Air consists of gases and tiny solid or liquid particles. In most cases these substances are harmless. In fact gases such as oxygen, nitrogen and carbon dioxide are essential to life.

Air pollution occurs when contaminants or excessive amounts of the natural components are released into the atmosphere from either human or natural sources. Vehicles, industry, commercial and domestic activities and natural sources such as bushfires and dust storms contribute pollutants to the atmosphere.

While the amount of air pollutants varies with vehicle type, motor vehicles are the major source of a number of air pollutants in urban areas.

Source : Austroads Roadfacts 2000

The key emissions from the transport sector that have an impact on air quality are

• carbon monoxide
• oxides of nitrogen
• hydrocarbons (also called volatile organic compounds)
• sulphur dioxide
• lead
• particulate matter
• air toxics (organic compounds and metals)

These are called the primary emissions. In the atmosphere, chemical reactions and sunlight can produce other pollutants. Many of these emissions also contribute to the enhanced greenhouse effect.