LA fires: Long-term exposure to wildfire smoke is poorly understood - and a growing risk

Millions of people across the Los Angeles area are being exposed to wildfire smoke as fires burn through homes and vehicles. The fires in January 2025 have burned thousands of structures, along with the building materials, furniture, paints, plastics and electronics inside them.

When materials like these burn, they can release toxic chemicals with the potential to harm people breathing the air downwind.

A 2023 study of smoke from fires in the wildland-urban interface – areas where urban neighborhoods bleed into the wildlands – found it contained a vast array of chemicals harmful to humans, including hydrogen chloride, polycyclic aromatic hydrocarbons, dioxins and a range of toxic organic compounds, including known carcinogens such as benzene, as well as toluene, xylenes, styrene and formaldehyde. The researchers also found metals in the smoke, including lead, chromium, cadmium and arsenic, which are known to affect several body systems, such as the brain, liver, kidney, skin and lungs.

The short-term effects of exposure to smoke like this can trigger asthma attacks and cause lung and cardiac problems.

But smoke can also have long-term effects, and those are less well understood. As an environmental toxicologist who focuses on wildfire smoke health effects, I, along with many of my colleagues, am increasingly concerned about the impact of long-term and repeated exposures to wildfire smoke that more people are now facing.

Long-term smoke exposure is increasing

Nationwide, the acreage burned in wildfires in the U.S. has nearly doubled each decade since 1990. That is changing how people are exposed to wildfire smoke.

Communities have found themselves blanketed in smoke for days and weeks at a time increasingly often. In 2023, massive wildfires in Canada repeatedly spread thick smoke into many U.S. communities. Controlled burns, which firefighters set to clear away flammable brush and reduce the severity of future wildfires, also add smoke to the air.

Wildfire smoke is now the leading source of PM2.5 – microscopic particulate matter than can penetrate into the lungs – in the western U.S.

This growing exposure increases the need to understand the long-term consequences of living and working in wildfire-risk areas.

Dose, duration and frequency matter

When scientists study the health risks of wildfire smoke, they tend to use analysis methods that were developed to assess health effects caused by low-level, chronic, urban air pollution exposures – picture car exhaust or smokestack emissions. However, these approaches fail to capture the dynamic and intense nature of wildfire smoke.

Researchers suspect there are differing consequences for people exposed to smoke at varying intensities and durations. Repeated exposure to wildfire smoke may also have compounding health effects over time.

To study the long-term impact of wildfire smoke, scientists need to know how much smoke people were exposed to, for how long and how often. That’s not an experiment anyone can conduct on humans in a lab, but the data can be gathered from communities being affected by wildfires.

Right now, however, this kind of data collection is rare.

Most studies that have explored long-term exposure, such as its impact on dementia or pregnancy, have used an average exposure over years rather than detailed data on exposures.

A few have focused on specific events. For example, a study of residents who had been exposed to six weeks of smoke during the 2017 Rice Ridge Fire near Seeley Lake, Montana, found their lung function was significantly reduced for at least two years after the fire. That was a forest fire, and while burning vegetation is bad, it’s generally thought to be less toxic than burning buildings.

Thinking differently about smoke exposure

Improving understanding of the long-term effects of wildfire smoke will require thinking differently about smoke.

If epidemiologists can begin clearly defining the negative health effects from wildfire smoke exposure in terms of dose, duration and frequency in their studies, taking into account the dynamic and episodic nature, then toxicologists can model these human experiences in animal experiments.

These experiments would have the potential to improve the understanding of the long-term health risks and then help scientists develop effective guidelines and strategies to mitigate harmful exposures.

Luke Montrose receives funding from the National Institute of Occupational Safety and Health.