Thermal Physiology 102: Human Thermal Limits

Pushing the Limits – Environmental Conditions.

What’s the hottest temperature humans can survive? Seems a straightforward question, but our ability to dissipate body heat through sweat makes the answer surprisingly complex. Despite a biological requirement to keep core temperature from rising more than a few degrees above 98.6^oF, it is possible for people to survive, and even work (albeit at reduced effort), in conditions exceeding 120^oF, provided sweat can evaporate and adequate drinking water is available.  However, human environmental limits are also constrained by other factors.

Relative humidity measures how much water vapor is in the air compared to how much water vapor the air can hold, affecting how easily sweat cools by evaporation. Aside from ambient temperature, humidity is the most important factor in determining the maximum human thermal tolerance. Crucially, relative humidity changes based on temperature, with warmer air able to hold more water vapor.[1] At low values of relative humidity, sweat freely evaporates, allowing survival in hotter conditions. Dry locations like El Paso can have high temperatures but relative humidity values below 10% during the hottest parts of the day.[2] In contrast, humid cities like Miami are both hot and have high humidity levels, with afternoon humidity averages in the mid-60% range (and occasionally higher).

Relative humidity and temperature are combined in an index called the wet bulb temperature. This index is measured by a thermometer covered in a saturated cloth sleeve; moisture from the sleeve evaporates, similar to sweat on our skin, cooling the thermometer (Fig. 1). Through evaporative cooling, the wet bulb temperature is nearly always lower (and, in dry conditions, sometimes much lower) than ambient temperature. The wet bulb index is the ideal index in determining the most extreme environmental conditions humans can survive because it factors in our ability to cool by sweating.  It also gives us a hard limit for human survival – above a wet bulb temperature of 95^oF, humans physiologically can no longer cool down.

Figure 1: A wet bulb globe temperature thermometer. This device has three measurement devices; the center instrument is a wet bulb thermometer. The cotton sleeve sticking up is saturated by a water reservoir and surrounds a thermometer. This “natural” version of the wet bulb thermometer is exposed to the air. Photo credit: Sr Airman Corey Hook (USAF), Defense Visual Information Distribution System.

This 95^oF wet bulb threshold is based on physics; heat flows from hot to cold. To cool our body core (usually 98.6^oF, but more on this below), our skin needs to be about 95^oF. This ~3.5^oF difference between core and skin creates a temperature gradient that allows circulating blood to move heat to the skin surface for dissipation into the environment. When the air surrounding our skin is cooler than 95^oF, heat can leave the skin by convection. If the air in contact with our skin is above 95^oF, heat can still leave the skin through sweat evaporation. However, if air in contact with our skin is above 95^oF and relative humidity is high, sweat can’t readily evaporate. In this hot and humid situation, the physics of heat flow leaves us with no method to dissipate heat off our skin surface and, therefore, prevents us removing heat from our body core.[3] (For a quick review of human heat dissipation methods, see our Thermal Physiology 101 article).

Scientists have recently questioned if this 95^oF wet bulb limit is too high- theoretical limits are only good if they hold up in the real world. Human subject testing is suggesting a lower wet bulb temperature limit for survival even in young, fit, healthy people – potentially as low as 88^oF in hot, humid environments.[4] These values come from measuring the core temperature of young volunteers walking on a treadmill in a lab chamber where temperature and humidity can be regulated. Above certain wet bulb temperatures, volunteer’s core temperature stops holding steady and starts to increase quickly, hinting at crucial thresholds where even relatively light work can lead to dangerous core temperature increases.

Wet bulb temperatures approaching 88^oF rarely occur naturally.[5] However, in areas where water can evaporate throughout the day (such as around coastal cities), wet bulb temperature can approach 88^oF. For example, the median extreme wet bulb temperature in Miami is a couple degrees warmer than the wet bulb temperature in Phoenix, despite Phoenix having much higher ambient temperature (Table 1). In our current climate, wet bulb temperatures approaching our physiological limits occur only briefly near the Persian Gulf coast, around the Red Sea, in parts of India and Pakistan, and along the southern Gulf of Mexico. In the United States, high wet bulb temperatures in the South explain high heat illness rates in these areas.

Table 1: Select ambient and wet bulb maximum temperatures. Values from Design Criteria Data from the National Centers for Environmental Information. Note wet bulb temperature is less than the ambient temperature due to evaporative cooling, and how this evaporative cooling is more effective in dry locations.

Human Heat Storage.

To understand more about human heat tolerance limits, we need to introduce the concepts of heat storage and compensable vs uncompensable heat production.

Heat storage in the body is a balancing act. When we exercise or work, our muscles generate waste heat, known as metabolic heat production.[6] In response, we activate mechanisms to dissipate this waste heat, scaling these mechanisms according to how quickly and drastically we increase metabolic heat production.

Functionally, this process works something like your home thermostat. The thermostat in both your house and your body has a fixed set point – at home, you might set this at 72^oF, while in the body it is around 98.6^oF. Your brain has a small area known as the hypothalamus that compares temperature readings from thermal receptors in your skin and central nervous system to your set point temperature. If these sensors register temperatures above the set point, cooling mechanisms kick in and continue until the set point temperature is reestablished, just as your home A/C turns on when a room temperature exceeds the programmed value. Kenefick et al, of the US Army Research Institute of Environmental Medicine, provide a detailed (and readable) overview of the process in this Sports Medicine paper.

