The wildfire danger we’re only just beginning to understand

Wildfires are rapidly encroaching upon urban areas, exposing residents to hazardous smoke containing burning plastics and metals. According to Hans Gutknecht/Media News Group/Los Angeles Daily News via Getty Images, wildfires were once considered confined to remote wilderness or forest edges, but are now increasingly urban. Urban development is rapidly expanding into the wilderness, bringing communities closer to wildfires.

Fires in urban settings release toxic chemicals, including burning metals, which pose numerous health risks such as higher cancer risks, cardiovascular or respiratory illnesses. The long-term effects of these chemicals include higher cancer risk and cardiovascular or respiratory illnesses, making wildfires more dangerous. The Hawk Fire in Nevada razed over 15,000 acres near Reno, destroying 47 homes before it was extinguished.

Last year's Los Angeles fires burned over 37,000 acres, including densely built residential neighborhoods, and displaced 205,000 people. In Boulder, Colorado, the 2021 Marshall Fire destroyed nearly 1,000 homes. This shifting geography of fires is increasing exposure to health risks, as over 46 million homes in the continental US are on the edge of fire-prone wildland.

PM2.5, released when vegetation burns, is associated with premature death, severe respiratory health damage, and cognitive impairment. The urban landscape has altered the composition of smoke, exposing us not only to particulate matter but also to chemical toxins from human structures and vehicles burning. While researchers are still learning about the long-term dangers of these new forms of smoke, early research suggests troubling risks, especially for aging populations.

In 2025, a total of 5.2 million acres of land have burned across the US, with 8.5 million acres already engulfed, well before the winter dry seasons that plague parts of the Southwest. Despite this, human housing continues to expand, with the wildland-urban interface growing substantially due to suburban construction. In 2020, 46% more houses were built in the high-risk WUI zone compared to 1990.

As suburbia expands into wilder regions, more houses are exposed to wildfires, and when houses and other human-made goods burn, they produce different and potentially more toxic forms of smoke.