Understanding asthma Patsy Westcott Asthma was known to doctors in ancient China and Greece, and today more than 100 million people worldwide have the condition.

In this article

What is asthma? Main symptoms

Possible causes The inheritance factor

Environmental factors

Asthma affects the small airways (bronchioles) that carry air in and out of the lungs. If you have asthma your airways can become inflamed, swollen and constricted (or narrowed) and excess mucus is produced.

More than 5.2 million people in the UK are being treated for asthma and about 1.1 million of these are children. Asthma affects approximately one in 12 adults and one in eight children in the UK. What this means is that there is a person with asthma in one in five households in the UK. It can affect almost anyone, at any age, anywhere.

An asthma 'attack' describes the symptoms of tightness in the chest, a wheezing or whistling noise in the chest, coughing and difficulty breathing that occur when the airways become narrowed, inflamed and blocked by plugs of mucus.

An attack can occur suddenly. However, many people with asthma learn to recognise the warning symptoms - such as an itchy nose or itchy skin, dizziness or light-headedness, or an irritating cough - that herald an attack.

Learning the warning signs can often alert someone with asthma in time to take preventive action.

Asthma is a chronic condition, which means attacks occur over a long period of time. Although there are times when acute episodes strike asthmatics, most people can say there are long periods during which they have few, if any, symptoms.

The main symptoms of asthma are:

• Coughing
• Wheezing
• Shortness of breath
• Tight feeling in the chest

It's becoming increasingly common in the developed world and is now the most common chronic condition in the west. Aspects of our modern environment, such as air pollution, processed foods and centrally heated, double-glazed houses (ideal breeding grounds for house dust mites) are thought to be contributing factors.

Asthma has many different causes. Scientists still don't know exactly what these are. You may have oversensitive airways, a family history of asthma or be allergic to one or more asthma triggers.

Some doctors believe the airways become oversensitive because cells in the lungs are damaged by viruses. Others believe the initial damage is caused by an allergic reaction causing the lungs to over-react to viral infections.

One of the most common predisposing factors for asthma are allergies to house dust mites, mould spores, pollen and pets, and sometimes food allergies. Most people find there are several things that can trigger their asthma.

Asthma tends to run in families that are prone to allergies. So, belonging to a family where some members have asthma and others have other allergies, such as eczema, hayfever or allergic rhinitis, makes a person more allergy-prone.

However, because there are so many factors involved, it can be difficult to predict exactly who in a family will develop asthma.

Although asthmatic and allergic tendencies are inherited, there is no single gene involved. Rather, there are a number of different ones that react with factors in your environment to trigger the onset of asthma.

Scientists are searching for the genes involved in asthma and this may eventually lead to a cure.

Environmental factors that increase the risk of developing asthma include:

• exposure to allergens during pregnancy (eg from foods in the mother's diet) that sensitise the unborn baby's immune system
• Infections such as colds during early life
• Being brought up in a house where there is a pet (especially a cat)
• Being introduced to certain foods such as cow's milk and eggs at a young age
• Being born at a time of year when the pollen count is high
• Being exposed to cigarette smoke in the uterus or early life - babies whose mothers smoke are twice as likely to develop asthma
• Air pollution

This article was last medically reviewed by Dr Rob Hicks in August 2006.
First published in May 2001.

Thanks to Royal Brompton & Harefield NHS Trust for allowing BBC Health to take photos.