Examination of morbidity displacement associated with extreme heat events in North Carolina

Extreme heat is one of the leading causes of weather-related deaths and illnesses across the United States. Quantifying the public health burden associated with extreme heat has been the subject of considerable research in recent years. This type of information is useful in determining the resources to allocate for preparedness and intervention programs aimed at combating the health effects associated with extreme heat and anticipating how the burden might increase or decrease in the future based on changes in regional climate, population demographics, as well as behavioral and technological adaptations.

The timing of deaths and illnesses associated with extreme heat presents an interesting challenge when trying to accurately quantify the overall health impact. For example, examination of time series of deaths often reveals that, following the spike in mortality coinciding with an extreme heat event, mortality decreases on the subsequent days below seasonal averages. The hypothesis is that this “mortality deficit” occurs because some of the individuals whose deaths were expected to occur during this time period had their deaths brought forward in time (i.e. displaced) by several days or weeks by the extreme heat event. These deaths accordingly represent days to weeks of life lost instead of months or years, which have different implications for the total burden of heat as well as the need for responders and care providers to allocate additional resources at monthly and seasonal time scales.

A recent study by Saha et al. (2014) examined the relationship between mortality displacement and the severity of heat events for seven cities across the United States. They found that between 35 and 75 percent of the mortality associated with the most extreme heat events was a result of short-term displacement. There has been comparatively minimal effort to examine the temporal nature of heat-related morbidity, specifically with an emphasis on displacement effects. The numbers of cases of heat-related emergency department (ED) visits are far greater than the number of deaths and therefore represent a large portion of the public health burden associated with extreme heat. Similar to Saha et al. (2014), we aim to quantify the effects of short-term displacement on ED visits related to extreme heat using data from the North Carolina Disease Event Tracking and Epidemiologic Collection Tool (NC DETECT). We consider different definitions for extreme heat events for consistency with National Weather Service and public health protocol in North Carolina, as well as explore differences in displacement effects across various landscapes (e.g. urban-rural contrasts) and diagnosis codes (e.g. cardiovascular and respiratory diseases). Given the contrasting nature of mortality (binary and finite) and morbidity (continuous and indefinite), new contributions to the knowledge base are expected related to the methodological treatment of diagnosis codes and the potential for return visits to the ED. In addition, greater knowledge of the time pattern in heat-related morbidity and the cumulative effects of heat on ED visits will contribute to the growing body of knowledge regarding links between heat and health outcomes and help care providers more specifically understand how they can prepare for hot summer weather.

References Saha, M. V., Davis, R. E., & Hondula, D. M. (2014). Mortality Displacement as a Function of Heat Event Strength in 7 US Cities. American Journal of Epidemiology, 179(4), 467-474.