Background
The authors write that "the health impact from extreme heat events is likely to increase because of greater risks of high temperatures from climate change and urbanization and because of greater social vulnerability arising from factors such as an aging population," citing Luber and McGeehin (2008).

What was done
Working with apparent temperature (AT) datasets compiled by the National Climatic Data Center (NCDC) for 187 U.S. stations (NCDC, 2011), Grundstein and Dowd calculated trends in the frequency of days with extreme maximum ATs across the United States for the period 1949-2010.

What was learned
The two researchers from the University of Georgia (USA) discovered that about 20% of the stations had experienced positive trends in 1-day extreme maximum AT, mostly in the western United States; and they report that another important finding was that there had been a 63% increase in stations with positive trends in 1-day extreme maximum ATs since a similar study was coducted by Gaffen and Ross (1998), who had analyzed the period 1949-1995.

What it means
In spite of the fact that there had been a significant increase in the number of metropolitan stations experiencing 1-day extreme maximum ATs over the period 1949-2010, and in spite of the fact that that trend had accelerated over the period 1995-2010, Grundstein and Dowd report that Davis et al. (2003) had found that heat-related mortality in major metropolitan areas had actually declined over the 1961-1998 period. And they say that Davis et al. argued that "this reduction in mortality is related to reduced vulnerability, principally as a result of increased use of air conditioning but also because of factors such as improved health care, implementation of heat watch/warning systems, and human biophysical acclimatization." However, they also report that "Sheridan et al. (2009) found that the decline in heat-related mortality has stopped across the majority of major metropolitan areas since the mid-1990s," and they say that the latter scientists contend that "observed decreases in heat-related mortality may not continue as air conditioning has reached near saturation across much of the United States and rising energy costs may limit the use of air conditioning."

The moral of this story would thus seem to be that people may be able to persevere against nature; but when a misdirected national energy policy is added to the mix, it's pretty much a losing battle for many of the country's citizens.

References
Davis, R.E., Knappenberger, P.C., Michaels, P.J. and Novicoff, M. 2003. Changing heat-related mortality in the United States. Environmental Health Perspectives 111: 1712-1718.

Gaffen, D.J. and Ross, R.J. 1998. Increased summertime heat stress in the U.S. Nature 396: 529-530.

Luber, G. and McGeehin, M. 2008. Climate change and extreme heat events. American Journal of Preventive Medicine 35: 429-435.

NCDC. 2011. U.S. Heat Stress Index. National Climatic Data Center [Available online at http://www.ncdc.noaa.gov/temp-and-precip/heat-stress.html.]

Sheridan, S.C., Kalkstein, A.J. and Kalkstein, L.S. 2009. Trends in heat-related mortality in the United States, 1975-2004. Natural Hazards 50: 145-160.