Nobody knows for sure what causes asthma, but we do know you're more likely to develop it if you have a family history of asthma, eczema or other allergies. You're twice as likely to develop asthma if your parents have it.
Modern lifestyles, such as housing and diet, also may have contributed to the rise in asthma over the last 30 years.
Every 10 seconds someone has a potentially life-threatening asthma attack, and the latest data shows that deaths from asthma are on the rise again.
What causes asthma?
There are many theories about what's caused the increase in the number of people with asthma.
One of the most popular is the "hygiene hypothesis". According to this theory, asthma is more common in western societies. Because western society is becoming cleaner, we have less exposure to allergens and pathogens.
When a person with asthma comes into contact with a "trigger", their airways become irritated. The muscles tighten, the airways narrow, and the lining of the airways gets inflamed and swollen.
The main symptoms are chest "wheeze" or noisy breathing, chest tightness and breathlessness. You may also develop a cough, particularly at night, but this is more common in children.
Boys under the age of two are more susceptible to asthma because their airways are narrower when they're younger. But they usually grow out of it, whereas girls are more likely to have asthma beyond puberty.
Obesity is also thought to make asthma more likely. Symptoms often get better when the person loses weight.
Find out more in Are we too clean for our own good?
Smoking and asthma
Smoking also has a definite impact. Parents' cigarette smoke will affect their child's lung function development, and it irritates the airways. People with asthma are advised not to smoke.
Research shows that smoking during pregnancy increases the risk of your child developing asthma. Children whose parents smoke are also more likely to develop the condition.
Once you have asthma, high levels of pollution and smoking may make it worse. But there's no proof that these triggers actually cause it.
How to help yourself
If certain things trigger your asthma, such as dust mites, minimise your exposure to them. Put mattress covers on your bed, use a damp cloth when you dust, don't have too many soft furnishings in your house, and put down laminate or wooden flooring instead of carpets.
Asthma triggers include pets, but studies show that getting rid of animals doesn't improve asthma. In fact, the emotional upset of getting rid of your pet may make your asthma worse. Keep your exposure to pets to a minimum in areas such as the bedroom, and consider not getting any new pets.
If you have symptoms more than three times a week and you need to use a reliever inhaler (usually blue), you should also use a preventer inhaler (usually brown).
But if you only have symptoms a few times a week when exercising, you can manage your symptoms safely with a reliever inhaler before you exercise.
Asthma is an inflammatory disease. This means preventative treatment is vital, and you must take it even when your asthma symptoms aren't present. This will ensure your asthma is well controlled.
Review your treatment with your asthma nurse or GP at least once a year as you might be able to reduce your dosage of medicine.
Find out more information about asthma treatments.
Taking steroids when you have asthma
Because asthma is caused by an inflammation of the airways, anti-inflammatory drugs such as steroids are sometimes used to treat it.
You may be concerned about the potential side effects of steroids, such as weight gain, stunted growth (in children) and weakened bones.
The risk of side effects if you or your child are using a steroid inhaler is lower than with steroid tablets because less of the medicine gets into your system. With both steroid inhalers and tablets, the risk of side effects increases if the dose is high and if you use them for long periods.
Generally, if inhaled steroids are prescribed carefully and at the lowest dose needed, the risk of side effects is outweighed by the ability to reduce your or your child's need for steroid tablets. Discuss the risks of steroid treatment with your doctor if you're concerned.
If you have queries about any aspect of asthma, you can call the Asthma UK helpline, which is a free telephone helpline staffed by asthma nurse specialists on 0300 222 5800, Monday to Friday, 9am to 5pm.
Article provided by NHS Choices