The changing risk and burden of wildfire in the United States

Abstract

Recent dramatic and deadly increases in global wildfire activity have increased attention on the causes of wildfires, their consequences, and how risk from wildfire might be mitigated. Here we bring together data on the changing risk and societal burden of wildfire in the United States. We estimate that nearly 50 million homes are currently in the wildland-urban interface in the United States, a number increasing by 1 million houses every 3 y. To illustrate how changes in wildfire activity might affect air pollution and related health outcomes, and how these linkages might guide future science and policy, we develop a statistical model that relates satellite-based fire and smoke data to information from pollution monitoring stations. Using the model, we estimate that wildfires have accounted for up to 25% of PM_2.5 (particulate matter with diameter <2.5 μm) in recent years across the United States, and up to half in some Western regions, with spatial patterns in ambient smoke exposure that do not follow traditional socioeconomic pollution exposure gradients. We combine the model with stylized scenarios to show that fuel management interventions could have large health benefits and that future health impacts from climate-change-induced wildfire smoke could approach projected overall increases in temperature-related mortality from climate change-but that both estimates remain uncertain. We use model results to highlight important areas for future research and to draw lessons for policy.

Keywords: air pollution; climate change; health impacts; wildfire.

Fig. 1.

Trends in the drivers and consequences of wildfire. (A and B) Increases in burned area in public and private US lands (A) (1) have been driven in part by rising fuel aridity, shown here over the western United States (4) (B). (C and D) The number of homes in the WUI has also risen quickly (C, our calculations; SI Appendix), which has contributed to rising suppression costs (D) incurred by the federal government. (E) Prescribed burn area has increased substantially in the South but is flat in all other regions (1). (F and G) Smoke days have increased throughout the United States (F), perhaps undermining decadal improvements in air quality across the United States (G). (H) We calculate an increasing proportion of overall ${\mathrm{P}\mathrm{M}}_{2.5}$ attributable to wildfire smoke, particularly in the West. Red and blue lines in each plot indicate linear fits to the historical data, with slopes reported in the upper left of each panel; all are significantly different from zero (P <0.01 for each), except for prescribed burn in regions outside the South. Red lines indicate underlying data are from published studies or government data, and blue lines indicate novel estimates from this paper.

Fig. 2.

The quantity, source, and incidence of wildfire smoke. (A and B) Average predicted micrograms per cubic meter of ${\mathrm{P}\mathrm{M}}_{2.5}$ attributable to wildfire smoke in 2006 to 2008 and 2016 to 2018, as calculated from a statistical model fitting satellite-derived smoke plume data. (C) Share of smoke originating outside the United States, June to September 2007 to 2014 (calculated from ref. 13), with a substantial amount of smoke in the Northeast and Midwest originating from Canadian fires and about 60% of smoke in the Northeast originating outside the country; nationally, $\sim$11% of smoke is estimated to originate outside the country. (D) The share of smoke originating in the western United States, June to September 2007 to 2014. Smoke originating in the western United States accounts for 54% of the smoke experienced in the rest of the United States. (E and F) Racial exposure gradients are opposite for particulate matter from smoke compared to total particulate matter: Across the coterminous United States, counties with a higher population proportion of non-Hispanic whites have lower average particulate matter exposure but higher average ambient exposure to particulate matter from smoke (P <0.01 for both relationships).

Fig. 3.

Health consequences of changes in smoke exposure depend on the assumed dose–response function and on the magnitude of management- or climate-driven changes in smoke. (A) Distributions of ${\mathrm{P}\mathrm{M}}_{2.5}$ for all grid-cell years in the contiguous United States, 2006 to 2018, under several stylized wildfire management strategies and climate change scenarios (see SI Appendix for details). The baseline distribution of total predicted ${\mathrm{P}\mathrm{M}}_{2.5}$ from all sources is in black. Gray distributions show alternative scenarios in which the timing and/or amount of overall smoke-related ${\mathrm{P}\mathrm{M}}_{2.5}$ is altered through management interventions or increased due to climate, including the (hypothetical) full elimination of smoke ${\mathrm{P}\mathrm{M}}_{2.5}$. (B and C) Annual number of avoided premature deaths in the US population age 65+ y for each management strategy, calculated by combining the ${\mathrm{P}\mathrm{M}}_{2.5}$ distributions in A with published long-run ${\mathrm{P}\mathrm{M}}_{2.5}$ exposure–response functions depicted in C (29, 31, 32).