The exact cause of asthma is unknown.
People with asthma have swollen (inflamed) and "sensitive" airways that become narrow and clogged with sticky mucus in response to certain triggers.
Genetics, pollution and modern hygiene standards have been suggested as causes, but there's not currently enough evidence to know if any of these do cause asthma.
A number of things can increase your chances of getting asthma.
Some people may also be at risk of developing asthma through their job.
Asthma symptoms often occur in response to a trigger.
Common triggers include:
Once you know your triggers, trying to avoid them may help control your asthma symptoms.
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 common causes of occupational asthma include:
Paint sprayers, bakers, pastry makers, nurses, chemical workers, animal handlers, timber workers, welders and food processing workers are all examples of people who may have a higher risk of being exposed to these substances.