In general, over the past 35 years, the U.S. populace has become systematically less affected by hot and humid weather conditions. All-causes mortality during heat stress events has declined despite increasingly stressful weather conditions in many urban and suburban areas. This relative "desensitization" of the U.S. metropolitan populace to weather-related heat stress can be attributed to a variety of factors, including improved medical care, infiltration of air conditioning, better public awareness programs relating the potential dangers of heat stress, and both human biophysical and infrastructural adaptations. Thus, heat-related mortality in the United States seems to be largely preventable at present (McGeehin and Mirabelli 2001; Semenza et al. 1996). Public health officials and primary care physicians should warn their patients of the dangers associated with high heat and humidity. This is particularly true for the most susceptible groups, which include the elderly and individuals being treated for circulatory and respiratory conditions, diseases that have the highest mortality rates.
With respect to projections of future heat-related mortality that might arise from greenhouse-gas-induced warming, urban warming, or other factors, it is clear that these projections must incorporate the observed reductions in heat vulnerability. However, many questions remain with respect to future heat-mortality impacts. If air conditioning is indeed the main cause of the observed declines, once air conditioning penetration approaches market saturation, will a significant heat-mortality impact remain in the United States? Will air conditioning availability extend to all socioeconomic classes? What is the impact of cheap energy on air conditioning use, and will future changes in energy markets and pricing inadvertently encourage people to endanger themselves during heat waves? In addition, the role of human biophysical adaptations to changing climates should be considered. One current hypothesis is that residents in the Northeast are less acclimatized to summer heat and humidity because of its lack of persistence, compared with southern cities, where summer thermal variability is low. At present, future temperature variability is difficult to predict, but it could impact mortality rates. Current research suggests that, in most of the United States, summer variability should decline as temperatures increase. But overall, it is obvious that there is no simple association between increased heat wave duration or intensity and higher mortality rates in the United States.
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