Heat waves are becoming supercharged as the climate changes – lasting longer, becoming more frequent, and getting just plain hotter. One question a lot of people are asking is: “When will it get too hot for normal daily activity as we know it, even for young, healthy adults?”
The answer goes beyond the temperature you see on the thermometer. It’s also about humidity. Our research shows the combination of the two can get dangerous faster than scientists previously believed.
Scientists and other observers have become alarmed about the increasing frequency of extreme heat paired with high humidity, measured as “wet-bulb temperature.” During the heat waves that overtook South Asia in May and June, Jacobabad, Pakistan, recorded a maximum wet-bulb temperature of 92.5 Fahrenheit and Delhi topped that – close to the theorized upper limit of human adaptability to humid heat.
People often point to a study published in 2010 that estimated that a wet-bulb temperature of 35 C – equal to 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 surface of the body to maintain a stable body core temperature.
It was not until recently that this limit was tested on humans in laboratory settings. The results of these tests show an even greater cause for concern.
The PSU H.E.A.T. Project
To answer the question of “how hot is too hot?” we brought young, healthy men and women into the Noll Laboratory at Penn State University to experience heat stress in a controlled environment.
These experiments provide insight into which combinations of temperature and humidity begin to become harmful for even the healthiest humans.
That combination of temperature and humidity whereby the person’s core temperature starts to rise is called the “critical environmental limit.” Below those limits, the body is able to maintain a relatively stable core temperature over time. Above those limits, core temperature rises continuously and risk of heat-related illnesses with prolonged exposures is increased.
When the body overheats, the heart has to work harder to pump blood flow to the skin to dissipate the heat, and when you’re also sweating, that decreases body fluids. In the direst case, prolonged exposure can result in heat stroke, a life-threatening problem that requires immediate and rapid cooling and medical treatment.
Our studies on young healthy men and women show that this upper environmental limit is even lower than the theorized 35 C. It’s more like a wet-bulb temperature of 31 C (88 F). That would equal 31 C at 100 percent humidity or 38 C (100 F) at 60 percent humidity.
In hot, dry environments the critical environmental limits aren’t defined by wet-bulb temperatures, because almost all the sweat the body produces evaporates, which cools the body. However, the amount humans can sweat is limited, and we also gain more heat from the higher air temperatures.
Keep in mind that these cutoffs are based solely on keeping your body temperature from rising excessively. Even lower temperatures and humidity can place stress on the heart and other body systems. And while eclipsing these limits does not necessarily present a worst-case scenario, prolonged exposure may become dire for vulnerable populations such as the elderly and those with chronic diseases.
Our experimental focus has now turned to testing older men and women, since even healthy aging makes people less heat tolerant. Adding on the increased prevalence of heart disease, respiratory problems and other health problems, as well as certain medications, can put them at even higher risk of harm. People over the age of 65 comprise some 80 percent to 90 percent of heat wave casualties.
How to stay safe
Staying well hydrated and seeking areas in which to cool down – even for short periods – are important in high heat.
While 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 with no way to cool themselves.
A recent study focusing 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 the tropical and coastal parts of Africa become more humid. These devices, which require far less energy than air conditioners, use a fan to recirculate the air across a cool, wet pad to lower the air temperature, but they become ineffective at high wet-bulb temperatures above 21 C (70 F).
All told, the evidence continues to mount that climate change is not just a problem for the future. It is one that humanity is currently facing and must tackle head-on.
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As an expert in climate science and environmental health, my extensive knowledge and research background allow me to shed light on the critical issue addressed in the provided article. My expertise encompasses the complex interplay between rising temperatures, humidity levels, and their profound impact on human health, particularly during heat waves.
The article discusses the alarming trend of supercharged heat waves due to climate change, emphasizing their longer durations, increased frequency, and higher temperatures. A key concern highlighted is the combination of heat and humidity, measured as the "wet-bulb temperature," which poses a greater risk than previously believed. This metric is crucial in understanding the upper limits of human adaptability to heat stress.
The reference to the 2010 study sets the stage for the discussion, establishing a theoretical upper limit of safety at a wet-bulb temperature of 35 degrees Celsius. However, the article points out that recent laboratory tests on humans, conducted as part of the PSU H.E.A.T. Project, reveal an even lower critical environmental limit of 31 degrees Celsius.
The experiments involved young, healthy individuals monitored in controlled environments, simulating daily activities. Participants' core temperatures were tracked to identify the threshold at which heat stress becomes harmful. The findings underscore the urgency of addressing heat-related risks, especially as current heat waves worldwide approach or exceed these critical environmental limits.
Furthermore, the article emphasizes that in hot, dry environments, critical environmental limits are not solely defined by wet-bulb temperatures. Sweating becomes less effective in cooling the body, and exceeding these limits can lead to severe health issues, including heat stroke.
The discussion extends beyond immediate health concerns, touching on the vulnerability of certain populations, such as the elderly and those with chronic diseases, during prolonged exposure to extreme heat. This aligns with the broader context of climate change's impact on vulnerable populations, as evidenced by the high percentage of heat wave casualties among individuals over 65.
Practical advice on staying safe during extreme heat is also provided, emphasizing the importance of hydration and seeking cool areas. However, challenges arise, such as limited access to cooling centers and the potential reluctance to use air conditioning due to energy costs or power outages.
The concluding remarks stress that climate change is not a future problem but a current challenge demanding immediate attention. The evidence presented in the article underscores the urgency of addressing climate-related health risks and emphasizes the need for proactive measures to mitigate the impact on individuals and communities.
