Doctors have identified many of the things that may trigger, or start, an asthma attack. Triggers are things that cause an asthma sufferer's airways to swell, or increase in size.

Different asthma patients have different triggers. Allergens are one of the most common triggers. These impurities in the air cause allergic reactions. Some of the more common allergens include animal hair, dust, mold and pollen.

Pollen is a fine dust that comes from grass, trees and flowers. Mold is a kind of fungus. It can grow on the walls or floors of homes. It is often in wet or damp areas like bathrooms, kitchens and basements.

The Environmental Protection Agency estimates that 21 percent of asthma cases in the United States have links to mold and dampness in homes.

Air pollution can also trigger asthma. Cigarette smoke is a major problem for asthma sufferers. So is air pollution from motor vehicles. Chemical sprays like air fresheners, hair spray, cleaning products and even strong beauty aids can trigger an asthma attack.

Some people cough, wheeze or feel out of breath during or after exercise. They are said to suffer from exercise-induced asthma. During the winter, breathing in cold air can trigger an asthma attack. So can colds and other infections of the respiratory system.

The Centers for Disease Control and Prevention says more than 25 million people in the United States have asthma. It says the disorder affects more than 7 million American children. Among adults, more women have the disease than men. However, it is more common among boys than girls.