Critical Environmental Limits for Human Thermoregulation in the Context of a Changing Climate.

Human-caused climate change has increased the average temperature of the Earth by over 1°C since the 19^th century with larger increases expected by 2100 due to continued human influence. This change in mean ambient temperature has had nonlinear effects, resulting in more high temperature extremes, i.e., heat waves, that have increased in frequency, duration, and magnitude. Additional occurrences of humid heatwaves have significantly affected human health due to the physiological strain associated with a relative inability for evaporative cooling. Inability to efficaciously cool the body, whether during passive heat exposure or physical activity, not only leads to elevated core temperatures but also places strain on the cardiovascular system, often exacerbating age-related co-morbidities. As part of the PSU HEAT (Pennsylvania State University - Human Environmental Age Thresholds) Project, a progressive environmental strain protocol has been developed to determine critical environmental limits - combinations of ambient temperature and humidity -- associated with uncompensable heat stress and intractable rises in core temperature (T_c). These human heat balance thresholds, well below those originally theorized by climatologists, have been surpassed in recent heatwaves and be exceeded on a more regular basis in the future, providing additional impetus to the urgency of adaptative measures and climate change mitigation.