Heat, Sweat, and Survival: The Differing Physiological Impact of Humid and Dry Heat

Not All Heat is Equal

If you had the choice between working in 104^oF desert heat or in an 86^oF jungle, which would you take?  86^oF is certainly much cooler than 104^oF, but we’ve all heard the saying “It’s not the heat, it’s the humidity”. According to a recent study by scientists in the U.S. and Australia, in this case, your choice won’t actually matter- working effort in both environments leads to an equal core temperature rise despite the difference in temperature!

This seemingly impossible finding hints at the differing physiological responses our bodies have to dry and humid heat. In this article, we explore the science behind why dry heat and humid heat affect our body’s ability to thermoregulate in different ways, with implications for how we protect ourselves from heat stroke in these very different environments.

It Might Feel Different, but...

You may have read the first paragraph and thought it can’t be right- no matter what the science says, being in a hot desert must feel different than in a warm jungle. Perhaps you’ve vacationed in both Las Vegas and Miami, two hot places with very different climates, and instinctively felt your body react differently. How can both environments elicit the same core temperature increase?

The study mentioned above, which included both men and women, had subjects walk on an inclined treadmill to create a working effort.[1]  In one round, the environment was set to 104^oF with little humidity- the sort of conditions expected in summertime Arizona or Nevada. In the other round, temperature was lower, about 86^oF, but very humid- conditions common along the Gulf Coast in the summer, coastal Venezuela, or Southeast Asia.

As a follower of Qore Performance, you likely know we all must maintain a core temperature within a very narrow range; ideally, around 98.6^oF. Although we expect and can tolerate a temporary slight increase when working in hot conditions, our bodies try hard to maintain our core temperature within a few degrees of this value. When our core temperature rises above 104^oF (and sometimes, at lower values), heat stroke becomes a very real possibility.

The need to maintain a stable core temperature is why a main finding of the study is so important: body core temperature increases were virtually identical for subjects in both environments. Starting with core temperatures around 98.6^oF, subjects in hot, dry conditions increased their core temperature by an average of 2.3^oF, while subjects in the warm, humid environment increased by around 2.2^oF - essentially the same.

Importantly, core temperature was not the only response evaluated. Test subjects were also evaluated for how much they sweated, with interesting findings. As you might expect, the hotter but drier conditions led to much higher sweat rates. In just 90 minutes, subjects finished the hot-dry rounds having lost, on average, 2.2% of their body weight in sweat (a quantity that places them at greater risk of heat stress). This was a full half liter more water loss than during warm-humid rounds, where body weight was only down about 1.4%. However, despite greater sweating during the hot-dry tests, subjects reported feeling sweatier during the warm-humid rounds, since sweat does not evaporate as easily in humid conditions.

Different Environments, Different Heat Loss Pathways

Understanding how two very different environments resulted in the same core temperature rise requires knowing how the body sheds heat. Since the test protocol was the same during both environments, the amount of metabolic heat the subjects generated and needed to dissipate remained the same. However, the dissipation pathways were different. As you may recall from earlier posts, metabolic waste heat leaves the body primarily through three methods: convection, radiation, and evaporation (the fourth method, conduction, was not a factor in this experiment).

To highlight how subjects transferred heat in different ways during the two tests, consider this. During both tests, the test protocol caused subjects to generate a bit over 400 watts of metabolic heat. During the hot-dry tests at 104^oF, the environment actually added an additional ~65 watts of thermal energy to each person. This is because heat flows from hot to cold, and, with body temperature starting at 98.6^oF, the person was cooler than the surrounding air, resulting in heat gain from the environment. In comparison, the warm-humid test at 86^oF allowed about 90 watts to dissipate from the (hotter) subjects into the (cooler) surrounding air. The result is a difference of ~500 watts of excess heat during the hot-dry tests but only ~300 watts excess heat during the warm-humid tests.

While these are real differences, both would still quickly become lethal if there wasn’t another method to dissipate heat. Fortunately, us humans have the ability to dissipate heat by sweat evaporation. In fact, in the hot-dry tests, sweating was the only method to dissipate heat. In the low humidity, hot environment, subjects sweated copiously and- importantly- sweat could evaporate easily so that, on average, over 440 watts of heat energy was transferred through sweating. The warm-humid test had much lower evaporative heat loss, coming in only around 230 watts. This is because sweat evaporation is much less effective in humid conditions.

