When I began studying thermal stress as a new PhD student, I came across a handful of studies that even today shape how I think about our physiological relationship with increasing heat. One such study, published in the top-tier journal Science Advances, is titled The emergence of heat and humidity too severe for human tolerance. For a scientific paper, it’s a catchy title, but it was the substance of the research that really fascinated me.
The article discussed parts of the globe where the combination of heat and humidity is rapidly approaching, and occasionally even exceeding, human thermal limits. Under such conditions, even healthy young people can’t avoid potentially fatal heat stroke if exposed long enough. The article essentially predicts places where heat stress is getting so extreme that the outdoor environment becomes deadly.
Of course, dangerous, even fatal, heat isn’t new. Well documented heat waves have been implicated in tens of thousands of deaths: around 70,000 across Europe in 2003 and nearly as many again in 2022, or the tens of thousands of Russian deaths from the combination of heat and wildfires in 2010. Closer to home, Phoenix, AZ, recorded its first official heat death of 2026 in April, while the broader Maricopa County (which includes Phoenix) has recently averaged 561 heat-related deaths every year. And, this year, heat fatalities are once again surging in Europe.
These fatalities often (but not exclusively) share a similar demographic profile: older adults, potentially in poor health or with chronic health conditions, often residing in buildings with poor or non-existent air conditioning or lacking the means to run air conditioning constantly. Many were homeless, unwilling or unable to access shelters during the hottest parts of the day and sleeping outdoors on nights that never offered much reprieve. For some, drug use (prescribed or illicit) was a contributing factor. They typically were not fit, young people in the prime of their life.
That is why the article in Science Advances really caught my attention. The researchers didn’t focus on the old, homeless, or young. Instead, their survival threshold was the limits for healthy, working-age people. If that population can’t survive, no one could. This article will deep dive into the parts of the world where extreme heat, and especially extreme humid heat, is rising to levels where even healthy, young adults are no longer safe outdoors.
It’s not (just) the heat, it’s the humidity
Before moving on, we need to cover some basics (stay with me, I promise they provide an important foundation for the rest of the article).
· This Thermal Physiology and the Environment series has focused on exertional heat stroke, occurring when metabolic heat load overwhelms our ability to cool down. Not so here. We will discuss classical heat stroke, where simple exposure to the (really hot) environment overwhelms the body’s capability to maintain a stable core temperature. No heat-generating physical activity needed.
· “Compensable” heat is a heat load the body can handle. Under compensable heat stress conditions, we can sweat (and sweat can evaporate) enough to maintain a core temperature somewhere below the threshold for heat stroke (usually defined as a core temperature above 104oF).[1]
· “Uncompensable” heat stress occurs when the environment (i.e., really hot conditions) or an activity (i.e., running or working really hard) creates a potentially dangerous rise in core temperature even as we try our best to maintain thermal equilibrium. Under uncompensable heat stress, core temperature will continue to rise unless the factors contributing to the heat load are removed (e.g., we move into an air-conditioned space, or we stop exerting so much physical effort).
· The wet bulb temperature is the thermal index generally used to define survivability thresholds for humans. The wet bulb temperature is the temperature of a shaded thermometer when covered by a wet cotton sleeve. This provides a good proxy for the temperature experienced by a sweating human since the water-soaked cotton sleeve evaporates and cools the thermometer in the same manner that a sweat-covered human is cooled. Here, “temperature” without any qualifier refers to ambient temperature (i.e., the air temperature, or “dry bulb” temperature), not wet bulb temperature.
· Survivability is the last term we need to define. No one is going to drop dead simply because we temporarily reach uncompensable heat stress (if this was true, you’d be dead the minute you walked into a sauna or started running hard on a hot day). Research literature often assumes an exposure time of about 6 hours when considering “survivability” (although it can vary). Spend 6 straight hours in a sauna and you’ll probably be dead!
