Climate Change Indicators: Heat-Related Deaths

About the Data

Indicator Notes

Several factors influence the ability of this indicator to estimate the true number of deaths associated with extreme heat events. It has been well documented that many deaths associated with extreme heat are not identified as such by the medical examiner and might not be properly coded on the death certificate. In many cases, the medical examiner might classify the cause of death as a cardiovascular or respiratory disease, not knowing for certain whether heat was a contributing factor, particularly if the death did not occur during a well-publicized heat wave. Furthermore, deaths can occur from exposure to heat (either as an underlying cause or as a contributing factor) that is not classified as extreme and therefore is often not recorded as such. Some statistical approaches estimate that more than 1,300 deaths per year in the United States are due to extreme heat, compared with about 750 deaths per year in the “underlying and contributing causes” data set shown in Figure 1.⁴ By studying how daily death rates vary with temperature in selected cities, scientists have found that extreme heat contributes to far more deaths than the official death certificates might suggest.¹⁶ This is because the stress of a hot day can increase the chance of dying from a heart attack, other heart conditions, or respiratory diseases such as pneumonia.¹⁷ These causes of death are much more common overall than dying directly from a heat-related illness such as heat stroke. Thus, this indicator very likely underestimates the number of deaths caused by exposure to heat.

Classifying a death as “heat-related” does not mean that high temperatures were the only factor that caused or contributed to the death, as pre-existing medical conditions can significantly increase an individual’s susceptibility to heat. Other important factors, such as the overall vulnerability of the population, the extent to which people have adapted and acclimated to higher temperatures, and the local climate and topography, can affect trends in heat-related deaths. Heat response measures, such as early warning and surveillance systems, air conditioning, health care, public education, cooling centers, infrastructure standards, and air quality management, can also make a big difference in reducing death rates. For example, after a 1995 heat wave, the city of Milwaukee developed a plan for responding to extreme heat conditions; during a 1999 heat wave, heat-related deaths were roughly half of what would have been expected.¹⁸

Future development related to this indicator should focus on capturing all heat-related deaths, not just those with a reported link to heat stress.

Starting in 2018, the source data began to provide separate counts for people who belong to more than one racial group. Prior to 2018, some of these individuals might have been classified as Black. This change has a small effect on the “non-Hispanic Black people” data in Figure 2, but not enough to change the shape of the line or the conclusions that can be drawn. The technical documentation for this indicator explains the change in more detail.

Data Sources

Data for this indicator were provided by the U.S. Centers for Disease Control and Prevention (CDC). The underlying cause data in Figure 1 are publicly available through the CDC WONDER database at: The contributing factor analysis in Figure 1 was developed by CDC’s Environmental Public Health Tracking Program, which provides a summary at: www.cdc.gov/nceh/tracking. The cardiovascular disease data in Figure 2 are publicly available through the CDC WONDER database at:

Technical Documentation

References

¹ Marvel, K., Su, W., Delgado, R., Aarons, S., Chatterjee, A., Garcia, M. E., Hausfather, Z., Hayhoe, K., Hence, D. A., Jewett, E. B., Robel, A., Singh, D., Tripati, A., & Vose, R. S. (2023). Chapter 2: Climate trends. In USGCRP (U.S. Global Change Research Program), Fifth National Climate Assessment.

² Hayden, M. H., Schramm, P. J., Beard, C. B., Bell, J. E., Bernstein, A. S., Bieniek-Tobasco, A., Cooley, N., Diuk-Wasser, M., Dorsey, M. K., Ebi, K., Ernst, K. C., Gorris, M. E., Howe, P. D., Khan, A. S., Lefthand-Begay, C., Maldonado, J., Saha, S., Shafiei, F., Vaidyanathan, A., & Wilhelmi, O. V. (2023). Chapter 15: Human health. In USGCRP (U.S. Global Change Research Program), Fifth National Climate Assessment.

³ Chu, E. K., Fry, M. M., Chakraborty, J., Cheong, S.-M., C. Clavin, Coffman, M., Hondula, D. M., Hsu, D., Jennings, V. L., Keenan, J. M., Kosmal, A., Muñoz-Erickson, T. A., & Jelks, N. T. O. (2023). Chapter 12: Built environment, urban systems, and cities. In USGCRP (U.S. Global Change Research Program), Fifth National Climate Assessment.

