Why not 35°C? Reasons for reductions in limits of human thermal tolerance and their implications
Year: 2024
Type: article
Abstract: was the hottest summer in recorded history globally, with heatwaves impacting nearly all regions of the world during the course of the year.It pushed the planet closer to the 1.5°C warming limit (for a single year) that most scientists agree needs to be the ceiling for climate change-induced warming before ecological degradation begins to rapidly increase.It also turned out that the most extreme conditions during the year put some regions close to the limits of what the human body can tolerate during extreme heat events.In our recent study using future projections from state-of-the-art climate model simulations [1], we found large swaths of the world's population, primarily located in South and East Asia, the Middle East, and sub-Saharan Africa, will be exposed to environmental heat stress that exceed limits of human heat tolerance with as little as 1.5-2°C of warming above preindustrial levels, another indicator of the drastic impacts climate change will place on human health.In worlds with warming over 2°C, annual hours of exposure to uncompensable environmental heat stress would exponentially increase in the regions above and would begin to emerge in other regions such as North and South America at 3°C and Australia at 4°C.The novelty of the study lies with the empirically obtained limits to human heat tolerance it used by recruiting young, healthy, unacclimatized subjects as part of the Penn State University Human Environmental Age Thresholds (PSU HEAT) study [2].Using data from those experiments, we found in an earlier study [3] that even in populations that should thermoregulate at near peak efficiency, wet-bulb temperature (T wb ) thresholds for heat stress compensability were closer to 30.6°C in warm-humid conditions (relative humidity values greater than 50% and temperatures 40°C or below) with a linear decrease across hotter and drier environments.These limits were significantly less than the widely cited 35°C T wb limit theorized as an "upper adaptability limit" to extreme heat via loose biophysical principles by Sherwood and Huber in a 2010 study, hereby referred to as SH10 [4].Here, we provide a brief explanation of the most evident reasons that physiological critical limits are lower than those based on biophysical theory (Figure 1).That is, why do the least vulnerable, healthiest humans not reach the theoretical SH10 upper limit for thermal balance that has been used to communicate extreme heat tolerance in the literature and mass media for over the past decade-plus?Additionally, we point out additional research that has now been completed, and those studies yet to be performed, to develop a fuller understanding of critical environmental thresholds for diverse populations across the world.Many of SH10's assumptions were based on interactions between the skin surface and the overlying atmosphere and how that affects the human energy balance.The human skin plays a large role in allowing the body to maintain homeostasis via its interactions with both the ambient environment and the body's internal core [5].As part of SH10's assumptions, a maximum, clamped skin temperature of 35°C was assumed, which would have maintained a core-to-skin temperature gradient to allow for efficient flow of heat away from the body given core temperatures between 37°C and 38°C.However, in empirical testing, skin temperatures ranged from 36°C to 38.5°C at the critical environmental limits (i.e.environments associated with excessive heat storage relative to heat loss and failure to maintain core temperature), increasing as a function of the dry-bulb temperature (T db ) [2].With higher skin temperatures associated with cutaneous vasodilation, dry heat gain, and reduced evaporative heat transfer, the efficiency of moving heat from core to skin was diminished.In environments in which T db exceeds skin temperature, the SH10 model does not
Source: Temperature
Language: English
Subfield: Physiology
Field: Medicine
Domain: Health Sciences
Open Access status: bronze
Funder: National Institute on Aging
Awards: G498543760, G980175704
OpenAlex ID: W4402209430