There are also natural sources such as wind-blown dust and smoke from bush fires. Some forms of air pollution create global problems requiring international solutions, especially upper atmosphere ozone depletion and acid rain. The enhanced greenhouse effect also falls into this category. This page only looks at urban air pollution. The most widespread pollutants are carbon monoxide , volatile organic compounds, hydrocarbons, oxides of nitrogen , sulfur dioxide , particles and lead .
Some pollutants can interact to form secondary pollutants, called 'smog'.
These six substances, which are found everywhere in cities are used as indicators of air quality. Air pollution can cause health problems. For example smog can aggravate existing respiratory ailments such as asthma and bronchitis, or increase the risk of respiratory problems, particularly on smog days. Elevated lead levels can affect the central nervous system, and may impair intellectual development in children. To protect the environment against harmful and unhealthy levels of air pollution, the government has adopted objectives to protect a range of beneficial uses, including the health and well-being of humans, plants and animals.