How hot is too hot?
Tony Wolf, assistant professor in the department of kinesiology at Penn State University, takes a look with human physiology in mind.
Dr. Tony Wolf is an environmental physiologist. His research is focused on understanding the impact of the environment on human health.
How Hot is Too Hot for the Human Body?
As the climate changes and heat waves become more frequent and severe, people often wonder – “When will it become too hot for normal daily activity?” Previous studies have theorized that the upper limit for human adaptability is a wet-bulb temperature of 35°C, or 95°F. Our research suggests that the combinations of temperature and humidity above which prolonged activity becomes relatively unsafe may be even lower.
To answer the question of “how hot is too hot?” we tested healthy young adults in a controllable environmental chamber. Each participant swallowed a small telemetry pill that monitored their deep body (core) temperature. They then performed light physical activity to simulate activities of daily living. As they performed this activity, we slowly increased either the temperature or the humidity in the chamber and monitored the participants’ core temperature.
Across a wide range of environments, participant core temperature remains relatively stable. Eventually, the combination of temperature and humidity reaches a threshold above which core temperature starts to rise. We call this point the “critical environmental limit,” and above this limit core temperature rises continuously, increasing risk of heat-related illness with prolonged exposure.
Our studies showed that the critical environmental limit is about 88°F wet-bulb temperature – equivalent to 88° at 100% relative humidity, or 100° at 60% humidity.
In our current climate, heat waves around the world are approaching – and sometimes exceeding – these limits. It is important to keep in mind these data are from healthy young adults, representing a best-case scenario. Our current focus is on testing older adults, given that some 80-90% of heat wave casualties occur in people over the age of 65.’
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[National Library of Medicine] – S. Tony Wolf