When and where can we expect environments that create uncompensable heat stress in young, healthy adults, even if they are resting in the shade? The answer is… it depends. Specifically, it depends on two main variables: temperature and relative humidity.[2]
High humidity degrades evaporation’s ability to cool a wet bulb thermometer. Suppose you have wet bulb thermometers in two environments. The first environment is 100oF but has low relative humidity of 10% (i.e., a desert), while the second is 100oF and 100% humidity (an extreme approximation of conditions along the Persian Gulf coast during peak summer). In the first environment, the wet bulb temperature can be as low as 64oF! (Don’t believe me? Check out this tool from Texas A&M). In the second, humid environment, the wet bulb temperature will be…100oF, because at 100% humidity, no cooling evaporation occurs.[3]
The huge difference is because water (or sweat) evaporates more easily in dry air. As humidity increases, sweat evaporates less easily. Once relative humidity reaches 100%, air is completely saturated and sweat evaporation stops.
What does this have to do with thermal survivability thresholds? As we’ve discussed in previous articles, we have a superpower unique in the animal kingdom: the ability to sweat copiously to cool off. If this sweat can evaporate, we can tolerate very high temperatures.
Your Thermal Threshold
With the basics behind us, we can explore our thermal threshold. Two research avenues provide answers.
The first avenue is based on physics. Humans maintain a body core temperature around 98.6oF. We generate ~100 watts of metabolic heat when performing light, sedentary activities, so we need an average skin temperature of around 95oF to create a gradient between internal organs and skin sufficient to dissipate metabolic heat.
From a physics standpoint, this means a wet bulb temperature of 95oF “for more than 6 hours leads to death even in fit, acclimated individuals”. Fortunately, such extreme environmental conditions don’t occur for that long.
The physics-based answer provides us with a clean upper thermal limit. Unfortunately, the real world is messy. Experimental studies have attempted to confirm a 95oF wet bulb temperature thermal threshold. Every time, experiments return an upper threshold limit well below 95oF.
Case in point: the research paper “Evaluating the 35°C wet-bulb temperature adaptability threshold for young, healthy subjects”. By carefully monitoring young, healthy volunteers (ages 18-34) in a variety of hot-dry and hot-humid environments, researchers found a wet bulb temperature around 88oF marks the shift from compensable to uncompensable heat stress when the test subjects pedaled on an exercise bike at very low intensity (mimicking metabolic heat production from daily activities like walking outside).[4]
Many studies corroborate this finding. But there’s one final (and important) nuance. The upper thermal limit for young, healthy people is around 88oF in a hot-humid environment, conditions where air temperature is around 90oF with 80% humidity. You might approach such conditions on the hottest, most humid hours of the year in Houston or New Orleans. The wet bulb thermal limit for uncompensable heat stress is actually lower when temperature gets really, really hot– like above 120oF– even if humidity is low.
Visually, the combination of heat and humidity that leads to uncompensable heat stress in young adults looks like this (the below is a composite graph based on data from several studies, including this one):
The dashed line is a fixed wet bulb temperature at 95oF across all combinations of temperature and humidity. The solid green line represents wet bulb thresholds determined by experiments. The top, representing conditions with 100% humidity, shows uncompensable heat stress starts at wet bulb temperatures around 87oF. This is the hot-humid end of the spectrum. Already, there’s an 8oF difference(!) between the human thermal threshold estimated by physics (the dashed line) and the one derived from lab studies using actual humans (the solid line).
However, the solid (experimentally derived) line doesn’t curve consistently like the dashed line. Instead, the solid line increasingly separates as ambient temperature rises and humidity decreases, ending at a temperature around 126oF- our upper thermal limit tipping us into uncompensable heat stress even when humidity is functionally zero.
Hot Spot Locations
We can finally return to the main question: where are we likely to experience conditions where even young, fit, healthy individuals are at risk of uncompensable heat stress? By now, you’ve probably guessed it’s locations where conditions get both really hot and humid.
The Persian Gulf area is one such region. The region lies at roughly the same latitude as Florida, with the critical difference that Florida is surrounded by water on three sides, benefiting from the ocean’s general cooling effect. The Persian Gulf is nearly surrounded by land and shallow as well, not offering nearly the same cooling benefit.[5] The Arabian peninsula and southern Iran are also largely deserts, heating up quickly and to extreme temperature (routinely exceeding 120oF, with records above 124oF in Saudi and 129oF in Iran). Deserts provide the extreme temperatures and, especially along the coast, the Persian Gulf provides the humidity.
