Air pollution in urban areas affects human health. Energy production, industrial processes and vehicles emit many different air pollutants into the atmosphere. For example acid rain, caused by emissions of sulphur and nitrogen oxides, causes not only soil degradation and lake acidification, but also faster corrosion of metals and decay of buildings constructed of limestone and marble.
There have been many attempts to reduce air pollution. Monitoring now occurs in endangered areas and international agreements are now in place which are helping to improve air quality on a global scale.