Wildfire Smoke May Increase Risk of New Cancer, Poor Outcomes of Existing Cancer

There is an emerging body of research that suggests exposure to wildfire smoke may increase the risk of developing cancer and dying from it.^1-3

Past studies have suggested that air pollution — specifically, fine particulate matter 2.5 microns in diameter or smaller (PM2.5) — may increase the risk of lung cancer, breast cancer, gastrointestinal cancers, and genitourinary cancers.^4-7

Research has also revealed associations between PM2.5 exposure and an increased risk of death among cancer patients, including those with lung, breast, gastrointestinal, gynecologic, central nervous system (CNS), and hematologic cancers.^8-10

Associations between wildfire smoke and cancer are less well studied. However, “the available studies … have demonstrated that short-term and long-term wildfire smoke exposure is associated with increased risk for several types of cancers, greater mortality from several cancers, worse survival following treatment of certain cancers, and more barriers to accessing cancer care,” said Raj Fadadu, MD, from the University of California, San Francisco.

Wildfire Smoke and Cancer Risks

In a study published in 2022, researchers examined associations between long-term wildfire smoke exposure and the incidence of various cancer types in more than 2 million people in Canada over a median follow-up of 20 years.¹

Compared to unexposed individuals, people who were exposed to wildfires within 50 km of their homes in the past 10 years had a higher risk of lung cancer (adjusted hazard ratio [aHR], 1.049; 95% CI, 1.028-1.071) and brain tumors (aHR, 1.100; 95% CI, 1.026-1.179). There was no association between wildfire smoke exposure and hematologic cancers, however.

Another study from 2022 showed that wildfire-related PM2.5 exposure was associated with an increased risk of cancer death.² Researchers evaluated more than 1.3 million cancer deaths in Brazil and found that the risk of death from all cancers increased with each 1-μg/m³ increase in wildfire-related PM2.5 exposure (relative risk [RR], 1.02; 95% CI, 1.01–1.03; P <.001).

Wildfire-related PM2.5 exposure was associated with an increased risk of death from several cancers, including:

• Nasopharyngeal cancer (RR, 1.10; 95% CI, 1.04–1.16; P =.002)
• Esophageal cancer (RR, 1.05; 95% CI, 1.01–1.08; P =.012)
• Stomach cancer (RR, 1.03; 95% CI, 1.01–1.06; P =.017)
• Colorectal cancer (RR, 1.08; 95% CI, 1.05–1.11; P <.001)
• Laryngeal cancer (RR, 1.06; 95% CI, 1.02–1.11; P =.003)
• Skin cancer (RR, 1.06; 95% CI, 1.00–1.12; P =.003)
• Breast cancer (RR, 1.04; 95% CI, 1.01–1.06; P =.007)
• Prostate cancer (RR, 1.03; 95% CI, 1.01–1.06; P =.019)
• Testicular cancer (RR, 1.10; 95% CI, 1.03–1.17; P =.002).

“Although the research is still quite sparse, there is increasing evidence that cancer survivors represent a vulnerable group for adverse health effects from wildfire smoke,” said Shehnaz Khursheed Hussain, PhD, from the University of California Davis Comprehensive Cancer Center in Sacramento.

“Patients with lung cancer, particularly those who have had recent lung surgery, or any cancer patient undergoing active treatment appear to be particularly vulnerable to adverse health effects from wildfire smoke,” Dr Hussain added.

In a study published this year, researchers investigated the association between wildfire exposure and long-term survival in 466,912 US patients who underwent surgical resection for stage I-III non–small cell lung cancer.³

Patients exposed to a wildfire had worse overall survival than unexposed patients. This was true for patients exposed within 3 months of hospital discharge (adjusted hazard ratio [aHR], 1.43; 95% CI, 1.41-1.45), between 4 and 6 months after discharge (aHR, 1.39; 95% CI, 1.37-1.41), and between 7 and 12 months after discharge (aHR, 1.17; 95% CI, 1.15-1.19).

Though it remains unclear exactly how wildfire exposure might increase the risk of cancer or cancer death, “studies have demonstrated various potential biological mechanisms underlying the pathway between general air pollution exposure and cancer development,” Dr Fadadu noted. “For example, air pollution has been shown to increase oxidative stress, cause both direct and indirect DNA damage, contribute to epigenetic changes, alter cellular metabolism, and promote a pro-inflammatory response, which can all promote carcinogenesis.”^4,11

“Wildfire smoke, particularly emissions generated from fires burning at the wildland urban interface, consists of volatile organic compounds and particulate matter mixtures that include a diversity of known carcinogens, such as polyaromatic hydrocarbons, benzene, formaldehyde, asbestos, and heavy metals,” Dr Hussain said.

Patient Guidance and Future Directions

Additional research is needed to develop evidence-based interventions for reducing the risk of cancer or cancer death related to wildfire exposure. However, “based on research conducted for other health outcomes, it would be reasonable for physicians to counsel patients to reduce smoke exposure by limiting time spent outdoors during wildfires, optimizing indoor air quality, and wearing a tight-fitting N95 mask when outdoors,” Dr Fadadu said.

