smog lasted 5 days from 5th to 9th December and caused 4000 deaths (principally among the old, the infirm, and those with respiratory diseases). The onset of fog was followed by acute respiratory symptoms. Almost exactly ten years later, December 3 to 7, 1962 London experienced another black fog, with 340 excess deaths. The improvement over the 1952 episode was laid to smoke reduction brought about by the Clean Air Act and public awareness of the harmful effects of smog, which restrained many respiratory cripples from going outdoors.

BHOPAL MIC GAS TRAGEDY (1984) The Methyl Isocyanate (MIC) gas leak in Bhopal during 1984 has been regarded the worst industrial accident in India, which is related to air pollution. Around 2,00,000 people were affected by the leak of poisonous Methyl isocyanate gas from a pesticide plant. The actual scenario of what went wrong at the Bhopal plant just after the midnight on the morning of December 3, 1984 is not exactly known. But several circumstantial evidences point to the total breakdown of the essential safety provisions within the plant. MIC can react with almost any chemical to generate considerable heat and CO2. The heat released accelerates the reaction and pressure goes on building up till it reaches an explosive level. The gas emitted from the factory spread over some 40 sq. kms area and affected people seriously as distant as 5 kms. MIC is invariably accompanied by Phosgene (COCl2) . The toxic effect of MIC is enhanced by COCl2.

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