⁴ Sarofim, M. C., Saha, S., Hawkins, M. D., Mills, D. M., Hess, J., Horton, R., Kinney, P., Schwartz, J., & St. Juliana, A. (2016). Chapter 2: Temperature-related death and illness. In USGCRP (U.S. Global Change Research Program), The impacts of climate change on human health in the United States: A scientific assessment (pp. 43–69).

⁵ Zanobetti, A., O’Neill, M. S., Gronlund, C. J., & Schwartz, J. D. (2012). Summer temperature variability and long-term survival among elderly people with chronic disease. Proceedings of the National Academy of Sciences of the United States of America, 109(17), 6608–6613.

⁶ USGCRP (U.S. Global Change Research Program). (2016). The impacts of climate change on human health in the United States: A scientific assessment (A. Crimmins, J. Balbus, J. L. Gamble, C. B. Beard, J. E. Bell, D. Dodgen, R. J. Eisen, N. Fann, M. D. Hawkins, S. C. Herring, L. Jantarasami, D. M. Mills, S. Saha, M. C. Sarofim, J. Trtanj, & L. Ziska, Eds.).

⁷ Berko, J., Ingram, D. D., Saha, S., & Parker, J. D. (2014). Deaths attributed to heat, cold, and other weather events in the United States, 2006–2010 (National Health Statistics Reports, Number 76). National Center for Health Statistics. www.cdc.gov/nchs/data/nhsr/nhsr076.pdf

⁸ CDC (U.S. Centers for Disease Control and Prevention). (2024). CDC WONDER database: All ages deaths by underlying cause [Data set]. Retrieved May 22, 2024, from

⁹ CDC (U.S. Centers for Disease Control and Prevention). (2024). Indicator: Heat-related mortality (Annual national totals provided by National Center for Environmental Health staff in June 2024) [Data set]. National Center for Health Statistics.

¹⁰ CDC (U.S. Centers for Disease Control and Prevention). (2024). CDC WONDER database: Multiple cause of death file [Data set]. Retrieved May 22, 2024, from

¹¹ Anderson, G. B., & Bell, M. L. (2011). Heat waves in the United States: Mortality risk during heat waves and effect modification by heat wave characteristics in 43 U.S. communities. Environmental Health Perspectives, 119(2), 210–218.

¹² CDC (U.S. Centers for Disease Control and Prevention). (1995). Heat-related mortality—Chicago, July 1995. Morbidity and Mortality Weekly Report, 44(31), 577–579.

¹³ NRC (National Research Council). (2011). Climate stabilization targets: Emissions, concentrations, and impacts over decades to millennia. National Academies Press.

¹⁴ CDC (U.S. Centers for Disease Control and Prevention). (2012). CDC WONDER database [Data set]. Retrieved August 1, 2012, from

¹⁵ NOAA (National Oceanic and Atmospheric Administration). (2012). National Centers for Environmental Information. Retrieved August 1, 2012, from www.ncei.noaa.gov

¹⁶ Medina-Ramón, M., & Schwartz, J. (2007). Temperature, temperature extremes, and mortality: A study of acclimatisation and effect modification in 50 US cities. Occupational and Environmental Medicine, 64(12), 827–833.

¹⁷ Kaiser, R., Le Tertre, A., Schwartz, J., Gotway, C. A., Daley, W. R., & Rubin, C. H. (2007). The effect of the 1995 heat wave in Chicago on all-cause and cause-specific mortality. American Journal of Public Health, 97(Suppl 1), S158–S162.

¹⁸ Weisskopf, M. G., Anderson, H. A., Foldy, S., Hanrahan, L. P., Blair, K., Török, T. J., & Rumm, P. D. (2002). Heat wave morbidity and mortality, Milwaukee, Wis, 1999 vs. 1995: An improved response? American Journal of Public Health, 92(5), 830–833. www.ncbi.nlm.nih.gov/pmc/articles/PMC1447169