The Relationship between Relative Extreme Temperature Events and Mortality in the United States

While there is ample evidence connecting extreme temperature events and human mortality across the world, most of this research has focused on the impacts of the hottest and coldest days of the year, which are generally confined to mid-summer and mid-winter. There has been far less research examining the association between health outcomes and cold and heat events that, while not the most extreme days of the year, are extreme relative to the time of year.

In this research, relative extreme temperature events are defined using the percentile thresholds relative to the climatology for a particular time of year. Using the recently defined Excess Heat Factor by Nairn and Fawcett, extreme heat events (EHE) and extreme cold events (ECE) are defined using absolute thresholds, and relative extreme heat events (REHE) and relative extreme cold events (RECE) using relative thresholds. REHE generally have a much broader seasonality than EHE, and are increasing in recent decades slightly more substantially.

All-cause mortality outcomes using a distributed lag nonlinear model are evaluated for the largest 51 metropolitan areas in the US for the period 1975-2010. Both the immediate impacts (0-day) as well as the cumulative 10-day and 20-day impacts are assessed for each of the extreme temperature event types.

For heat events, both relative and absolute, the greatest mortality increases occur with a 0-day lag, with the subsequent days showing below-expected mortality (harvesting) that decreases the overall cumulative impact. For EHE, increases in mortality are still statistically significant out to 10 days at around half of all metropolitan areas, whereas for REHE there are few statistically significant results left. Thus, for relative heat events (in spring in particular), it appears as though the vast majority of the increase in mortality is short-term displacement.

For cold events, both relative and absolute, there is little mortality increase on day 0, but the impacts increase in subsequent days. Overall cumulative impacts are statistically significant at more than half of the stations for both ECE and RECE. In examining monthly variability, in around a dozen cities, focused on the southeastern US, where early season RECE has the greatest magnitude increase in mortality of all cold events, suggesting that there may be a lack of acclimatization increasing mortality in early season (particularly December) events.