Reductions in weather-related mortality over time and regional differences in mortality responses are related to several factors. Health care has continued to improve significantly since the 1960s owing to advances in medical knowledge (Goldman and Cook 1984; Seretakis et al. 1997). Urban planners and architects have increasingly factored summer relief from heat stress into their designs, including more shaded outdoor areas and ready access to potable water. Public health officials, government agencies, and the media have taken more proactive measures to address potential mortality threats on unusually hot and humid days, including the recent implementation of heat watch-warning systems (Kalkstein et al. 1996; McGeehin and Mirabelli 2001). Furthermore, human biophysical acclimatization to high temperatures could also play a role in reduced mortality, both within season (Greenberg et al. 1983; Kalkstein 1993; Marmor 1975; Seretakis et al. 1997) and over longer periods of time (Bonner et al. 1976; Frost and Auliciems 1993; Keatinge et al. 2000; Wyndham et al. 1976).
It is likely that air conditioning has been a critical factor in reducing heat-related mortality. Air conditioning has permeated many businesses, automobiles, and households over the last 20 years, especially in cooler regions where it had once been considered more of a luxury than a necessity (McGeehin and Mirabelli 2001). To date, it has been difficult to quantify the role of air conditioning in reducing mortality because of multiple, confounding factors. In one case-control study, Kilbourne et al. (1982) determined that access to air conditioning reduced heat stroke by 400%. In a large cohort study comparing households with and without air conditioning in the early 1980s, Rogot et al. (1992) identified a 42% lower death rate for air-conditioned households during hot months. Kalkstein (1993) estimated the impact of air conditioning by comparing mortality trends on days with "offensive" air masses (high mortality days in which air conditioning use would be maximum) versus all other days. For New York City, Kalkstein estimated a 21% reduction in mortality resulting from air conditioning use. Separate analyses of the impact of air conditioning on mortality during the 1995 Chicago heat wave indicate that moving from unventilated, indoor locations to air conditioning reduced the mortality risk of individuals by a factor of about 5-6 (Chan et al. 2001; Semenza et al. 1996). Although there is little disagreement that air conditioning reduces summer mortality rates, estimates of the actual impact on mortality rates vary markedly.
To examine the impact of air conditioning availability in more detail, we used data on the percentage of households with available air conditioners according to the Energy Information Agency (2003) for the years 1980, 1981, 1982, 1984, 1987, 1990, 1993, and 1997. Data on air conditioner use are available for nine census divisions covering the United States. We averaged the data from the available years within the period 1980-1989 and 1990-1998 to produce decadal mean values of air conditioner use within each of the nine regions. We compared these values with the annual excess heat-related mortality data for these two decades averaged across all of the MSAs within eight of the nine census regions (there were no cities with one of the regions).
In all regions except one, the mortality decline from the 1980s to the 1990s was coupled with increased air-conditioning penetration (Figure 4). The lone exception is the Mountain region, which includes the climatically dissimilar MSAs of Phoenix and Denver, each of which exhibited much different decadal mortality trends. Excluding the Mountain region, on average for U.S. cities, excess mortality was reduced by 1.14 deaths/year (per standard million) for every percentage increase in home air conditioning availability. Overall, there is a fairly strong inverse relationship between air conditioning and mortality rates. Air conditioning saturation is almost complete in the West South Central, South Atlantic, and West North Central regions, where 10 of the 13 cities exhibited no threshold ATs in the 1990s. Mortality rates were highest in the Pacific and Northeast regions where air conditioning use has become more commonplace only recently. Given this general relationship, one would anticipate significant mortality declines until the time when 100% air-conditioning saturation is approached for the entire United States. Afterward, the net impacts of high heat and humidity on mortality remain an open question. But contemporary analyses should focus on cities in the southern U.S. regions where air conditioning is present in most homes. The impacts of heat waves on mortality there may provide some case studies of how future populations might respond to heat stress events under full air-conditioning saturation conditions.
This cursory analysis implicitly assumes that air conditioning completely accounts for the observed mortality changes. In this article, our goal is not to attribute the observed declines in heat-related deaths to specific causes. Air conditioning is one of the major factors, but other technologic and biophysical changes, including those outlined earlier in this discussion, will most likely have some influence as well.
