Wildfire Smoke and Health Research

For most of the last 30 years, the science of fine-particle pollution was a science of tailpipes, smokestacks, and power plants. Then the American West started burning longer and harder, and in 2023 Canadian wildfire smoke turned New York City orange. Wildfire smoke is now one of the most active areas of air pollution research, and one of the most uncomfortable: even as regulated sources clean up, smoke from fires keeps erasing the gains.

Background

Wildfire smoke is a complex mix of fine particulate matter (PM2.5), carbon monoxide, nitrogen oxides, volatile organic compounds (VOCs), and polycyclic aromatic hydrocarbons. PM2.5 is the dominant health concern. Chemical analyses by groups at NOAA, UC Davis, and Colorado State have shown that smoke aerosol differs in composition from urban PM2.5: more organic carbon, more reactive oxygen species, and a long tail of partially combusted woody material.

A 2023 Nature paper by Marshall Burke (Stanford) and colleagues found that wildfire smoke now contributes up to 25 percent of PM2.5 in parts of the contiguous U.S. in some years, and has reversed decades of air quality progress across many western counties.

What the studies found

Three lines of evidence stand out.

Respiratory effects are large and acute. Rosana Aguilera and colleagues at UC San Diego analyzed 14 years of Southern California hospital admissions and published in Nature Communications in 2021. Each 10 microgram per cubic meter increase in PM2.5 from non-smoke sources raised respiratory admissions by about 1 percent. The same increase from wildfire smoke raised them by 1.3 to 10 percent depending on age group. The finding that smoke particles are more toxic per microgram than ambient PM2.5 has been replicated in studies from Brazil (Ye et al., Nature Communications 2022) and Australia.

Cardiovascular effects are real but smaller per event. A 2020 study by Wettstein et al. in the Journal of the American Heart Association analyzed California ED visits during the 2015 to 2017 fire seasons and found increases in ischemic heart disease, dysrhythmia, and heart failure visits on smoke days, especially among adults over 65. A 2024 Nature Communications study by Tian Tian and colleagues in urban China found cardiovascular admissions rose with landscape-fire PM2.5 even after adjusting for background pollution.

Medium-term and birth effects. Researchers including Colleen Reid (CU Boulder) and Sam Heft-Neal (Stanford) have linked seasonal wildfire smoke exposure during pregnancy to lower birth weight and preterm birth. A 2022 California analysis estimated 3 percent of preterm births in the state were attributable to wildfire smoke exposure in heavy fire years.

Why it matters

Wildfire smoke breaks the assumption baked into most air quality regulation: that controlling emissions of regulated sources will steadily improve air quality. The EPA's 2024 PM2.5 standard explicitly excludes "exceptional events" (mostly wildfires) from compliance calculations, which has the side effect of obscuring real exposure for the people who breathe it.

Smoke also reaches further than people expect. The 2023 Canadian fires sent PM2.5 above 200 micrograms per cubic meter in New York and Philadelphia, both more than 1,000 miles from the source. Plumes from the 2020 California fires reached Europe.

Open questions

Researchers are still trying to nail down why wildfire PM2.5 appears more toxic, whether the effect varies by what burned (grassland vs. conifer vs. structures), and how to design clean air shelters and HEPA filtration programs that actually protect the most exposed. Long-term cohort studies of firefighters and people in repeatedly affected communities are ongoing.

Sources

• Wildfire smoke impacts respiratory health more than fine particles from other sources (Aguilera et al., Nature Communications 2021)
• Impacts of Fine Particulate Matter From Wildfire Smoke on Respiratory and Cardiovascular Health in California (Heaney et al., GeoHealth 2022)
• Cardiovascular and cerebrovascular emergency department visits associated with wildfire smoke exposure in California in 2015 (Wettstein et al., JAHA 2018)
• Short-term exposure to wildfire-related PM2.5 increases mortality risks and burdens in Brazil (Ye et al., Nature Communications 2022)
• Landscape fire PM2.5 and hospital admissions for cause-specific cardiovascular disease in urban China (Tian et al., Nature Communications 2024)
• Health Effects of Wildfire Smoke Exposure (Annual Review of Medicine, 2024)
• The changing risk and burden of wildfire in the US (Burke et al., PNAS 2021)