Air Pollution

I.INTRODUCTION
Air Pollution, contamination of the atmosphere by gaseous, liquid, or solid wastes or by-products that can endanger human health and the health and welfare of plants and animals, or can attack materials, reduce visibility, or produce undesirable odours. Among air pollutants emitted by natural sources, only the radioactive gas radon is recognized as a widespread major health threat, although gases and particles from volcanic eruptions can cause serious more localized problems. A by-product of the radioactive decay of uranium minerals in certain kinds of rock, radon seeps into the basements of homes built on these rocks, posing a risk of lung cancer to residents.
Each year industrially developed countries generate billions of tons of pollutants. The most prevalent and widely dispersed air pollutants are described in the accompanying table. The level is usually given in terms of atmospheric concentrations (micrograms of pollutants per cubic metre of air) or, for gases, in terms of parts per million, that is, millilitres of gas per thousand litres of air. Many come from directly identifiable sources; sulphur dioxide, for example, comes from electric power plants burning coal or oil. Others are formed through the action of sunlight on previously emitted reactive materials (called precursors). For example, ozone, a dangerous pollutant in smog, is produced by the interaction of hydrocarbons and nitrogen oxides under the influence of sunlight. Ozone also causes serious crop damage. On the other hand, the discovery in the 1980s that air pollutants such as fluorocarbons are causing a loss of ozone from the Earth's protective ozone layer has caused the phasing out of these materials. A further category of air pollution is heavy metals, present as particulates and arising from many industrial processes.

II.METEOROLOGY AND HEALTH EFFECTS
Pollutant concentration is reduced by atmospheric mixing, which depends on such weather conditions as temperature, wind speed, and the movement of high and low pressure systems and their interaction with the local topography, for example, mountains and valleys. Normally, temperature decreases with altitude. But when a colder layer of air settles under a warm layer, producing a temperature or thermal inversion, atmospheric mixing is retarded and pollutants may accumulate near the ground. Inversions can become sustained under a stationary high-pressure system coupled with low wind speeds.
Periods of only three days of poor atmospheric mixing can lead to high concentrations of hazardous materials in high-pollution areas and, under severe conditions, can result in injury and even death. An inversion over Donora, Pennsylvania, in 1948 caused respiratory illness in over 6,000 people and led to the deaths of 20. Severe pollution in London took 3,500 to 4,000 lives in 1952 and another 700 in 1962. Release of methyl isocyanate into the air during a temperature inversion caused the disaster at Bhopal, India, in December 1984, with at least 3,300 deaths and more than 20,000 illnesses. The effects of long-term exposure to low concentrations are not well defined; however, those most at risk are the very young, the elderly, smokers, workers whose jobs expose them to toxic materials, and people with heart or lung disease. Other adverse effects of air pollution are injury to livestock and crops.
Often, the first noticeable effects of pollution are aesthetic and may not necessarily be dangerous. These include visibility reduction due to tiny particles suspended in air, or bad odours, such as the rotten egg smell produced by hydrogen sulphide emanating from pulp and paper mills.

III.SOURCES AND CONTROL
The combustion of coal, oil, and petrol accounts for much of the airborne pollutants. About 60 per cent of the sulphur dioxide and 20 per cent of the nitrogen oxides emitted into the atmosphere in the United Kingdom are produced by fossil-fuel-fired electric power plants. About 70 per cent of the carbon monoxide and 50 per cent of the nitrogen oxides come from burning petrol and diesel in cars and lorries. Other major pollution sources include iron and steel mills; smelters; municipal incinerators; oil refineries; cement plants; and nitric and sulphuric acid plants.
Potential pollutants may exist in the materials entering a chemical or combustion process (such as sulphur in coal), or they may be produced as a result of the process itself. Carbon monoxide, for example, is a typical product of internal-combustion engines. Methods for controlling air pollution include removing the hazardous material before it is used, removing the pollutant after it is formed, or altering the process so that the pollutant is not formed or occurs only at very low levels. Car exhaust pollutants can be controlled by burning the fuel as completely as possible, by recirculating fumes from fuel tank, carburettor, and crankcase, and by changing the engine exhaust to harmless substances in catalytic converters. Industrially emitted particulates may be trapped in cyclones, electrostatic precipitators, and filters. Pollutant gases can be collected in liquids or on solids, or incinerated into harmless substances.


