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If you have asthma, the tubes that carry air in and out of your lungs become swollen and narrow. This makes it difficult to breathe. You may have had asthma since you were a child, or you may have gotten it as an adult. Some people get asthma for the first time later in life, in their 60s or 70s.

There are many treatments that work well. They can help you breathe more easily. And they can keep you from getting symptoms.

• About 22 million American adults have asthma.¹
• Asthma can make you feel breathless. Your chest may feel tight and you may cough or wheeze.
• With the right treatment, asthma can be controlled well.
• Even if your asthma is mild, it's important to get the right treatment.
• An asthma attack can end up being very serious. So, if you can't control your symptoms with the medications you usually use, you should see a doctor right away.
• You may be able to avoid the things that make your asthma worse.
• Having asthma shouldn't keep you from holding down a job and being active.
• Learning about asthma can help you to control it better.

To understand what happens in asthma, it's useful to know what your lungs do and how they work.

Your lungs sit in the center of your chest, behind your ribs. Their main job is to get oxygen from the air into your body.

Your blood vessels carry oxygen from your lungs to the rest of your body. Every cell in your body needs oxygen to work.

• When you breathe in, air goes into your lungs.
• The main tube that goes from your throat to your lungs is called your windpipe (trachea).
• Your windpipe divides into two tubes called bronchi (each individual tube is called a bronchus).
• Inside each lung, the air moves down a network of tubes called bronchioles.
• At the end of each tube is a tiny pouch (called an alveolus) surrounded with blood vessels.
• Oxygen in the air passes through these pouches into the blood vessels.
• Carbon dioxide passes the other way back into your lungs. This is a waste product that you get rid of when you breathe out.

If you have asthma, you can't always breathe normally. The tubes inside your lungs get narrower, so you can't get enough air in and out of your lungs. This makes you gasp for breath. Or it may make you cough or wheeze.

To learn more, see What are the symptoms of asthma in adults?

Three things happen in your lungs to make the air passages narrower:

• Muscles in the walls of your airways get tighter
• The walls of the airways become swollen
• Mucus is released into the airways, partially blocking them.

We're not exactly sure why some people have asthma and others don't. But two things seem to be important:

• The genes you inherit from your parents (asthma often runs in families)
• An infection or other things in the air. For example, you may have first had asthma symptoms when you had a cold or a chest infection. Or you may be allergic to certain things in the air. These things are called allergens. Common allergens are pollen, house dust mites, mold and bits of fur and skin (dander) from pets.

After you've had asthma symptoms once, your airways are extra-sensitive to things that make them swollen. Whenever you come into contact with an asthma trigger (see below), your immune system overreacts. Your airways become inflamed and full of mucus. This is what causes your symptoms.

It might be that the thing that first gave you asthma symptoms (for example, dog hair) always brings on asthma symptoms. Or you may find that other things give you symptoms as well.

Certain things can bring on your asthma symptoms. These things are called triggers. It's useful to know what triggers your symptoms. If you can avoid these triggers, you may be able to keep your asthma symptoms from happening. The most common triggers are:¹

• Allergens (such as house dust mites, bits of fur and skin from animals or pollen)
• Smoke and air pollution
• Exercise
• Certain medications
• Things you use in your job
• Illnesses (such as a cold)
• Stress.

To read more about these triggers, see Some common triggers for asthma.

Some people are more likely to get asthma than others. And asthma tends to run in families.

For example, if a child has one parent with asthma, they are twice as likely to have asthma themselves than a child whose parents don't have asthma.²

Diseases run in families because of genes that are passed from parents to their children. But there is no single gene that causes asthma. It happens because of a combination of many different genes. Some of these genes change the way your immune system works. But we don't know how these genes work together to cause asthma.³

If you inherit asthma from your parents, you may also inherit eczema, hay fever or both.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.