If the rate of our body’s heat production equals the rate of heat loss, heat storage change in the body is zero, i.e., we maintain a stable temperature. There is an equation for this:

Left of the equal sign is body heat storage (the “delta” symbol denotes change). To keep heat storage change at zero (required for most warm-blooded animals like us humans), the sum of the values on the right side must also equal zero.

• M is metabolic heat production, or the heat produced by our body and muscles, and is always positive (the “+” sign).
• E is heat loss from sweat evaporation, which will always be negative (the “-“ sign) because sweat evaporation only cools our body.
• Ra, Cv, and Cd are heat exchange between the body and the environment by radiation, convection, and conduction, respectively; the “+/-” (plus or minus) denotes that heat can flow in either direction. Typically, heat flows from the body to the environment, so a ““ sign is used, but in hot environments where air and surfaces like buildings and asphalt is greater than 95^oF, heat flows from the environment to the body, so a “” sign is used.

As long as  is zero, heat production in the body is “compensable” and our core temperature does not rise.  When metabolic heat production (M) goes up, such as during physical labor, heat losses from the other factors on the right side of the equation (Ra, Cv, C, E) need to compensate. Our body turns up the sweat rate, reroutes blood flow to the skin, expands blood vessels near the skin surface, and increases heart rate to shuttle heat from the core to the skin faster.

Since it depends on our surrounding environment, we cannot control heat loss from radiation or conduction (aside from using an ICEPLATE® Curve enhanced vest or backpack or directly applying cold water or ice to our skin, or moving into an air conditioned room). We have some ability to increase heat loss from convection by increasing air flow over our skin, such as by using a fan. Our body’s primary response to keep heat storage change zero is increasing our sweat rate, which as we’ve noted is less effective in humid environments. We also have a limit to just how much sweat we can produce – a healthy, well-conditioned person can sweat a bit more than 3 liters and hour and keeping up this sweat rate requires constantly drinking water to replenish what is lost.

Pushing the Heat Storage Limits. 

These studies suggest the core temperature elite, well acclimatized athletes can endure is quite high, but such elevated temperatures cannot be maintained indefinitely. Sustained above about 104^oF, proteins in tissue begin to denature, or break down. Dedicated heat acclimation strategies increase the production of heat shock proteins that have a protective effect at the cellular level, and many elite athletes now incorporate thermal training in their workout programs. For non-elite athletes, the upper limit to core temperature is less clear. Human studies on the absolute maximum threshold is constrained for the obvious reason that no ethical research program will subject participants to a study designed to end with heat stroke!

Keeping Cool By Turning Off The Fan?

The human heat equation implies some interesting contradictions. One relevant, and hotly contested, is the role of fans for cooling. If skin surface temperature needs to be maintained around 95^oF, and heat flows from hot to cold, using a fan in any temperature above 95^oF should add to net body heat. Does this make using a fan in extreme temperatures counterproductive, perhaps even dangerous?

The answer is nuanced. Fan use (or other methods to increase airflow) in extreme temperatures is debated in the scientific community. Increasing airflow over the skin in temperatures below 95^oF promotes convective heat loss by preventing the formation of a warm boundary layer of air against the skin (by blowing it away). In temperatures above 95^oF, this is flipped; a boundary layer of air is replaced with hotter air by the fan. This has led to suggestions that fan use is not effective for personal cooling in hot weather. Authoritative agencies differ widely in recommending fan use; the Center for Disease Control suggests avoiding fan use in temperatures above 90^oF when indoors, while the World Health Organization puts this threshold at 104^oF.  The UK National Health Service splits the difference by recommending fan use below 95^oF.

When pressed to clarify these widely different recommendations, scientists respond with the unsatisfactory answer of “it depends”. Most of the time, using a fan or other form of airflow is beneficial for heat loss. Periods when fans aren’t useful depend heavily on humidity levels, so no single temperature value is always correct. Heat gained by blowing hot air over the skin is normally more than offset by heat loss increased by more sweat evaporation caused by the increased air flow. The additional evaporation occurs because the moving air replaces a skin-adjacent boundary layer of sweat-saturated air. Biophysical modeling studies suggest that, for health people who sweat normally, fan use is likely beneficial in temperatures above 102^oF, and above 107^oF if humidity is low.

 ---

About the author: Erik is a doctoral candidate at Duke University where he conducts 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). 

[1] In normal hot temperatures we experience, a rise of 1.8^oF (1^oC) increases the air’s ability to hold water vapor by about 7%. Technically, this relationship isn’t linear, but this 7% is a good approximation for the temperatures we experience on earth.

[2] I know from personal experience working out in the Chihuahuan desert north of El Paso that you are likely to end the day encrusted in salt from evaporated sweat but rarely end up drenched because sweat evaporates into the dry air so quickly.

[3] The exception, of course, is if we are using an adaptive technology like the ICEPLATE EXO® system, which allows heat to flow from skin to a cold ICEPLATE® by conduction.

[4] And even lower values in hot, dry environments, showing that high temperatures limit survival even in moderate and low humidities. The interaction of temperature and humidity on thermoregulation is a complex, evolving field of science.

[5] In addition to heat and humidity, atmospheric stability is required for really high wet bulb temperatures. In many parts of the world, heat energy draws humidity up into the atmosphere (think afternoon thunderstorms), preventing extreme humidity at ground level.

[6] Metabolic heat is also produced when we are resting, but generally at levels low enough that maintaining stable core temperature is not a challenge.

[7] It’s important to note that 107^oF is the highest value I’ve seen in any literature, was documented in elite and properly conditioned athletes, and likely represents the absolute maximum core temperature humans can survive without damage!