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Exposure to wildfire smoke may cause long-term health effects, research suggests.

Rick Rycroft/AP

What we don’t know about wildfire smoke is likely hurting us

SEATTLE—The worst of Australia’s most recent bout of raging fires may be drawing to a tentative close, but the long-term effects may be just beginning, experts say. As wildfires increase in prevalence and severity with each year, scientists are pouring more time and research into the effects the billowing clouds of smoke may have on human health. Three experts from the Environmental Protection Agency (EPA), the U.S. Forest Service, and the University of California (UC), Davis, discussed some of these public health consequences here today in a presentation at the annual meeting of AAAS, which publishes Science. Here are some of the take-homes:

Why is wildfire smoke so dangerous?

Like other types of smoke from building fires or even cigarettes, wildfire smoke contains a mixture of particulate matter, carbon monoxide, and volatile chemicals. These components differ depending on what material is feeding the flames. Smoke from fires that burn through poison oak and poison ivy may contain traces of irritants from those plants. Smoke can also pick up chemicals from plastic and other humanmade materials when wildfires burn through cities or housing developments, says Wayne Cascio, a cardiologist and director of EPA’s Center for Public Health and Environmental Assessment. 

More research is needed, Cascio adds, to determine the exact conditions that might contribute to more or less harmful smoke. And while the chemical content of wildfire smoke may not always differ substantially from other types of smoke, wildfires are a totally different kind of event by nature; the smoke can travel far and fast, cloaking urban areas in a toxic blanket that can sometimes be seen from space.

The most dangerous part of wildfire smoke, says Lisa Miller, an immunologist at UC Davis, is the particulate matter. Wildfire smoke contributes about 40% of fine particulate matter pollution in our atmosphere, and these tiny specks of solid material can be smaller than 2.5 microns—miniscule enough to wreak havoc in human bodies. “Material of this size can readily enter the deep lung and the bloodstream,” she says. 

What do we know about the long-term health effects of wildfire smoke?

Wildfire smoke can increase respiratory conditions such as asthma in the short term, but there’s limited research on its long-term effects. After California’s Humboldt wildfires ravaged thousands hectares of land in June 2008, sending a blanket of smoke across California, Miller, who heads the respiratory diseases unit at the California National Primate Research Center, saw the opportunity for a long-term study. 

After the smoke from the fires abated, and then twice over the next several years, Miller and her team tracked changes in the immune system and lung function in monkeys at the center. Initially, Miller expected the animals to develop asthma or other common respiratory ailments. But instead, she detected something more insidious: After exposure to smoke, the baby monkeys’ lungs stiffened, with the tissue becoming thicker and more rigid than that of monkeys born the following year. More than 10 years later, the monkeys born in smoke still have abnormally small, stiff lungs. 

In their adolescence (around 3 years for a rhesus macaque), the monkeys also showed signs that the smoke affected their immune systems, Miller says, although the effects dwindled with age. The immune damage wasn’t limited to one generation, either. Recently, Miller conducted a similar study on the offspring of the smoke-exposed macaques and found that the new babies showed signs of the same weakened immune response their smoke-exposed mothers had demonstrated in their adolescence.

Of course, there are significant differences between Miller’s monkeys and humans who may be exposed to wildfire smoke. For one thing, the monkeys spent all of their time outside, while humans may retreat indoors to limit smoke exposure. Still, “the monkeys may serve as a sentinel for health outcomes in susceptible populations,” she says. 

Who is most at risk?

Predictably, older people, children, and pregnant women are most at risk. This translates to just more than 100 million Americans, around one-third of the population, Cascio says. That number is also likely to grow in the future, as the country’s population of older people increases, wildfires increase in severity, and more people move to semi-urban areas where wild spaces and cities merge.

People who work outdoors and around wildfire smoke are also at elevated risk, says Joe Domitrovich, an exercise physiologist and wildfire firefighter with the United States Forest Service. “Wildland firefighters are spending 100 days each summer fighting these fires,” he says. Domitrovich and colleagues have conducted a number of studies to gauge the effect the smoke has on their bodies. Although the research is ongoing, their studies have already shown that retired wildland firefighters are at higher risk of lung cancer and cardiovascular disease. 

What precautions can people take?

“When smoke inundates your area, people should try to stay indoors,” Cascio says. “Don’t exercise, because increasing your physical activity increases your respiratory rate, and consequently, your smoke exposure. If you have air conditioning it should be running so the filtering can improve the indoor air quality.” He also advises being aware of indoor air quality as well during these times; that means avoiding incense, candles, and particularly smoky cooking.