Scientists know that wildfire smoke can exacerbate conditions like asthma and COPD, increase the risk of heart attack and stroke, hinder concentration, reduce the body’s ability to fight infections, and cause inflammation targeting the lungs, kidneys, liver, and likely other organs. But what about more lasting, even permanent, effects? Can exposure to even short periods of intense wildfire smoke leave permanent scars inside your body?
Though it’s a relatively young area of scientific investigation, the answer appears to be yes; though the potential damage depends on age, distance from the fire, the quantity of smoke exposure, and even the fire’s characteristics. “The issue with wildfires is that it’s all over the map in terms of what’s being burned,” says Lisa Miller, an immunologist at the University of California Davis who is studying long-term effects of wildfire smoke exposure on rhesus monkeys. “It’s a chemical mess.”
“While the impact of air pollution in general on human health has been known for some time, we have only recently started to understand how impactful wildfire smoke is on human health,” Anthony White, a neuroscientist at QIMR Berghofer Medical Research Institute in Herston, Australia, says. “This issue is magnified by the fact that it can be difficult to distinguish between effects of ambient air pollution, and health impact specifically from wildfire smoke, especially when that smoke pollution occurs sporadically and without warning.”
More than just air pollution
What scientists are learning about persistent effects of wildfire smoke is largely from animal studies, short-term research on wildfire smoke, and research on air pollution and wood-burning kitchen fires. Most of that research measures exposure to PM2.5, particles measuring 2.5 micrometers—about 30 times smaller than the diameter of a human hair.
“While we don’t have a lot of evidence from long-term exposures to wildfire smoke yet, it’s safe to extrapolate a lot of what we know from urban air pollution [effects] on health,” Ana Rappold, an environmental statistician at the Environmental Protection Agency says. But air pollution research only tells part of the story because wildfire smoke differs not only from air pollution but from one wildfire to the next.
Its composition changes depending on what’s being burned, both the biomass—trees, shrubs, grass, animals—and any other fuel, such as homes and businesses, says Stephanie Cleland, an environmental health epidemiologist at the EPA.
In addition to PM2.5, wildfire smoke contains other toxic chemicals and volatile organic compounds that vary depending on the fuel, the temperature of the burn and even how old the smoke is. “It’s likely that it might lead to different types of health effects or a different severity; and you’re exposed to multiple things at the same time, which isn’t always the case with typical ambient air pollution,” Cleland says. Plus, with the greater density of PM2.5 concentrations and the various gases from wildfires, “you could have a synergistic effect,” Rappold adds.
The brain typically has greater protection than other organs because of the blood brain barrier, a tight network of blood vessels that strictly regulates what can pass through, much like a bouncer who decides who can enter a nightclub. But the blood brain barrier is not completely impermeable. Adam Schuller, an environmental toxicologist at the Colorado State University, has described three ways pollutants might reach the brain: particles travel in oxygenated blood from the lungs directly to the brain; particles directly enter the brain along the olfactory tract; or inflammatory factors triggered by an inflammatory response in the lungs invade the brain.
Once there, particulate matter can damage neurons both directly, through the buildup of harmful, unstable molecules called free radicals, or indirectly, White says, by triggering immune cells to release molecules that impair or kill neurons and disrupt the connections that allow brain cells to communicate and store memories, even if the neurons don’t die.
Cleland and Rappold have identified short-term cognitive effects from this exposure by comparing scores of more than 10,000 American adults on Luminosity—a brain training mobile app—when some were in areas affected by wildfire smoke. They found that people exposed to medium or heavy density smoke—based on satellite data from NOAA—performed more poorly, with slightly lower attention scores, than those exposed to light-density or no smoke. The designations of “medium” and “heavy” density do not correlate directly to Air Quality Index (AQI) numbers, but what New York City experienced in early June would qualify as medium or heavy, Rappold says.
Pairing those findings with what’s known about other types of smoke exposure suggests long-term cognitive effects are likely. There’s strong evidence that ambient air pollution and kitchen fire woodsmoke increases the risk of Alzheimer’s disease and other dementias, and evidence that air pollution increases depression risk.
“We may see additional effects of wildfire smoke on other neurological changes, but that requires more studies and more people to generate solid study outcomes,” White says. “We are also currently trying to determine how much of an impact wildfire smoke has on dementia compared to exposure to ambient air pollution.”
Emerging research also suggests that exposure to air pollution during pregnancy can increase the risk of autism spectrum disorders and attention deficit hyperactivity disorder (ADHD) in the developing fetus. Most research on wildfire smoke exposure and pregnancy has focused on preterm birth and birthweight, but wildfire smoke contains the same PM2.5 that air pollution does, so theoretical risk of long-term impacts on the fetus are possible.
