Laboratory tests show that the core temperature of the human body begins to rise at wet bulb temperatures below the theoretical safe limit
Heat waves are becoming supercharged as the climate changes â that last longer, become more frequent, and just get hotter. A question many people ask is, “When will it be too hot for normal daily activities as we know them, even for healthy young adults?”
The answer goes beyond the temperature you see on the thermometer. It is also a matter of humidity. Our to research shows the combination of the two can becomes dangerous faster than scientists previously believed.
Scientists and other observers have been alarmed by the increasing frequency of extreme heat associated with high humidity, measured as “wet bulb temperature”. During the heat waves that hit South Asia in May and June 2022, Jacobabad, Pakistan recorded a maximum wet bulb temperature 33.6 C (92.5 F) and Delhi in the lead â close to the theoretical upper limit of human adaptability to damp heat.
People often point to a study published in 2010 who estimated that a wet bulb temperature of 35C – equal to 95F at 100% humidity, or 115F at 50% humidity – would be the upper safe limit, beyond which the human body can no longer cool itself by evaporating sweat from the surface of the body to maintain a stable core temperature.
Only recently has this limit been tested on humans in the lab. The results of these tests point to an even more concerning cause.
To answer the question “how hot is it?” we brought young and healthy men and women into Penn State University’s Noll Laboratory experience heat stress in a controlled environment.
These experiments provide insight into temperature and humidity combinations that are beginning to become harmful to even the healthiest of humans.
Each participant swallowed a small telemetry pill, which monitored their deep or core body temperature. They then sat in an environmental chamber, moving just enough to simulate minimal activities of daily living, such as cooking and eating. The researchers slowly increased the room temperature or humidity and monitored when the subject’s core temperature began to rise.
This combination of temperature and humidity by which a person’s core temperature begins to rise is called the “critical environmental limit.â Below these limits, the body is able to maintain a relatively stable core temperature over time. Above these limits, the core temperature continuously increases and the risk of heat-related illnesses during prolonged exposures increases.
When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate heat, and when you also sweat, it decreases body fluids. In the worst case, prolonged exposure can lead to heatstroke, a life-threatening condition that requires immediate and rapid cooling and medical treatment.
Our studies of healthy young men and women show that this upper environmental limit is even lower than the theorized 35C. This is more like a wet bulb temperature of 31C (88F). This would equate to 31 C at 100% humidity or 38 C (100 F) at 60% humidity.
Current heat waves around the world are approaching or even exceeding these limits.
In hot, dry environments, the critical environmental limits are not defined by wet bulb temperatures, because almost all the sweat produced by the body evaporates, which cools the body. However, the amount humans can sweat is limited and we also draw more heat from higher air temperatures.
Keep in mind that these thresholds are solely based on preventing your body temperature from rising excessively. Even lower temperatures and humidity can put stress on the heart and other body systems. And while exceeding these limits does not necessarily present a worst-case scenario, prolonged exposure can become disastrous for vulnerable populations such as the elderly and those with chronic conditions.
Our experimental focus has now turned to testing older men and women, because even healthy aging makes people less tolerant of heat. The increased prevalence of heart disease, respiratory problems and other health conditions, as well as certain medications, may put them at even greater risk of harm. People over 65 include 80 to 90% of victims of the heat wave.
Comment: The heat in Delhi is unbearable. This is what the climate crisis looks like
Staying well hydrated and looking for places to cool off â even for short periods â is important in hot weather.
As more and more cities in the United States get bigger cooling centers to help people escape the heat, there will still be many people who experience these dangerous conditions with no way to cool down.
Even those who have access to air conditioning may not turn it on because of the high energy cost â a common phenomenon in Phoenix, Arizona â or because of large-scale power outages during heat waves or wildfires, as is becoming increasingly common in the western United States.
A recent study on heat stress in Africa found that future climates will not be conducive to the use of even low-cost cooling systems such as “swamp coolers” as tropical and coastal regions of Africa become wetter. These devices, which require much less power than air conditioners, use a fan to recirculate air through a cool, moist pad to lower the air temperature, but they become inefficient at high wet bulb temperatures. above 21C (70F).
All told, the evidence continues to mount that climate change is not just a problem for the future. This is a situation humanity is currently facing and must confront head-on.
Larry Kenney is a professor of physiology, kinesiology and human performance, Daniel Vecellio is a geographer-climatologist and postdoctoral researcher, Rachel Cottle is a doctoral student in exercise physiology and Tony Wolf is a postdoctoral researcher in kinesiology, all at Penn State.