Asthma and Smog: Does Air Pollution Cause Asthma?

in Asthma
Published: July 2, 2010

smogFrom the archives: Air pollution irritates this lung condition. But could smog cause asthma? 

THE smog hanging over downtown Toronto was so thick that summer day that you couldn’t see the CN Tower from the midtown restaurant where Sara La Rocque was waiting tables at an outdoor patio.

One day later, Sara, a creative writing student, couldn’t breathe properly. Although she had experienced asthma briefly as a child, this was different. It was like breathing through a straw. Sara quit her job.

As the province of Ontario lumbered through a record summer of smog advisories, Sara stayed indoors. She was miserable. It wasn’t until six weeks later that she could even walk around the block with the family’s golden retriever (she is not allergic to dog dander) or the Burmese mountain dog.

Sara’s pretty sure what triggered her condition: “My asthma is smog-induced,” she says. “I think that’s what caused it.” A few years ago, most scientists would have doubted her analysis. The common wisdom was that air pollution could only exacerbate symptoms in people already living with asthma.

But now, a handful of mavericks in the scientific world are building a case to prove Sara’s point – that pollution might not just worsen asthma, but cause it. Not that these asthma researchers can yet say how this might happen. That’s still under study.

When it comes to asthma, theories abound as to why it develops, starting with the hygiene hypothesis. This suggests that our urban society is too germ- and virus-free, causing the underworked immune systems of those who inherit the allergic tendency to react to proteins – such as inhaled pollen or dust mites – that should be harmless. The immune system’s over-reaction results in airway inflammation and allergic asthma attacks.

There are also new indications that antibiotics in early life and obesity may be contributing factors. But some scientists keep coming back to the relationship between asthma and air pollution, particularly to that dense layer of smog that blights our cities all too often in the summer.

Somewhere in that haze of pollution, they believe, lies an answer to the mystery of why asthma gets switched on with such frequency in the urban world.

Over the past decade, scientists have found compelling evidence that air pollution irritates the lungs and triggers attacks in those who already have asthma. Some research indicates it can also worsen asthmatic flare-ups to allergens such as pollen, dust mites or pet dander.

“Air pollution remains one of the most under-appreciated contributors to asthma exacerbation,” wrote George Thurston, an associate professor of environmental medicine at New York University School of Medicine, in an article in the Journal of Allergy and Clinical Immunology (JACI).

A classic instance: When a strike closed a steel mill in the Utah Valley for the winter during the mid-1980s, researchers found that admissions of children to hospital for asthma and pneumonia were cut in half – and they climbed right back up the following winter after the steel mill had reopened. That’s a vivid example of pollution’s effect on asthma, and there are plenty more.

California Studies See Traffic Link

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.”

Risks Near Major Roadways

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.

In May 2006, 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 neighborhoods. Then the USC researchers looked more closely at the children who played a lot of sports outdoors, and found something interesting.

Sports, Smog and Asthma

They zeroed in on 273 kids who participated regularly in three or more sports. The sporty kids in high ozone neighborhoods were over three times as likely to develop asthma as kids who didn’t play sports in those neighborhoods. 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.

Which Pollutants Cause Asthma?

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.”

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