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The Asthma Section

Asthma and Smog: Does Air Pollution Cause Asthma?


The Pollutants

Even without the skepticism, those who suspect that air pollution causes asthma face a big challenge: Which pollutant or pollutants could be responsible? That’s a tough call because there are so many ingredients in the smog that infiltrates our cities, especially in the summer.

Emissions from vehicles – a main contributor to smog – are a key culprit. As Harvard School of Public Health’s Joel Schwartz put it in a 2004 article in Pediatrics: “The overwhelming weight of the recent evidence suggests that traffic pollution is associated with the risk of developing asthma.”

But that assertion needs to be narrowed down again: which pollutant in the vehicle exhaust that’s part of the smog? In his research, Gauderman measured levels of nitrogen dioxide (NO2), emitted by cars and trucks, outside the homes of over 200 kids.

His team found that for every increase of 5.7 parts per billion in average nitrogen dioxide, the risk of getting asthma by age 10 increased 83 per cent in kids. But it’s not clear whether NO2 caused the increased cases of asthma – or whether it might have been one of the other gases or particulates in the exhaust.

Nevertheless, the evidence linking diesel fumes to the development of asthma has been convincing enough to move governments to act. New regulations in Canada and the U.S. have been introduced to reduce the amount of sulfur in diesel to less than 15 parts per million from 500 ppm. The regulations will be fully in effect in the fall of 2006.

Researchers are also looking closely at the impact of ozone, the invisible gas that McConnell’s group found was linked to the onset of asthma in the sporty California kids.

Ozone is formed when several gases mix in the presence of sunlight – including nitrogen oxide, which comes mainly from vehicles and coal-fired power plants, and Volatile Organic Compounds (VOCs), which comes from gassing up cars and is given off by products such as paints and cleaning fluids.

Scientists have amassed a powerful body of evidence to show that ozone triggers asthmatic trouble in people who already have the disease. During the 1996 Olympic Summer Games in Atlanta, for instance, the city restricted traffic during the 17-day Games, which cut peak ozone levels by 28 per cent. The result: A 42 per cent decline in children’s asthma attacks.

But no one was able to suggest that ozone might lead to the development of asthma until the USC group’s groundbreaking study on those athletic kids. “This study needs to be replicated elsewhere,” Thurston observed in the Journal of Allergy and Clinical Immunology. “But it does suggest that higher long-term exposure to air pollution might well cause the induction of asthma.”

So does it? No one knows for sure. Gauderman says he’s not suggesting anyone will have an “allergy” to the pollution itself. But lab studies indicate that certain pollutants can cause chronic inflammation in the airways and make some people more likely to react to pollens or other allergens. In other words, pollution may be an important contributor to the onset of the disease.

As for Sara, she’s walking her dogs again, and ready to cope with summer in the city. She’s not planning to wait tables outdoors this summer, however. Given the effect pollution has on her asthma, she’s looking for an office job. Medication is helping a lot, but Sara knows what to expect on the smoggiest days: “I’ll have to stay inside.”

For the related sidebars to this article, see the Summer 2006 issue of Allergic Living magazine.
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Allergic Living acknowledges the assistance of the OMDC Magazine Fund, an initative of the Ontario Media Development Cooperation.