Antonella Zanobetti, principal research scientist in the Department of Environmental Health at the Harvard T.H. Chan School of Public Health, discusses a new study that found that people appear to adapt over time as temperatures creep higher, but also may face increased mortality risk from extreme temperature swings—and their level of risk may depend on where they live.

What did you discover that hadn’t been known before?

It’s long been known that there is an effect of both heat and cold on mortality and that heat-related mortality is lower in warmer U.S. cities, and cold-related mortality is lower in colder cities. But, until now, no one had looked at how temperature changes over time were affecting health across the nation.

We looked at the relationship between mortality and temperature by region, from the 1960s until now, to see how that relationship is changing. We looked at six different U.S. regions, grouping cities into those regions according to their seasonal temperatures and humidity. We found that, as average summer temperatures increased, the effect of very warm temperatures on mortality decreased. But we found the opposite effect with very cold temperatures—as average winter temperature increased, so did mortality. We also found that temperature-related mortality varied by region. For example, along the Pacific coast, the climate is milder than in other parts of the country, so when there’s an extreme temperature event there, people appear to be more susceptible.

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