During an attack, the airways narrow because the muscles in their walls are squeezing them. This is called bronchoconstriction. In addition, more and thicker mucus is secreted into the airways, and their inner lining becomes inflamed and over-sensitive to any irritants. A severe asthma attack, if not treated in time, can prove fatal. It is hard to know exactly what causes airway inflammation and asthma.
Genetic factors play a part.
Some people have an underlying predisposition. There is strong evidence that exposure to allergens in early life increases their risk of developing asthma. Asthma is also more likely to occur in someone with relatives who suffer from some form of allergy. Asthma can also be caused by exposure to some occupational sensitisers . But it is not always clear what triggers an asthma attack.