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About 22 million American adults have asthma.¹

Asthma is more common now than it used to be.² There are probably several reasons why.

• Some doctors think that children who don't get many colds and other infections are more likely to get asthma than children who often get colds.³ This is called the hygiene hypothesis. The idea is that getting colds and other infections keeps the immune system busy. If your immune system isn't kept busy, it may overreact to harmless things, such as pollen. This may cause asthma.
• Another theory is that our homes contain more triggers (such as house dust mites) that may cause asthma.³ This may happen because we have central heating and air out our homes less than we used to. There are also more man-made chemicals in our food. These cause asthma in some people.

Asthma is more common in children than in adults:¹

• About 9 in 100 children have asthma
• About 7 in 100 adults have asthma.

Asthma usually starts in childhood. But not everyone who has asthma as a child will still have it as an adult. Up to three-quarters of children grow out of it.⁴ However, children who get symptoms more often are more likely still to have asthma in later life.

In the United States, asthma is slightly more common in black people than in white people or Hispanic people (who are not Puerto Rican).¹ And it's slightly more common in women than in men. Although as children, boys are more likely to get asthma than girls.

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.