Exposure to wildfire smoke was associated with significantly elevated risks of several cancers, including lung, colorectal, breast, bladder, and blood cancers, according to evidence from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial presented in a poster and press briefing during the 2026 American Association for Cancer Research (AACR) Annual Meeting.1
“Wildfires are becoming more frequent and severe in the United States and globally,” presenting author Shuguang Leng, MBBS, PhD, Associate Professor, University of New Mexico (UNM) Comprehensive Cancer Center, Albuquerque, commented in an AACR press release, citing evidence during his presentation that the number of large fires has tripled in the last 50 years.2 He continued, “wildfire smoke has emerged as a major source of ambient air pollution, reversing decades of improvement achieved under the Clean Air Act.”
Shuguang Leng, MBBS, PhD
Large fires release toxicants and particulate matter that “do not just affect the communities next to the burning sites—[they] can travel hundreds of miles…,” Dr. Leng explained. Exposure varies regionally, with the West and Midwest generally more affected than the East Coast, although stagnant or increasing trends have also been observed in eastern states since 2016.3 A study mapping wildfire smoke across the country showed that in the 5 years that followed (2016–2020), annual average levels of wildfire smoke increased compared with those documented a decade prior (2006–2010), with nearly two-thirds of the contiguous United States experiencing measurable smoke exposure of approximately 1 μg/m3 or higher.4
Acute or episodic exposure to wildfire smoke is already known to induce or exacerbate respiratory and cardiovascular conditions; for example, a study of the 2023 Canadian wildfires found that spikes in air pollution in New York City were associated with increased hospital visits for asthma symptoms.5 Whereas evidence on the long-term health effects remains limited, he explained that wildfire smoke may contribute to chemical carcinogenesis at multiple stages: initiating cancer through DNA mutations caused by carcinogens; promoting tumor growth through inflammation that alters the tumor microenvironment; and advancing disease progression by depositing black carbon particles in lymph nodes, potentially impairing immune function.
Qizhen Wu, MBBS, PhD
“The main purpose of our study was to examine whether long-term exposure to wildfire smoke was associated with the risk of developing cancer in the general population,” Dr. Leng said in the press release. Based on the findings, Qizhen Wu, MBBS, PhD, the presentation’s first author and a postdoctoral fellow at UNM Comprehensive Cancer Center, stated, “For the general public, the key message is that wildfire smoke is not only a short-term respiratory or cardiovascular concern—chronic exposure may also carry long-term cancer risks,” even at relatively low levels.
Study Details
The current analysis used data from the PLCO Cancer Screening Trial, a multicenter randomized controlled trial designed to evaluate whether certain screening examinations reduce mortality from prostate, lung, colorectal, and ovarian cancers. Participants were recruited at 10 centers across the United States between 1993 and 2001, with 91,460 consenting to inclusion in the geospatial linkage study (mean age = 62.4 years). The cohort continues to be followed for cancer incidence, chronic health conditions, and mortality; for the present analysis, cancer incidence data were available through 2018.
For the general public, the key message is that wildfire smoke is not only a short-term respiratory or cardiovascular concern— chronic exposure may also carry long-term cancer risks.— QIZHEN WU, MBBS, PhD
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The investigators examined associations between wildfire smoke exposure and cancer incidence. Exposure was assessed monthly using satellite-derived smoke plume data, available beginning in 2006, from the National Oceanic and Atmospheric Administration (NOAA) Hazard Mapping System. This data was coupled with high-resolution air pollution estimates at 1 × 1 km2 spatial resolution. Wildfire smoke–related PM2.5 (ie, fine particulate matter measuring less than or equal to 2.5 μm in aerodynamic diameter) and black carbon concentrations were defined as deviations above local background levels. Exposure estimates were assigned based on participants’ residential locations and modeled as a time-varying variable using 36-month moving averages preceding each month; this approach was guided by prior evidence suggesting that approximately 3 years of air pollution exposure may contribute to the growth of EGFR-positive lung adenocarcinoma.6
The analysis focused on incident lung, colorectal, breast, ovarian, bladder, and hematopoietic cancers, as well as melanoma.
