It's not clear exactly what causes asthma, although it is likely to be a combination of factors.

Some of these may be genetic. However, a number of environmental factors are thought to play a role in the development of asthma. These include air pollution, chlorine in swimming pools and modern hygiene standards (known as the "hygiene hypothesis").

There is currently not enough evidence to be certain whether any of these can cause asthma, although a variety of environmental irritants, such as dust, cold air and smoke, may make it worse.

Who is at risk?

Although the cause of asthma is unknown, a number of things that can increase your chances of developing the condition have been identified. These include:

• a family history of asthma or other related allergic conditions (known as atopic conditions) such as eczema, food allergy or hay fever
• having another atopic condition
• having bronchiolitis (a common childhood lung infection) as a child
• childhood exposure to tobacco smoke, particularly if your mother also smoked during pregnancy 
• being born prematurely, especially if you needed a ventilator to support your breathing after birth
• having a low birth weight as a result of restricted growth within the womb

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

Asthma triggers

In people with asthma, the small tubes (bronchi) that carry air in and out of the lungs become inflamed and more sensitive than normal.

This means that when you come into contact with something that irritates your lungs (a trigger), your airways become narrow, the muscles around them tighten, and there is an increase in the production of sticky mucus (phlegm).

Asthma symptoms can have a range of triggers, such as:

• respiratory tract infections – particularly infections affecting the upper airways, such as colds and the flu
• allergens – including pollen, dust mites, animal fur or feathers
• airborne irritants – including cigarette smoke, chemical fumes and atmospheric pollution
• medicines – particularly the class of painkillers called non-steroidal anti-inflammatory drugs (NSAIDs), which includes aspirin and ibuprofen, and beta-blockers sometimes given for high blood pressure or some types of heart disease
• emotions – including stress or laughing
• foods containing sulphites – naturally occurring substances found in some food and drinks, such as concentrated fruit juice, jam, prawns and many processed or pre-cooked meals
• weather conditions – including a sudden change in temperature, cold air, windy days, thunderstorms, poor air quality and hot, humid days
• indoor conditions – including mould or damp, house dust mites and chemicals in carpets and flooring materials
• exercise
• food allergies – including allergies to nuts or other food items

Once you know your asthma triggers, you may be able to help control your condition by trying to avoid them.

Want to know more?

• Asthma UK: asthma triggers A-Z

Occupational 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 commonly reported causes of occupational asthma include exposure to:

• 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.

Occupations that are commonly associated with the condition include paint sprayers, bakers and pastry makers, nurses, chemical workers, animal handlers, welders, food processing workers and timber workers.

Want to know more?

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