Air pollution episodes in Melbourne vary with the season:
- From late spring to early autumn, the main problem is photochemical smog (‘summer smog’).
- From late autumn to winter, fine particles can build up in the air, reducing visibility (‘winter smog’).
Melbourne is fortunate in that the prevailing winds and weather systems move quickly, tending to disperse pollution. However, under certain light wind conditions, the air and the pollution it contains are recirculated, creating smog.
The main pollutants involved in producing summer smog are hydrocarbons and oxides of nitrogen, which come from cars and factories. On warm days from November to March, sunlight can cause this mixture to react chemically, producing photochemical smog. The resultant smog contains harmful pollutants such as ozone and fine particles. Read Summer smog in Victoria (publication 1188) for more information.
The concentration of ozone is used to indicate smog levels.
We define a summer smog event as being when the ozone level averaged over one hour reaches 0.1 parts per million (ppm) at any monitoring station. This is specified in Victoria’s State Environment Protection Policy (Ambient Air Quality), based on the National Environment Protection (Ambient Air Quality) Measure. The standards and goals for ozone are designed to protect human health and vegetation.
High ozone concentrations may cause eye irritation. People suffering allergic disorders (such as asthma and hay fever), chronic disorders (such as bronchitis) or cardiac conditions are at risk when ozone concentrations exceed the standard.
International studies show that high concentrations of ozone can reduce crop yields of citrus fruits, grapes, potatoes and soy beans. Prolonged exposure to summer smog also tends to crack rubber, weaken synthetic fibres and fade paints and dyes.
On cool days with light winds, typically from April to August, Melbourne can be affected by the buildup of fine particles in the air.
The main sources of these airborne particles are domestic wood heaters and open fireplaces, industrial emissions, motor vehicles and open burning of agricultural stubble and grass, as well as fuel reduction burning in forested areas. Barbecues, lawn mowers and road dust also add to airborne particle levels.
Winter smog is composed mainly of fine particles that scatter sunlight and reduce visibility. We define a winter smog event as occurring when any of these results is recorded at two or more monitoring stations:
- the visibility in dry air between 7 am and 7 pm is below 20 kilometres
- the 24-hour PM10 (mass of particles smaller than 10 mm in 1 m3 of air) level is above 50 mg/m3
- the concentration of any other primary pollutant (carbon monoxide, etc) is above the standard.
Fine particles can affect our health, particularly the respiratory system. Airborne particles also soil buildings and fabrics. Particle deposits on electrical insulators can cause a short circuit or flashover at electrical contacts. This can cause fuses to blow and create a fire risk.