Higher ambient exposure to a type of inhalable, fine particulate matter was found to be associated with an increased risk of cancer-related death for patients who were diagnosed with non–small cell lung cancer (NSCLC) while living in California—a state where wildfires are becoming more prevalent, according to an abstract presented during the 2025 ASCO Annual Meeting.1
“Paradoxically, higher wildfire-dominated…exposure was associated with improved survival particularly among those patients with stage IV disease and among those treated with immunotherapy, which we thought warrants further investigation,” commented medical oncologist Surbhi Singhal, MD, Assistant Professor at the University of California Davis Comprehensive Cancer Center in Sacramento.
Background
“We know that wildfires emit multiple hazardous components,” Dr. Singhal remarked. Among the pollutants found in wildfire smoke is fine particulate matter. It measures less than or equal to 2.5 μm in aerodynamic diameter and is referred to as PM2.5.
“As the size and frequency of wildfires increase, our findings have important public health and clinical implications for patients with NSCLC.”— SURBHI SINGHAL, MD, AND COLLEAGUES
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Beyond wildfires, emission of PM2.5 arises from multiple sources, including vehicle exhaust, power plants, and agriculture. Ambient exposure has been found to be associated with an increased risk of developing lung cancer, according to Dr. Singhal, and it may be linked to mortality. Prior to this analysis, the impact of wildfire-dominated PM2.5 exposure on the survival of patients with NSCLC remained unknown.
Study Details
Using the California Cancer Registry, the investigators identified 18,585 patients who were diagnosed with NSCLC between 2017 and 2020; their PM2.5 exposure was examined over the 12-month period after diagnosis (vulnerable 12-month treatment window). Exposure was calculated by using a validated method to fuse PM2.5 measurement data from meteorologic monitoring, emergency smoke monitoring, satellite, official air monitoring, low-cost sensors, and predictive smoke modeling; dividing the state of California into a 1-square-kilometer grid; calculating the 24-hour average PM2.5 measurement for the centroid of each grid cell; and mapping the patient home address at the time of diagnosis to the closest grid centroid.
Two PM2.5 exposure models were used: mean annual PM2.5 exposure (ie, ambient levels) and the number of days of exposure to PM2.5 levels of at least 55 μg/m3 (ie, wildfire spike levels).
The primary endpoint of this observational cohort study was cancer-related death. All-cause death was evaluated as a secondary endpoint.
Key Results
Referring to a dot map of PM2.5 exposure among patients with newly diagnosed NSCLC according to wildfire season, Dr. Singhal stated: “During a highly active wildfire [season], we had several patients who had at least 10 days of [exposure to] PM2.5 [levels of] greater than 35 or greater than 55 μg/m3, but no patients in our cohort had that degree of exposure during a minimally active wildfire season.”
A total of 6,097 lung cancer–related deaths were reported over the study period. Based on the first model, higher mean annual PM2.5 exposure was associated with an increase in the hazard of cancer-related death of 19.8% (hazard ratio [HR] = 1.198). This relationship appeared to be consistent across all subgroups.
“Interestingly,” Dr. Singhal stated that, based on the second model, for every 10-day period of exposure to PM2.5 levels of at least 55 μg/m3, the hazard of cancer-related death decreased by 7%. A hazard ratio of 0.933 was reported.
“There were a few subgroups that were especially interesting, including those with tobacco exposure [current or former: HR = 0.914], immunotherapy [given: HR = 0.878], and stage IV disease [HR = 0.888],” Dr. Singhal commented. “We looked closer at that population of [those with] stage IV [disease] with [exposure to] PM2.5 [levels] of greater than 55 μg/m3. Among patients who smoked tobacco and had immunotherapy, we saw a specifically decreased risk of cancer-related death [by 21%; HR = 0.790].”
When asked whether she could provide a biologic hypothesis for the results observed with the second model, Dr. Singhal shared this reply: “There are very few preclinical data in this space. Perhaps there is some sort of modulation of the immune microenvironment that allows for synergy with the immunotherapy; [however], this is all theoretical…, and there is not a great explanation for that finding.”
A limitation of the study is that it used the patient home address at the time of diagnosis to calculate PM2.5 exposure and did not account for possible relocation during the vulnerable 12-month treatment window. Additionally, the investigators were unable to examine how PM2.5 exposure may impact survival in those with driver mutations, as the California Cancer Registry did not capture molecular data between 2017 and 2020.
“As the size and frequency of wildfires increase, our findings have important public health and clinical implications for patients with NSCLC,” the investigators concluded.
DISCLOSURE: Dr. Singhal has served as a consultant or advisor to Bristol Myers Squibb, Caris Life Sciences, and OncoHost. For full disclosures of the other study authors, visit coi.asco.org.
REFERENCE
1. Singhal S, Riess JW, Alaniz M, et al: Association between wildfire-dominated PM2.5 exposure and non-small cell lung cancer survival in California. 2025 ASCO Annual Meeting. Abstract 10520. Presented May 31, 2025.