IV.LARGE-SCALE EFFECTS
The tall smokestacks used by industries and utilities do not remove pollutants but simply boost them higher into the atmosphere, thereby reducing their concentration at the site. These pollutants may then be transported over large distances and produce adverse effects in areas far from the site of the original emission. Sulphur dioxide and nitrogen oxide emissions from Britain and other industrialized countries of Western and Central Europe are causing acid rain in Norway and Sweden. The pH level, or relative acidity, of many freshwater lakes has been altered so dramatically by acid rain that entire fish populations have been destroyed. Sulphur dioxide emissions and the subsequent formation of sulphuric acid can also be responsible for the attack on limestone and marble at large distances from the source. There are also claims that acid rain is causing widespread damage to forests in Europe, but the precise role is unclear and earlier predictions of large-scale forest death are unfounded.
The worldwide increase in the burning of coal and oil since the late 1940s has led to ever-increasing concentrations of carbon dioxide. The resulting “greenhouse effect”, which allows solar energy to enter the atmosphere but reduces the reemission of infrared radiation from the Earth, may well lead to a warming trend that might affect the global climate and lead to a partial melting of the polar ice-caps. Possibly an increase in cloud cover or absorption of excess carbon dioxide by the oceans (in the so-called carbon cycle) would check the greenhouse effect before it reached the stage of polar melting. Nevertheless, many research reports released during the 1990s have indicated that the greenhouse effect is definitely under way and that the nations of the world should be taking immediate steps to deal with it. In June 1999 a massive cloud of air pollution, roughly the size of the United States, was discovered 1 km to 3 km (’ mi to 2 mi) above the Indian Ocean. The thick brown haze included soot, sulphates, nitrates, mineral dust, and significant amounts of gases such as carbon monoxide and sulphur dioxide. Scientists believe it was created by human activities, especially the burning of fossil fuels, and could have a significant impact on the regional and global climate, as well as plant and animal life.

V.ACTION BY GOVERNMENTS
Various countries have set standards in legislation in the form of concentration levels that are believed to be low enough to protect public health. Source emission standards are also specified to limit the discharge of pollutants into the air so that air-quality standards will be achieved. However, the nature of the problem requires the implementation of international environmental treaties, and to this end 49 countries agreed in March 1985 on a United Nations convention to protect the ozone layer. This “Montreal Protocol”, which was renegotiated in 1990 and 1992, called for the phaseout of certain chlorocarbons and fluorocarbons by the end of the century and provides aid to developing countries in making this transition. In addition, several international protocols have been aimed specifically at reducing the incidence of acid rain. In December 1999 the Montreal Protocol announced that almost all production and consumption of virgin ozone depleting substances had been phased out in the developed world. Similar control measures were introduced for developing countries in July 1999, and it was anticipated that all developing countries would be able to meet their freeze targets and subsequent obligations under the protocol.
Concern over trans-boundary air pollution, including acid rain, in Europe has led to the United Nations Economic Commission for Europe (UNECE) developing air quality guidelines, called Critical Loads and Levels, which represent thresholds below which it is believed that damage will not occur to different ecological systems. Critical Loads are based on the amount of acidity that an ecosystem can tolerate being deposited indefinitely. Critical Levels are concentrations of ozone, sulphur dioxide, and nitrogen oxides over different averaging times and applicable to different categories of vegetation. Massive cuts are being made in emissions in Europe in order to move towards these thresholds. In contrast, in many developing countries, pollutant concentrations are rising very rapidly due to increased industrialization and motor traffic; concern in such places is primarily with impacts on human health in cities. The World Health Organization has published air quality guidelines designed to protect health.