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The Asthma Section

Asthma and Smog: Does Air Pollution Cause Asthma?

California Studies Link Traffic to Onset

A group of researchers from California are now taking the relationship a step beyond the theory that air pollution triggers or worsens asthma in those who have already been diagnosed.

In a series of studies, scientists from the University of Southern California have been following thousands of California schoolchildren over several years to examine how their health is affected by the key components of smog, such as emissions from cars and trucks and ground-level ozone.

Their findings indicate that living beside a roadway greatly increases a child’s chance of developing asthma in the first place. What’s more, they’ve found that kids who take part in three or more sports breathe in high concentrations of ozone while playing, and are more likely to develop asthma than equally sporty kids playing in cleaner air.

These studies suggest that “air pollution is related to the onset and not just the exacerbation (of asthma),” says Jim Gauderman, associate professor of preventive medicine at USC’s Keck School of Medicine and one of the study co-authors. Does that mean that pollution causes asthma? “That kind of definitive statement is quite a way off,” Gauderman acknowledges in an interview.

For one thing, the pathology of how pollution might lead to the development of asthma is not clear, and scientists don’t even know which of the airborne chemicals is doing the damage.

Proving the case that pollution causes asthma will require plenty more studies – research with human populations to confirm the California findings, plus more studies of animals in the lab to show how pollution may result in asthma, Gauderman says. “Then we can talk cause.”

To even suggest that it might lead to its onset is a highly controversial proposition. Earlier studies found no evidence that air pollution increased the risk of getting asthma. What’s more, the 58 per cent increase in the prevalence of childhood asthma since 1980 happened at a time when North America’s air became cleaner, not dirtier.

As Noreen Clark, a public health professor at the University of Michigan, put it in an often cited 1999 review: “Studies illustrating the causal effect between outdoor air pollution and asthma prevalence are scant.” Clark’s skepticism is the prevailing view.

But the USC scientists are challenging that thinking. The previous researchers didn’t see the link between asthma and pollution, Gauderman says, because they relied on community-wide air monitors to assess levels of pollution. “And locally increased levels not captured by a community-wide monitor may be more important.”

Pollutants in the air can vary considerably depending on where you are in a community, and whether you’re breathing in a lot of dirty air while playing sports outdoors. By looking more closely at people who breathe in more polluted air – either because they live in a pocket of pollution or because they’re playing sports and breathing harder – the link between air pollution and asthma becomes more visible.

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