Abstract
Warming and drying meteorological conditions associated with anthropogenic climate change have increased the risk of extreme heat and wildfire in many regions around the world. Extreme heat and wildfire smoke fine particulate matter (smoke PM 2.5 ) individually contribute to substantial global morbidity and mortality burdens, while emerging evidence suggests that co‐occurring heat and smoke events may have synergistic impacts that exacerbate adverse health outcomes. Despite the potential for a high societal burden, these co‐occurring events are an underexplored climate‐related hazard. To quantify the co‐occurrence of extreme heat and wildfire smoke, we combined daily estimates of heat index (a combination of temperature and relative humidity) and smoke PM 2.5 over the western United States to document the spatiotemporal patterns of the frequency, duration and intensity of individual and co‐occurring extreme events from 2006 to 2020. We found 130 million person‐days of exposure to co‐occurring exceedances over the 15‐year study period. These events were found most often in late summer when the temporal distributions of heat and smoke PM 2.5 exceedances typically coincide, and spatially from northern California to western Montana where extreme heat and smoke most often overlap. To the best of our knowledge, this study presents the largest database of population‐level exposure to co‐occurring extreme heat and wildfire smoke events. We also show that the specific definitions of extreme heat and smoke events can substantially affect both the number and spatial distribution of co‐occurring extreme events, with implications for future epidemiological or climate studies.