Narrator: This is Science Today. The prevalence of asthma - especially in children - has been rising at an alarming pace over the years. Clinical pulmonary specialist, Susan Janson of the University of California, San Francisco says it's still unclear why the prevalence is so high.
Janson: There are a number of hypothesis about it. It seems to be diagnosed more and more often. Are we looking for it better, so are we making more diagnoses? The other hypotheses are children are kept indoors much more than they used to be, so they're in the environment where the allergens are and they're not outside all the time.
Narrator: Janson says the most important allergens for both children and adults with asthma are indoor allergens.
Janson: Which we call perennial because they're there all the time. Cats, dogs, dust mites, cockroach mites - whatever those things in the household that could just live there - molds; those are all important. More important than the outdoor ones just because they're there constantly until the situation is corrected.
Narrator: For Science Today, I'm Larissa Branin.