COMPOUNDS:

• OZONE: Ozone is a gas that is formed when nitrogen oxides react with a group of air pollutants known as 'reactive organic substances' in the presence of sunlight. (See factsheet on air toxics.) The chemicals that react to form ozone come from sources such as: motor vehicle exhaust, oil refining, printing, petrochemicals, lawn mowing, aviation, bushfires and burning off. Motor vehicle exhaust fumes produce as much as 70% of the nitrogen oxides and 50% of the organic chemicals that form ozone.
  
  NOx + VOC + Sunlight = OZONE
  
  
• NITROGEN OXIDES: Nitrogen oxides or NOx is a generic term for a group of gases that contain nitrogen and oxygen in varying amounts. NOx results from combustion, and the primary manmade sources are automobiles, electric utilities and other industrial, commercial or residential sources that burn fuels. NOx occur naturally as well. A common NOx is nitrogen dioxide (NO2), which is one of the main ingredients in forming both ground-level ozone and acid rain. Ground level ozone can trigger serious respiratory problems in people and acid rain causes lakes and streams to become unsuitable for many fish. NOx can be transported over long distances with the help of strong winds, which means its effects are not contained to locations near the polluting source.
  
  
• Carbon monoxide (CO) is also a by-product of combustion and is formed when the carbon in fuel is not burned thoroughly. People with heart disease are most likely to suffer from low exposure to CO and it can be deadly at extremely high levels.
  
  
• CO and NOx are not considered greenhouse gases, but they do impact the concentrations of methane, ozone, CFC substitutes (HCFC), which are main greenhouse gases. Therefore, they are considered indirect greenhouse gases.
  
  
• The natural amount of ozone in the lower atmosphere is generally around 0.04 parts per million (ppm), and that amount is not harmful to human health. Vegetation can also emit organic chemicals that help form ozone. In Melbourne, up to 20% of these organic chemicals can come from vegetation and up to 64% in Brisbane.
  
  
• In most Australian towns and cities, the amount of ozone in the air does not exceed the national standards (see below). Only larger cities, like Australia's capital cities, have occasions when there is enough ozone in the air for it to be a risk to human health. In larger cities, the level of ozone exceeds the national standard several times a year. The highest levels are found most often in Sydney and Melbourne, but Brisbane and Perth can also experience high levels.