Background
Loss of life during and soon after a heat wave is often followed by a period of reduced mortality, which suggests that people who succumb to heat stress would have died within a very short time even in the absence of unseasonably high air temperatures.  "For these people," in the words of the authors, "the added stress of heat may be merely the final physiological insult that precipitates death," a pattern that is typically observed in high-income populations, where deaths are dominated by chronic cardio-respiratory diseases in the elderly.

But what about low-income countries, where a high proportion of deaths are at younger ages and where infections and acute respiratory disease are common?  In these situations one might expect less short-term displacement of deaths and greater mortality in children who in the absence of undue heat stress would have lived much longer.

What was done
To test this hypothesis, Hajat et al. studied the history of heat-wave related deaths in three cites of contrasting wealth (gross national income per capita) - Delhi (India), Sao Paulo (Brazil) and London (England) - based on daily counts of all-cause mortality (excluding violent deaths) plus counts by age and cause-of-death, which were derived from mortality registries for the four-year period January 1991 to December 1994.  This they did by examining "time-series of daily mortality data in relation to daily ambient temperature using Poisson models and adjusting for season, relative humidity, rainfall, particulate air pollution, day of the week and public holidays," using "unconstrained distributed lag models to identify the extent to which heat-related excesses were followed by deficits (mortality displacement)."

What was learned
For each city, an increase in all-cause mortality was observed for same-day and previous day temperatures greater than 20°C, with the excess being greatest in Delhi and smallest in London.  In Delhi, the excess of deaths persisted for three weeks, while in London they prevailed for only two days and were followed by deficits that led to the sum of the two effects being zero by day eleven.  In Sao Paulo, as might have been expected, the pattern of deaths was intermediate between these two extremes.  Summed over the course of 28 days, the risk of death associated with heat stress was 2.4% per degree greater than 20°C in Delhi, 0.8% in Sao Paulo and a negative 1.6% in London.

What it means
Hajat et al. conclude that "populations in low-income countries where life-threatening infections are still common may have the greatest vulnerability to the effects of heat," and they say that "those most susceptible to heat are likely to remain susceptible if there is not due attention paid to infectious disease, diarrheal illness, and other major causes of early mortality in these poor populations."  We agree with these assessments and suggest that the best way to provide the attention Hajat et al. say is lacking is to not hamper the abilities of these countries to grow their economies in the same way we grew ours, i.e., via the use of abundant fossil fuels, which can now be done much more cleanly than was the case when we did it, and which can simultaneously provide a wealth of other benefits.