Hazard ratios (HRs) were estimated using Cox proportional hazards models, with proportional hazards assumptions verified. Models were stratified by study center, accounted for participant-level clustering, and adjusted for age, sex, race and ethnicity, educational attainment, smoking status, pack-years of smoking, body mass index, and trial arm. The investigators additionally used restricted cubic spline analyses to assess exposure-response relationships.
Wildfire Smoke Exposure and Cancer Incidence
Among PLCO participants with linked wildfire smoke exposure data, the investigators identified incident cases of lung (n = 1,758), colorectal (n = 800), breast (n = 1,739), ovarian (n = 222), bladder (n = 896), and hematopoietic (n = 1,696) cancers, as well as melanoma (n = 1,127), during the decade following 2006. Median monthly exposures during the same time period were 0.349 μg/m3 (interquartile range [IQR] = 0.214 μg/m3) for wildfire smoke–related PM2.5, 0.008 μg/m3 (IQR = 0.007 μg/m3) for wildfire smoke–related black carbon, and 2.444 days (IQR = 1.826 days) for the number of wildfire smoke plume days.
The investigators observed significant associations between PM2.5 exposure (per 1 μg/m3 increase) and the risk of developing the following cancer types:
- Lung: HR = 1.924 (95% confidence interval [CI] = 1.176–3.146), 92% greater risk;
- Colorectal: HR = 2.311 (95% CI = 1.110–4.814), 131% greater risk;
- Breast: HR = 2.092 (95% CI = 1.344–3.258), 109% greater risk;
- Bladder: HR = 3.485 (95% CI = 1.656–7.335), 249% greater risk;
- Hematopoietic: HR = 1.629 (95% CI = 1.020–2.603), 63% greater risk.
Dr. Leng highlighted that bladder cancer demonstrated the strongest magnitude of association. Results for plume-day counts appeared generally consistent with those observed for PM2.5 exposure, whereas associations with black carbon exposure were reported only for breast and bladder cancers. No associations were documented for ovarian cancer or melanoma across the evaluated exposure metrics.
Additional Analyses
The investigators also conducted histology-specific analyses, as certain subtypes may have distinct etiologies; for example, Dr. Leng noted that lung squamous cell carcinoma is more strongly associated with cigarette smoking, whereas lung adenocarcinoma is also frequently seen in the context of environmental exposures. In the current study, lung adenocarcinoma demonstrated a significant association with wildfire smoke–related PM2.5 exposure (HR = 2.364, 95% CI = 1.150–4.860; P = .019), whereas the association for lung squamous cell carcinoma did not reach statistical significance (HR = 2.615, 95% CI = 0.981–6.966; P = .055).
“We also run into some surprising findings,” Dr. Leng remarked, noting that proximal colorectal cancer (HR = 4.344, 95% CI = 1.822–10.353; P < .001) and muscle-invasive bladder cancer (HR = 19.423, 95% CI = 5.622–67.107; P < .001) appeared particularly associated with wildfire smoke exposure.
During the question-and-answer session of the press briefing, Dr. Leng emphasized that additional research is needed to better understand why certain cancer types and subtypes appear more susceptible to wildfire smoke exposure, including studies of tissue-level mechanisms using approaches such as artificial intelligence and spatial transcriptomics.
Also during the question-and-answer session, former AACR President and AACR Academy President Elizabeth M. Jaffee, MD, FAACR, of Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University, Baltimore, who served as moderator, asked whether cancer incidence differed by smoking status. Dr. Leng responded that stratified analyses did not reveal substantial differences, suggesting that “wildfire smoke might increase or affect cancer risk regardless.”
