Heat-related mortality has consistently declined on a decadal basis (Figure 2). In 19 of our 28 study cities, total annual heat-related (population-adjusted) mortality was statistically significantly lower in the 1990s than in our 1960s-1970s decade. On average, for the 28 cities, the number of excess deaths on hot and humid days declined from 41.0 ± 4.8 deaths/city/year (mean ± SE) in the 1960s-1970s, to 17.3 ± 2.7 in the 1980s, to only 10.5 ± 2.0 in the 1990s. Because 12 of the 28 cities showed no evidence of the existence of a threshold AT in the 1990s, mortality rates in these locations were unrelated to high ATs despite a widespread increase in summer ATs (Gaffen and Ross 1998).
The temporal and spatial patterns of excess deaths merit some attention. In the 1960s-1970s, every MSA except Tampa, Florida, exhibited statistically elevated mortality on hot and humid days. By the end of the 1980s, 6 of the 28 MSAs had no threshold AT (and thus no excess deaths), and an additional 11 locations showed statistically significant mortality declines relative to the 1960s-1970s. Thus, by the 1980s, mortality rates declined in 41% of the cities that had elevated mortality one "decade" earlier. Most of the cities with no elevated mortality in the 1980s are in the southern United States (Phoenix, AZ, Houston, TX, Miami, FL, Charlotte, NC, Norfolk, VA)--with Denver, Colorado, being the exception. Most of the cities with significant declines by the 1980s but with elevated death rates are in the northeastern quadrant of the United States. Through the 1990s, the general trend toward lower mortality rates continued. Dallas, Texas; Minneapolis, Minnesota; Kansas City, Missouri; St. Louis, Missouri; Cincinnati, Ohio; and Washington, DC, all exhibited no elevated death rates on high AT days in the 1990s, whereas deaths in Baltimore, Maryland, declined relative to the 1980s but remained significantly elevated. Therefore, by the end of our period of record, 12 of the 28 MSAs had no excess deaths linked to high ATs.
In general, cities along the southern tier of the United States, particularly in the Southeast, exhibited the weakest heat-mortality relationships. Most MSAs in the Northeast and Great Lakes regions have had mortality declines over time but still showed statistically significant mortality increases after high AT days in the 1990s. By contrast, none of the West Coast cities have seen significant mortality declines; in fact, excess death rates in Seattle, Washington, have actually increased compared with the 1960s-1970s. A few other locations exhibit mortality patterns that are outliers relative to neighboring cities, such as New Orleans, Louisiana; Atlanta, Georgia; and Buffalo, New York, where the decadal death rates are more comparable with West Coast locales.
Summertime trends in 1600 hr LST ATs from 1964-1998 exhibit statistically significant increases in 9 of the 28 cities (Figure 3). These increases, which can arise from a combination of higher temperatures and/or increasing humidity (Gaffen and Ross 1998, 1999; Knappenberger et al. 1996; Robinson 2000; Schwartzman et al. 1998), are concentrated in the southern United States. However, isolated increasing trends are also evident outside of this general region in San Francisco, California, and New York, New York. Of the remaining 19 MSAs, there is a general tendency toward higher ATs over time, although the regression slopes are not statistically significant and therefore are indistinguishable from no long-term change. Only two stations, Minneapolis and Kansas City, have decreasing summer ATs, and neither decline is significant. Given the general trend toward temporally increasing heat and humidity across most U.S. metropolitan areas, one would expect increasing heat stress to exposed individuals over time. This expectation should be tempered by regional adaptations in which individuals attempt to limit their exposure to high heat and humidity, so the number of exposed individuals should vary over time and space.
A comparison of the interactions between AT trends and mortality shows little overall pattern (Table 2). Of the nine MSAs with significantly increasing summer ATs, six had statistically significant mortality declines in the 1990s (relative to the 1960s-1970s), two showed no change (Atlanta and San Francisco), and one had no AT-mortality relationship over the entire period of record despite significantly increasing heat stress (Tampa). Both of the MSAs with declining ATs also exhibited mortality declines (Minneapolis and Kansas City).
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