Everyone in Seeley Lake was inhaling smoke for 49 days in a row. The smoke plume had parked itself over the houses in the small Montana rural community where a wildfire had started outside. The quality of the air fell. There were multiple instances where the airborne particulate matter concentration was higher than what the monitors could detect.
Christopher Migliaccio, an immunology associate professor at the University of Montana, saw a chance to do something very few people have ever done before: investigate the effects of exposure to smoke from wildfires. In order to obtain data, he and his team hastily gathered funds and drove to Seeley Lake.
In 2017, that was the case. For two years following the fires, the researchers checked in with the residents to see how their lungs were functioning. They were surprised that even with the high dosage of smoke, the worst effects did not appear right away. Rather, lung function appeared to decline over time. Approximately 10% of the cohort experienced lung function below the lower limit of normal immediately following the fires. Approximately 46% did by the one-year mark. The majority of those individuals continued to have abnormally poor lung function at the two-year mark. Migliaccio told me, “We were really taken aback.” Before COVID struck, he and his colleagues planned to stay with the residents for a third year. Rather, they experimented with exposing mice to smoke from wildfires in a lab setting. Their results pointed to a similar outcome: The worst effects took time to present.
There is only one smoke event that Migliaccio’s work can discuss. However, more Americans are repeatedly dealing with this kind of situation. Large-scale smoke exposures from wildfires were comparatively uncommon until recently. However, that is evolving: In most U.S. states, decades of progress on improving air quality are being erased or even reversed by more frequent and intense wildfires. The number of people exposed to unhealthy air from wildfire smoke increased 27-fold nationwide between 2012 and 2022; in some regions, one out of every four days with unhealthy air is now a smoke day. According to Joan Casey, an environmental epidemiologist at the University of Washington whose research helped establish a connection between exposure to wildfire smoke and an elevated risk of dementia, “it is the exposure that is impacting air quality across the U.S. now more than any other pollution source.”
However, the implications of that for human health have barely been understood by science, let alone policy. “Our developmental stage is preschool,” Casey stated. The effects of repeated exposure to smoke on the body and mind are still mostly unknown, but the scant research that is available suggests that it is not good.
Numerous studies demonstrate that exposure to smoke increases the risk of heart attacks and respiratory distress; acute effects of smoke inhalation cause hospitalization and cause people to miss work and school. For months or, in the case of the Seeley Lake cohort, years, those risks may persist.
As more people breathe smoky air on a regular basis throughout their lives, the issue may no longer be what happens when someone gets a large dose of smoke, but rather what happens when they are exposed to it repeatedly. How much is known about the long-term effects of smoking or, worse, the long-term effects of smoking for an extended period of time? Marianthi-Anna Kioumourtzoglou, an epidemiologist and environmental engineer at Brown University as of next week, told me, “Very little.”
According to Kioumourtzoglou and Casey, the best model for estimating wildfire-smoke exposure was created just recently and only goes back to 2006 due to satellite imagery and monitoring constraints. In order to ascertain whether smoke may have played a role in the occurrence of a specific disease in a population, researchers can examine data spanning no more than 20 years. Many diseases, including some types of cancer, can have longer latency periods. Furthermore, from a chemical perspective, each fire is a different nightmare. The majority of the trees burned in the Seeley Lake fire. The toxicity profile of the smoke is drastically altered when entire neighborhoods of vehicles, homes, and parking lots are added. Toxic compounds from burning plastics and other man-made materials swirled through the air during the January fires in Los Angeles that destroyed entire neighborhoods.
Scientists are well aware of the negative health effects of common ambient air pollution, such as the particulate matter emitted by factories and tailpipes, but smoke from wildfires is chemically different and probably more harmful. From a research standpoint, its intricacy is also intimidating: The toxicity of the smoke seems to be influenced even by the kinds of trees that are burned. For example, to simulate various regional forests in a lab study, researchers burned peat, pine needles, and various types of wood. They discovered that eucalyptus smoke was the most harmful to the lungs and that pinewood smoke was the most mutagenic, indicating that it may be more likely to cause cancer than other woods. The duration of the smoke’s presence in the atmosphere may also be significant. According to some research, smoke gets more toxic with age, which is bad news for those who live downwind of smoke—for example, during Canadian wildfires in parts of the United States.
Additionally, exposure to wildfire smoke is spiky, occurring in spurts with intervals in between, in contrast to our comparatively constant exposure to ambient air pollution. According to Kioumourtzoglou, that makes modeling challenging and raises a number of issues that require further study: Does breathing extremely high levels of smoke for three days or lower levels for three weeks have a greater negative impact on one’s health? How does their exposure to smoke change depending on their stage of life—as an asthmatic adult, a child whose lungs were still developing, or a fetus in utero?
Numerous efforts to even begin to address these questions rely on luck and the quick thinking of scientists like Migliaccio, who take advantage of the opportunity to investigate a nearby fire. For example, Lisa Miller, a UC Davis student studying pulmonary immunology and toxicology, was in her office during the summer of 2008 when smoke from wildfires began to settle over the area. Outside her office, the air resembled dense winter fog. Suddenly, she remembered the rhesus monkeys she had been studying at the Primate Research Center; they had spent the entire time outdoors in their natural habitat. As excellent human health models, these primates served as a case study for the effects of smoke exposure.
