Over the past three decades, anthropogenic emissions of chemical compounds
into the atmosphere have caused many environmental and health problems.
Some chemicals, such as chlorofluorocarbons (CFCs), are produced deliberately
and end up in the atmosphere by accident from equipment or goods. Others,
such as sulphur dioxide (SO2) and carbon monoxide (CO), are
unavoidable by-products of burning fossil fuels. Urban air pollution,
acid rain, contamination by toxic chemicals (some of them persistent and
transported over long distances), depletion of the stratospheric ozone
layer and changes in the global climatic system are all important environmental
threats to ecosystems and human wellbeing.