In an individual, they may occur from hour to hour and day to day. This condition is due to inflammation of the air passages in the lungs and affects the sensitivity of the nerve endings in the airways so they become easily irritated. In an attack, the lining of the passages swell causing the airways to narrow and reducing the flow of air in and out of the lungs. A study in the South Atlantic Island of Tristan da Cunha, where one in three of the 300 inhabitants has asthma, found children with asthmatic parents were much more likely to develop the condition. Exposure to tobacco smoke and exposure to chemical irritants in the workplace are additional risk factors.
Urbanization appears to be correlated with an increase in asthma.
The nature of the risk is unclear because studies have not taken into account indoor allergens although these have been identified as significant risk factors. One such incident in London, UK, in June 1994 saw 640 people rushed to emergency departments in the throes of full-blown asthma attacks. A similar incident happened in Melbourne, Australia. Many experts have blamed climatic conditions such as thunderstorms, which break up pollen grains, releasing starch granules that trigger attacks. Because asthma is a chronic condition, it usually requires continuous medical care.