cause

• exact cause of asthma is not known and is likely to be a combination of factors
• may be genetic, as it often runs in families and people who have allergies are at higher risk
• number of environmental and social factors are thought to play a role in the development of asthma and allergies
• exposure to tobacco as a child
• triggers such as dust, air pollution and chemicals such as chlorine in swimming pools
• exposure to smoking while in the womb
• being born prematurely (before 37 weeks) or with a low birthweight
• modern hygiene standards, 'too hygienic' don't build up resistance