Clinicians should consider educating patients with cancer about the potential risks associated with wildfire smoke exposure and advise them to monitor the Air Quality Index (AQI) and avoid prolonged or heavy exertion outdoors when the AQI level is “Unhealthy for Sensitive Groups” or higher, Dr Hussain suggested.¹²

More broadly, health systems should be strengthened to ensure access to high-quality cancer care for vulnerable populations, and public health measures should include strategies to protect these groups from the adverse effects of wildfire smoke exposure, she added.

“Wildfires can exacerbate existing social and economic inequities, which can place vulnerable populations at higher risk than normal for a range of health outcomes,” Dr Hussain noted.

“For me, this emphasizes that future research and action around mitigating the effects of wildfire smoke must consider the effects of social determinants of health; for example, community interventions like clean-air shelters can reduce exposure to air pollution during wildfire events and may be required in communities with unhoused populations or communities with low economic resources,” she said.

Dr Hussain also cited the need for more research to investigate the “cancer-promoting and treatment-inhibiting effects of continued exposure to wildfire smoke across the course of care and throughout survivorship after a cancer diagnosis.” She and her team are currently exploring such topics in a pilot study.¹³

Topics to be explored include the potential impact of wildfire smoke exposure on the pulmonary-toxic and cardiotoxic effects of chest radiation, surgeries, and certain chemotherapies used in cancer treatment, and whether wildfire smoke exposure suppresses the immune system’s ability to control cancer growth or metastases, Dr Hussain said.

Along with clinical and public health measures to address the impact of wildfire smoke exposure on individuals and communities, there is a need for efforts prioritizing “high-impact ecological interventions related to land use management in order to prevent wildfires from increasing in both severity and frequency globally,” Dr Fadadu said.

Disclosures: Dr Hussain and Dr Fadadu said they have no relevant disclosures.

References

1. Korsiak J, Pinault L, Christidis T, Burnett RT, Abrahamowicz M, Weichenthal S. Long-term exposure to wildfires and cancer incidence in Canada: A population-based observational cohort study. Lancet Planet Health. 2022;6(5):e400-e409. doi:10.1016/S2542-5196(22)00067-5

2. Yu P, Xu R, Li S, et al. Exposure to wildfire-related PM2.5 and site-specific cancer mortality in Brazil from 2010 to 2016: A retrospective study. PLoS Med. 2022;19(9):e1004103. doi:10.1371/journal.pmed.1004103

3. Zhang D, Xi Y, Boffa DJ, Liu Y, Nogueira LM. Association of wildfire exposure while recovering from lung cancer surgery with overall survival. JAMA Oncol. 2023;9(9):1214-1220. doi:10.1001/jamaoncol.2023.2144

4. Turner MC, Andersen ZJ, Baccarelli A, et al. Outdoor air pollution and cancer: An overview of the current evidence and public health recommendations. CA Cancer J Clin. 2020;70(6):460-479. doi:10.3322/caac.21632

5. White AJ, Fisher JA, Sweeney MR, et al. Ambient fine particulate matter and breast cancer incidence in a large prospective US cohort. J Natl Cancer Inst. Published online September 11, 2023. doi:10.1093/jnci/djad170

6. Myers RL, Tammemagi MC, Brauer M, et al. Evaluation of outdoor air pollution exposure (PM2.5) in female non-smoking lung cancer patients. Presented at WCLC 2022. August 6-9, 2022. Abstract 2268.

7. Wei Y, Danesh Yazdi M, Ma T, et al. Additive effects of 10-year exposures to PM2.5 and NO2 and primary cancer incidence in American older adults. Environ Epidemiol. 2023;7(4):e265. doi:10.1097/EE9.0000000000000265

8. Wong CM, Tsang H, Lai HK, et al. Cancer mortality risks from long-term exposure to ambient fine particle. Cancer Epidemiol Biomarkers Prev. 2016;25(5):839-845. doi:10.1158/1055-9965.EPI-15-0626

9. Ou JY, Hanson HA, Ramsay JM, et al. Fine particulate matter air pollution and mortality among pediatric, adolescent, and young adult cancer patients. Cancer Epidemiol Biomarkers Prev. 2020;29(10):1929-1939. doi:10.1158/1055-9965.EPI-19-1363

10. Ou JY, Hanson HA, Ramsay JM, et al. Fine particulate matter and respiratory healthcare encounters among survivors of childhood cancers. Int J Environ Res Public Health. 2019;16(6):1081. doi:10.3390/ijerph16061081

11. Swanton C, Hill W, Lim E, et al. Mechanism of action and an actionable inflammatory axis for air pollution induced non-small cell lung cancer: Towards molecular cancer prevention. Presented at ESMO 2022; September 9-13, 2022. Abstract LBA1.

12. US Environmental Protection Agency. Air quality guide for particle pollution. Published February 2023. Accessed November 2, 2023.

13. Watry G. Exploring the link between wildfire smoke and cancer risk. UC Davis Health. Published August 11, 2022. Accessed November 2, 2023. 

This article originally appeared on Cancer Therapy Advisor