In fact, the Guiness Book of World Records states the Saudi town of Dhahran, just across from Bahrain on the Persian Gulf coast, holds the world record for highest dew point (95oF), set on a day with an ambient temperature of 108oF.[6] Extrapolating these values gives an estimated wet bulb temperature over 96oF! You’d be well advised to stay indoors, with working air conditioning, in such conditions.
So, the Persian Gulf area along the coast experiences heat stress conditions closest, and potentially even briefly exceeding, human thermal thresholds. A close runner up is nearby in South Asia, especially the Indus river valley in Pakistan, along northern India, and into Bangladesh. A 2020 study found wet bulb temperatures in these areas routinely exceed 84oF (and are often much higher), values approaching the experimentally derived value for uncompensable heat stress.
Unlike Persian Gulf nations, the Indian subcontinent is more populous and less wealthy. Unlike oil-rich Dubai or Kuwait City, air conditioning is far less common. It’s no surprise that India and Pakistan are hotspots for heat related fatalities. This region is unique in that extreme thermal stress is experienced relatively early in the spring, as heat and humidity build prior to summer monsoons.
The Persian Gulf experiences the most extreme heat stress, and heat stress in South Asia affects the largest number of people. Other locations that deserve mention:
· Southeast Asia, including parts of Thailand, Cambodia, Vietnam, Laos, Malaysia, Singapore, the Philippines, and Indonesia, all experience extreme humid heat.
· Eastern China, seasonally during the summer months and closer to the coast (similar to the U.S. Gulf coast region).
· Parts of the west coast of Africa around the Gulf of Guinea.
· South America along the Caribbean Sea, especially areas along the Colombia and Venezuela coastline and Panama.
· The Gulf Coast of the United States.
Locations where heat and humidity create extreme thermal stress. Most areas are located along the coast, where oceans contribute to humidity extremes. This map is generalized but contextually useful; for a more detailed map, see Figure 1 in Raymond et al article.
These are not the only locations where extreme wet bulb temperatures occur,[7] but they are locations repeatedly discussed in scientific literature. As the climate warms, they are also the places most likely to begin experiencing sustained periods where conditions exceed human tolerance.
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About the author: Dr. Erik Patton holds a PhD from Duke University where he conducted research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
[1] To be more accurate, compensable heat stress doesn’t always require sweating to cool you down. Compensable heat stress is your normal state of being; outside of hot environments, other heat transfer methods (radiative, convective) alone are generally sufficient to maintain the body’s thermal equilibrium without sweating. For example, I’m writing this in an air-conditioned office and not sweating a bit – I’m under compensable heat stress conditions.
[2] In keeping with assumptions made in most of the scientific literature and lab studies cited, we ignore an important third variable, solar radiation, by assuming everyone is in the shade.
[3] To confuse matters a bit more, at 100oF with 10% relative humidity the “feels like” temperature (also known as heat index) is about 94oF. This is still lower than the actual air temperature but nowhere close to the 64oF wet bulb temperature. This is because a wet bulb thermometer is fully saturated and able to draw on a constant water supply, whereas the heat index makes assumptions to model a human body (you aren’t ever completely saturated, and sweating rate doesn’t always keep up with maximum evaporation potential in really dry conditions).
[4] It’s interesting to me that typical peak wet bulb temperatures in most inhabited regions approach 88oF, the same wet bulb temperature that marks the shift between compensable and uncompensable heat stress. Coincidence or evolutionary adaptation to our ancestors’ most extreme thermal environment?
[5] The currently-famous and contested Strait of Hormuz being the only outlet to the Indian Ocean.
[6] Fact-checked against NOAA’s Integrated Surface Database. It’s true.
[7] Some places, like central Africa along the equator, have few weather stations and/or lack historical records and so are not often included in research analysis.
