non-survivable heat stress conditions are already occurring

The heat stress limits for human survivability have been long defined by a 6-h exposure to a wet-bulb temperature of 35oC. This definition has been employed regularly by the climate community to understand the threat of heat stress on humans. However, we recently developed a physiology-based model demonstrating that environmental heat stress thresholds, when accounting for limitations to evaporative cooling via sweating, and realistic hyperthermic thresholds for heat stroke, may be cooler and drier than previously thought. The potential consequences for older people, in whom sweating responses are further diminished, are thought to be especially severe. For the first time, we employ our physiology-based heat stress limit model to determine whether non-survivable thresholds (defined by temperature and humidity) were surpassed during six historical events where conditions were climatologically extreme and/or high heat-related mortality was reported. We focus on six-hourly environmental limits for two age groups that are either completely exposed or have sought shade. Our results show that non-survivable limits are already occurring during present-day extreme heat events, all of which are well below the theorised 35oC wet-bulb temperature threshold. Of particular concern is the regular exceedance of deadly thresholds for older people outdoors across all events. Moreover, extremely hot yet dry conditions are just as deadly as hot and humid conditions, despite much lower wet-bulb temperatures than 35oC. For future climatological assessments, we emphasise the importance of employing physiology-derived methods to accurately assess the risk of potentially deadly heat stress.