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finding 2.2 : even-small-differences-seasonal-average-temperatures-result-illness-death
Days that are hotter than usual in the summer or colder than usual in the winter are both associated with increased illness and death [Very High Confidence]. Mortality effects are observed even for small differences from seasonal average temperatures [High Confidence]. Because small temperature differences occur much more frequently than large temperature differences, not accounting for the effect of these small differences would lead to underestimating the future impact of climate change [Likely, High Confidence].
This finding is from chapter 2 of The Impacts of Climate Change on Human Health in the United States: A Scientific Assessment.
Process for developing key messages: The chapter was developed through technical discussions of relevant evidence and expert deliberation by the report authors at several workshops, teleconferences, and email exchanges. The authors considered inputs and comments submitted by the public, the National Academies of Sciences, and Federal agencies. For additional information on the overall report process, see Appendices 2–3.
The content of this chapter was determined after reviewing the collected literature. The authors determined that there was substantial literature available to characterize both observed and projected mortality from elevated temperatures, with sufficient literature available to also characterize mortality from cold as well as cold-related hospitalizations and illness. Populations of concern were also considered to be a high priority for this chapter. As discussed in the chapter, there were limitations in terms of the state of the literature on understanding how future adaptation will influence climate-related changes in temperature-related mortality, addressing the impact of temperature on rural populations, and examining health-related endpoints other than mortality and morbidity.
Description of evidence base: Two well-recognized conclusions from the literature are that extreme temperatures lead to illness and premature death and that these extreme temperatures are best described in relation to local average seasonal temperatures rather than absolute temperature values. Epidemiological studies find an increase in mortality at temperatures that are high related to the local average.a6714dce-b324-4324-a88e-d31d31fa2d95 ad5fb3ba-9924-4df9-a68f-1e94822f78f9 bdea0759-701d-4183-9966-cee3ce977e08 32af3968-aefd-4a28-810f-aed7277d6f3a b9525432-c24b-4aee-9b0f-1783af53f48d 48d08f16-c2bb-4591-831d-22c262d869fb Based on absolute temperatures, Anderson and Bell 2011 found that cities in the South and Southeast were the least sensitive to heat, demonstrating acclimatization.a6714dce-b324-4324-a88e-d31d31fa2d95
Illness has been linked with hot daily average temperature6dd31085-f435-4888-bc08-d55718abd744 7be7dd5f-5970-45ec-a14c-1fed7851517b ac0d1490-bc44-4738-af3d-90d6a499dcbe c3c2dea7-0420-4f9a-b18b-41ac5641255a ff5550b2-6d4a-436c-866f-232c6d8f7ad6 and apparent temperature, among other metrics.da62c30f-01b4-4dc6-b3c6-197bd77eaf30 6b8418a6-978f-4196-8c50-c8ce246910ad 176b5759-b07b-4209-b8ff-8c55768c6aee 9e5154e8-cddf-4c83-ac36-1c43a2d88d7b Across studies, adverse health episodes were most strongly associated with exposures to high temperatures that occurred on the same day or the previous day.da62c30f-01b4-4dc6-b3c6-197bd77eaf30 ac0d1490-bc44-4738-af3d-90d6a499dcbe However, a cumulative effect of heat was also observed at periods of up to one week after exposure, tapering off beyond seven days.c3c2dea7-0420-4f9a-b18b-41ac5641255a 3735f126-9595-4fee-a491-0510b8fc9cd2 Cardiovascular and respiratory illness has been most commonly examined in relation to extreme heat, but the association is more varied for illness than for mortality due to effects across age groupsc3c2dea7-0420-4f9a-b18b-41ac5641255a c7fdbd26-6a4b-48e4-a94e-56cf436e0b2fand differences in morbidity risk associated with emergency room records versus hospital admissions.6dd31085-f435-4888-bc08-d55718abd744 7be7dd5f-5970-45ec-a14c-1fed7851517b 7ca0e947-163a-46f3-9274-cea209b94510 ac0d1490-bc44-4738-af3d-90d6a499dcbe 6b8418a6-978f-4196-8c50-c8ce246910ad 9a85ffc5-67e9-465a-8af8-d82b15d5c98b 176b5759-b07b-4209-b8ff-8c55768c6aee c3c2dea7-0420-4f9a-b18b-41ac5641255a c7fdbd26-6a4b-48e4-a94e-56cf436e0b2f
The evidence for mortality is clearest for extreme temperatures, as addressed in threshold-based studies,bdea0759-701d-4183-9966-cee3ce977e08 but studies that account for smaller changes in temperature found mortality changes even for small deviations of temperature.1dd78be0-0355-46a2-81b7-9a55154cc564 e805bfdc-c4c2-43a0-b2e5-5a66945c74e4 This is consistent with studies showing a U-shaped relationship of temperature and mortality—while there may be some plateau near the optimal temperature, the plateau is often small, and not always coincident with the seasonal average temperature.1dd78be0-0355-46a2-81b7-9a55154cc564 7513f4d2-671e-4a73-b3a3-cd354fa62c29 However, some of the individuals who die in response to elevated temperatures were already near death, and so the temperature event is sometimes considered to have “displaced” the death by a matter of days rather than created an additional death. Studies have found that this effect is generally below 50% of the total deaths, and is much smaller than that (10% or less) for the most extreme events, such as the 2003 European heat wave.bdea0759-701d-4183-9966-cee3ce977e08 8fcb835f-34bc-483b-ace3-650ef439b7b4 2cee671a-e17f-4e66-b37d-0c29a35f7210 6d040a6e-0001-4475-81a6-06a198e63b28 In contrast, one recent study found that in seven U.S. cities mortality displacement was greater than 80% for small temperature deviations and around 50% even for the 3% of warmest events in the study sample.eaabcdba-02ea-478b-899b-d0924862128b
New information and remaining uncertainties: This finding reflects consideration of a number of recent studiesb9525432-c24b-4aee-9b0f-1783af53f48d 6b3cd0ec-1e3e-42e8-ad82-5c12ed7ab0e8 not referenced in the recent 2014 NCA.61fd6e32-63d0-4f5a-bbbb-f68262376a37 There is a consensus of studies linking extreme temperatures and mortality, and a growing body of literature demonstrating that smaller differences in temperature are also linked with mortality. However, the mortality displacement effect, and the fact that deaths that do not occur during an identified heat wave are less likely to be directly attributed to extreme heat, contribute to continuing uncertainty about the magnitude of the effect of temperature on mortality.
Assessment of confidence based on evidence: There is very high confidence in the relationship between extreme temperatures and premature deaths due to the consistency and strength of the literature, particularly given the different study designs that produce this result. There is high confidence that small temperature deviations from normal temperatures contribute to premature mortality due to high agreement among those studies that have examined the issue. Though some studies indicate that for these small temperature differences, mortality displacement may play a larger role than for more extreme temperatures. Fewer studies have examined the role of these smaller temperature differences in projections, but the directionality of the effect is clear, so the determination of the authors was that not including this effect would likely lead to an underestimate of future mortality, with high confidence.
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- Heat, heat waves, and hospital admissions among the elderly in the United States, 1992–2006 (da62c30f)
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- Association of summer temperatures with hospital admissions for renal diseases in New York State: A case-crossover study (ff5550b2)
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