An introduction to wet-bulb temperature, and to the question of how hot is too hot for humans.
W. Larry Kenney, Penn State; Daniel Vecellio, Penn State; Rachel Cottle, Penn State, and S. Tony Wolf, Penn State
Extreme heat has been breaking records across Europe, Asia and North America, with millions of people sweltering in heat and humidity well above “normal” for days on end.
Death Valley hit a temperature of 128 degrees Fahrenheit (53.3 degrees Celsius) on July 16, 2023 – not quite the world’s hottest day on record, but close. Phoenix broke a record heat streak with 19 straight days with temperatures above 110 F (43.3 C), and had more in the forecast, accompanied by several nights that never got below 90 F (32.2 C). Globally, Earth likely had its hottest week on modern record in early July.
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 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 gets dangerous at lower levels than scientists previously believed.
Scientists and other observers have become alarmed about the increasing frequency of extreme heat paired with high humidity.
People often point to a study published in 2010 that theorized that a wet-bulb temperature of 95 F (35 C) – equal to a temperature of 95 F at 100% humidity, or 115 F at 50% 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.
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 environmental chamber.
These experiments provide insight into which combinations of temperature and humidity begin to become harmful for even the healthiest humans.
Each participant swallowed a small telemetry pill that continuously monitored their deep body or core temperature. They then sat in an environmental chamber, moving just enough to simulate the minimal activities of daily living, such as showering, cooking and eating. Researchers slowly increased either the temperature in the chamber or the humidity in hundreds of separate experiments and monitored when the subject’s core temperature started to rise.
That combination of temperature and humidity at which the person’s core temperature starts to continuously rise is called the “critical environmental limit.”
Below those limits, the body is able to maintain a relatively stable core temperature over long periods of time. Above those limits, core temperature rises continuously and the 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 95 F (35 C). It occurs at a wet-bulb temperature of about 87 F (31 C) across a range of environments above 50% relative humidity. That would equal 87 F at 100% humidity or 100 F (38 C) at 60% humidity.
Current heat waves around the globe are exceeding those critical environmental limits, and approaching, if not exceeding, even the theorized 95 F (35 C) wet-bulb limits.
In the Middle East, Asaluyeh, Iran, recorded an extremely dangerous maximum wet-bulb temperature of 92.7 F (33.7 C) on July 16, 2023. India and Pakistan have both reached hazardous levels in recent years, as well.
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.
A recent paper from our laboratory showed that heart rate begins to increase well before our core temperature does, as we pump blood to the skin. 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. 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% to 90% of heat wave casualties.
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.
Even those with access to air conditioning might 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 becoming more common in the western U.S.
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.
W. Larry Kenney, Professor of Physiology, Kinesiology and Human Performance, Penn State; Daniel Vecellio, Geographer-climatologist and Postdoctoral Fellow, Penn State; Rachel Cottle, Ph.D. Candidate in Exercise Physiology, Penn State, and S. Tony Wolf, Postdoctoral Researcher in Kinesiology, Penn State
This article is republished from The Conversation under a Creative Commons license. Read the original article. Some illustrations and edits added by EarthSky.
The lead author of this article, W. Larry Kenney, discusses the impact of heat stress on human health with PBS NewsHour.
Note: EarthSky editors added the following information.
It’s well-known that the world’s average temperatures are climbing. We are also experiencing more extreme weather events as the world warms. According to the Columbia Climate School:
The study found that worldwide, wet-bulb readings approaching or exceeding 30 C (86 F) on the wet bulb have doubled since 1979. The number of readings of 31 (87.8 F) – previously believed to occur only rarely – totaled around 1,000. Readings of 33 (91.4 F) – previously thought to be almost nonexistent – totaled around 80.
Scientists have noted that we seem to be much closer to a tipping point than previously believed. They also said some communities are closer to sustained intolerable heat than they expected. Steven Sherwood, a climatologist at the Australia’s University of New South Wales, said that scientists:
… previously believed we had a much larger margin of safety.
