Clinical studies have demonstrated that high levels of sulphur dioxide and nitrogen dioxide make asthma symptoms worse. These levels can occur in Britain during severe winter smogs. Ozone is a summer pollutant, formed on hot sunny still days, when sunlight triggers chemical reactions between the nitrogen oxides, volatile organic compounds and carbon monoxide emitted from motor vehicles. At high levels, ozone can cause short-term breathing difficulties in healthy people and can also make attacks worse for people with asthma. These pollutants may increase susceptibility to respiratory tract infections.
Particulates are the tiny particles of carbon in black smoke.
The smallest particulates can penetrate deep into the lungs, past the defense mechanisms of the respiratory tract. The current consensus is that air pollutants can make existing health problems worse but are not thought to be the cause of these health problems. Asthma is a chronic and potentially serious lung disease affecting some three million people in Britain and causing nearly 2,000 deaths each year. It is caused by a combination of environmental factors such as exposure to allergens and genetic susceptibility. It is aggravated by other environmental factors such as exposure to cold air, tobacco smoke, viruses, and indoor and outdoor air pollutants.