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Heat-Related Morbidity and Mortality

Extreme Heat Media Toolkit

Impacts on Risk

Both increased average temperatures and increasingly frequent and severe extreme heat events produce increased risks of heat-related illness and death that can be significant The European heat wave of 2003 caused more than 35,000 excess deaths. Human susceptibility to heat-related illness depends on several different factors, from physiologic adaptation to the local environment to socioeconomic status, and the impact of these changing exposures will depend on the vulnerability of exposed populations. As noted above, host factors such as age and the burden of other serious illnesses such as heart disease and diabetes that might exacerbate heat-related problems are important. In the United States, the number of individuals 65 years of age and older (who are more susceptible to heat effects) is expected to increase from 12.4% in 2000 to 20% in 2060. Socioeconomic factors also determine vulnerability; economically is advantaged and socially isolated people face higher burdens of death from heat. Cities and climate are co-evolving in a manner that will certainly amplify both the health effects of heat and the vulnerability of urban populations to heat-related deaths by magnifying the increased temperatures caused by climate change as compared to adjacent rural and suburban locales. The urban built environment can both exacerbate and alleviate the effects of heat. For example, high concentrations of buildings in urban areas cause what is known as the urban heat island effect: generating as well as absorbing and releasing heat, resulting in urban centers that are several degrees warmer than surrounding areas. Expanding parks and green spaces and increasing the density of trees in and around cities can help to reduce this effect. It is estimated that 60% of the global population will live in cities by 2030, greatly increasing the total human population exposed to extreme heat. Researchers comparing annual heat-related deaths for the city of Los Angeles in the 1990s to those projected for the mid- and late 21st century have concluded that heat-related deaths will increase, perhaps up to seven-fold. Another study assessing 21 U.S. cities estimates that for most of the cities, summer deaths will increase dramatically and winter deaths will decrease slightly, even with acclimatization. This shift to higher summer heat-related deaths will likely outweigh the extra winter deaths averted. Climate change is projected to increase the average number of summertime heat-related deaths, with the greatest increases occurring in mid-latitude major cities where summer climate variability is greatest. Noting that the number of current heat-related deaths in U.S. cities is considerable in spite of mortality displacement (reduced mortality in the months following a heat event due to increased early deaths of critically ill people who would have died in the near-term regardless) and the increased use of air conditioning, a substantial rise in weather-related deaths is the most likely direct health outcome of climate change. It is difficult to make valid projections of heat-related illness and death under varying climate change scenarios. A review of past changes in heat-related deaths found few significant relationships for any decade or demographic group, and suggested that improved medical care, air conditioning use, and other adaptation efforts were the causes of reduced death, stating that despite increasing stressful weather events, heat–related deaths are preventable, as evidenced by the decline of all-cause mortality during heat events over the past 35 years. Overall, research suggests that under a climate change scenario using current anthropogenic emissions trends, there will be a small increase in the overall U.S. heat-related death rate by the end of the 21st century. A standardized definition and methodology for identifying heat-related health outcomes is needed for surveillance and to evaluate temperature-related illness and death.

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