Heat is the main weather-related cause of death in the United States. Observations of increasing heat and humidity and comparable future forecasts have led to projections of increased future mortality in U.S. cities on hot, humid days. Prior and ongoing research by the authors have demonstrated that adaptations have resulted in decadal-scale declines in mortality on hot, humid days despite increasing heat stress. However, this prior research did not examine the impact of heat waves—consecutive days of stressful conditions. Physiological studies demonstrate that heat mortality is often linked to the human body’s inability to recover during a sequence of consecutive, stressful days. Our goal is to examine the changing mortality response to heat waves over time in a sample of large U.S. cities.

Daily mortality counts were collected from National Center for Health Statistics archives from 1964–1998 and age-standardized to account for demographic changes. Six major U.S. metropolitan areas were examined: Chicago, Dallas, Minneapolis, Philadelphia, Phoenix, and St. Louis. These locations were selected because prior research demonstrated differential mortality responses to heat stress. Surface synoptic temperature and dew point temperature observations were collected for each city and morning and afternoon apparent temperatures were computed for comparison with daily mortality counts. Both data sets were temporally filtered to identify both heat waves and mortality episodes relative to the seasonally-varying baselines. Heat waves were identified using two methods: 1) consecutive day periods of high apparent temperatures and 2) consecutive day periods of above normal mortality during the warm season. By examining both the reputed “cause” and response, it is possible to better refine the relationships between variables.

Preliminary results suggest that heat wave impacts have been declining temporally in most cities. Relatedly, the apparent temperature threshold associated with heat wave deaths has risen over time. These results suggest that various adaptations, including, but not limited to, increased air conditioning penetration, has effectively mitigated against heat-wave mortality in many cases. Furthermore, there is some evidence for a “mortality displacement” effect in which mortality is substantially reduced on days following major heat wave mortality, but this response is neither temporally consistent nor is it comparable between cities. Ongoing and future research is designed to better refine changes in seasonal lagged relationships between heat and mortality, the net impact of mortality displacement on total mortality counts during heat waves, and possible impacts of high heat and humidity in locations were air conditioning saturation is almost complete.