Wildfires are rapidly expanding into cities and suburbia, exposing us to toxic smoke now filled with burning plastics and metal. | Hans Gutknecht/Media News Group/Los Angeles Daily News via Getty Images Wildfires are popularly considered wild — restricted to the remote wilderness or the edge of forests, and to most, a problem for the dry and hot states of the Western US. But over the past few years, and now across the country, these fires are getting less wild and far more urban. Key takeways Urban construction is rapidly expanding into the wilderness, putting more communities closer to wildfires. Fires in urban areas expose us to burning metals and other substances that release numerous toxic chemicals. The long-term effects of those chemicals — including higher cancer risk and cardiovascular or respiratory illnesses — make wildfires more dangerous. This summer, fires in Spokane, Washington, demolished over 800 homes in a suburban neighborhood, and earlier in the season, the northeast coast was engulfed in smog from fires in Ontario. The Hawk Fire in Nevada razed more than 15,000 acres near Reno, destroying 47 homes before it was extinguished. The Los Angeles fires of last year remain some of the most devastating in recent memory, with over 37,000 acres burned — much of it densely built residential neighborhoods — and displacing 205,000 people from their homes. In Boulder, Colorado, the 2021 Marshall Fire destroyed nearly 1,000 homes. This changing geography of fires is affecting the health risks they present, too. When vegetation burns, it releases particulate matter, or PM 2.5, and with over 46 million homes in the continental US — this is approximately one-third of all homes — on the edge of fire-prone wildland, many more of us are being exposed to air filled with that soot. This alone is a major health risk. According to the World Health Organization, PM 2.5, is associated with premature death, severe damage to respiratory health, and cognitive impairment. It is especially dangerous for aging populations: a study published in February found that accumulated exposure to wildland smoke was associated with greater hospitalization risk for various cardiovascular diseases in older Americans, including irregular heart rhythms and heart failure, even when they are only exposed to moderate levels of PM 2.5. But the urban landscape has also changed the very composition of the smoke that increasingly hangs over all of us. As human structures and vehicles burn, the fires expose us not just to particulate matter but also to the chemical toxins of urbanization itself, from plastic to metal oxides and vehicle fuel. The changing nature of this smoke, in fire zones but also in surrounding regions as it drifts outward, exposes us to a new class of health risks. While researchers are still learning what exactly those risks might entail, early research suggests long-term dangers that are deeply troubling, especially as exposed populations age. The changing fire frontier In 2020, the worst California fire year on record ate up 4.2 million acres of land, most of it wild, spewing huge amounts of PM 2.5 into the air as the blazes raged. Emergency rooms, already short-staffed by the ongoing pandemic, buckled under the burden immediately. Northern California’s Stanford Health Care system saw a 12 percent jump in hospital admissions over the weeks following the fires, and a 43 percent increase in cardiovascular conditions. In Oregon that same year, 10 percent of emergency room visits were due to asthma-like conditions induced by smoke as fires migrated north. Unfortunately, in a world of warming temperatures and drying conditions, that story is becoming all too common. In 2025, a total of 5.2 million acres of land burned across the US, and this year that number has already climbed to 8.5 million, before we’ve even hit the winter dry seasons that plague parts of the Southwest. Despite this, the global footprint of human housing continues to expand, and with it the danger. For centuries, human development has breached wildfire territory. However, over the last three decades, the zone that links the wild to cities and towns, called the wildland-urban interface (WUI), has grown substantially due to suburban construction. The USDA Forest Service found a 46 percent increase in the number of houses in the risky WUI zone between 1990 and 2020. By 2020, a third of US homes had been built in this highly vulnerable landscape. The math is simple: as suburbia expands into wilder regions, the more houses are exposed to wildland, and the fire risk grows greater. And when houses and other human-made goods burn, different and potentially more toxic forms of smoke can be produced. The Los Angeles County fires in January 2025, which saw large urban regions devastated by flames, were an unfortunate case study for these risks. The Palisades and Eaton fires began on the northern edge of the LA foothills, where dense housing meets more wild, and more fire-prone, terrain. In a retrospective analysis, Underwriters Laboratories’ Fire Safety Research Institute detailed how the urban-wildland makeup of Los Angeles County has not only put more communities physically in fire-prone