Summertime in North America is becoming smoke season. Last summer, when a haze from catastrophic Canadian wildfires hung over the continent—turning Montreal, where I lived at the time, an unearthly gray and my home city of New York a putrid orange—plenty of people seemed untroubled by this reality. Relatively few people wore masks; infamously, an outdoor yoga class continued on a skyscraper terrace in Manhattan. Research has long shown that exposure to the tiny particles that make up wildfire smoke is a major health hazard; it kills thousands of people prematurely each year and is linked to a range of maladies. Yet the message—that smoke is a legitimate health emergency—seems not to be getting through.

Now, in mid-June, the smoke is creeping back. Ninety-four fires are currently burning in Canada, of which seven are uncontrolled. Last month, officials in Minnesota and Wisconsin issued air-quality warnings when smoke drifted south. The West is expecting an intense fire season. And smoke travels far beyond burn sites: Research from UC Davis published this month found that 99 percent of North America was covered by smoke at some point from 2019 to 2021, and that almost every lake on the continent spent at least 10 days a year under such haze.

New evidence is starting to show more clearly just how devastating a public-health crisis this is. Smoke from California wildfires prematurely killed more than 50,000 people from 2008 to 2018, according to research published last week in the journal Science Advances. The researchers estimated that the health expenses of that exposure totaled $432 billion. And a recent analysis from the National Bureau of Economic Research (NBER) found that, given the march of climate change, smoke-related deaths in the U.S. will rise considerably: In the worst-case scenario, by mid-century, cumulative excess deaths from wildfire-smoke exposure could top 700,000, a two-thirds increase over current numbers. Measured in economic terms, pegged to the price people put on avoiding real health risks, these deaths amount to monetary damage on par with that of all other previous climate-related damage in the U.S. combined.

Among the hazards of wildfire smoke, researchers know the most about tiny particles called PM2.5, which are small enough to slip into the bloodstream and infiltrate the lungs and other organs, causing inflammation and increasing the risk of a cascade of interrelated problems, including cognitive issues, breathing and heart conditions, and premature death. But wildfire smoke contains far more than one form of pollutant; its dangers are likely as complex a cocktail as whatever is burning. Smoke from a burning tree looks different than smoke from a burning town, and in a wildfire there may be both, with perhaps a few industrial sites thrown in. “There’s a lot of chemicals in that. There’s all sorts of things in the pollution that you might not see in other sources of PM2.5,” says Marissa Childs, an environmental-health researcher at the Harvard T. H. Chan School of Public Health who was a co-author of the NBER paper. “We’re still unclear on what that means for health.” But no one expects it to be anything good.

The health hazards of smoke don’t yet show up in the cost-benefit analyses of climate policy, either, says Minghao Qiu, a researcher at Stanford University who studies air quality and climate change and was the lead author of the NBER paper. The social cost of carbon, for example, a metric meant to help weigh whether a climate policy is cost-effective, tries to estimate the societal damages of one extra ton of emissions by accounting for mortality related to extreme temperature, agriculture outputs, labor productivity, and other such factors, Qui told me. But measures like that do not at present include wildfire-smoke deaths. A large part of the climate-damage pie is simply missing.

Until recently, air quality in the U.S. had been improving for decades, thanks to legislation regulating industrial sources of PM2.5. But fires are eating away at those gains. About a quarter of the PM2.5 pollution in the U.S. is now connected to wildfire smoke—“maybe 50 percent of [it] in the West in a bad year,” Qiu said. The bad year he has in mind is 2020, California’s worst season on record. Climate change will turn that from an outlier into a norm. “Every year in the 2050s will look somewhat like 2020,” he said. And even a season that’s not the worst on record poses a danger: One revelation from the work he and his colleagues did, Qiu said, was that “there really is no safe level” of smoke—even a relatively low level can increase a county’s mortality rate dramatically. Perhaps because of this dynamic, from 2011 to 2020 almost half of wildfire-smoke deaths happened in the eastern United States. The East might have fewer, smaller wildfires and lower smoke concentrations overall, but more people live there. And if more people are exposed to even low levels of smoke, mortality rates rise. (Qiu expects this particular dynamic to shift as western fires intensify further.)

Yet despite the risks, most Americans are left to deal with the threat on their own. The CDC recommends staying home, closing windows, and running an air filter, or—if you must go outside—wearing a respirator. But not everyone can stay inside without fear of losing their jobs; the federal government has done little more than urge employers to have a plan for their outdoor workers in a smoke event, and only three states—California, Oregon, and Washington—have rules regulating on-the-job smoke exposure. The CDC also recommends that all Americans follow the directions of local emergency managers, but New York City Mayor Eric Adams was widely criticized for having neither a plan nor any fast instructions for New Yorkers when last summer’s smoke crisis hit. If a government’s main policy approach is to suggest that people figure it out with little tangible support, “that’s going to have unequal impacts,” Childs told me.

The Clean Air Act, which was largely crafted in the 1960s and ’70s, considers wildfire an “exceptional event,” leaving it beyond the burden of regulation. But now, with wildfire smoke representing a larger share of the PM2.5 to which Americans are exposed, that logic may no longer hold. As more frequent wildfires bear down on the American West and as temperatures rise across the country, fires will negate some of the air-quality gains from combatting other forms of air pollution, such as emissions from cars and power plants. Regaining that lost ground will be impossible without curbing one of the primary underlying causes of today’s supercharged fires: our use of fossil fuels.

This is all new, in a way.

“It took us a long time in the research perspective to come to a consensus that wildfire smoke is increasing,” Childs told me. Now it’s clear that it is. The open question is what governments will do about it—how cities, states, and the country will try to protect people from the smoke, or try to change the trajectory of a future in which it grows only more common.