(ISTOCKPHOTO)Although asthma affects 23 million people in the United States, including more than 6 million children, the cause of the respiratory condition is still a bit mysterious. Experts arent exactly clear why some people develop asthma and others remain unaffected by asthma triggers like pollen and pets.

However, much is known about the risk factors for the disease—some factors you can change and others you cannot. For example, asthma is more common in boys than girls and in children in urban areas than other parts of the country; plus, it is more likely to be diagnosed in people who are obese than in those who are slender.

About 9% of the U.S. population under age 18 has asthma. Studies involving twins have found that a childs chance of developing asthma is anywhere from 50% to 80% dependent on genes, but the environment in which a child lives and grows is also very important. Exposure to dust, pet dander, pollution, exhaust, and secondhand smoke can all increase a childs risk for asthma. These allergens and irritants can also trigger the sudden exacerbation of asthma symptoms known as an asthma attack.