There is no single cause of asthma, but certain factors may increase the likelihood of developing it. These include genetic factors and the environment.

Who is at risk of developing asthma?

Factors that are known to increase the likelihood of developing asthma include:

• a family history of asthma or other related allergic conditions (known as atopic conditions), such as eczema, food allergy or hay fever 
• developing another atopic condition such as a food allergy
• having bronchiolitis as a child (a common lung infection among children) 
• being exposed to tobacco smoke as a child, particularly if your mother smoked during pregnancy 
• being born prematurely (especially if you needed a ventilator) 
• being born with a low birth weight (less than 2kg or 4.5 pounds)

Want to know more?

• Asthma UK: What causes asthma?

Asthma triggers

The symptoms of asthma can have a range of triggers, but they do not affect everyone in the same way. Once you know your asthma triggers, you can try to avoid them.

• Infections of the airways and chest. Upper respiratory infections, which affect the upper airways, are often caused by cold and flu viruses and are a common trigger of asthma. 
• Allergens. Pollen, dust mites, animal fur or feathers, for example, can trigger asthma.
• Airborne irritants. Cigarette smoke, chemical fumes and atmospheric pollution may trigger asthma.
• Medicines. The class of painkillers called non-steroidal anti-inflammatory drugs (NSAIDs), including aspirin and ibuprofen, can trigger asthma for some people, although are fine for most. Children under 16 years of age should not be given aspirin. 
• Emotional factors. Asthma can be triggered by emotional factors, such as stress or laughing.
• Foods containing sulphites. Sulphites are naturally occurring substances found in some food and drink. They are also sometimes used as a food preservative. Food and drinks that are high in sulphites include concentrated fruit juice, jam, prawns and many processed or pre-cooked meals. Most people with asthma do not have this trigger, but some may. Certain wines can also trigger asthma in susceptible people.
• Weather conditions. A sudden change in temperature, cold air, windy days, poor air quality and hot, humid days are all known triggers for asthma.
• Indoor conditions. Mould or damp, house dust mites and chemicals in carpets and flooring materials may trigger asthma.
• Exercise. Sometimes, people with asthma find that their symptoms are worse when they exercise.
• Food allergies. Although uncommon, some people may have allergies to nuts or other food items, known as an anaphylactic reaction. If so, these can trigger severe asthma attacks.

Want to know more?

• Asthma UK: Asthma triggers A-Z.
• Food Standards Agency: Intolerance to additives.

What happens during an asthma attack?

During an asthma attack:

• The bands of muscles around the airways tighten.
• There is increased inflammation in the linings of the airways, which swell.
• The airways produce sticky mucus or phlegm, which can further narrow the airways.

The passages of the airways narrow, making it more difficult for the air to pass through and, therefore, more difficult to breathe. This can cause the characteristic wheezy noise, although not everyone with asthma will wheeze. In a life-threatening attack, there may not be a wheezy sound.

An asthma attack can happen at any time. However there are usually warning signs for a couple of days before. These include symptoms getting worse, especially during the night, and needing to use the reliever inhaler more and more.

Asthma can be life threatening. See Asthma - treatment to find out more about how to manage your asthma. Speak to your doctor or asthma nurse for further advice.

If you or someone else is having a severe asthma attack and cannot breathe, dial 999 immediately for emergency medical treatment.