More than 120 million Americans — one-third of the U.S. population — have been living under air quality alerts this summer, with citizens in New York City, Chicago, and Detroit at times experiencing some of the unhealthiest air in the world. The hazy conditions, fed by an unprecedented surge in Canadian wildfires likely fueled by climate change, has grounded planes, canceled outdoor sporting events, and filled emergency rooms with asthma patients.

Although some cities are experiencing relief this week, the 1,000-plus blazes raging in boreal forests from British Columbia to Nova Scotia mean that Americans in the Midwest and eastern United States can expect more waves of eye-stinging, throat-burning smoke.

That smoke is more than a nuisance: It contains dangerous particulate matter that we breathe into our lungs, and scientists are urgently calling for more research to understand the health effects of increasingly frequent exposures. They’re also worried that the widely used air quality index is insufficient to alert us to the particular threat of wildfire smoke, which early research suggests may be more harmful than other types of air pollution.

It’s well-established that short, intense exposures to wildfire smoke can exacerbate respiratory problems like asthma and chronic obstructive pulmonary disease (COPD) and aggravate pre-existing heart issues, causing surges in hospital visits. Much less is known, though, about the accumulated risks of living and breathing in even mildly smoky conditions day after day.

“Right now, we don’t have a good understanding of what the long-term effects of prolonged exposures to wildfire smoke are,” said Ana Rappold, an epidemiologist at the Environmental Protection Agency. It’s a data gap she and other scientists are trying to fill. Research teams are looking at changes to lung function, cognition, and gene expression after smoke exposure and potential impacts on developing fetuses and infants, efforts that have taken on new urgency with recent events.

“All the predictions have been saying there will be more smoke,” Rappold said. “For those of us in this field, it’s a little bit scary to see what people have been talking about for years and years actually starting to happen here on the East Coast.”

Days- or weeks-long stretches of thick smoke have already been an increasingly common feature of life in the western U.S. In states including Idaho, Montana, and Wyoming, wildfires have in recent years reversed the substantial air quality gains achieved by the Clean Air Act of 1970 — especially when it comes to the most dangerous particulates, called PM2.5. “For the last four decades, we’d been making steady progress on PM2.5 and other pollutants,” said Daniel Jaffe, an environmental chemist at the University of Washington whose lab led the research. “Then around 2012 things started changing and it just keeps getting worse and worse.”

Particulate matter is a term for solid particles or liquid droplets suspended in the air; PM2.5 is the tiniest and farthest traveling of these. With a diameter of 2.5 micrometers or smaller, less than 1/20th the width of a human hair, they can stay airborne for long periods of time and float over hundreds of miles. At those sizes, particles can also get past the slimy, sticky defenses of a person’s nasal passages and be inhaled deep into the lungs. From there, they can enter the bloodstream and lodge in different tissues including the heart, kidneys, and brain, damaging cells, causing inflammation, and increasing the risk of heart attack, stroke, and infection.

These public health concerns are the reasons governments regulate PM2.5. For decades, the chief sources of PM2.5 in the U.S. have been smokestacks, factories, and vehicle tail pipes and tires slapping off microscopic flecks of rubber. Most of what researchers know about the health impacts of particulate matter come from studying this kind of air pollution. They know a lot less about wildfire-generated PM2.5, but emerging evidence suggests it’s even more problematic, especially for lung health.

Researchers have found that wildfire-specific PM2.5 is more likely to send people over the age of 65 to the hospital for respiratory issues than similar concentrations of PM2.5 from other sources, like car exhaust. In Colorado, increases in wildfire PM2.5caused higher spikes in emergency room visits for asthma than did other kinds of air pollution.

Similar results from a study in Southern California last year found that wildfire PM2.5 increased unplanned hospital visits for all respiratory visits by 3% and by 10% for asthma specifically. Yet another recent study, conducted by researchers at the University of California, San Diego, found PM2.5 created by wildfires was associated with an increase in hospitalizations for respiratory conditions up to 10 times higher than for PM2.5 from other sources. (So far, researchers have not observed similar increases in hospital admissions for cardiovascular events, which are often linked to high exposures of PM2.5 from burning fossil fuels.)

“The health effects really do seem to be different, and it’s a difference we can’t ignore,” said Mary Rice, director of the Beth Israel Deaconess Medical Center Institute for Lung Health in Boston.

