For 49 straight days, everyone in Seeley Lake was breathing smoke. A wildfire had ignited outside the small rural community in Montana, and the plume of smoke had parked itself over the houses. Air quality plummeted. At several moments, the concentration of particulate matter in the air exceeded the upper limit of what monitors could measure.

Christopher Migliaccio, an associate professor of immunology at the University of Montana, saw an opportunity to do what few have ever done: study what happens after people get exposed to wildfire smoke. He and his team quickly cobbled together funding and drove out to Seeley Lake to get data.

That was in 2017. The researchers followed up with residents for two years after the fires, checking on their lung function. To their surprise, the worst effects didn’t show up immediately, despite the heavy dose of smoke. Instead, people’s lung function seemed to deteriorate later. Right after the fires, about 10 percent of the cohort had lung function that fell below the lower limit of normal. By the one-year mark, about 46 percent did. At the two-year mark, most of those people still had abnormally poor lung function. “We were very surprised,” Migliaccio told me. He and his colleagues had intended to follow the residents for a third year, but then COVID hit. Instead, they tried exposing mice to wildfire smoke in a controlled lab environment. Their results pointed to a similar outcome: The worst effects took time to present.

Migliaccio’s work can speak to only a single smoke event. But it is the type of event that more people in the United States are dealing with, over and over again. Until recently, wildfires that exposed large populations to smoke were a relatively rare occurrence. But that’s changing: More frequent and intense wildfires are erasing or even reversing decades of gains made in American air quality in the majority of U.S. states. Across the country, from 2012 to 2022, the number of people exposed to unhealthy air from wildfire smoke increased 27-fold; one out of every four unhealthy air days in parts of the country is now a smoke day. “It is the exposure that is impacting air quality across the U.S. now more than any other pollution source,” Joan Casey, an environmental epidemiologist at the University of Washington whose work helped show a link between wildfire-smoke exposure and increased risk of dementia, told me.

Yet science—to say nothing of policy—has hardly caught up with what that means for human health. “We’re in the preschool stage of development,” Casey said. What cumulative smoke exposure can do to a body and mind remains largely a mystery, but the few studies that do exist point to nothing good.

Plenty of research shows that respiratory distress and heart attacks spike in the event of smoke exposure; acute impacts of breathing smoke send people to the hospital and make them miss work and school. Those risks can linger for months afterward, or, in the case of the Seeley Lake cohort, for years.

Now that more people are regularly breathing smoky air over their lifetime, though, the relevant concern may no longer be what happens when a person gets one big dose of smoke; rather, it may be what happens when they are exposed many times. How much does anyone know about the long-term consequences of exposure to smoke or, worse, the long-term consequences of long-term exposure to smoke? “Very little,” Marianthi-Anna Kioumourtzoglou, an environmental engineer and epidemiologist at—as of next week—Brown University, told me.

The best model for assessing wildfire-smoke exposure was developed only recently, Kioumourtzoglou and Casey told me, and, because of satellite imagery and monitoring limitations, goes back to only 2006. That means researchers have at most 20 years of data to look at in determining whether smoke could have contributed to a population’s incidence of a particular illness. Latency periods for plenty of diseases, including some cancers, can be longer. Plus, every fire is its own unique nightmare, chemically speaking. The Seeley Lake fire burned mostly trees. Add to that whole neighborhoods of cars and houses and parking lots, and the toxicity profile of the smoke changes significantly. During the Los Angeles fires in January, which burned down entire neighborhoods, noxious compounds from burning plastics and other man-made materials swirled through the air.

Scientists know a lot about the harms of regular ambient air pollution (such as the particulate matter that spews from tailpipes and factories), but wildfire smoke is chemically different—and likely worse, from a health standpoint. Its complexity is also daunting from a research perspective: Even the types of trees burned appear to affect the smoke’s toxicity. In a lab study, for instance, researchers burned peat, pine needles, and several types of wood to simulate different regional forests. They found that pinewood smoke was the most mutagenic, suggesting that it might be more likely to cause cancer than other woods, and that eucalyptus smoke was the most toxic to lungs. How long the smoke stays in the air may matter too. Some research suggests that smoke becomes more toxic as it ages, which is bad news for people living downwind from smoke—in parts of the U.S. during Canadian wildfires, for instance.

And unlike our relatively steady exposure to ambient air pollution, exposure to wildfire smoke is spiky, coming in bursts, with pauses in between. That makes it hard to model, Kioumourtzoglou told me, and also introduces many questions, each of which needs research attention: Is the health impact worse if a person breathes a very high level of smoke for three days, or if they breathe a lower level for three weeks? How does the point in life at which they are exposed—as an adult with asthma, a child whose lungs were still developing, a fetus in utero—change how the smoke affects them?

Many of the attempts to even start to answer these questions depend on chance and the swift action of researchers like Migliaccio, who seize on the chance to study a fire close at hand. In the summer of 2008, for instance, Lisa Miller, who studies pulmonary immunology and toxicology at UC Davis, was in her office as wildfires sent smoke settling over the region. The air outside her office looked like thick winter fog. She  suddenly thought about the rhesus monkeys she studied at the primate research center; they had been outside in their habitat the whole time. These primates were great models for human health, so they became a case study for what happens to a smoke-exposed body.

