Links between air pollution and cancer risk

An environmental exposure researcher on how the air we breathe can raise risk of lung and other cancers — and what to do about it
Tony Do and Bonnie Liu, visiting from Florida, take a selfie with the smoky Seattle skyline on August 10, 2017. The smoky skies last week were caused by wildfires in British Columbia, Canada. Photo by Robert Hood / Fred Hutch News Service

Residents of the Pacific Northwest are breathing easier this week after more than 10 days of smoke-filled skies. The combination of wildfires in British Columbia and unusually stagnant, hot air resulted in a dramatic — if temporary — plummet in regional air quality.

But for the rest of the western United States, wildfire season is just getting underway. And climate experts predict that as the planet heats up, the fires will continue to start earlier in the year, burn harder and last longer than in decades past.

Worsening wildfires can wreak obvious, immediate havoc on neighboring human and natural habitats. And by dumping particulates into the air, they might also boost residents’ risk of cancer down the road, said Fred Hutchinson Cancer Research Center’s Dr. Parveen Bhatti, an expert on environmental factors that affect cancer risk.

Generally, those of us in the Northwest don’t need to worry about sucking in dirty air because Seattle is “a fairly clean city in terms of air pollution,” Bhatti said.

But researchers do know that prolonged exposure to air pollution — be it from traffic, industry or regular bouts of heavy smoke — is bad for your health.

In 2013, the International Agency for Research on Cancer, part of the World Health Organization, classified air pollution as a carcinogen, or cancer-promoting agent. Multiple large studies have found a clear association between air pollution and an increased risk in lung cancer, and individual studies have indicated possible increased risks of bladder, breast, and some other cancers as well.

The good news is that air quality in the U.S. is mostly decent and actually better than it used to be, thanks to the Clean Air Act of 1970. The bad news is there are exceptions: Air pollution levels are on the rise in many low-income, urban areas of the world, according to the WHO, and some parts of the U.S. still have poor air quality.

The nitty-gritty of dirty air

The American Cancer Society’s Cancer Prevention Study II, which began in 1982 and enrolled 1.2 million participants in the U.S., has drawn links between regional differences in air pollution and increased risk of lung cancer, Bhatti said. That’s true even among nonsmokers. A 2011 analysis from that large study saw that increases in the type of air pollution known as particulate matter — tiny, airborne particles given off by wildfires, industry, and traffic — also increased deaths due to lung cancer among those who had never smoked.

Environmental exposure researcher Dr. Parveen Bhatti Fred Hutch file

Bhatti and his colleagues are currently analyzing data from the Women’s Health Initiative — a large, long-term research study that involved more than 161,000 postmenopausal women in the U.S. — to see if the link to increased cancer risk holds true in that specific population as well.

To understand the specifics of this increased risk of cancer — and the possible biology behind it — you have to first understand what air pollution is and how researchers classify it. There are several types of air pollutants that can harm human health and the environment, according to the Environmental Protection Agency, but the type that appears to most influence cancer risk is known as particulate matter.

As the name implies, these are literally tiny particles thrown into the air as a result of less-than-clean burning — and, unlike some modern, fuel-efficient cars, wildfires burn really dirty. Tiny bits of wood and ash get into the air, and into our lungs.

One kind of particulate Bhatti and his colleagues track in epidemiology studies is called PM 2.5, referring to the smallest bits, which measure less than 2.5 microns across. (That’s about one-thirtieth the width of an average human hair.)

“They’re really tiny particles,” Bhatti said. “The reason we are particularly concerned with those is because those penetrate to the deeper parts of the lung and can actually get into your circulation.”

A recent analysis of data from several studies found that an overall, long-term increase in the concentration of these particles of 10 micrograms per cubic meter of air (slightly less than a doubling of the average yearly level considered “clean” air by California state standards) is linked to a 9 percent increase in lung cancer cases.

That boost is significant, but it’s a drop in the pond of lung cancer cases compared to cigarette smoking. Regular smoking increases the risk of lung cancer by 1,000 to more than 2,000 percent, according to the American Lung Association.

“On an individual basis, smoking is a much more brutal exposure,” Bhatti said. “Air pollution doesn’t even come close. However, given how widespread the exposure to air pollution can be, it has the opportunity to negatively impact the health of many more people.”

The two likely work in similar ways to trigger cancer. Like smoking, particulate matter in the air leads to inflammation in the lung, which is known to spur cancer formation.

How air pollution might trigger cancers in other parts of the body is a bit more of a mystery, however. Certain chemicals toxic to human DNA often stick to the small particles, and it’s possible those chemicals are triggering mutations deep in our cells that spur the formation of cancer, Bhatti said. But the particles themselves might also inflict damage.

What if summer wildfires get worse?

While they can worsen asthma and other lung conditions as well as heart disease, isolated incidents of wildfire smoke like the one that recently blanketed the Pacific Northwest are unlikely to significantly affect anyone’s risk of cancer, even if you were outside all day long, Bhatti said.

“As we think of with most exposures, it takes years of exposure to lead to an increased risk,” he said.

The question of cancer risk gets murkier, however, when you factor in that wildfire season in the Western U.S. has grown from an average of five months to more than seven months long since the 1970s as our planet heats up, and the average number of large wildfires per year is also on the rise.  

“With climate change … if it becomes that most of the summer months we have smoke-filled air, that’s where we become concerned that we’re getting this regular exposure over time that could really then contribute to increased long-term health risks like cancer,” Bhatti said.

If air pollution from wildfires does become a more regular occurrence, Bhatti said individuals can follow standard public health advice for poor air-quality days to help reduce their long-term risk of lung cancer.

Most of the advice boils down to avoiding as much of the dirty air as you can: Skip outdoor exercise (but keep exercising, Bhatti said, because that lowers your risk of many cancers too). Stay inside buildings and cars with doors and windows closed and preferably some kind of air-filtration system or recirculating air conditioner running. Go to local cooling centers or other indoor spaces with A/C or, if you can, just get out of town until air quality improves.

But ultimately, the onus is on our local and federal governments to keep monitoring air quality and take steps to stem pollutants and climate change itself, Bhatti said.

“We really need to make sure our local government and the federal government pays attention and is taking action to make sure that these types of things don’t get worse so that we’re not dealing with this on an annual basis, which leads to that long-term exposure,” he said. “That’s where I think the major impact needs to come from.”

Rachel Tompa is a former staff writer at Fred Hutchinson Cancer Center. She has a Ph.D. in molecular biology from the University of California, San Francisco and a certificate in science writing from the University of California, Santa Cruz. Follow her on Twitter @Rachel_Tompa.

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