Long-term exposure to fine particulate matter from wildfire smoke was associated with a higher risk of death among older patients with lung cancer, and wildfire-related particulate matter appeared more harmful than nonwildfire air pollution at the same exposure level. Zhang et al reported the findings from a large SEER-Medicare cohort study in The Lancet Oncology.
Wildfire smoke is becoming an increasingly important source of fine particulate matter, or PM2.5, in the United States. Previous toxicology studies have suggested that wildfire-related PM2.5 may be more harmful than particulate matter from other sources, but large-scale data on its effects on survival among patients with lung cancer have been limited.
Study Details
The retrospective cohort study included 414,016 patients aged 65 years or older with histologically confirmed non–small cell or small cell lung cancer in the SEER-Medicare database. Patients were diagnosed between 2006 and 2019 and were followed annually from diagnosis until death, loss to follow-up, or the end of 2019.
Researchers estimated wildfire-related and nonwildfire PM2.5 exposure based on patients’ residential ZIP codes. They then examined whether average exposure over the previous 3 years was associated with the risk of death, accounting for factors including age, cancer stage, treatment, comorbidities, and neighborhood characteristics.
Median follow-up was 2 years, representing more than 1.2 million person-years. During the study period, 319,928 patients, or 77.3%, died. Nearly 89% of patients had non–small cell lung cancer.
Key Findings
Both wildfire-related and nonwildfire PM2.5 were linked to higher mortality. For each 1-µg/m³ increase in concentration, wildfire-related PM2.5 was associated with a 7.8% higher risk of death (hazard ratio [HR] = 1.0782), compared with a 1.7% higher risk with nonwildfire PM2.5 (HR = 1.0171). The difference was statistically significant.
Average nonwildfire PM2.5 concentrations were about 23.5 times higher than wildfire-related concentrations, which helps explain why nonwildfire PM2.5 accounted for more excess deaths overall. However, when the two were compared at the same exposure level, wildfire-related PM2.5 was associated with a greater increase in mortality risk. The investigators estimated 696 excess deaths per year from wildfire-related PM2.5 and 3,465 from nonwildfire PM2.5.
“Wildfire-related PM2.5 showed greater toxicity than nonwildfire PM2.5 per 1-μg/m³ increase in exposure,” the investigators wrote.
Longer and more frequent exposure to wildfire smoke was also linked to higher mortality, with more smoke days and repeated smoke events over 3 years both associated with a higher risk of death.
The association between wildfire-related PM2.5 and mortality was generally consistent across age, race or ethnicity, cancer stage, histology, comorbidity burden, residential area, tobacco use disorder status, and receipt of radiotherapy. The association was stronger among women than men and among patients who had not received surgery or chemotherapy.
Study Implications
The investigators suggested that the greater toxicity of wildfire-related PM2.5 may be related to its higher proportion of smaller particles and greater concentrations of carcinogens such as polycyclic aromatic hydrocarbons and heavy metals.
However, they cautioned that the observational design cannot establish causation. Exposure was estimated at the ZIP code level rather than measured individually, the analysis was limited to adults aged 65 years or older, detailed smoking history was unavailable for the main analysis, and residual confounding may remain.
“Our findings suggest the need to reduce wildfire-related PM2.5 exposure in clinical care settings for patients with lung cancer, especially during wildfire episodes,” the investigators wrote. They suggested measures including high-efficiency particulate air filtration, protective masks, and improved indoor ventilation.
Min Zhang, PhD, of the Department of Environmental Medicine at the Icahn School of Medicine at Mount Sinai, is the corresponding author of the study.
DISCLOSURE: The study was supported by the National Institute of Environmental Health Sciences, National Cancer Institute, National Institutes of Health, National Center for Advancing Translational Sciences, Centers for Disease Control and Prevention’s National Program of Cancer Registries, and Stony Brook University. For full disclosures of the study authors, visit thelancet.com.
