| Devastating health effects from recent heatwaves, as well as projected changes in climate, highlight the importance of understanding the relationship between temperature and mortality. Many studies have linked weather to mortality; however, the role of such critical factors as regional variation, susceptible populations, acclimatization, and heatwave effects remain unresolved.;We applied time-series models to a national set of US communities, allowing a non-linear relationship between temperature and mortality. Second-stage analysis was used to relate cold, heat, and heatwave effects to community-specific variables. A hierarchical model was used to determine the impact of heatwave intensity, length, and timing on individual heatwave effects. We considered exposure timeframe, susceptibility by age and cause of death, and confounding from pollutants. We tested sensitivity to heatwave definition, temperature metric, and other modeling choices.;Heat-related mortality was associated with a shorter lag (average of same and previous day), with an overall increase in mortality risk of 2.56% (95% posterior interval: 1.98%--3.14%) comparing the 99th to 90th percentile temperatures for the community. Cold-related mortality was most associated with a longer lag (average of current day up to 25 days previous), with a 4.27% (3.24%--5.31%) increase in risk comparing the 1st to 10th percentile temperature for the community. Both heat and cold effects were robust to air pollution control. Overall, mortality risk increased 4.40% (3.24%--5.58%) on heatwave days compared to non-heatwave days, after controlling for the effects of single days of heat. Individual heatwaves that were more intense, longer, or earlier in the summer tended to have higher effects. Spatial heterogeneity in effects indicates that weather-mortality relationships from one community might not be relevant in another community. Larger spatial heterogeneity for absolute temperature estimates (comparing risk at specific temperatures) than for relative temperature estimates (comparing risk at community-specific temperature percentiles) provides evidence for acclimatization. We identified susceptibility based on age, socioeconomic conditions, urbanicity, and central air conditioning.;Heat, cold, and heatwaves all increase mortality risk nationally. Acclimatization, individual susceptibility, community characteristics, and individual heatwave characteristics all influence temperature-related effects on mortality. Findings have implications for decision-makers and researchers estimating health effects from climate change. |
