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Exposure to wildfire smoke was associated with a significantly increased risk of lung, colorectal, breast, bladder, and blood cancer, according to results from a study presented at the , held April 17-22.

Although it’s known that wildfire smoke (WFS) contains a wide array of toxins, including carcinogens like polycyclic aromatic hydrocarbons, the whole-body impacts of WFS in real-world settings remain unclear, especially when it comes to cancer incidence, according to Qizhen Wu, the presentation’s first author and a postdoctoral fellow at the University of New Mexico (UNM) Comprehensive Cancer Center.

Wu explained that the toxic compounds in WFS have the potential to disrupt a variety of biological systems—not just in the lungs, the site of initial exposure, but in the blood as well, which can then spread carcinogens throughout the body. He also noted that smoke exposure is, itself, an inflammatory event with systemic implications for carcinogenesis.

“Wildfires are becoming more frequent and severe in the United States and globally, and WFS has emerged as a major source of ambient air pollution, reversing decades of improvement achieved under the Clean Air Act,” said Shuguang Leng, associate professor at the UNM Comprehensive Cancer Center and the study’s senior author. “The main purpose of our study was to examine whether long-term exposure to WFS was associated with the risk of developing cancer in the general population.”

Wu, Leng, and colleagues analyzed cancer incidence data from the Prostate, Lung, Colorectal, and Ovarian (PLCO) Cancer Screening Trial, which tracks the cancer incidence of participants: adults from across the U.S. with no history of prostate, lung, colorectal, or ovarian cancers who enrolled between 1993 and 2001.

Within the PLCO trial, 91,460 participants were evaluable for WFS exposure. From 2006 to 2018, Wu, Leng, and colleagues identified 1,758 lung cancer cases, 800 colorectal cancer cases, 1,739 breast cancer cases, 242 ovarian cancer cases, 896 bladder cancer cases, 1,696 blood cancer cases, and 1,127 melanoma cases.

Using a statistical analysis method that allows scientists to examine nonlinear risk associations, the researchers confirmed that WFS exposure was significantly associated with an increased risk of developing lung, colorectal, breast, bladder, and blood cancer. There was no evidence of deviation from a linear dose-response relationship. Associations with ovarian cancer and melanoma were not significant.

“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,” Wu said. “Notably, increased cancer risk may occur even at relatively low levels of wildfire smoke PM2.5 commonly experienced by general populations.”

Wu noted that further investigation was warranted for specific aspects of WFS, including its origin and its contents, which could have different implications for cancer risk across the continent depending on which geographic populations were exposed to which WFS sources. Wildfires from different regions may contain different compounds from burning in varying proportions, and the chemical transformations that occur in smoke as it drifts may also impact biological effects, he said.

“As wildfires continue to increase in frequency and intensity, understanding their long-term health impacts is becoming increasingly important,” Leng said. “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.”