What causes asthma?

It's difficult to say for sure what causes asthma, and the causes vary from person to person.

What we do know is that research shows you're more likely to develop asthma if:

  • you have a family history of asthma, eczema or any allergies - for example, evidence shows that if one or both of your parents have asthma, you are more likely to have it
  • you have eczema or an allergy, such as hay fever (an allergy to pollen)
  • you had bronchiolitis (a common childhood lung infection) as a child
  • you were born prematurely, especially if you needed a ventilator to help you breathe after birth
  • your birth weight was low because you didn't grow at a normal rate in the womb (this can be caused by various factors)
  • your mother smoked while she was pregnant with you - research has shown that smoking during pregnancy significantly increases the risk of a child developing asthma
  • your parents smoke (or smoked while you were a child)
  • you spend time around people who smoke - research shows that being exposed to second-hand cigarette smoke means you're more likely to develop asthma
  • you have been exposed to certain substances at work - this is known as occupational asthma
  • you're an adult female - hormones can affect asthma symptoms, and some women first develop asthma before and after the menopause

Why are more people getting asthma these days?

It's thought that many of the lifestyle choices we make today - our housing, diet, and more hygienic environment for instance - may have added to the increasing numbers of people with asthma over the past few decades. Some researchers are currently investigating the theory that improved hygiene conditions have reduced the number of childhood infections. Fewer infections may mean the immune system doesn't develop as well. This lowered immunity can increase the risk of asthma. Environmental pollution, including traffic fumes and chemicals from power plants, can also make asthma symptoms worse and may play a part in causing some asthma.

What's the difference between a 'cause' and a 'trigger' of asthma?

When we talk about the 'cause' of asthma, we mean the underlying reason why you get it in the first place. There are different theories to explain what these possible causes are (as explained above) but if you've got asthma, it's impossible to know for certain what caused it in your individual case.

When we talk about an asthma 'trigger', we mean anything that starts your asthma symptoms or makes your asthma symptoms worse. You may find, for example, that visiting someone with a pet or spending time in a dusty room sets off your symptoms. Other common triggers include exercise, pollen, cold weather or cigarette smoke. You can find out more about asthma triggers and, importantly, how to manage them here.

When does asthma appear?

Asthma can appear at any age. Symptoms usually start during childhood, but it's not uncommon for adults to get it. Some adults develop it after a viral infection. If you get asthma in adulthood, it's known as 'adult-onset' or 'late-onset' asthma. Certain things found in the workplace, such as chemicals or dust from flour or wood, can also lead to asthma symptoms. This is known as occupational asthma.

Last reviewed May 2015