Doctors and public health officials have noted with alarm the increase in asthma in the United States and other western countries during the second half of the 20th century. Many theories have been proposed for why so many children are affected: pollution, dust, household animal exposure, even living in homes that are too clean.

But epidemiologist Rene Goodwin from Columbia University in New York says there are only four things proven to cause the onset of asthma. "There's a family history of asthma, there's a family history of allergies, and then there's the exposure to cockroach allergen, and then there's exposure to environmental tobacco smoke."

Goodwin examined trends of smoking in the United States and compared them to reported rates of asthma. She took data from a study that followed 4,500 children for 15 years. "We took the rates of asthma in each age group under 18 and graphed that," she explains. "And then the US Department of Agriculture provides data on cigarette tobacco consumption from 1900 to 2000 and we took that information and graphed that as well."

As rates of cigarette smoking increased, Goodwin saw a corresponding rise in asthma. She also found that asthma rates were 2.5 times greater in children whose mothers smoked a half a pack a day indoors than in those children whose mothers smoked fewer cigarettes.

She says that while rates of smoking have been dropping across the United States, they're not dropping uniformly. "I found data that suggests that the same populations where you see asthma rising the most you see smoking continuing to rise? young women are the biggest group of new smokers. And also, while there are some declines in smoking overall, the decline is most steep in men."

Goodwin says environmental tobacco smoke is becoming more prevalent in developing countries where cigarette manufacturers are looking for new markets, and advertising heavily. She urges epidemiologists to start tracking the incidence of asthma in those countries to see if it rises along with rates of smoking.

Goodwin's research appears in the Annals of Allergy, Asthma and Immunology.