Wildfire Smoke Linked to Potential Risks for Some Cancers
Wildfire smoke exposure may be associated with increased risks for multiple types of cancer, suggests an analysis of prospective cohort data from over 90,000 individuals.
To determine how this widespread pollution might be affecting cancer risk, senior author Shuguang Leng, MBBS, PhD, and colleagues analyzed data from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial. That prospective national study enrolled approximately 154,000 participants between 1993 and 2001 and tracked cancer incidence through 2018. Of these, 91,460 participants had wildfire smoke exposure data and were included in the analysis.
During the 2006-2018 exposure period, the investigators identified incident cases of 242 ovarian, 800 colorectal, 896 bladder, 1696 hematopoietic, 1739 breast, and 1758 lung cancers, as well as 1127 melanoma cases. The median 36-month moving average for wildfire smoke PM2.5 (fine particulate matter) across the cohort was 0.37 µg/m3.
Wildfire smoke exposure was significantly associated with increased risks for lung, colorectal, breast, bladder, and hematopoietic cancer, according to the results of the study presented by Leng at American Association for Cancer Research (AACR) Annual Meeting 2026.
Each 1 µg/m3 increase in the 36-month moving average of wildfire smoke PM2.5 was associated with a 63% higher risk for hematopoietic cancer (HR, 1.63; 95% CI, 1.02-2.60), a nearly twofold higher risk for lung cancer (hazard ratio [HR], 1.92; 95% CI, 1.18-3.15), more than twofold higher risks for breast cancer (HR, 2.09; 95% CI, 1.34-3.26) and colorectal cancer (HR, 2.31; 95% CI, 1.11-4.81), and a more than threefold higher risk for bladder cancer (HR, 3.49; 95% CI, 1.66-7.34). No significant associations were observed for ovarian cancer or melanoma.
The investigators quantified wildfire smoke exposure at each participant’s residence on a monthly basis using three measures: near-ground wildfire smoke PM2.5, wildfire smoke black carbon, and satellite-derived wildfire smoke plume-day counts, with measurements available from 2006 until first cancer diagnosis or last contact.
Given evidence that 3 years of air pollution exposure can influence the development of epidermal growth factor receptor-positive lung adenocarcinoma, the team modeled exposure as a time-varying variable using 36-month moving averages preceding each month. HRs were estimated using Cox proportional hazards models stratified by study center, with restricted cubic splines applied to evaluate dose-response relationships. Models were adjusted for age, sex, race and ethnicity, education, smoking history, BMI, and trial arm.
All five cancer types linked with wildfire smoke exposure showed linear dose-response relationships, Leng noted, “which means the higher the exposure, the higher the cancer risk.”
Results based on wildfire smoke plume-day counts were generally consistent with those for PM2.5, while associations for black carbon exposure were observed only for breast and bladder cancers.
With wildfires on the rise, these findings suggest that the resulting smoke may become a “major driver for cancer burden in the US in the coming decades,” said Leng, of the University of New Mexico Comprehensive Cancer Center, Albuquerque, New Mexico.
“Wildfire smoke has become a major source of air pollution in the United States,” he continued. Large fires in the US are three times more common than they were 50 years ago, and the “tons of toxicants and particles” released by these fires “can travel hundreds of miles to affect communities far away.”
The investigators also conducted histology-specific analyses, finding that adenocarcinoma showed the strongest association with wildfire smoke among lung cancer subtypes. Among colorectal cancers, proximal tumors appeared more sensitive to wildfire smoke exposure, while among bladder cancers, the association was strongest for muscle-invasive disease.
Wildfire Smoke Exposure Expected to Rise
Under even the most conservative climate projections, wildfire smoke exposure in the US is expected to rise over the next 20-30 years, Leng said.
Annual average wildfire smoke PM2.5 levels, currently estimated at around 0.5 µg/m3, could rise to 1 µg/m3. Based on the study’s dose-response data, this would correspond to substantially greater cancer risk.
There will be “a much larger area” of the US exposed “at a much higher dose,” Leng predicted.
Mitigating the Risks of Wildfire Smoke
This is a “strong hypothesis-generating study,” Jun Wu, PhD, professor of environmental and occupational health at the UC Irvine Program in Public Health, Irvine, California, told Medscape Medical News.
“This is one of the first large, prospective US cohort studies to examine wildfire smoke specifically in relation to cancer risk, especially cancer sites beyond the lung,” Wu said. “A major strength is that the PLCO platform has around 91,000 participants with longitudinal follow-up and detailed covariate data, including smoking history, which is often a weak point in previous air pollution-cancer studies.”
According to Wu, who was not involved in the analysis but recently published data linking wildfire smoke exposure to preterm birth, the reported risks for colorectal, breast, bladder, and hematopoietic cancers represent novel contributions to the literature. However, she cautioned against viewing the specific HRs as a precise estimates of risk due to wide confidence intervals.
The findings should encourage individuals, public health officials, and clinicians to mitigate the risks of wildfire smoke, Wu said.
Specifically, she suggested that public health assessments expand beyond acute outcomes like emergency department visits to include long-term endpoints such as cancer, while community clean-air shelters need to be made more widely available.
She advised clinicians to incorporate wildfire exposure into routine patient histories and to provide vulnerable patients — such as those with asthma, chronic obstructive pulmonary disease, heart failure, or pregnancy — with smoke-season action plans.
Risk mitigation begins with awareness, according to Wu, who advised individuals check their local air quality index on AirNow.gov or PurpleAir.
On smoky days, she suggested prioritizing indoor air quality by keeping windows closed and running air purifiers. If going outside on such days is necessary, she suggested an N95 or KN95 mask, as these offer “meaningful protection,” while cloth and surgical masks do not.
These preventive steps may have once been out of the ordinary, Wu said, but the risk for wildfire smoke exposure is becoming a part of everyday life.
“The common thread is a shift in framing,” Wu said. “Wildfire smoke has traditionally been treated as an acute event, but the emerging evidence points to a chronic environmental exposure. Both our clinical and public health systems have room to grow into that reality.”
The analysis was funded by the National Institutes of Health. The investigators and Wu reported having no conflicts of interest.
This article was previously published on Medscape.