There is evidence of an adaptation response in the spatial patterns of mortality declines. In the 1980s, most of the cities with no elevated mortality were in the southern United States where high summertime heat and humidity are common; for example, there was a lack of excess mortality in Phoenix and Houston, where temperatures and ATs can often reach very high levels. Apparently, the populace in and around these cities has largely adapted to these uncomfortable conditions, no doubt by incorporating a combination of the factors cited above. Through the 1990s, cities with no identifiable threshold ATs included several midwestern cities that were weather sensitive one decade earlier. This pattern suggests that adaptations to heat and humidity originally seen in the southeastern United States have spread northward (Davis et al. 2003). In effect, the mortality response in northern cities in the 1990s has become more like that seen in southern cities in the 1980s. The lack of mortality declines in the western United States, where ATs typically do not reach uncomfortable levels, remains a mystery. Possible confounding factors include the representativeness of the weather observation sites in MSAs that encompass mountainous terrain, changing demographics related to rapid immigration, and air quality impacts (Davis et al. 2003). It is perhaps noteworthy that the Pacific Coast and Mountain regions have the lowest percentage of residential air conditioning availability in the United States (Figure 4). Resolution of the western U.S. AT-mortality relationships remains a topic for future investigation.
Our analysis has not addressed the mortality impacts of weather variability. One current hypothesis is that individuals are stressed during the summer by significant temperature changes, particularly minimum temperatures. High minimum temperature variability has been linked to higher mortality rates in northeastern and northern interior cities (Chestnut et al. 1998; Kalkstein 1993, 2000). This observation could partially account for the spatial pattern of decadal mortality declines across the United States, because mortality rates in the 1990s remain elevated in the Northeast and West Coast, where summer temperature variability typically is higher because of air mass changes associated with more frequent frontal passages. In an effort to provide a cursory examination of possible impacts of variability on our observed mortality declines, we calculated the trends in the summer (June, July, and August average) 1600 hr LST AT standard deviation from 1964 through 1998. Only 3 of our 28 MSAs exhibited statistically significant (p 
0.05) trends, and the directions of the trends were inconsistent (increasing variability in Houston and New Orleans; declining variability in Minneapolis). Our findings indicate that temporal changes in variability have played little role in the observed mortality declines. With respect to possible future changes in temperature variability that might result from a warming climate, Robeson (2002) examined the relationship between mean air temperature and air temperature variance across the United States. In general, Robeson found increasing temperatures to be associated with reduced temperature variance. In the summer, this relationship is statistically significant for minimum temperatures in the southeastern quarter of the United States and for maximum temperatures in the western interior regions. Very few significant positive mean-variance relationships were observed. These results suggest that, given a background warming, air temperature variance should generally decline across the United States, a hypothesis supported by Michaels et al. (1998) in their analysis of July maximum and minimum temperatures. However, the fundamental question of the differential mortality impact of prolonged exposure to high heat and humidity compared with highly variable weather conditions remains unresolved.
Finally, there appears to be no relationship between temporal climate trends in AT and mortality responses (Table 2). This calls into question the utility of efforts linking climate change forecasts to future mortality responses in the United States (Chestnut et al. 1998; Kalkstein and Greene 1997; NAST 2000). Most of these and similar projections implicitly assume that the historical relationship between AT and mortality is constant. However, this and related research suggest that adaptations (in all forms) preclude the assumption of a stationary time series; therefore, any projections of future mortality rates linked to climate change must explicitly account for temporal changes in heat-related death rates (Davis et al. 2002, 2003). We intentionally did not attempt to account for temporal changes in the urban heat island, as is common in many climatological studies. We hoped to use ATs that were representative of the ambient conditions experienced by the populace within each MSA. The observed trends in AT (Figure 2) are likely related to a variety of causes, including increasing greenhouse gas levels, urbanization effects, land use changes, and simple natural climate variability. Regardless of the cause of the observed changes in background heat and humidity, the pattern of changes is unrelated to the observed reductions in mortality.
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