Heat wave mortality in large U. S. cities

Extreme heat and humidity has been identified as the leading weather-related cause of death in the United States. However, despite rising temperature and humidity in major U. S. metropolitan centers over the course of the past 40 years—a result of urbanization and climate change—we have documented in prior research a decline in the sensitivity to extreme heat in U. S. urban populations. We conclude from these observations that the processes of adaptation, both technological and sociological, have more than compensated for increasing weather stress and further suggest that the heat-related mortality impacts of future climate changes in the United States will be minimal.

However, our previous work examined only the impacts of high heat and humidity on daily mortality and made no attempt to delineate or explore the impacts of extended heat events. As the impacts of “heat waves” may be significantly different from those resulting simply from isolated extremely hot days, in this work, we explored the relationship between mortality and temperature within extended high heat and humidity events.

We acquired daily mortality counts for 28 major U. S. cities from National Center for Health Statistics archives from 1964-98 and standardized by age and population size. These mortality rates were compared to morning apparent temperature for each city. "Heat waves" were defined as four or more consecutive days of above normal apparent temperatures in which at least one day exceeded the mean by at least one standard deviation. "Mortality events" were defined as those instances when the total mortality within a heat wave exceeded the expected mortality by 1.5 standard deviations or more. Mortality and heat events were then analyzed to determine the extent to which they coincided temporally and as to whether these relationships systematically changed over our period of record.

Generally, we found that while heat waves are still a public health concern, higher heat and humidity is necessary to elicit a mortality response now than 30-40 years ago.