For a long time it was thought that air pollution could not cause asthma to develop in previously healthy people. Some recent experiments challenge this belief. Scientists have shown that people exposed to ozone or nitrogen dioxide are more likely to react to allergens such as grass pollens and housedust mites than those who are not exposed. Thus a combination of air pollution and allergens could cause some cases of asthma. Across the board, other factors including allergens, colds, cigarette smoking and exposure to fumes and dust at work are probably more important.
Lung cancer is more common in cities than in the country.
Fortunately the radon levels in Australian homes are quite low. Certain chemicals found in diesel exhaust and in emissions from heavy industry can also cause lung cancer. Although these chemicals have been detected in outdoor air, both in Sydney and Melbourne, the levels are too low to cause many cases of lung cancer. One of the most alarming things during the London fog in 1952 was that prize animals at the Smithfield Show developed respiratory distress and died. A considerable number of laboratory experiments since then have confirmed that certain animals are particularly susceptible to air pollution.