Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project)

Abstract

A wet-bulb temperature of 35°C has been theorized to be the limit to human adaptability to extreme heat, a growing concern in the face of continued and predicted accelerated climate change. While this theorized threshold is based in physiological principles it has not been tested using empirical data. This study examined the critical wet-bulb temperature (Twb, crit) at which heat stress becomes uncompensable in young, healthy adults performing tasks at modest metabolic rates mimicking basic activities of daily life. Across six experimentally determined environmental limits, no subject's Twb, crit reached the 35°C limit and all means were significantly lower than the theoretical 35°C threshold. Mean Twb, crit values were relatively constant across 36-40°C humid environments and averaged 30.55±0.98 °C but progressively decreased (higher deviation from 35°C) in hotter, dry ambient environments. Twb, crit was significantly associated with mean skin temperature (and a faster warming rate of the skin) due to larger increases in dry heat gain in the hot-dry environments. As sweat rates did not significantly differ among experimental environments, evaporative cooling was outpaced by dry heat gain in hot-dry conditions, causing larger deviations from the theoretical 35°C adaptability threshold. In summary, a wet-bulb temperature threshold cannot be applied to human adaptability across all climatic conditions and where appropriate (high humidity), that threshold is well below 35°C.

Utility of the Heat Index in defining the upper limits of thermal balance during light physical activity (PSU HEAT Project)

“…This was not true for the increased activity associated with LightAmb conditions. It is well-documented that increased metabolic activity lowers critical environmental limits when quantified by temperature or another index such as WBGT (Vecellio et al 2021;Wolf et al 2021aWolf et al , 2021b. We conclude that the HI adequately describes physiologically determined upper limits of heat balance during minimal metabolic workloads, but that the addition of anything more than this limited internal heat production causes deviations from efficacious HI utility in this regard.…”

“…Dynamics of heat stress response were shown to be explained not only by core (rectal) temperature but also by skin temperature, sweat rate, skin wettedness, and skin blood flow amongst other physiological variables. We have previously shown that there was large variability in sweat rates and skin temperatures amongst subjects who completed these studies (Vecellio et al 2021), which would likely differ from one calculated measure that would come from the Fiala model. Additionally, measures of all these variables at 30 and 120 min seemed to play an important role.…”

Robust increase in population exposure to heat stress with increasing global warming

“…TW of 35 • C has been shown to be the upper limit of human survivability, when the thermoregulation of the inner body becomes inefficient and can lead to heat strokes after a few hours of exposure [8]. Recently, [9] demonstrated, with empirical physiological observations, that the limit of survivability is well below 35 • C, and that it varies depending on the environment (humid or dry). Vanos et al [10] also highlighted that this threshold for deadly heat assumes very specific conditions.…”

Increasing Health Risks During Outdoor Sports Due To Climate Change in Texas: Projections Versus Attitudes

“…However, we note that other health impacts are likely to occur at or before reaching these temperatures, especially if physical activity is involved (Vanos, Baldwin, et al, 2020). For example, recently, Vecellio et al (2022) found that the 35°C T WB threshold may be too high in more realistic settings and varies depending on humidity; the critical threshold for heat tolerance in an experimental group of young, healthy adults was found to be 30-31°C T WB for warm-humid conditions and ranged from 25 to 28°C T WB for various hot-dry conditions.…”

“…Further, health impacts are likely at or before reaching the most extreme heat thresholds, especially if physical activity is involved. Vecellio et al (2022) found that the 35°C limit may be too high in both warm-humid and hotter, drier environments (conditions which essen tially encompass most Texas cities) and found a critical threshold for heat exposure closer to 30.55 ± 0.98°C in warm-humid climates due to increased skin heating and limited variation in sweat rates. Thus, the observed status of attitudes toward practice cancellation in response to heat is concerning: wet-bulb temperatures and heat indices are projected to increase dramatically in the future throughout Texas (Lutsko, 2021).…”