What was done
Heat-related mortality in Massachusetts (USA) during the months of May through September from 1990 to 2008 was determined by the authors, based on (1) death certificate data from the Massachusetts Department of Public Health's Registry of Vital Records and Statistics, and on (2) daily maximum apparent temperature - a function of temperature and humidity - which was measured at 102 locations and interpolated across space via kriging and subsequently aggregated to produce 29 municipality groups (MGs), i.e., spatial units composed of municipalities that had minimal variation in population, employing a 0-day lag, because it produced the best correlation between apparent temperature and mortality anomalies.

What was learned
Hattis et al. report that average state-wide mortality anomalies were 5.11, 6.26 and 7.26 deaths on days exceeding the 85th, 90th and 95th percentiles of apparent temperature, respectively, which days they classified as hot, very hot and extremely hot, respectively. However, they write that these results "may be overstated," because "previous studies indicate that high apparent temperatures are associated with pollution episodes, which also impact mortality," citing Kovats and Hajat (2008), while further reporting that "a study of Mexico City found that controlling for air pollution and respiratory epidemics decreased the impact of apparent temperature on mortality by 50%," citing O'Neil et al. (2005). In addition, they found that a linear stepwise regression showed that the percent African-American population and the percent elderly population (those above the age of 65) were positively associated with an MG's mortality anomaly on days exceeding the 85th percentile of apparent temperature. And, last of all, they determined that "in spite of the urban heat island effect," their measure of urbanization "was not associated with higher rates of heat-related mortality."

What it means
In light of their several findings, the three researchers conclude that "at least in Massachusetts, an area's demographics may be more important to its heat-related mortality than its level of urbanization."

References
Kovats, R.S. and Hajat, S. 2008. Heat stress and public health; a critical review. Annual Review of Public Health 29: 41-55.

O'Neil, M., Hajat, S., Zanobetti, A., Ramirez-Aguilar, M. and Schwartz, J. 2005. Impact of control for air pollution and respiratory epidemics on the estimated associations of temperature and daily mortality. International Journal of Biometeorology 50: 121-129.