**Patrick L. Kinney**

Department of Environmental Health, Boston University School of Public Health, 715 Albany Street, Talbot 4W, Boston, MA 02118, USA; pkinney@bu.edu; Tel.: +1-617-358-2469

Received: 30 May 2018; Accepted: 21 September 2018; Published: 18 October 2018

**Abstract:** High temperatures have large impacts on premature mortality risks across the world, and there is concern that warming temperatures associated with climate change, and in particular larger-than-expected increases in the proportion of days with extremely high temperatures, may lead to increasing mortality risks. Comparisons of heat-related mortality exposure-response functions across different cities show that the effects of heat on mortality risk vary by latitude, with more pronounced heat effects in more northerly climates. Evidence has also emerged in recent years of trends over time in heat-related mortality, suggesting that in many locations, the risk per unit increase in temperature has been declining. Here, I review the emerging literature on these trends, and draw conclusions for studies that seek to project future impacts of heat on mortality. I also make reference to the more general heat-mortality literature, including studies comparing effects across locations. I conclude that climate change projection studies will need to take into account trends over time (and possibly space) in the exposure response function for heat-related mortality. Several potential methods are discussed.

**Keywords:** heat-related mortality; climate change; trends over time
