Critical Environmental Limits for Young, Healthy Adults (PSU HEAT)

Abstract

The average temperature of the Earth has been steadily increasing for decades and is expected to continue increasing throughout the 21st century. Along with the increase in average global temperatures, the frequency, duration, and severity of heat waves have increased. Thus, there is a critical need for research targeted at understanding the specific environmental conditions associated with increased risk of heat‐ and humidity‐related morbidity and mortality.PurposeTo establish critical environmental limits for healthy young adults at two metabolic rates that reflect activities of daily living and light activity. We hypothesized that critical environmental limits would be shifted toward lower combinations of temperature and humidity at higher compared to lower rates of metabolic heat production.MethodsTwenty‐five (12M/13F; 24 ± 4 yrs) subjects were exposed to progressive heat stress in an environmental chamber at two rates of metabolic heat production chosen to represent minimal activity (MinAct) or light ambulation (LightAmb). Progressive heat stress comprised either (1) constant dry‐bulb temperature (Tdb; 36, 38, or 40 °C) and increasing ambient water vapor pressure (Pa; Pcrit trials), or (2) constant Pa (12, 16, or 20 mmHg) and increasing Tdb (Tcrit trials). Chamber Tdb and Pa, and subject gastrointestinal temperature (Tgi), were continuously monitored throughout each trial. After a 30‐min equilibration period, progressive heat stress superseded until subject heat balance could no longer be maintained and a clear rise in Tgi was observed. Each subject was tested during MinAct and LightAmb in 2 or 3 randomized environmental conditions, for a total of 4–6 trials per participant.ResultsBy design, oxygen consumption (V̇O2: 0.46 ± 0.10 vs. 0.80 ± 0.16 L/min; 1.8 ± 0.3 vs. 3.2 ± 0.3 METs), metabolic heat production (82.9 ± 12.5 vs. 133.3 ± 14.8), and sweat rate (142.8 ± 80.2 vs. 230.8 ± 84.9) were lower for MinAct compared to LightAmb (all p < 0.0001). There were no differences in these variables (p ≥ 0.05) among environmental conditions for either MinAct or LightAmb intensities. Higher metabolic heat production (p < 0.001) during LightAmb compared to MinAct trials resulted in significantly lower critical environmental limits across all Pcrit and Tcrit conditions (all p < 0.001).ConclusionsThese data are the first to define critical environmental limits for the maintenance of heat balance in healthy young adults during minimal to light physical activity reflecting activities of daily living and light household tasks. By establishing critical environmental limits in a young, healthy population, the findings of this study will be used as a benchmark for future studies in aged and other vulnerable populations, and can be used to develop guidelines, policy decisions, and evidence‐based alert communications to minimize adverse impacts of extreme heat events.