Miller and her team studied the monkeys for the next 15 years. They found that those that were exposed to wildfire smoke as infants became adolescents with smaller, stiffer lungs than their peers born the following year, which resulted in poorer lung function and worse immune regulation. When the researchers exposed blood samples from both populations of adolescent monkeys to bacterial infection, the samples from the smoke-exposed animals responded more weakly, indicating that their immune system wasn’t working as well. The smoke-exposed monkeys also slept far less, she told me: “It was absolutely stunning.” Some research suggests that smoke can affect humans the same way: In 2022, a large study in China concluded that human children who had been exposed to air pollution early in life also had poorer-quality sleep. High-quality sleep is important to neurodevelopment in children, and poor sleep is associated with a range of negative health consequences across a lifetime.

What we can learn from Miller’s monkeys is limited; they spend 24 hours a day outdoors, unlike humans, and they get constant medical care and perfectly tailored diets, also unlike humans. Still, rhesus monkeys are some of the closest animals to us, physiologically, and on a basic level, smoke exposure in infancy seems to have affected these monkeys’ health for their entire life, Miller said.

Standard air pollution is known to have a negative impact on virtually all aspects of fertility, and for people who wish to conceive children, smoke may pose a hazard too. After wildfires in Oregon prompted an air-quality emergency in Portland in 2020, researchers looked at the sperm quality of 30 people who had their semen analyzed at a fertility clinic before and after the fires. The study was small, but the trend was clear: Motility—how well the sperm swims—went down for most of the participants. A nearly identical study in Seattle, which is still awaiting peer review, yielded a similar result. And Luke Montrose, an environmental toxicologist at Colorado State University, told me that he’s seen similar results in bull sperm: He and his colleagues got records from a cattle-breeding facility in Colorado that tests bull sperm with many of the same metrics used for human sperm at a fertility clinic. Sperm that isn’t up to shape gets discarded by the facility; after a wildfire in the area, more of the cattle sperm got thrown out, Montrose found. Quality must have gone down.

The results on the bull sperm are preliminary and are awaiting peer review. But in the meantime, Montrose is enrolling male firefighters in a study to learn whether their fertility is affected by their job (which forces them to breathe much higher doses of smoke than the general population). When Montrose and his colleagues exposed mice to a very high dose of wood smoke in a lab—simulating what they estimate would be the equivalent of 15 years on the job for a wildland firefighter—the mouse sperm was significantly altered at the epigenomic level, where gene expression is altered without changing the underlying DNA sequence. “Normally, in a study like this, you see a handful of sites being changed,” Montrose told me; he and his colleagues found changes at more than 3,000 different sites, reflecting about 2,000 different genes. Montrose wonders what this far higher level means for the mice’s fertility and for their offspring. Whether these changes are positive or negative, it appears that smoke can alter, on a deep level, the very cells involved in reproduction. “It’s intriguing, but we still don’t quite know what it means,” Montrose said.

If smoke can affect health early in life, it also can affect life’s end. Breathing smoke can cause inflammation, which is a key pathway for many neurological disorders, and research is now turning up associations between smoke and conditions that strike older people, such as Parkinson’s, Casey told me. Smoke also causes premature death: more than 50,000 people in California died prematurely from wildfire smoke between 2008 and 2018, according to one estimate, and more than 11,000 people in the U.S. do so each year, according to another. Climate change is only accelerating those dynamics. As I’ve written before, the National Bureau of Economic Research found last year that in a worst-case warming scenario, deaths from wildfire-smoke exposure in the U.S. could top 27,000 a year by the middle of the century. That is, smoke could kill 700,000 people from now until 2055.

Burning fossil fuels has locked us into a downward spiral: Warmer temperatures mean more fires. Already, summer 2025 is poised to be a fiery one in California, only half a year after fires devastated Los Angeles. Canada has already been burning for weeks, sending smoke billowing down through the U.S. The smoke is coming for us all—each of us is now more likely to encounter it in the coming years.

That terrible reality means that researchers will have more opportunities to understand what smoke does to us. Susan Cheng, a cardiologist at Cedars-Sinai Medical Center, in Los Angeles, is now part of a major multi-institutional study of people exposed to the L.A. fires. As a cardiologist, she’s well aware of the extreme heart risks associated with smoke inhalation. Breathing regular pollution over a long period can accelerate heart disease by prematurely aging blood vessels and accelerating plaque buildup in the coronary artery; at least one recent study found that people’s risk of heart failure and other serious cardiac problems can persist for months after smoke exposure. If that’s any indication, fire smoke is a major heart hazard.

“We really need to be closely tracking and following this,” Cheng told me. “Otherwise, we will be facing a major information gap, and trying to, in hindsight, put the pieces together.” She pictures asking, years down the line, “How did people get this way? How did our patients end up with these accelerated aging processes, accelerated development of these different chronic conditions?” Studies like hers—which began in January and will follow a cohort of more than 13,000 Angelenos for the next 10 years—aim to answer those questions now, before even more of the country starts having to ask them.