Children who play outside in neighborhoods with high levels of air pollution are much more likely to develop asthma than those who play in areas with cleaner air, suggesting that pollution may cause asthma, according to a major study being published today.
The five-year study of 3,535 children in California contradicts earlier studies that concluded that air pollution does not cause the disease but aggravates the lungs of people who have asthma.
A host of experiments in the past decade has pointed to a variety of explanations - including dust mites, cockroaches, molds, pollens, pets, obesity and increased awareness among doctors - for the rapidly rising rates of asthma around the world, especially in urban areas.
Asthma rates have more than doubled in the past 20 years, and today more than 5 million children and 10 million adults in the United States have asthma, including about 10 percent of the children in Baltimore's elementary schools.
But until today's article by Dr. Rob McConnell of the University of Southern California in the British journal Lancet, no large-scale study has blamed vehicle exhaust and other forms of air pollution as a cause of the epidemic.
The new focus on pollution may be especially timely because of concerns by officials in several Northeastern states that the Bush administration may act to weaken regulations restricting emissions from power plants.
"I think this is a very important discovery," said Baltimore's health commissioner, Dr. Peter Beilenson. "Asthma is a growing cause of emergency room visits in Baltimore and elsewhere. And this study may give new impetus against any efforts to weaken the nation's clean air laws."
As part of an $18 million project funded by California and the federal government, McConnell and nine other scientists followed for five years more than 3,500 children ages 9-16, from 12 middle-income communities in Southern California. Many of the children participated in organized sports such as soccer, basketball and tennis.
Half of the neighborhoods where the children lived had low levels of ozone, nitrogen dioxide and particulate pollution. The other half had high levels of air pollution, according to the study.
No children had asthma before the study began in 1995. But the researchers found that 265 children developed asthma during the observation period, and the majority of these were children who played outdoors in the more polluted areas.
Children who played at least three sports outdoors in areas with high ozone pollution were three times more likely to develop asthma than children who played no sports in these areas, according to the study. But playing sports outside had no effect on asthma rates in communities with low pollution. Asthma was also more common among children who spent several hours a day outdoors in polluted areas, regardless of whether they played in team sports. Children who spent a lot of time outside in areas with high ozone levels were 40 percent more likely to develop asthma than children living in the same areas who spent little time outside, said McConnell. This was not true in areas with little air pollution.
"This is a new and significant finding - that not only does pollution aggravate asthma, it causes asthma," said Dr. Joel Schwartz, a professor at the Harvard School of Public Health.
Dr. Alan Leff, an asthma sepecialist at the University of Chicago, disagreed about the significance of McConnell's study. "There is no evidence whatsover that pollution causes asthma," said Leff. "All this study shows is that children who exercise in polluted areas are more likely to have their doctors detect symptoms of asthma than children who exercise in nonpolluted environments."
This discrepancy could be explained, Leff said, by the tendency of doctors in areas with high asthma rates to be more sensitive to the symptoms of the disease and more likely to detect and diagnose it. He believes that pollution aggravates asthma and that doctors may be more likely to detect the disease in polluted areas because the cases are more severe.
Leff said a far more convincing study by Dr. Erika Von Mutius of asthma in Germany in the 1990s reached the opposite conclusion.
Von Mutius set out with the hypothesis that children who grew up in the poorer and more polluted East Germany would suffer from more asthma than children in the wealthier and cleaner West Germany. But she found the opposite: that children in more polluted East Germany had lower asthma rates. Von Mutius was forced to give up her theory in favor of what she later called "the hygiene hypothesis," which argues that children who grow up in cleaner environments may be more susceptible to asthma because their lungs and bodies are not used to fighting dirt, irritants and microbes.