It’s not clear exactly what causes asthma, although it is likely to be a combination of factors.
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.
Although the cause of asthma is unknown, a number of things that can increase your chances of developing the condition have been identified. These include:
Some people may also be at risk of developing asthma through their job.
In people with asthma, the small tubes (bronchi) that carry air in and out of the lungs become inflamed and more sensitive than normal. This means that when you come into contact with something that irritates your lungs (a trigger), your airways become narrow, the muscles around them tighten, and there is an increase in the production of sticky mucus (phlegm). Asthma symptoms can have a range of triggers, such as:
Once you know your asthma triggers, you may be able to help control your condition by trying to avoid them.
In some cases, asthma is associated with substances you may be exposed to at work. This is known as “occupational asthma”. Some of the most commonly reported causes of occupational asthma include exposure to:
You may be at an increased risk of developing occupational asthma if you are regularly exposed to substances such as these through your work.
Occupations that are commonly associated with the condition include paint sprayers, bakers and pastry makers, nurses, chemical workers, animal handlers, welders, food processing workers and timber workers.
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Source: NHS UK
http://www.nhs.uk/Conditions/Asthma/Pages/Causes.aspx