Chronic exposure has been linked not only to respiratory ailments, but also cancers, heart disease and stroke. The building-blackening particles are so light they float on air. Fossil-fuel burning is by far the main man-made source: cars, trucks, tractors and boats or industrial processes like power generation and cement-making. Natural sources include bacterial activity, volcanic eruptions and lightning. High ozone levels can cause breathing problems, trigger asthmatic attacks and worsen respiratory disease.

Source: Sunday Times September 27, 2016 12:11 UTC