Smoke’s particle size is key to its health consequences

The ongoing wildfires in the western US have spewed so much smoke that they’ve darkened the skylines of cities as far away as New York and Washington, DC. For those living near the fires, the health effects of breathing the smoke—especially when indoor activities are restricted because of the COVID-19 pandemic—are a top concern.

“It’s really tempting to think that wildfire smoke is a more natural form of pollution and therefore not as harmful” as sources such as traffic or energy production, says Sarah B. Henderson, a senior environmental health scientist at the British Columbia Centre for Disease Control. Science, however, says otherwise.

The key metric for smoke is how much particulate matter is in the air: few particles are good; many particles are hazardous. But beyond counting particles, what other factors control smoke’s short- and long-term health effects? Experts who study the effects of pollution on human health say that smaller particles are more hazardous, regardless of their sources—such as combustion from engines, power plants, or wildfires; dust generation; or chemical reactions of other pollutants. Whether the particles’ chemical composition makes a difference is unclear.

The health effects of wildfire smoke are an increasing concern worldwide, because even as many countries have clamped down on other sources of pollution, the frequency and severity of wildfires have increased because of climate change (Proc. Natl. Acad. Sci. U.S.A. 2017, DOI: 10.1073/pnas.1617464114). In Australia, last year’s bushfires burned at least 11 million hectares—an area roughly the size of Cuba. As of C&EN’s deadline, more than 1.6 million hectares have burned in California so far this year.

And in Lebanon and North Africa, the traditional wildfire season of May through October has expanded into a near-constant fire season. The situation in Lebanon is made worse because the country is also plagued by high levels of pollution from other sources, such as diesel generators, says Najat Saliba, a chemistry professor at the American University of Beirut. “I don’t open my windows at home anymore,” she says.

Wherever particulate matter originates, “what we can say for sure is that breathing combustion-related particles leads to respiratory and cardiovascular diseases, adverse birth outcomes, and diabetes,” says economist Arden Pope, whose research at Brigham Young University covers the health effects of air pollution. Particle size also comes into play, with particulate matter 2.5 µm in diameter or smaller (PM_2.5) causing greater harm. “Particles that are larger than 2.5 µm in diameter don’t seem to have as big a health impact because they’re larger and so they don’t penetrate as far into the lungs,” Pope says.

Unfortunately, forest-fire smoke is chiefly made up of PM_2.5. In one study, for example, analysis of smoke from wildfires in Portugal in 2010 found PM_2.5 concentrations ranged from 0.69 to 25 mg/m³, while particles between 2.5 and 10 µm had concentrations of just 0.048 to 3.1 mg/m³ (Atmos. Environ. 2013, DOI: 10.1016/j.atmosenv.2013.01.062).

While researchers agree that particulate size matters, opinions are split as to whether particles’ chemical composition makes a difference. A particle’s composition can vary widely depending on its origin, whether from wildfire, other biomass burning, engines, construction, sandstorms, or atmospheric reactions.

According to a US Environmental Protection Agency spokesperson, who would only answer questions through email, wildfire smoke is a complex mixture composed of many individual chemicals that vary depending on what exactly is being burned. While there has been extensive research on these components, the data haven’t shown that any individual chemical is more associated with negative health effects. The same is true for chemical components of particles from other sources.“People always think that the chemical composition must be important,” says Michael Brauer, a professor at the School of Population and Public Health at the University of British Columbia. “But this has been studied for more than 20 years, and there’s not that much consistency except to say that any combustion pollution is worse than mechanical things such as road dust and sandstorms.”

The chemical composition of particulates “is interesting as a scientific question, but from a policy perspective we know that if we reduce the overall concentration of small particulate pollution, then there will be health benefits regardless of the composition,” Brauer says.

In fact, Brauer fears that if scientists and government regulators get too bogged down in the detail of which compounds within particulates are the most linked to poor health, then industrial polluters will shirk responsibility. “The argument can detract from actually tackling the problem of pollution beyond wildfires, because car manufacturers will say it’s one thing and the steelworks will say it’s another, and so on,” Brauer says. “What we need to do is concentrate on reducing it overall.”

Not everyone agrees. “I understand that perspective, but I take a different view,” Pope says. “It would be interesting and potentially helpful for public health policy to know which particles are worse.” If a specific chemical did turn out to be significantly worse for human health than others, then regulators could focus on lowering emissions of that pollutant.

Until there are more data on the effects of chemical composition, the focus will remain on size as the most useful indicator of particulate matter impacts on health. Some experts argue that regulators should take size a step further and start measuring a new category of “ultrafine” particles that are 0.1 µm or smaller. However, a 2020 policy assessment carried out by the EPA concluded that there is insufficient evidence of health effects linked to human exposure of ultrafine particles, and the agency confirms that it has no plans to revise its monitoring. A 2019 analysis funded by the European Commission concluded that “existing literature shows inconsistent associations between [ultrafine particle] concentration and health impact,” although it recommends that future air policy discussions continue to consider the topic.

“Ultrafine particles don’t last very long in the atmosphere, or they tend to be very localized, so to be honest, you couldn’t have a very useful or informative system that measures ultrafine particles,” Henderson says.

This could change in coming years, Henderson says, if studies begin to emerge that suggest the chemical composition of particles makes a difference or ultrafine particles pose more of a threat to human health in the long term. “These are really difficult questions to answer,” she says, but “we’re getting to a place where we can tease out more answers because we have more and more data from a lifetime of pollution exposure.”

Benjamin Plackett is a science journalist based in London.