Atmosphere

The atmosphere consists of layers of air  covering the Earth.

Plants and animals rely on the gases in the atmosphere. Plants need CO₂ for the process of photosynthesis and release oxygen as a result. Animals -- including human beings -- need oxygen to live and release CO₂ as they exhale. The CO₂ is then taken up again by plants, forming a cycle of life in the biosphere.

In the modern world, however, industrial activity, the burning of fossil fuels, waste treatment, intensive agriculture and many other economic activities lead to air pollution. Air pollution is harmful to humans, as well as to plants, animals and their natural habitats. It even brings about changes in the Earth's climate.

It was long believed that air pollutants, once released, were diluted to negligible concentrations in the atmosphere. Measurements have shown this belief to be flawed, if not misleading.

High concentrations of primary pollutants can occur within and around emission areas. Nearly all large particles are deposited locally. Local weather is an important factor in determining short-term pollution levels. However, local emissions may have regional and global implications. For example, CFCs emitted at the surface are responsible for ozone layer depletion that occurs at an altitude of 20-30 kilometres in the stratosphere.