Nowadays the main problem is motor vehicle exhaust. Petrol engines produce oxides of nitrogen and unburnt volatile organic compounds. Under the influence of sunlight, these substances combine with oxygen in the air to produce ozone, the main gas in smog. This ozone near the ground should not be confused with the ozone layer high in the atmosphere which protects us from harmful ultraviolet rays. Another gas, nitrogen dioxide, is responsible for the brown colour of smog.
Many Australian cities experience smoggy days during the summer.
Fine particles in the air cause the hazy winter days seen in our cities. These particles come from a wide variety of sources including petrol and diesel exhaust, road dust, smoke and fly ash from industry, wind blown topsoil, backyard incinerators, bushfires, pollens from grasses and other plants, etc. The particles of greatest concern are those which are small enough to be breathed into the lungs. There is no question that the old style of air pollution could kill people. Most of these people were elderly and already had heart or lung diseases.