zones, but also has historically made the geographic region itself more vulnerable to fires. As those fires spread into LA’s more urban areas in 2025, the smoke they produced became more dangerous, largely because of what was burning: more than 16,000 buildings, household items, and electric vehicles. The resulting emissions were thus far more complex than typical vegetation-only fires. “What’s burning is now very different,” said Jackie Goodrich, an epidemiology professor at the University of Michigan. “So now modern homes have tons of plastic, metals, and all of our electronic appliances, and electric vehicles have these batteries that have a lot of rare metals and other toxic materials we don’t even know about.” As a result, these emissions contain higher concentrations of volatile organic compounds (VOCs), metals, and carbon monoxide. PFAS, commonly dubbed “forever chemicals,” are also released through fire extinguishing foam and damaged firefighting gear, which traditionally contains PFAS to protect the skin from fuel that may soak through. These chemicals are associated with adverse health effects, including higher risk of certain cancers and weakening of the immune system. Many of these compounds are dangerous carcinogens. These urban fires have a dangerous impact on indoor air quality, too, as chemicals and metals are absorbed into soft surfaces like pillows or textiles. Studies conducted in the aftermath of the LA fires found that post-wildfire VOC levels were elevated inside evacuated homes during the months following the fires. All of this goes to show that Americans can face the health risks of WUI smoke exposure, even when their homes emerge from the blaze relatively undamaged. These dangers mount over time and across populations Studies examining hospital admission rates or emergency room visits offer some perspective on the short-term health effects of wildfires. But work by Goodrich and University of Arizona public health professor Jeff Burgess is providing new insight into long-term harm by using a subset of the population repeatedly exposed to and vulnerable to fires: the people who fight them. In a study published last year, Goodrich and her fellow researchers examined blood samples from fire responders working in mixed urban and wild areas. They found that exposure to smoke limited the genetic expression of a tumor suppressor known to inhibit gastric cancer cell growth. It also affected regulatory expression similarly associated with tumor proliferation across cancer types, as well as neurodegenerative diseases. Burgess has run similar studies across 33 states with 9,000 participating firefighters, allowing the researchers to examine changes in microRNA levels, blood proteins, and DNA methylation (which controls whether genes are on or off). His research on a small cohort of new fire recruits found altered DNA methylation at various sites in the genome, many of which are similarly linked to cancers, neurological disease, and cardiovascular disease. It is still unclear whether these biological warning signs lead directly to cancer, Burgess said, largely due to the natural latency period between exposure to toxic substances and actual onset of disease. But as participants age, he expects to see a higher incidence of cancer, especially as exposure to big fires accumulates. Firefighters have more prolonged exposure to these toxins than most, of course. But the health effects of wildland-urban fires can extend beyond self-selecting groups, especially as more people move into regions exposed to fire. Residents in the WUI are exposed to the same particulate matter and chemical slurry as firefighters. Goodrich suggests that this could lead to a higher risk of asthma and respiratory problems for children, as well as cardiovascular health incidents in adults. Examining longer-term gene expression effects, however, is still more difficult in the general population than it is in the firefighters that her lab studies. While firefighters are often exposed to the same chemicals over and over again, it can be harder to get a clear sense of how toxins affect the average civilian who encounters them more sporadically. That said, researchers at UCLA have been keeping a close eye on background toxic chemicals in the aftermath of the LA fires. In a paper published in June, they identified high levels of chromium 6, a carcinogenic metal nanoparticle that can cross cell membranes easily, in wildfire cleanup zones for up to two months following the actual fire. Abnormal concentrations of these heavy metals were also found up to six to nine miles downwind from the zones themselves. Even following the disaster and immediate recovery periods of urban destruction, the public health dangers can persist: toxic compounds like VOCs can contaminate waterways, and damaged air filtration systems can make it harder to protect surviving homes against future incidents. As wildfires become less confined to the chaparral and forests and more likely to rage through suburban streets, we’ll all have to prepare to face the orange haze of fire season. And as we do, we’ll have to confront new dangers that may persist long after the blaze dies down.