Why do people get asthma?

It’s difficult to say for sure why people get asthma. But thanks to research, we’re clear about some of the risk factors that make asthma more likely.

What causes asthma is different to what triggers asthma:

  • The causes are the underlying reasons why someone gets asthma in the first place.
  • Triggers are things like dust mites or pollen that can make asthma symptoms worse.

Here we look at what causes asthma, and where it’s possible for you to lower the risk. The good news is that some of these risk factors are things you can do something about.

Asthma and allergies tend to run in families

If there’s asthma, eczema, hay fever or other allergies in your family it makes asthma more likely.

If you have asthma yourself, your child is much more likely to have asthma too, particularly if both parents have asthma. There’s slightly more chance of asthma being passed on by the mother than the father.

Children with allergies can go on to develop asthma

If your child tends to get allergies they’re said to be ‘atopic’. Being prone to allergies is usually something that runs in families. And if your child has one allergy they’re more likely to get another one.

For example, children with eczema, and a family history of allergy, are more at risk of developing asthma than other children.  

“GPs and researchers talk about the ‘atopic march’,” says Dr Andy Whittamore, Asthma UK’s in-house GP. “This is when allergies appear in children in a certain order, depending on their age.

“Sometimes allergies overlap, and sometimes your child will switch from one allergy to another. A small child with eczema may go on to have a food allergy, and then, as they get older, hay fever, and then asthma.” 

Smoking increases the risk of your child developing asthma

Research has shown that smoking during pregnancy, and smoking around your baby or child, both significantly increase the risk of a child developing asthma or other breathing problems.

And smoking also has a part to play in adult onset asthma.

Being born early

A child is more at risk of asthma if they were born prematurely (before 37 weeks), especially if they needed a ventilator to help them breathe after the birth.

A low birth weight (when a baby is born small for their gestational age) can also be a risk factor for asthma.


Bronchiolitis is caused by a virus, and usually affects babies and young children under two years old. It leads to swelling in the lungs and airways. This makes it harder to breathe, and your child will cough and wheeze.

If your child has had bronchiolitis a lot, they could be more at risk of developing asthma as they get older.

Your baby will be more at risk of bronchiolitis if you smoke.

Exposure to triggers at work

Occupational asthma is a type of asthma caused by certain things found in the workplace, such as chemicals or dust from flour or wood.

If you haven’t had asthma before and then get it because of the work you do, and if your symptoms improve when you’re not at work, you probably have occupational asthma.

Occupational asthma is a common cause of adult onset asthma.

Female hormones

Hormones can play a part in triggering late onset asthma and some women first develop asthma during or after the menopause.

Pollution plays a part in causing asthma

Environmental pollution, including traffic fumes and chemicals from power plants, can make asthma symptoms worse and may play a part in causing asthma.

Studies suggest that children living near very busy roads are more likely to develop asthma.

Why are more people getting asthma and allergies?

Some researchers put the increase in asthma and other allergic conditions over the last few decades down to the fact that we live in much cleaner, more urban conditions. This means we have less contact with the ‘friendly bacteria’ that thrive in more rural, natural environments.

Along with fewer childhood infections, this has resulted in lowered immunity, and more chance of allergies, including asthma.

The 'hygiene hypothesis'

The idea that we’re missing out on exposure to useful microbes early in life began to be considered a while back with a theory known as the ‘hygiene hypothesis’. This was based on evidence that children growing up in large families, in unhygienic homes, had fewer allergies, including asthma.

More recent research suggests babies exposed to ‘friendly bacteria’ in the first few months of their lives are thought to have less risk of developing asthma and allergies.

This is why some studies show that children growing up on farms have fewer allergies, and other studies show that having a dog in the house when your baby is very small can protect them from allergies and asthma. The studies are based on exposure to friendly microbes in babies less than two or three months old. 

But being around animals, or being in a natural environment, may not necessarily protect your child against asthma – other factors need to be taken into account, such as if there’s a family history of allergy and asthma. 

“It’s important to remember this isn’t about us trying to be less clean and hygienic in the hope that it will cut the risk of developing asthma,” says Dr Andy Whittamore, Asthma UK’s in-house GP.

“The causes of asthma are much more complex. And it's worth remembering that good hygiene can help us avoid certain asthma triggers. Washing your hands well, for example, is an important part of preventing colds and flu which are a top trigger for asthma symptoms.”

Last reviewed October 2018

Next review due October 2021

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