Asthma is an allergic reaction, provoked by any one of 200 irritants, or allergens, such as the house dust mite, cat dander, pollen, or certain foods. Attacks can also be brought on by exercise, emotional disturbance and infections like bronchitis. The underlying causes are not known, although there appears to be a genetic component. Prevention, by avoiding exposure to allergens, would be the ideal solution, but this is far from easy and in many cases is impossible.

While there is no evidence that air pollution causes asthma, it can aggravate the condition in people who are already susceptible. Hence during periods of poor air quality, there is a rise in the number of hospital admissions due to acute asthma attacks.

Asthma is a narrowing of the airways of the lungs, made worse by allergens and eased by medication. The inflamed airways become irritable, leading to bouts of coughing, wheezing, breathlessness and chest tightness. The symptoms can be reversed by treatment, but they can be severe and are sometimes fatal.

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