Toxicology studies have suggested that these differences likely lie in the unique make-up of wildfire PM2.5. In one, researchers found that mice exposed to particulate matter collected during the 2008 Camp Fire in Northern California experienced a rush of white blood cells, inflammation, and fluid into their lungs. They estimated that microgram for microgram, wildfire PM2.5 was 10 times more toxic than particulate matter from ambient air in the same region.

Wildfires can burn at temperatures up to 1,500 degrees Fahrenheit, torching whatever is in their path. The smoke they generate is a turbulent miasma of combusted carbon-based materials — which vary, depending on the types of trees, grasses, or buildings in the area — noxious gasses, and aerosolized dust and dirt and earth metals whipped up by the fire’s furious convections. As the smoke travels, it also interacts with sunlight and other atmospheric elements, forming new molecules and evolving its chemical composition with each passing hour and mile.

“Not all smoke is created equally,” said Ian Gilmour, chief of the EPA’s Cardiopulmonary and Immunotoxicology Branch. For the last decade, his lab has been trying to understand what kind of pollutants wildfires generate depending on how hot they’re burning, what kind of fuels they’re igniting, and how far they’re traveling.

More recently, his team has started looking at what happens when those different chemical compositions get into human cells or the lungs of lab animals. They’ve found that under different burning conditions, eucalyptus (common in California) and peat (found more on the East Coast) were more toxic to respiratory tissues than other biofuels, but pine led to more changes in cells’ DNA — a potential precursor to cancer.

Air quality regulations in the U.S. and elsewhere assume that all particles of roughly the same size are all about equally toxic. But what these studies and others suggest is that may not be entirely accurate. This makes the matter of assessing whether wildfire PM2.5 is more harmful than other sources a pressing public health concern.

“It doesn’t make sense to me that we rely all the time on a mass-based standard,” said Gilmour. “Chemistry is important, and the chemistry is different depending on what sources are around and the atmospheric conditions. That has to play a role, and part of our job is trying to shed a light on it.”

In the last few years, some provincial governments of Canada have updated the tools they use to communicate the quality of outdoor air to be more responsive to the unique dynamics and health risks of wildfire smoke. Whereas in the U.S. we have the Air Quality Index, or AQI, Canadians have the AQHI for Air Quality Health Index, which rates the unhealthiness of the air on a scale from 1 to 10.

Historically, the AQHI took three pollutants into account — PM2.5, ground-level ozone, and nitrogen dioxide — using a three-hour average of each tied to its association with mortality to generate the different risk levels. But in 2018, British Columbia began piloting a modified version of the model based on one-hour averages that places more weight on PM2.5.

“Prior to implementing these changes, people complained all the time that when it was smoky outside they didn’t see changes in the index — it wasn’t matching their experience of the environment,” said Sarah Henderson, the scientific director of Environmental Health Services at the British Columbia Centre for Disease Control. The new version, called AQHI+, was officially adopted in British Columbia in 2019. Since then, a number of other Canadian provinces have followed suit. AQHI+ takes into account not just added mortality risk, but also the increase in asthma-related hospital visits and inhaler prescriptions.

“Wildfire smoke elicits a stronger respiratory response in the population than air pollution from other sources,” Henderson said. “That was the rationale for adopting these tools.” She’s not convinced it’s because of the unique composition of wildfire PM2.5, and suspects that other, unmeasured parts of the mixture might be even more powerful irritants. But those substances are nearly impossible to measure without sophisticated laboratory instruments, so it makes sense to use PM2.5 as a proxy. “From a public health perspective, that’s good enough,” she said.

While there is ample evidence that wildfire smoke has negative effects on the health of all humans, but especially young children, developing fetuses, and people with pre-existing heart and lung conditions, researchers say there is still a need to get more granular about what exactly it’s doing inside people’s bodies.

“The reason we need to know is because a lot of what we’re recommending people for how to protect themselves is based on traffic pollution, and we don’t know that it should actually be the same for wildfire smoke,” said Chris Carlsten, head of respiratory medicine at the University of British Columbia, where he also directs the Air Pollution Exposure Laboratory. “We need to understand how best to protect people and not just tell them how to avoid it or hope it goes away. Because it’s not going to.”

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