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

TL;DR

Long-standing evidence links long-term exposure to all-source $PM_{2.5}$ and mortality, but whether wildfire events exacerbate this risk was unknown. A national cohort of 60,999,431 Medicare beneficiaries aged 65+ from 2007–2016 is analyzed, linking annual all-source $PM_{2.5}$ exposure (2006–2015) and daily wildfire $PM_{2.5}$ data (2006–2015) to mortality outcomes using stratified Poisson models with nonlinear exposure–response and three wildfire-day strata, with extensive adjustment for area-level covariates and stratification by poverty and region. The results show that wildfire-day counts modify the $PM_{2.5}$–mortality relationship, with greater amplification of risk as wildfire days increase, particularly at lower baseline $PM_{2.5}$ and in certain high-poverty or region-specific contexts. These findings provide a realistic, policy-relevant mechanism by which wildfire smoke heightens mortality risk beyond chronic PM$_{2.5}$ exposure and have implications for air-quality standards and wildfire preparedness.

Abstract

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.

Figures (4)

• Figure 1: Wildfire event distribution. The plot represents the distribution of the three wildfire event day strata over the contiguous US for the years 2006, 2010, and 2015. The three wildfire event day strata are (1) 0 to 20 days with non-zero wildfire PM$_{2.5}$ per year, (2) 21 to 35 days with non-zero wildfire-PM$_{2.5}$ per year, and (3) more than 35 days with non-zero wildfire PM$_{2.5}$ per year.
• Figure 2: Mortality hazard ratio with 95% confidence intervals and all-source PM$_{2.5}$ distribution. The plots illustrate the exposure-response curve and distribution of all-source PM$_{2.5}$ values, ranging from the $1^{st}$ to the $99^{st}$ percentile of the entire cohort all-source PM$_{2.5}$ distribution. The top plots represent the hazard ratio of mortality compared to the current NAAQS for annual all-source PM$_{2.5}$(9 $\mu g/m^3$) for the entire cohort and stratifying by the three wildfire days strata. The lower plots show the distribution of all-source PM$_{2.5}$ for the entire cohort, as well as stratified by the three wildfire day strata.
• Figure 3: Mortality hazard ratio with 95% confidence intervals and all-source PM$_{2.5}$ distribution stratifying by poverty level. The plots illustrate the exposure-response curve and distribution of all-source PM$_{2.5}$ values, ranging from the $1^{st}$ to the $99^{st}$ percentile of the entire cohort all-source PM$_{2.5}$ distribution. The top plots represent the hazard ratio of mortality compared to the current NAAQS for annual all-source PM$_{2.5}$(9 $\mu g/m^3$) stratifying by poverty level. The lower plots represent the distribution of all-source PM$_{2.5}$ for the two poverty-based strata.
• Figure S1: Mortality hazard ratio with 95% confidence intervals and all-source PM$_{2.5}$ distribution stratifying by region. The plots illustrate the exposure-response curve and distribution of all-source PM$_{2.5}$ values stratifying by region, ranging from the $1^{st}$ to the $99^{st}$ percentile of the entire cohort all-source PM$_{2.5}$ distribution. The top plots represent the hazard ratio of mortality compared to the current NAAQS for annual all-source PM$_{2.5}$(9 $\mu g/m^3$) stratifying by region. The lower plots represent the distribution of all-source PM$_{2.5}$ for each region.