Asthma - Causes

The exact cause of asthma is unknown.

People with asthma have inflamed (swollen) and "sensitive" airways that become narrow and clogged with sticky mucus in response to certain triggers.

Factors such as a genes, air pollution, chlorine in swimming pools and modern hygiene standards have been suggested as possible causes, but there's not currently enough evidence to be certain whether any of these do cause asthma.

Who's at risk?

While the cause of asthma is unknown, there are a number of things that can increase your chances of developing it. These include:

• a family history of asthma or related allergic conditions (known as atopic conditions) such as eczema, food allergy or hay fever
• having another atopic condition yourself
• having bronchiolitis (a common childhood lung infection) as a child
• exposure to tobacco smoke as a child
• your mother smoking during pregnancy 
• being born prematurely or with a low birth weight

Some people may also be at risk of developing asthma through their job.

Asthma triggers

Asthma symptoms often occur in response to a trigger.

Common triggers include:

• infections – particularly infections of the upper airways, such as colds and flu
• allergens – including pollen, dust mites, animal fur ("dander") or feathers
• airborne irritants – including cigarette smoke, fumes and pollution
• medicines – particularly painkillers called non-steroidal anti-inflammatory drugs (NSAIDs), which include aspirin and ibuprofen, and beta-blockers
• emotions – including stress or laughter
• food additives – including sulphites (often found in pickled products, wine, beer and dried fruit) and tartrazine (a yellow food colouring)
• weather conditions – including sudden changes in temperature, cold air, windy days, thunderstorms and hot, humid days
• indoor conditions – including mould or damp and chemicals in carpets and flooring materials
• exercise
• food allergies – including allergies to nuts and other foods

Once you know your triggers, trying to avoid them may help control your asthma symptoms.

Want to know more?

• Asthma UK: asthma triggers

Work-related asthma

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:

• isocyanates (chemicals often found in spray paint)
• flour and grain dust
• colophony (a substance often found in solder fumes)
• latex
• animals
• wood dust

You may be at an increased risk of developing occupational asthma if you are regularly exposed to substances such as these through your work.

Paint sprayers, bakers and pastry makers, nurses, chemical workers, animal handlers, welders, food processing workers and timber workers are all examples of people who may have a higher risk of being exposed to these substances.

Want to know more?

• Asthma UK: occupational asthma 
• Health and Safety Executive: asthma at work