Changing heat-related mortality in the United States.

Heat is the primary weather-related cause of death in the United States. Increasing heat and humidity, at least partially related to anthropogenic climate change, suggest that a long-term increase in heat-related mortality could occur. We calculated the annual excess mortality on days when apparent temperatures--an index that combines air temperature and humidity--exceeded a threshold value for 28 major metropolitan areas in the United States from 1964 through 1998. Heat-related mortality rates declined significantly over time in 19 of the 28 cities. For the 28-city average, there were 41.0 +/- 4.8 (mean +/- SE) excess heat-related deaths per year (per standard million) in the 1960s and 1970s, 17.3 +/- 2.7 in the 1980s, and 10.5 +/- 2.0 in the 1990s. In the 1960s and 1970s, almost all study cities exhibited mortality significantly above normal on days with high apparent temperatures. During the 1980s, many cities, particularly those in the typically hot and humid southern United States, experienced no excess mortality. In the 1990s, this effect spread northward across interior cities. This systematic desensitization of the metropolitan populace to high heat and humidity over time can be attributed to a suite of technologic, infrastructural, and biophysical adaptations, including increased availability of air conditioning.

Decadal changes in summer mortality in U.S. cities.

Recent studies suggest that anthropogenic climate warming will result in higher heat-related mortality rates in U.S. cities than have been observed in the past. However, most of these analyses assume that weather-mortality relationships have not changed over time. We examine decadal-scale changes in relationships between human mortality and hot, humid weather for 28 U.S. cities with populations greater than one million. Twenty-nine years of daily total mortality rates, age-standardized to account for underlying demographic changes, are related to afternoon apparent temperatures ( T(a)) and organized by decade for each city. Threshold T(a) values, or the T(a) at and above which mortality is significantly elevated, are calculated for each city, and the mortality rates on days when the threshold T(a) was exceeded are compared across decades. On days with high T(a), mortality rates were lower in the 1980s and 1990s than in the 1960s and 1970s in a majority of the cities. Regionally, northeastern and northern interior cities continue to exhibit elevated, albeit reduced, death rates on warm, humid days in the 1980s and 1990s, while most southern cities do not. The overall decadal decline in mortality in most cities is probably because of adaptations: increased use of air conditioning, improved health care, and heightened public awareness of the biophysical impacts of heat exposure. This finding of a more muted mortality response of the U.S. populace to high T(a) values over time raises doubts about the validity of projections of future U.S. mortality increases linked to potential greenhouse warming.