LAUSR

Heat waves are one of the most lethal environmental hazards worldwide,1,2 and their frequency, intensity, and duration are increasing.3 There were >70,000 excess deaths during the 2003 European heat wave4 and >55,000 excess deaths during the 2010 heat wave in western Russia.5 In the United States, the 2021 Pacific Northwest heat wave overwhelmed emergency departments, where on the hottest day there were 1,038 emergency department visits for heat-related illness; in comparison, only 9 such visits occurred on the same day in 2019, in the absence of a heat wave.6 Owing to a variety of potential physical, psychiatric, and sociological factors, individuals age ≥65 y have the highest excess morbidity and mortality during heat waves.4,6,7 In some regions, temperature extremes are occurring even faster than projected just a few years ago.8,9 Thus, there is an urgent need for cheap, effective, and sustainable evidence-based cooling solutions to be deployed for those most at risk. Although in-home air conditioning (AC) is an obvious and effective countermeasure, its availability is region dependent; for example, only ∼ 5% of households in the United Kingdom are equipped with AC.10 Furthermore, the purchase and operating costs of units are often unaffordable for those most vulnerable to heat exposures,11,12 and widespread adoption of AC is a major contributor to greenhouse gas emissions.13 An alternative option for those who do not have access to in-home AC is to visit a cooling center, a cool site, or air conditioned building designated as a safe location during extreme heat.14 Although epidemiological evidence suggests that visiting a cooling center may reduce mortality risk,15 it is unclear whether that is due to direct effects of the cooling or simply due to population differences between those who can and cannot access these centers. For example, being bedbound is associated with a >6-fold risk during heat waves.15 In this issue of Environmental Health Perspectives, Meade et al.16 publish findings from a comprehensive study investigating primarily the thermal (rectal/core body temperature) response to a simulated 9-h heat wave in older adult volunteers (age range, 64–79 y). Using an environmental chamber set at 40°C air temperature and 9% relative humidity (heat index of 37°C), 20 control (no-cooling) participants rested in the heat for 9 h, whereas 20 (the “cooling group”) were provided a 2-h cooling break in 23°C air temperature between hours 4 and 6. In the control (nocooling) group, core temperature increased by ∼ 1:0 C from baseline and plateaued after 6 h. In the cooling group, core temperature was 0.8°C lower after 2 h cooling (i.e., hours 4–6) in comparison with the control group. However, core temperature rose to the same value as controls within only 2 h of reentering the 40°C environment. Their data highlight the powerful but short-lived benefits of air cooling for relieving thermal strain. Based on the lower core temperature measured in the cooling group during hours 4–6,Meade et al.16 state, “our data lendmechanistic support for epidemiological reports indicating up to a 66% reduction in the odds of heat-related mortality in older adults who visited cooled locations during heat waves.”This statement is noteworthy. Although their data show an expected reduction in core temperature and heart rate following removal from the heat, in the author’s opinion, no data are presented that provide insight regarding the pathophysiological mechanisms involved in heat-related mortality or how such mechanisms are affected by brief cooling periods. For instance, there were no differences between groups in markers of systemic inflammation or end core temperature, which are relevant markers linked to the onset of heat stroke.17,18 Thus, it remains unclear whether a brief (2-h) respite from the heat is protective against the negative health impacts of heat waves. The rapid increase in core temperature following reexposure to the heat (during hours 6–9) calls into question whether brief exposure to a cooling center should be recommended as a first line of defense during heat waves. Carefully designed animal studies may be required to understand the dose–response relationship between cooling duration and protection from heat stroke, and the underlying mechanisms involved. Interventions such as water dousing, limb immersion, electric fans, or the application of wetted clothing are important strategies to help mitigate thermal strain in the absence of AC.19 Although AC produced a more robust cooling effect than other methods,16 major benefits of alternative “home-based” methods are that they are cheap, simple to implement, sustainable, and usable for bedbound patients. It is important to note, however, that the efficacy of electric fans for removing body heat depends on the air temperature and humidity combination.20,21 Nonetheless, the use of cooling centers is largely restricted to those who can travel, making them unlikely to protect the most vulnerable, such as those confined to bed or living with psychiatric illness. In addition, as Meade et al. acknowledge, an individual’s core temperature will likely rise in the process of traveling to a cooling center, especially if walking in the heat or if public transportation does not have AC. Thus, they note, traveling to a cooling center may offset some of the proposed benefits—an important consideration that was not simulated in Meade et al.’s experimental protocol.16 Despite these limitations, the study by Meade et al.16 is of undeniable importance, especially in the context of climate change. Prior to this study, the physiological responses of older individuals to visiting a cooling center (although simulated) were unknown. Future studies should seek to a) determine the core temperature and heart rate responses of those who use cooling centers during a real-world heat wave, b) determine the minimum cooling time needed to offset the initiation of heat stroke to a clinically meaningful level, and c) determine the physical, social, and Address correspondence to Josh Foster. Email: [email protected] or [email protected] The author declares he has no conflicts of interest. Received 1 February 2023; Revised 1 March 2023; Accepted 7 April 2023; Published 1 June 2023. 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