People with asthma have more sensitive airways and their lungs respond more to the effects of air pollutants such as particles, sulphur dioxide, nitrogen oxides and ozone.

Outdoor pollutants not especially associated with asthma include carbon monoxide and lead. Carbon monoxide is more likely to affect people with heart disease than people with asthma. Lead has become much less of a problem as an air pollutant since most vehicles now run on unleaded petrol.

Airborne particles

Particles suspended in air are a complex mixture of solids and aerosols and their composition and size depends largely on their source, which can be natural (from pollens, bacteria and fungi) or man-made (primarily from combustion sources such as power stations, petrol-and diesel-powered motor vehicles, wood heaters, fireplaces and incinerators).

Respirable particles (PM₁₀, particles with diameters less than 10 micrometre) and fine particle (PM_2.5, particles with diameters less than 2.5 micrometres) have been attracting recent attention, largely because they can penetrate and be retained in the deepest structures of the lung.

Sulphur dioxide

Although natural sources such as volcanoes contribute to ambient levels of sulphur dioxide, the combustion of sulphur-containing fossil fuels is the primary source of this gas.

Nitrogen oxides

The major source of nitrogen oxides is the combustion of fossil fuels in power stations and motor vehicles. In sunlight, nitric oxide rapidly changes into the secondary pollutant, nitrogen dioxide, although high temperatures (for example, in power plants and gas heaters and stoves) can also lead to nitrogen dioxide being emitted directly.

Ozone

Ozone is a highly reactive gas formed in the lower atmosphere by chemical reactions between nitrogen oxides, oxygen and volatile organic compounds in the presence of sunlight. The pollutants produced are referred to as ‘photochemical smog’ and ozone is the most important pollutant in this group. Ozone in the upper atmosphere is protective by screening out ultraviolet rays.

Air quality standards

In 1998, the National Environment Protection Council made a National Environment Protection Measure (NEPM) for outdoor air quality which sets national air quality standards for the six major pollutants: particles (PM₁₀), sulphur dioxide, nitrogen dioxide, ozone, carbon monoxide and lead. These standards have been set to protect human health.

Ambient air quality goals recommended by the National Health and Medical Research Council are shown in Appendix 1.

Table 2. NEPM air quality standards and goals

(Note: Table 2 only available on this website)

Air quality index

The Air Quality Index is calculated by converting the measured pollutant concentrations into index values. The lower the index is, the better the quality of our air. There are five categories in the index: Very Good (0-33), Good (34-66), Fair (67-99), Poor (100-149), Very Poor (>150). Air quality is quite good in most Australian cities with only occasional incidences of exceeding the particle and ozone standards.

Table 3. City Specific comparison of criteria air pollutants and compliance to NEPM standards (2000-2001) with number of exceedances in brackets

(Note: Table 3 only available on this website)