These summarized numbers don’t reflect the full range of variability in the study; those inclined to math will find the generalized numbers I list for the warm-humid report result in a bit more thermal energy retained in the test subjects. In the actual study, excess heat storage in subjects was found to be virtually the same across both test environments, leading to nearly identical core temperature increases. However, it exemplifies the differences in how heat loss occurs in two different environments: very well through sweat in hot-dry conditions, with sweating much less effective in humid conditions. 

When Heat and Humidity Become Lethal

Based on physics, environments with a wet bulb temperature above 95oF are lethal to humans. The “wet bulb” index is a reading taken by covering a thermometer with a saturated cloth to allow evaporative cooling, similar to human sweat. When the temperature, even with this evaporative cooling, remains above 95^oF, there isn’t enough of a temperature gradient between a human body and the environment to allow heat to dissipate fast enough to prevent a lethal rise in internal temperature.

A leading lab at the Pennsylvania State University has devoted research to refining the combinations of heat and humidity that are lethal for young, healthy adults. The results are fascinating, finding lethal conditions exist even at lower temperatures. (No one dies in these tests; testing is terminated once it is clear heat storage in the body is rapidly exceeding dissipation! Core temperature trends are then extrapolated to determine when death by heat stroke would occur.) Through human lab studies, these researchers have found wet bulb temperatures exceeding about 88^oF result in “uncompensable heat gain” when performing light physical activity- i.e., the body cannot dissipate heat fast enough to prevent uncontrolled core temperature rise.

Above the curve, the shaded area represents a zone of heat and humidity lethal to resting people, if exposed for sufficient time. For reference, the “X” represents a combination of ~95^oF and 100% relative humidity (or a wet bulb temperature of 95^oF). The “+” is less well defined, since individual sweat ability factors in, but is thought to be slightly more than 130^oF in very dry (~5% relative humidity) conditions. Image created by the author as a generalized example; for detailed charts based on lab experiments, see Cottle et al or Wolf et al.

Wet bulb temperatures approaching these values can be generated by a combination of air temperature around 95^oF with relative humidity above 70% (moderately hot and very humid, like heatwaves on the Indian subcontinent or in parts of the Philippines). Closer to home, wet bulb temperatures approaching this value have been experienced during Houston, Miami, and New Orleans heatwaves (wet bulb temperatures ~84^oF).

Even below a lethal threshold, more humid air degrades performance. One study focused on stationary bike time trials found up to 6% slower performance in a 97^oF compared to cooler trial runs at 80^oF. The study also found that increasing relative humidity from ~33% to ~66% further reduced performance by over 3%. Taken together, an athlete could reasonably expect a 10% performance degradation on a hot, humid day compared to a warm, drier day.

Other results from this lab corroborate these findings. At any given temperature, people reach critical thermal limits faster when humidity levels are higher. Conversely, when relative humidity is low, we can tolerate much higher temperatures. It all comes down to the ability of sweat to evaporate off our skin, carrying away excess heat.

In our dynamic world, temperature extremes are now approaching human thermoregulatory limits more frequently, and staying near them longer. In addition, in all but the coldest parts of the planet, water vapor in the air (the driver of humidity) is increasing at a faster rate than air temperature, since warmer air holds exponentially more water. Scientists note this means dangerous levels of increasing humid heat are driven more by increases in humidity, with air temperature playing a secondary role. Adaptation to these new conditions is critical to keep us moving, working, and playing.

So What?

That’s great science, but what does it mean in real life? In hot-dry conditions, it shows the vital importance of sweat and, by extension, maintaining hydration. When humidity levels are low, sweat is free to evaporate. Even in very hot conditions, this allows us to dissipate huge amounts of thermal energy through sweat evaporation from our skin – provided, of course, we have enough drinking water to replace that sweat! Tools like ICEFLASK™ and the ICEPLATE® Gen 3 water bladder address this challenge.

In humid environments, the situation is more nuanced. Sweat doesn’t evaporate as easily when it is humid, reducing its effectiveness to cool our bodies. If the environment isn’t extremely hot, we can still shed some heat without sweating, but in warm and hot weather it is unlikely to be sufficient to balance our internal thermal load. With sweat evaporation reduced, we run the risk of overheating. Conductive thermal solutions that directly draw away body heat help us maintain stable temperature. This is how Qore Performance’s IceAge ecosystem boosts our ability to handle the heat in humid conditions, directly drawing away body heat. In both dry and humid hot conditions, products like ICEVENTS® promote airflow over our bodies, assisting sweat evaporative to promote maximum sweat cooling efficiency.

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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 (120^oF) to Arctic conditions in central Alaska (-42^oF).