The first appearance of life on earth around 3,500 million years ago initiated major changes to the chemical composition of the atmosphere changes which were inextricably linked to the evolution of the biosphere – the thin film of air and water and soil and living organisms enclosing the earth.

Indeed livestock2 has referred to the unique planetary atmosphere of the earth as an extension of the biosphere, with processes within the biosphere actively maintaining and controlling the composition of the air to provide a stable environment for life on earth. All forms of life have contributed to these processes and until very recent times the human species was just another minor contributor.

However, in the last 150 years, particularly since the 1940s, activities such as fossil ‘fuel’ burning and large-scale destruction of forests have had an increasing impact on the global atmosphere. The ability of the biosphere control systems to adjust to these changes and still maintain the present conditions for life on earth is by no means certain.

Earth’s atmosphere contains several well-defined layers but air pollution occurs within the troposphere, the region extending from the surface of the earth up to the Tropopause at an altitude of about 10-15 km.

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The troposphere is composed of the atmospheric planetary boundary layer, roughly the lowest 500 m, and the free troposphere.

The planetary boundary layer is the region governing atmospheric transport and dispersion processes and represents the extent of the influence of the earth’s surface on wind fields in the atmosphere. Turbulent air motion in the boundary layer transports heat and water vapour, as well as pollutants, from the earth’s surface to the free atmosphere.

Air pollution is usually defined from the human perspective but in the Gaian words described by Lovelock, natural and anthropogenic pollution are not distinguishable and the term becomes irrelevant.

The World Health Organization (WHO) defines air pollution as “Limited to situations in which the outdoor ambient atmosphere contains materials in concentrations which are harmful to man and his environment.” The air at the earth’s surface consists primarily of nitrogen and oxygen (78 and 21 percent respectively). Much of the remaining 1 per cent is water vapour and carbon dioxide.

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The activity of humans in generating energy, manufacturing goods and disposing of wastes results in the release into atmosphere of a number of pollutants that may alter plant metabolism and induce diseases.

This global outlook notwithstanding, for densely populated urban societies the experience of air pollution from human activities is very real and has been a cause for concern from early times. Poorly ventilated houses amidst smoky industries combined to make life uncomfortable for early civilizations and classical writings contain references to poor air quality. Air pollution now has different sources but still a common problem in cities throughout the world.