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The Asthma Section

Asthma and Smog: Does Air Pollution Cause Asthma?

In May, for instance, the USC researchers reported new findings that children who live within 75 metres of a roadway are 50 per cent more likely to develop asthma than children who live farther away.

This was a study of over 5,000 kids age 5 to 7. The biggest increase in risk was seen in kids with no family history of asthma who had lived near the roadway since before the age of 2. (Kids who moved near the roadway after age 2 did not increase their risk of asthma.) According to the group led by USC epidemiologist Rob McConnell, this indicates that infants, or even fetuses, could be especially vulnerable to the effects of traffic pollution.

“We conclude that living in a residence with more nearby traffic increases the risk of childhood asthma,” the researchers reported in the journal Environmental Health Perspectives. In fact, breathing fresh vehicle exhaust was responsible for nearly 60 per cent of the asthma reported in kids living within 75 metres of a major roadway.

“Because a substantial number of southern California children live near a major road, this exposure is potentially an important public health problem that could be remediable by transportation and residential development policy and by more effective control of vehicular emissions,” they wrote.

In 2002, the USC researchers suggested in another report that an ingredient of smog – ozone – might cause asthma to develop. McConnell and fellow USC scientists tracked more than 3,500 children aged 9 to 16 who had no history of asthma. The kids lived in 12 communities with varying levels of pollution.

Overall, those living in high pollution neighborhoods were not, in fact, more likely to get asthma than kids in lower pollution neighbourhoods. Then the USC researchers looked more closely at the children who played a lot of sports outdoors, and found something interesting.

They zeroed in on 273 kids who participated regularly in three or more sports. The sporty kids in high ozone neighbourhoods were over three times as likely to develop asthma as kids who didn’t play sports in those neighbourhoods. The children who played lots of sports in cleaner air, on the other hand, did not have a higher risk of asthma.

These were not cases of exercise-induced asthma. “Our results show that playing multiple team sports in a high ozone environment is associated with development of physician-diagnosed asthma,” McConnell’s team reported in The Lancet. “The results are consistent with a large increased risk both for new onset asthma and for exacerbation of previously undiagnosed asthma.”

But it will take a lot more evidence to convince the skeptics that outdoor air pollution can cause asthma. If breathing something in the air is the explanation for the asthma epidemic, it’s more likely to be in the air indoors than outdoors, says Dr. Allan Becker, an allergist with the Children’s Hospital in Winnipeg and a professor of pediatrics at the University of Manitoba.

He’s one of more than 100 researchers who are part of AllerGen, a Canadian network of scientists and institutions mobilizing to improve the quality of life for allergic disease sufferers. Over two-thirds of all asthma starts in the first five years of life, Becker notes, and children at that age spend 90 per cent of their time indoors. So it makes sense that something inside – what children breathe or what they eat – is contributing to the rise in asthma.

Bolstering that theory is the change in house construction since the 1970s, when the asthma epidemic began. Since that time, homes have been built to reduce the leakage of air from the outside, significantly reducing the amount of air flowing through cracks around the windows.

To Becker, indoor air is a major suspect in the mystery of what is driving the asthma epidemic, and it will be examined in a major research project that AllerGen is set to begin next year.

Next Page: Which Pollutants Cause Asthma?

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