Changing heat-related mortality in the United States - Research Article

Environmental Health Perspectives, Nov, 2003 by Robert E. Davis, Paul C. Knappenberger, Patrick J. Michaels, Wendy M. Novicoff

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 [ or -] 4.8 (mean [ or -] SE) excess heat-related deaths per year (per standard million) in the 1960s and 1970s, 17.3 [ or -] 2.7 in the 1980s, and 10.5 [ or -] 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. Key words: air conditioning, apparent temperature, climate change, global warming, heat index, heat stress, human bioclimatology, human mortality, weather stress. Environ Health Perspect 111:1712-1718 (2003). doi: 10.1289/ehp.6336 available via http://dx.doi.org/[Online 23 July 2003]

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Heat waves are the most prominent cause of weather-related human mortality in the United States (Changnon et al. 1996). In northern U.S. cities, human mortality increases significantly on unusually hot and humid days (Bridger et al. 1976; Davis et al. 2002, 2003; Kalkstein and Davis 1989; Kalkstein and Greene 1997; Oechsli and Buechley 1970). Mortality increases are evident in total daily deaths as well as among the elderly subgroup (Applegate et al. 1981; Greenberg et al. 1983; Henschel et al. 1969; Jones et al. 1982; Kilbourne 1997; Kunst et al. 1993; Lye and Kamal 1977; Oechsli and Buechley 1970). Although a fraction of these deaths are directly attributable to heat, the majority are ascribed to causes of death not commonly considered to be weather related, such as circulatory and respiratory diseases (Bull and Morton 1978; Ellis et al. 1980; Keatinge et al. 1986; Larsen 1990a, 1990b). Increases in total and elderly mortality have also been associated with hot weather in Eurasia (Donaldson et al. 2003; Katsouyanni et al. 1993; Keatinge et al. 2000; Kunst et al. 1993; Laschewski and Jendritzky 2002; Nakai et al. 1999).

Atmospheric concentrations of human-produced greenhouse gases have increased significantly since the onset of the Industrial Revolution (Keeling and Whorf 1994). When the effects of the most important gases--carbon dioxide, methane, chlorofluorocarbons, ozone, and nitrous oxides--are combined, the current "effective" C[O.sub.2] concentration of approximately 450 ppm is more than 50% higher than the earth's natural, preindustrial background level and represents a 30% increase since 1960 (Houghton et al. 1990, 1996, 2001). Evaluations of global surface temperature histories, after accounting for urban warming biases and other influences, indicate that the globe has warmed approximately 0.67[degrees]C since 1900 (Folland and Parker 1995; Jones 1994). Some scientists argue that this increase is directly attributable to increasing greenhouse gas levels (Arrhenius 1896; Hansen et al. 1998; Houghton et al. 2001; Manabe and Wetherald 1975). Furthermore, based upon scenarios of future increases in greenhouse gas emissions, climate models estimate a globally averaged temperature rise of 1.4-5.8[degrees]C between now and the year 2100 (Boer et al. 2000; Boville et al. 2001; Houghton et al. 2001; Mitchell and Johns 1997; Stouffer and Manabe 1999). In the United States, the air temperature has increased 1.0[degrees]C since 1964 (the first year in this analysis), and model projections suggest 3-5[degrees]C of warming by 2100 [National Assessment Synthesis Team (NAST) 2000].

Given the historic linkage between high temperatures and death, these climate model temperature projections have led scientists and public health officials to forecast significant increases in mortality from greenhouse warming in the United States in the early twenty-first century (Chestnut et al. 1998; Gaffen and Ross 1998; Kalkstein and Greene 1997; NAST 2000). The ultimate impact of climate change will depend upon the extent of biophysical adaptations and the implementation of effective and widely available countermeasures (Chestnut et al. 1998; Donaldson et al. 2003; Kalkstein and Greene 1997; Keatinge et al. 2000; McGeehin and Mirabelli 2001; Seretakis et al. 1997). During the past several decades, the U.S. populace has been confronted with an increase in the annual number of heat-stress events, particularly in urban and suburban areas (Gaffen and Ross 1998). Projections of longer, more intense heat waves, more isolated hot days, higher minimum temperatures, and higher dew point temperatures arising from human influences on climate suggest a continuation of this trend. However, most, if not all, of the forecasts of increasing mortality are based on steady-state weather-mortality models that implicitly assume that weather-mortality relationships have not varied significantly over time. In contrast, we hypothesize here that mortality associated with warm and humid days has systemically declined over time (Davis et al. 2002, 2003; Donaldson et al. 2003).