Long-term impact of PM2.5 on mortality is exacerbated when wildfire events occur

There is extensive evidence that long-term exposure to all-source PM2.5 increases mortality. However, to date, no study has evaluated whether this effect is exacerbated in the presence of wildfire events. Here, we study 60+ million older US adults and find that wildfire events increase the harmful effects of long-term all-source PM2.5 exposure on mortality, providing a new and realistic conceptualization of wildfire health risks.

💡 Research Summary

The authors conducted a nationwide cohort study of 60,999,431 Medicare beneficiaries aged 65 years and older across the contiguous United States from 2007 to 2016 to examine whether the number of wildfire‑related fine particulate matter (PM₂.₅) days modifies the association between long‑term all‑source PM₂.₅ exposure and all‑cause mortality. Annual average all‑source PM₂.₅ concentrations were derived from the Di et al. ensemble model and aggregated to ZIP‑code level. Wildfire PM₂.₅ estimates came from a validated 10‑km² grid model (Childs et al.) and were used to count the number of days per year with non‑zero wildfire PM₂.₅. These counts were categorized into three strata: 0–20, 21–35, and >35 days, approximating tertiles of exposure.

The analytical framework employed stratified Poisson regression models within each wildfire‑day stratum. Mortality counts were modeled as a function of annual all‑source PM₂.₅ using natural splines to capture non‑linear exposure‑response relationships. Models were adjusted for a comprehensive set of confounders: individual‑level age categories, race/ethnicity, sex, and Medicaid eligibility; ZIP‑code or county‑level socioeconomic variables (e.g., poverty proportion, median income, home value, education, housing ownership, population density); two county‑level health risk factors (average BMI and smoking prevalence); four ZIP‑code meteorological variables (summer and winter average maximum temperature and relative humidity); fixed effects for US Census region (Northeast, Midwest, South, West) and calendar year; and an offset for person‑years.

Key findings: Across the entire cohort, a rise in annual all‑source PM₂.₅ from the current National Ambient Air Quality Standard (NAAQS) of 9 µg/m³ to 12 µg/m³ was associated with a hazard ratio (HR) of approximately 1.04 (95 % CI 1.02–1.06). In the low‑wildfire stratum (0–20 days), the HR was similar, indicating modest additional risk. In the middle stratum (21–35 days), the HR increased to roughly 1.06, while in the high‑wildfire stratum (>35 days) the HR rose sharply to >1.12, demonstrating a clear effect‑modification by wildfire exposure.

Subgroup analyses revealed that ZIP codes with higher poverty rates (<15 % vs. ≥15 % of residents below the poverty line) exhibited steeper exposure‑response curves at lower PM₂.₅ levels, suggesting heightened vulnerability among socio‑economically disadvantaged populations. Region‑specific analyses showed heterogeneous patterns: the West and South, where wildfire frequency is greatest, displayed the most pronounced amplification of mortality risk with increasing wildfire days, whereas the Northeast and Midwest showed more muted effects.

Methodological strengths include the massive sample size, high‑resolution exposure assessment, and rigorous adjustment for a wide array of individual and area‑level confounders, which together enhance causal inference. Limitations involve potential residual confounding from unmeasured individual behaviors (e.g., indoor air filtration), exposure measurement error inherent in model‑based PM₂.₅ estimates, and the binary definition of wildfire days (any non‑zero concentration) that does not capture intensity variations.

The study provides compelling evidence that long‑term health impacts of ambient PM₂.₅ are substantially exacerbated in regions and years with frequent wildfire smoke. Consequently, regulatory standards based solely on average PM₂.₅ concentrations may underestimate risk in fire‑prone areas. Public health policies should integrate wildfire preparedness, targeted interventions for high‑poverty communities, and adaptive air‑quality management that accounts for both chronic background pollution and episodic wildfire smoke exposure. This integrated approach is essential for mitigating mortality risk in the context of a warming climate that is projected to increase wildfire frequency and severity.