Exposure to wildfire smoke was associated with a significantly increased risk of lung, colorectal, breast, bladder, and blood cancers, according to research presented at the 2026 American Association for Cancer Research (AACR) Annual Meeting, held April 17-22.
Wildfire smoke (WFS) is known to contain a wide range of toxins, including carcinogens such as polycyclic aromatic hydrocarbons, but the systemic effects of WFS in real-world settings, particularly regarding cancer incidence, remain unclear, said Qizhen Wu, Ph.D., lead author of this presentation and postdoctoral fellow at the University of New Mexico (UNM) Comprehensive Cancer Center.
Dr. Wu explained that the toxic compounds in WFS can disrupt a variety of biological systems, and are found not only in the lungs, the initial site of exposure, but also in the blood, where carcinogens can spread throughout the body. He also pointed out that smoke exposure itself is an inflammatory event that has systemic effects on carcinogenesis.
As wildfires increase in frequency and severity in the United States and around the world, WFS has emerged as a leading source of air pollution, reversing decades of improvements achieved under the Clean Air Act. The main objective of our study was to examine whether long-term exposure to WFS is associated with the risk of developing cancer in the general population. ”
Shuguang Leng, MBBS, Ph.D., Associate Professor, UNM Comprehensive Cancer Center, senior author of the study
Wu, Leng and colleagues analyzed cancer incidence data from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial. The study will track cancer incidence among participants, adults across the United States with no history of prostate, lung, colorectal, or ovarian cancer who were enrolled between 1993 and 2001.
To quantify the participants’ exposure to WFS, the researchers assessed fine particulate matter (PM2.5) and black carbon from ground-based air pollution data from the participants’ areas of residence. The research team used satellite imagery from 2006 to match PM2.5 and black carbon data to WFS exposure events, and also used satellite imagery to calculate the number of days that participants’ areas were exposed to WFS plumes (defined as plume days). This analysis accounted for monthly wildfire smoke exposure until the participant was first diagnosed with cancer or last had contact with the study.
In the PLCO trial, 91,460 participants were evaluable for WFS exposure. Researchers calculated a monthly updated 36-month rolling average of each participant’s exposure to WFS, defined as micrograms per cubic meter (μg/m3) of PM2.5 and black carbon and days of exposure for WFS plume day counts, from 2006 to 2018. The median values of these moving averages were 0.37 μg/m3 for WFS PM2.5, 0.0083 μg/m3 for WFS black carbon, and 1.94 days for WFS monthly plume days.
During the same period, Wu, Leng et al. identified 1,758 lung cancer cases. 800 cases of colorectal cancer. There were 1,739 breast cancer cases. There were 242 ovarian cancer cases. There were 896 cases of bladder cancer. 1,696 blood cancer cases. and 1,127 melanoma cases.
Using statistical analysis techniques that allow scientists to examine nonlinear risk associations, researchers determined that WFS exposure was significantly associated with increased risk of developing lung, colorectal, breast, bladder, and blood cancers. There was no evidence of deviation from a linear dose-response relationship. Associations with ovarian cancer and melanoma were not significant.
The researchers also found that the risk of developing these cancers increased with every 1 μg/m3 increase in the 36-month moving average of WFS PM2.5. For every 1 μg/m3 increase in WFS PM2.5 exposure, the likelihood of developing cancer significantly increased. The risk of lung cancer increased by 92%. 131% increased risk of colorectal cancer. 109% increased risk of breast cancer. 249% increased risk of bladder cancer. and a 63% increased risk of blood cancer.
The association between increased exposure levels and risk for various cancers was similar for WFS plume days. However, the association between increased WFS black carbon exposure and cancer risk was significant only for breast and bladder cancers.
“The key message for the public is that wildfire smoke is not only a short-term respiratory and cardiovascular concern, but chronic exposure may also carry long-term cancer risks,” Wu said. “Notably, increased cancer risk can occur even at relatively low levels of wildfire smoke PM2.5 experienced by the general population.”
Dr. Wu noted that specific aspects of WFS, such as its origin and content, require further investigation and may have different impacts on cancer risk across the continent depending on which geographic populations are exposed to which WFS sources. Wildfires in different regions can contain different compounds from combustion in different proportions, and chemical changes that occur as the smoke drifts can also influence biological effects, he said.
“As wildfires continue to increase in frequency and intensity, understanding their long-term health effects is becoming increasingly important,” Wren said. “Further research is needed, but we hope these findings will help raise awareness and support future research into the long-term health effects of wildfire smoke.”
Limitations of this study include that satellite imagery data were only available from 2006 onwards, which precluded analysis of the role of wildfire smoke in cancer development, as there is a multi-year lag between cancer onset and cancer diagnosis. Additionally, location-based analyzes assumed that participants were within their residential area during all measured exposure periods and did not take into account time spent indoors and outdoors.
This study was funded by the National Institutes of Health.
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American Association for Cancer Research
