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Largest study to date of rare genetic variants and asthma risk finds few associations. Despite a strong suspected link between genetics and asthma, commonly found genetic mutations account for only a small part of the risk for developing the disease -- a problem known as missing heritability.
Some of these may be genetic. However, a number of environmental factors are thought to play a role in the development of asthma. These include air pollution, chlorine in swimming pools and modern hygiene standards (known as the  hygiene hypothesis ). There is currently not enough evidence to be certain whether any of these can cause asthma, although a variety of environmental irritants, such as dust, cold air and smoke, may make it worse.