For the next fifteen years, Miller and her group conducted research on the monkeys. Compared to their peers born the following year, those exposed to wildfire smoke as infants grew up to be adolescents with smaller, stiffer lungs, which led to worse immune regulation and lung function. Blood samples from the smoke-exposed animals reacted less strongly to bacterial infection than samples from the other populations of adolescent monkeys, suggesting that their immune systems were not functioning as well. She told me, “It was absolutely stunning,” adding that the monkeys exposed to smoke slept much less. According to some research, smoke may have the same effects on people: A significant study conducted in China in 2022 found that children who were exposed to air pollution at a young age also slept less well. Children’s neurodevelopment depends on getting enough good sleep, and getting too little sleep is linked to a number of lifelong health issues.
There is little we can learn from Miller’s monkeys because, unlike humans, they are constantly outside, receive round-the-clock medical attention, and have diets that are precisely suited to their needs. According to Miller, rhesus monkeys are among the most physiologically similar animals to humans, and their health appears to have been impacted by smoke exposure during infancy throughout their lives.
Smoke may also be harmful to those who want to conceive, and standard air pollution is known to negatively affect almost every aspect of fertility. Researchers examined the sperm quality of 30 individuals who had their semen examined at a fertility clinic before and after wildfires in Oregon caused an air-quality emergency in Portland in 2020. Although the study was small, the trend was evident: for the majority of participants, motility—the ability of the sperm to swim—went down. The same conclusion was reached by a nearly identical study in Seattle that is currently pending peer review. Additionally, Luke Montrose, a Colorado State University environmental toxicologist, informed me that he has observed comparable outcomes in bull sperm: Records from a Colorado cattle breeding facility that tests bull sperm using many of the same metrics used for human sperm at a fertility clinic were provided to him and his colleagues. The facility discards sperm that isn’t suitable; Montrose discovered that more cattle sperm was discarded following a nearby wildfire. The quality must have declined.
Peer review is still pending for the preliminary findings regarding the bull sperm. However, Montrose is currently recruiting male firefighters for a study to find out if their work, which exposes them to far higher levels of smoke than the general population, has an impact on their fertility. The mouse sperm was markedly changed at the epigenomic level, where gene expression is changed without altering the underlying DNA sequence, when Montrose and his colleagues exposed mice to a very high dose of wood smoke in a lab setting, simulating what they estimate would be the equivalent of 15 years of work as a wildland firefighter. Montrose told me, “Usually, in a study like this, you see a handful of sites being changed.” However, he and his colleagues discovered changes at over 3,000 sites, representing roughly 2,000 different genes. Montrose questions how the mice’s fertility and progeny will be affected by this much higher level. It seems that smoke can profoundly change the very cells involved in reproduction, whether these changes are positive or negative. We still don’t fully understand what it means, but it’s fascinating,” Montrose remarked.
If smoking can have an impact on health in early life, it can also have an impact on death. Inflammation is a major contributing factor to many neurological disorders, and studies are now linking smoking to diseases that affect the elderly, like Parkinson’s, Casey informed me. Smoke also causes premature death: one estimate states that between 2008 and 2018, wildfire smoke killed over 50,000 people in California and over 11,000 people annually in the United States. Climate change is only accelerating those dynamics. As I’ve previously reported, the National Bureau of Economic Research discovered last year that, under the worst warming scenarios, the number of deaths in the United States from exposure to wildfire smoke could surpass 27,000 annually by the middle of the century. In other words, between now and 2055, smoke could kill 700,000 people.
We are stuck in a downward spiral because burning fossil fuels causes more fires as temperatures rise. Just six months after fires decimated Los Angeles, California is already on the verge of a fiery summer in 2025. Smoke is already pouring into the United States from Canada’s weeks-long fire. We are all at risk of experiencing the smoke in the years to come.
Because of this awful reality, scientists will have more chances to learn more about the effects of smoke on humans. An important multi-institutional study of individuals exposed to the L.A. fires now includes Susan Cheng, a cardiologist at Cedars-Sinai Medical Center in Los Angeles. She is well aware of the serious risks smoking poses to the heart because she works as a cardiologist. By prematurely aging blood vessels and speeding up plaque buildup in the coronary artery, prolonged exposure to pollution can hasten heart disease; at least one recent study found that people’s risk of heart failure and other severe cardiac issues can last for months after being exposed to smoke. Fire smoke is a serious heart hazard, if that’s any indication.
Cheng informed me that “we really need to be tracking and following this closely.” “If not, we’ll be dealing with a significant information gap and attempting to piece things together after the fact.” Years later, she imagines herself asking, “How did people get this way? How did our patients acquire these various chronic conditions and accelerated aging processes?” In order to address those questions before more of the nation is forced to do so, studies like hers, which started in January and will track a cohort of over 13,000 Angelenos for the next ten years, are being conducted.