The interactive world map below shows the highest wet-bulb temperatures recorded as of 2020, according to location. Image via Jeremy Hinsdale; adapted from Raymond et al., Science Advances, 2020. Used with permission.
Bottom line: Wet-bulb temperature is a measure of heat that considers humidity as well (and other factors). It’s a good indicator of when temperature and humidity have combined to create dangerous conditions for those who must be outdoors, or indoors, without a way to cool off.
Source: Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project)
Record temps as extreme heat hits worldwide in 2023
High Wet Bulb Globe Temperatures (WBGT) are forecast to hit Louisiana, United States, over the next few days. The image below shows a forecast for August 2, 2023, 18 UTC, with WBGT as high as 35°C forecast for a location 10 miles South East of Abbeville, Louisiana, U.S.
The above image shows a Wet Bulb Globe Temperature of 35°C (95°F) forecast for August 11, 2023, 19 UTC, for a location near Baton Rouge, Louisiana, U.S.
Heat is the leading cause of weather-related deaths in the United States, as illustrated by the above image (credit: NOAA ). Heat fatalities may be conservative figures. Recent research finds that where heat is being listed as an official cause of death, this likely underestimates the full toll of these events. Extreme heat can trigger heart attacks and strokes. In addition, some heart disease risk factors, such as diabetes—as well as heart medications, such as diuretics and beta blockers—can affect a person’s ability to regulate their body temperature and make it difficult to handle extreme heat. The study finds that extreme heat accounted for about 600-700 additional deaths from cardiovascular disease annually. A recent study estimates that extreme heat accounted for 12,000 premature deaths in the contiguous U.S. from 2000 to 2010, and a recent analysis calculates that the summer 2022 heatwave killed 61,000 people in Europe alone.
The image below shows a temperature (°F) forecast for August 1, 2023, from Climate Reanalyzer
The temperature at that location at the time was 35.2°C or 95.4°F, lower than the temperature on the land surrounding the Gulf, but the relative humidity at that spot over the water was 78%, and that combination led to this very high 'feels like' temperature.
This constitutes a warning. The sea, rivers and lakes are traditionally seen as places to go to, to cool off. However, high temperatures combined with high humidity over water bodies can result in conditions that go beyond what humans can bear.
The image below show a high reading on the 'Misery Index', the perceived ('feels like') temperature that is used by nullschool.net , combining wind chill and the heat index (which in turn combines air temperature and relative humidity, in shaded areas). A Misery Index temperature of 56.1°C or 133.1°F was recorded at a location off the coast of the United Arab Emirates (green circle) on August 5, 2023.The image below, earlier discussed here , expands risk assessment beyond its typical definition as the product of the severity of impact and probability of occurrence, by adding a third dimension: timescale, in particular imminence.
Imminence alone could make that the danger constituted by rising temperatures needs to be acted upon immediately, comprehensively and effectively. While questions may remain regarding probability, severity and timescale of the dangers associated with climate change, the precautionary principle should prevail and this should prompt for action, i.e. comprehensive and effective action to reduce damage and improve the situation is imperative and must be taken as soon as possible.
Rapidly rising temperatures constitute tipping points in several ways
Firstly, there is a biological threshold beyond which rising temperatures become lethal for humans, as discussed above.
Secondly, as Gerardo Ceballos describes in the video below and in a 2017 analysis , there is a biological tipping point that threatens annihilation of species via the ongoing sixth mass extinction. Researchers such as Gerardo Ceballos (2020) Kevin Burke (2018) and Ignation Quintero (2013) have for years warned that mammals and vertebrates cannot keep up with the rapid rise in temperature. Humans are classified as vertebrate mammals, indicating that we will not avoid the fate of extinction, Guy McPherson (2020) adds.