Elizabeth M. Jaffee, MD, FAACR
Although no age-stratified analyses were conducted, when Dr. Jaffee asked whether the investigators had examined associations with early-onset cancer incidence, Dr. Leng suggested that wildfire smoke exposure “might be an important contributor [among nonsmokers]…, especially [in] the places that repeatedly get affected by wildfire smoke at higher concentrations”; however, he told The ASCO Post that “early-onset tumors remain relatively rare. Therefore, even with more than 90,000 participants in the PLCO cohort, statistical power is insufficient to robustly evaluate associations with early-onset cancer incidence.”
The investigators additionally characterized PM2.5 exposure–response associations across cancer types. Restricted cubic spline analyses demonstrated approximately linear relationships between this metric and the risks of lung, colorectal, breast, bladder, and hematopoietic cancers.
The Findings in Context
“We found evidence to support that wildfire smoke exposure may be associated with increased risk of several cancer types and in a linear dose-response manner,” Dr. Leng emphasized in his concluding remarks, noting that annual mean wildfire smoke–related PM2.5 concentrations were approximately 0.5 μg/m3 during 2011 to 2020 but are projected to approach or exceed 1.0 μg/m3 during 2046 to 2055 under three climate scenarios.7 “Even under the conservative situation, there is a much larger area affected at a much higher dose,” he said.
KEY POINTS
- Wildfire smoke exposure may be linked to increased risks of several cancer types in a linear dose-dependent manner.
- Associations were found to vary across subtypes, with the strongest effects observed for proximal colorectal cancer and muscle-invasive bladder cancer.
- In coming decades, wildfire smoke may represent a major driver of cancer burden in the United States.
However, the current study had several limitations. Because satellite-derived wildfire smoke exposure data were only available beginning in 2006, the investigators could not assess the role of wildfire smoke in cancer initiation given the often years-long interval between cancer initiation and diagnosis. In addition, the location-based analysis assumed that participants remained within their residential areas during each exposure period.
“As wildfires continue to increase in frequency and intensity, understanding their long-term health impacts is becoming increasingly important,” Dr. Leng noted in the press release, adding during his presentation that wildfire smoke may become a major driver of cancer burden in the United States in coming decades. “While more research is needed, we hope these findings will help raise awareness and support future studies on the long-term health effects of wildfire smoke.”
DISCLOSURE: The study was funded by grants from the National Institutes of Health. Dr. Leng and Dr. Wu reported no conflicts of interest. Dr. Jaffee reported the following nonrelevant relationships: She serves as Chief Medical Advisor for the Lustgarten Foundation; is co-founder of Abmeta Therapeutics and Adventris; has served on advisory boards for Mestag Therapeutics, SURGE Therapeutics, Break Through Cancer, HDT Bio, NEUVOGEN, Candel Therapeutics, and NeoTx; and has received industry grants from Genentech and Agenus.
REFERENCES
1. Wu Q, Sinclair L, Pankratz VS, et al: Wildfire smoke and cancer risk in the United States: Evidence from the PLCO Trial. 2026 AACR Annual Meeting. Abstract 6252. Presented April 21, 2026.
2. Kenward A, Sanford T, Bronzan J: Climate Central: Western Wildfires: A Fiery Future. Available at https://assets.climatecentral.org/pdfs/westernwildfires2016vfinal.pdf. Accessed May 14, 2026.
3. Burke M, Childs ML, de la Cuesta B, et al: The contribution of wildfire to PM2.5 trends in the USA. Nature 622:761-766, 2023.
4. Childs ML, Li J, Wen J, et al: Daily local-level estimates of ambient wildfire smoke PM2.5 for the contiguous US. Environ Sci Technol 56:13607-13621, 2022.
5. Chen K, Ma Y, Bell ML, et al: Canadian wildfire smoke and asthma syndrome emergency department visits in New York City. JAMA 330:1385-1387, 2023.
6. Hill W, Lim EL, Weeden CE, et al: Lung adenocarcinoma promotion by air pollutants. Nature 616:159-167, 2023.
7. Qiu M, Li J, Gould CF, et al: Wildfire smoke exposure and mortality burden in the USA under climate change. Nature 647:935-943, 2025.
