Critical Environmental Limits for Middle-Aged Adults (PSU HEAT Project)

Abstract

Critical environmental limits are the combinations of temperature and humidity above which heat balance cannot be maintained for a given metabolic heat production. Our lab has previously established critical environmental limits for young and older adults; however, these limits have yet to be established for middle-aged adults. The purpose of the present study was to (1) identify critical environmental limits for middle-aged adults, and (2) investigate age-related differences in critical environmental limits at a low metabolic rate reflecting activities of daily living across the adult age spectrum. Twenty-six older (71±5 yrs, 14F/12M), 36 middle aged (51±9 yrs, 26F/10M), and 27 young adults (23±3 yrs, 13F/14M) were exposed to progressive heat stress in a controlled environmental chamber during minimal activity. Progressive heat stress for the determination of critical environmental limits comprised either (1) constant dry-bulb (Tdb) temperature at 34°C with progressively increasing water vapor pressure (Pa) by 1 mmHg every 5 minutes (Pcrit trial), or (2) constant Pa at 12 mmHg with progressively increasing Tdb by 1°C every 5 minutes (Tcrit trial). Critical environmental limits in hot-dry environments were higher in young compared to middle aged (49.3±2.3 °C vs. 45.4±4.0 °C; p = 0.002) and older adults (49.3±2.3°C vs. 43.2±4.0°C; p < 0.001), but were not different between middle aged and older adults (45.4±4.0 °C vs. 43.2±4.0°C; p = 0.125). Critical environmental limits in warm-humid conditions were lower in older adults compared to young (23.1±6.1 mmHg vs 31.7±1.5 mmHg, p < 0.001) and middle aged (23.1±6.1 mmHg vs 29.0±5.1 mmHg, p = 0.004), but were not different between the young and middle aged (31.7±1.5 mmHg vs 29.0±5.1 mmHg, p = 0.220). Age was correlated with critical environmental limits in Tcrit (R2 = 0.34; p < 0.001) and Pcrit (R2 = 0.51; p < 0.001) trials based on linear and curvilinear regression, respectively. Critical environmental limits for middle aged adults performing minimal activity in warm-humid and hot-dry environments are intermediate to young and older adults. These data demonstrate an age-related decline in critical environmental limits that can be characterized by linear decline in hot-dry environments and by curvilinear decline in warm-humid environments. Supported by NIH Grant R01 AG067471. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.