Death Valley hit a temperature of 128 degrees Fahrenheit (53.3 degrees Celsius) on July 16, 2023 – not the world’s hottest day on record, but close.
Phoenix broke record heat streak with 19 days straight with a temperature over 110 °F (43.3 °C), and there was more in the forecast, including several nights that never got below 90 °F (32.2 °C). In the world, the Earth probably had its share the hottest week on modern record in early July.
There are heat waves becoming overloaded as the climate changes – living longer, becoming more frequent and getting hotter.
One question many people are asking is: “When does it become too hot for normal daily activities as we know it, even for healthy young adults?”
The answer goes beyond the temperature you see on the thermometer. It’s also about humidity.
Our research it is designed to come up with the combination of the two, measured as “wet bulb temperature.” Together, heat and humidity put people at greatly increased risk, and the combination becomes dangerous at lower levels than scientists previously believed.
Limits of human adaptability
Scientists and other observers fear an increasing frequency of extreme heat and high humidity.
People often focus on a study published in 2010 which theorizes that there was a wet bulb temperature of 95 °F (35 °C) – equal to a temperature of 95 °F at 100 percent humidity, or 115 °F at 50 percent humidity – would be the upper limit of safety, beyond which the human body can no longer cool itself by evaporating sweat from the body surface to maintain the core body temperature.
Only recently has this limit been tested on humans in laboratory settings. The results of these tests show cause for even greater concern.
The PSU HEAT Project
To answer the question “how hot is too hot?” we brought young, healthy men and women into the Noll Laboratory at Penn State University experience heat stress in a controlled environment chamber.
These experiments provide insight into what makes combinations of temperature and humidity harmful to even the healthiest of people.
Each little participant is swallowed telemetry pill who continuously monitored their deep body or heart temperature. They then sat in an environmental chamber, moving just enough to simulate the minimal activities of daily life, such as showering, cooking and eating.
Researchers slowly increased the room temperature or humidity in hundreds of separate experiments and monitored when the subject’s core temperature began to rise.
The ” ” is calledcritical environmental limit.”
Below these limits, the body is able to maintain a relatively constant core temperature for long periods of time. Above these limits, the core temperature rises continuously and the risk of heat-related illnesses increases with prolonged exposures.
When the body is overheated, the heart has to work harder to pump blood flow to the skin to dissipate the heat, and when you are sweating too, body fluids are reduced. In the most severe case, prolonged exposure can lead to heat stroke, a life-threatening problem that requires rapid and immediate cooling and medical treatment.
Our studies of healthy young men and women show that this is the upper environmental limit even lower than the theoretical 35 °C. It occurs at a wet bulb temperature of about 87 °F (31 °C) over a range of environments above 50 percent relative humidity. That would equal 87 °F at 100 percent humidity or 100 °F (38 °C) at 60 percent humidity.
Dry vs moist environments
Current heat waves around the globe are breaching those crucial environmental limits, and approaching, if not exceeding, even the theoretical 95 limit. °F (35 °C) wet-bulb boundaries.
In the Middle East, Asaluyeh, Iran, made a particularly dangerous record maximum wet bulb temperature of 92.7 °F (33.7 °C) on July 16, 2023. India and Pakistan both have also reached dangerous levels in recent years.
In hot, dry environments, the critical environmental limits are not defined by wet bulb temperatures, as almost all sweat produced by the body evaporates, cooling the body. However, the amount that humans can sweat is limited, and we also get more heat from the higher air temperatures.
Keep in mind that these reductions are based entirely on keeping your body temperature from rising too high. Even lower temperatures and humidity can stress the heart and other body systems.
A recent paper from our lab demonstrated that heart rate begins to increase long before our heart temperature does, as we pump blood to the skin. And while exceeding those limits isn’t necessarily the worst-case scenario, prolonged exposure can be devastating for vulnerable populations like the elderly and those with chronic diseases.
Our experimental focus has now turned to testing older men and women, as healthy aging makes people less refractory. The increased prevalence of heart disease, respiratory and other health problems, as well as certain medications, can put them at even higher risk of harm. People over the age of 65 include some 80% to 90% of accidental heat waves.
How to be safe
It is important to stay hydrated and seek areas to cool down – even for short periods – during high heat.
Although more cities in the United States are expanding cooling centers to help people escape the heat, there will still be many people who will experience these dangerous conditions no way to cool themselves.
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Ethose with access to air conditioning may not turn it on because of the high cost of energy – a common occurrence in Phoenix – or because of large scale power outages during heat waves or wildfires, as is increasingly common in the western US.
All that being said, the evidence continues to point to climate change not being just a problem for the future. It is one that humanity is facing right now and it needs to be tackled head on.
This is an update to an article first published July 6, 2022.
W. Larry KenneyProfessor of Physiology, Kinesiology and Human Performance, Penn State; Daniel VecellioGeographer-climatologist and Postdoctoral Fellow, Penn State; Rachel Cottle, Ph.D. Candidate in Exercise Physiology, Penn Stateand S. Tony WolfPostdoctoral Researcher in Kinesiology, Penn State