Critical Environmental Limits for Older Adults: How Hot is Too Hot?

Abstract:

Adults over the age of 65 are the most vulnerable population during severe environmental heat events. Excessive thermal strain resulting from prolonged elevations in ambient temperature and humidity, as well as prolonged activity in hot environments, challenges the aging thermoregulatory and cardiovascular systems. Both sweating capacity and the ability to increase skin blood flow are blunted in older adults. An important gap in our understanding of the impact of extreme weather on aging human populations is when, i.e., at what set of environmental conditions does age make a difference? Over a wide range of environments core temperature (T_c) equilibrates proportional to metabolic heat production but independent of ambient temperature and humidity. The delineation between compensable heat stress (heat gain ≤ heat loss capacity; T_c equilibrates) and uncompensable heat stress (heat gain > heat loss capacity; T_c continues to rise) defines critical environmental limits (combinations of ambient temperature and humidity) beyond which heat balance is not possible and T_c will continue to rise. Using a unique experimental design, we determined critical environmental limits for a group of older (62-80 yrs) and young (19-26 yrs) women during low-level activity. In addition, partial calorimetry was used to determine age-specific values for critical evaporative coefficients (K_e′) that can be used to model thermal responses of the elderly. Compared to the young women, the older women had significantly constrained (lower) critical environmental limits, in part due to lower sweating rates. For utility, these limits can be plotted on a standard psychrometric chart (see figure). Age-specific values of K_e′, derived by partial calorimetry, were likewise lower for the older women (9.1 vs 15.5 W · m⁻² · mmHg⁻¹ ; p<0.05). Constrained psychrometric limits and lower critical evaporative coefficients lend biophysical clarity to decreased thermoregulatory capacity of older adults in the heat. These limits (myriad combinations of temperature and humidity) and associated critical evaporative coefficients can be used to model responses of older individuals in hot environments and are directly translatable for evidence-based policy decisions, to prepare for impending heat events, and to implement appropriate safety interventions.