Thirdly, there are further tipping points, e.g. social-political ones. On the one hand, it would be good if people became more aware, as this could prompt more people into supporting the necessary action. On the other hand, as temperatures keep rising, there is also a danger that panic will break out, dictators will grab power and civilization as we know it will collapse abruptly, as warned about earlier, e.g. in 2007
Conclusion
• Heat-related mortality in Europe during the summer of 2022 - by Joan Ballester et al.
• Biological annihilation via the ongoing sixth mass extinction signaled by vertebrate population losses and declines - by Gerardo Ceballos, Paul R. Ehrlich and Rodolfo Dirzo (2017)
Discussed at: https://www.facebook.com/groups/arcticnews/posts/10160875637104679 • As Temperatures Spike, So Do Deaths from Heart Disease (2022 News release)• Association of Extreme Heat and Cardiovascular Mortality in the United States: A County-Level Longitudinal Analysis From 2008 to 2017 - by Sameed Khatana et al. (2022)• Co-extinctions annihilate planetary life during extreme environmental change, by Giovanni Strona and Corey Bradshaw (2018)Discussed at: https://www.facebook.com/groups/arcticnews/posts/10156903792219679 • When will we die?• Climate Reanalyzer - Hourly Forecast Maps• PBS video - Too HOT and HUMID to Live: Extreme Wet Bulb Events are on the Rise• Nullschool• How agriculture hastens species extinction | 60 Minutes (CBS News) | Gerardo Ceballos• Vertebrates on the brink as indicators of biological annihilation and the sixth mass extinction - by Gerardo Ceballos, Paul Ehrlich, and Peter Raven (2020)
• Rates of projected climate change dramatically exceed past rates of climatic niche evolution among vertebrate species - by Ignatio Quintero et al. (2013)
https://onlinelibrary.wiley.com/doi/abs/10.1111/ele.12144Discussed at: https://www.facebook.com/groups/arcticnews/posts/10158460232764679 • Pliocene and Eocene provide best analogs for near-future climates - by Kevin Burke et al. (2018)
• Earth is in the Midst of Abrupt, Irreversible Climate Change - by Guy McPherson (2020)
Discussed at: https://www.facebook.com/groups/arcticnews/posts/10160004947844679
• Transforming Society• Climate Plan• Climate Emergency Declaration
• Ten Dangers of Global Warming• Extreme heat stress
Discussed at: https://www.facebook.com/groups/arcticnews/posts/10156972951354679
WBGT is a measure used by weather.gov to warn about expected heat stress when in direct sunlight. It takes into account the effect of temperature, relative humidity, wind speed, and solar radiation on humans.As temperatures and humidity levels keep rising, a tipping point can be reached where the wind factor no longer matters, in the sense that wind can no longer provide cooling. The human body can cool itself by sweating, which has a physiological limit that was long described as a 35°C wet-bulb temperature. Once the wet-bulb temperature reaches 35°C, one can no longer lose heat by perspiration, even in strong wind, but instead one will start gaining heat from the air beyond a wet-bulb temperature of 35°C.Accordingly, a 35°C wet-bulb temperature (equal to 95°F at 100% humidity or 115°F at 50% humidity) was long seen as the theoretical limit, the maximum a human could endure. 2020 study (by Raymond et al.) warns that this limit could be regularly exceeded with a temperature rise of less than 2.5°C (compared to pre-industrial). A 2018 study (by Strona & Bradshaw) indicates that most life on Earth will disappear with a 5°C rise. Humans, who depend for their survival on many other species, will likely go extinct with a 3°C rise, as illustrated by the image below, from an earlier post • Wet Bulb Globe Temperature• National Weather Service - Wet Bulb Globe Temperature: How and when to use it• The emergence of heat and humidity too severe for human tolerance - by Colin Raymons et al. (2020)• Brief periods of dangerous humid heat arrive decades early• Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects (PSU HEAT Project) - by Daniel Vecellio et al. (2022)Discussed at: https://www.facebook.com/groups/arcticnews/posts/10159973158374679 • NOAA - Weather Fatalities 2022• The Effects of Heat Exposure on Human Mortality Throughout the United States - by Drew Shindell (2021)