Effects on the immune system and lungs
The limited research available on persistent wildfire smoke health effects suggests the lungs and immune system can especially suffer. When wildfires enveloped the Seeley Lake region of Montana in 2017, Chris Migliaccio, a toxicologist at the University of Montana, began studying the effects of the wildfire smoke on nearby residents. Initially, he did not find any significant effects on their lung function two months after the exposure. But one and two years later, residents did have significant decreases in lung function—the ability to empty their lungs quickly. The pandemic prevented longer term follow-up.
One of the only very long-term studies on wildfire smoke effects comes from rhesus monkeys at an NIH-supported lab in Davis, California. Though scientists there had not planned to study wildfire smoke in the animals, nearby wildfires in June and July of 2008 sent plumes of smoke over the monkey’s outdoor enclosures, exposing them to 10 days of PM2.5 levels above the EPA’s daily standard. Lisa Miller, the immunologist, has been studying the effects of exposure on monkeys born that spring, who were just 4 to 6 months old—the human equivalent of about 2 to 3 years old—when the smoke blew through.
When Miller cultured monkeys’ blood in the lab and exposed it to bacteria, the immune cells had a defective response, suggesting they wouldn’t be capable of mounting a robust defense to a bacterial infection. But that was 15 years ago, and Miller is still seeing abnormal immune function in those monkeys today.
She has also discovered shifts in the monkeys’ circadian rhythms—the internal body clock that governs sleep-wake cycles—and “pretty stunning” changes in the monkeys’ lung structure seen in CT scans. The monkeys produce higher levels of cortisol in response to stress and sleep less, and their lungs are stiffer and have smaller volume. “They have what seems to be evidence of interstitial lung disease,” a collection of conditions that cause scarring in the lungs, she says.
While monkeys are not a perfect proxy for humans, they are as close as it gets, and Miller’s evidence is consistent with another study that looked at influenza rates and wildfire smoke exposure over eight years in Montana. The researchers found that exposure to PM2.5 during wildfire season (July to September) was linked to worse flu rates in the subsequent flu season. Air pollution, and therefore wildfire smoke, also carries greater risks for those with heart disease, Rappold says, but scientists are only just starting to examine long-term effects of smoke exposure on the heart.
Though it’s the best evidence available on long-term exposure, Miller’s research still only reveals effects from a single fire. “It’s important for the public to understand the difference in wildfire smoke versus fireplace smoke and the fact that not all wildfires have the same effect,” Miller says.
Reducing your risk
A core principle of toxicology is that the “dose makes the poison,” but dose—or the density of PM2.5 and other gases in the case of wildfire smoke—is just one factor. Duration and frequency matter too, explains Luke Montrose, an environmental toxicologist at the Colorado State University.
“Thinking about how you can tweak those levers in your own life, reducing your dose, duration and frequency,” can help you find ways to reduce the risk of health effects, Montrose says. One way to do that is to use a handheld air quality monitor or to check local AQI levels. Just as you might check the weather before heading out for a hike or other activity, “it’s getting people in the mindset that they should look at the air quality before they go outside to know whether they should be outside at all,” Montrose says.
The advantage of handheld monitors, which Rappold and Cleland recommended as well, is that they also tell you about indoor air quality. Public health advice often advises people to stay inside during days with poor air quality, but “wildfire smoke is capable of penetrating and disrupting indoor air quality,” Schuller says. Using a HEPA filter, a MERV-13 filter or even a DIY Corsi-Rosenthal box can improve indoor air quality and reduce health effects. If your indoor air is better than what’s outside, staying inside with air conditioning can protect you from the heat as well.
“We don’t know yet how the combination of wildfire smoke and extreme heat affects human health,” since both have been treated separately in research so far, White says. “These are both important stresses on the body, and the combination may have outcomes that are as yet unknown.”
During the worst days, a mask can help reduce exposure to PM2.5 even if it cannot filter out gases. One research review suggests surgical masks reduce exposure by 20 percent and N95s masks by 80 percent. Adjusting your workout routine on particularly bad days is worth considering as well. Strenuous exercise forces pollution particles deeper into your lungs and fills them up more than shallower breathing during rest, Montrose says. And the increased oxygen you take in with each breath can deliver a higher dose of pollutants.
Government officials cannot stop the smoke, but they can alert the public to the risks and use and share resources like the EPA’s Smoke-Ready Toolbox for Wildfires. “There is a need to get the message to the public that smoke from wildfires may have more long-term effects,” White says. “Providing clear public health warnings to the community when smoke is likely to impact them is vital to allow people to make plans to avoid exposure as much as possible.”
One or two days of orange apocalyptic skies may not leave permanent effects—we don’t know yet—but as wildfires’ frequency and geographic range increase, timespans of smoky skies could last more than a few days and happen more often.
“Unfortunately, we really don’t have an answer for how much and how long we can be exposed to wildfire smoke before it has long term effects on health,” White says. “But perhaps the most important fact is that the more we study air pollution and wildfire smoke, the more we learn that lower amounts can still have toxic effects, and therefore, the less exposure you can get, the better.”