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.

These include:

• having an allergy-related condition, such as eczema, a food allergy or hay fever – these are known as atopic conditions
• having a family history of asthma or atopic conditions
• having had bronchiolitis – a common childhood lung infection
• exposure to tobacco smoke as a child
• your mother smoking during pregnancy
• being born prematurely (before 37 weeks) or with a low birth weight

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:

• infections like colds and flu
• allergies – such as to pollen, dust mites or animals
• smoke, fumes and pollution
• medicines – particularly anti-inflammatory painkillers like ibuprofen and aspirin
• emotions, including stress, or laughter
• weather – such as sudden changes in temperature, cold air, wind, thunderstorms, heat and humidity
• mould or damp
• exercise

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

Asthma + Lung UK: asthma triggers

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

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.

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