Some of the apparent increases may not be real but may have occurred because doctors got better at making the diagnosis. But increased reporting seems unlikely to account for all the new cases. Theories come and go, and when you come right down to it, no one really knows why some people develop asthma and others don't.

In "The Asthma Epidemic," an article published last week in The New England Journal of Medicine, doctors tried to sort out various theories about the causes of asthma and explain why rates have risen. But there are no clear-cut answers.

Like other chronic diseases, asthma is probably caused by multiple genes and environmental exposures, and it can have quite different causes in different people. About half the cases are thought to stem from allergies and the rest from other problems that can irritate and inflame the airways, causing them to close.

Genetic changes in the population cannot explain the increasing rates, though, because such changes occur too slowly to account for the rapid increases in asthma, the authors said, suggesting that environmental factors are more likely candidates. But what has changed enough in the environment to explain spiking asthma rates?

The authors of the article, from University Children's Hospital in Munich, Germany, review many study findings. They report that one clear risk factor is secondhand tobacco smoke. Exposure to it does increase asthma risk in infants and small children. But how would that explain the increases, when overall, parents today smoke less than previous generations?

People frequently blame air pollution for causing asthma, but its role is not entirely clear. Pollution does make asthma worse in people who already have the disease, but it's not known whether pollution also makes asthma develop in the first place. And in any case, air pollution in the United States has decreased in the last few decades. Living in a place with high vehicle exhaust may make asthma worse, but the evidence is "relatively weak," the researchers report.

Dust mites, microscopic insects that live in bedding and furniture, were long blamed for causing asthma to develop in infants and small children and have led to a booming industry of mattress covers, air filters and guilt-ridden parents tethered to dust mops and vacuum cleaners. But recent studies have questioned the connection. Once children have asthma, though, the mites and their droppings may make the symptoms worse.

Cat dander has become a complete puzzle, with some studies finding that exposure early in life leads to asthma, and others saying it protects against asthma. At this point, nobody knows which study to believe, but most experts agree that when people already have asthma, being around cats can make it worse.