Thermal Energy Part 2: Heat Stress Quantified

The Absolute Limit

Last year, a fascinating study tried to determine the absolute thermal limit of human survival. Unlike many prior thermophysiology studies, volunteer subjects remained at rest during testing and freely drank water. The study’s intent was to find the environmental conditions at which young, healthy individuals can no longer survive, even if resting in shade.

Testing protocol called for volunteers to enter a 107^oF heat chamber where humidity was slowly increased until their core temperature reached an inflection point. At this inflection point, subject’s core temperature began to rapidly increase. The temperature-humidity combination where the inflection occurred marks a transition from compensable to uncompensable heat stress. Put plainly, the study found the environmental condition where subject’s biological processes were no longer adequate to keep them cool.

Testing did not stop there. Equipped with information on the conditions that created a transition to uncompensable heat stress, subjects returned to the chamber for three more session. One was a control session at lower temperature, while the other two tested the subject’s ability to thermoregulate over nine hours in conditions slightly above or below what was identified as the uncompensable inflection point. The extreme nine-hour duration of the test protocol- the longest such study I am aware of- ensured subject’s core temperate could, if possible, stabilize somewhere below the 104^oF threshold associated with heat stroke.

During the session slightly above uncompensable conditions, eight of twelve subjects were pulled from testing before completing nine hours. These individuals all reached protocol safety limit of 102.5^oF core temperature or showed signs of heat illness like nausea and dizziness. By analyzing the rate of core temperature increase, researchers concluded that heat stroke- and possibly death- would occur, on average, after 9.8 hours of exposure.[1]

Aside from the test duration, this study is unique because temperature was a constant 107^oF during testing both above and below the uncompensable inflection point. To find fatal environmental conditions, researchers instead manipulated humidity. The finding that heat stroke would occur after 9.8 hours was based on a wet bulb temperature of about 92.6^oF, not ambient temperature alone.

Wet Bulb Temperature (T_wb) – A Scientific Index

Wet bulb temperature (T_wb) is a common thermal index used in human heat research for the simple reason that, in hot conditions, temperature alone does a poor job defining human survivability limits. As far back a 1905, researcher J.S. Haldance noted in dry, shaded conditions, men can remain in temperatures over 120^oF “without causing much inconvenience”.  Haldance also noted this is not true in humid conditions when sweat cannot evaporate from the skin.

We’ve explored humidity’s role in thermoregulation in earlier posts (see this video for a short explainer). Sweating cools us when thermal energy from our body is drawn into sweat during the transition to water vapor. This phase change consumes large amounts of thermal energy, with the effect that sweat evaporation efficiently transfers body heat to the environment.

A wet bulb temperature accounts for the sweat evaporation process. T_wb is measured by placing a saturated cotton sleeve over a thermometer. The thermometer cools by evaporation at the cotton sleeve just like our skin surface cools from sweat evaporation. Since evaporation consumes thermal energy, T_wb never exceeds ambient temperature.

However, the amount of cooling is directly related to humidity levels. In very dry conditions, such as desert environments, evaporation results in a T_wb much lower than ambient temperature. On the other hand, T_wb in conditions of 100% relative humidity will equal ambient temperature, since no evaporation (and therefore no cooling) is possible.

Since T_wb mimics the human ability to thermoregulate through sweat, it is extensively used in studies on human thermal limits. Based on physics, the absolute limit to human survivability was once proposed to be T_wb of 95^oF, equal to the temperature your skin surface must not exceed to maintain a thermal gradient sufficient to remove metabolic heat generated from basic biological processes (i.e., the body heat produced as a byproduct of organ function).

Recently, researchers have refined the absolute T_wb limit for human survival through experiments. The study discussed earlier is one example, finding the threshold for young, healthy adults is closer to a T_wb of 90^oF (and a T_wb value of 92.6^oF is fatal in less than 10 hours). Research protocols that include light working efforts, such as walking, find T_wb threshold limits even lower, around 82^oF.

It’s fortunate that T_wb is rarely recorded above 95^oF, but projections in coming decades assess that, in some parts of the world, it will start to occur much more frequently. Already, extreme humid heat values now occur more than twice as often as they did in 1979!

An interesting example of research on human thermal survivability limits is the “heat death line” proposed by Schickele in 1946. Her work for the US military explored environmental conditions during 157 heat-related Army recruit deaths in training during World War 2. The resulting chart, shown below, shows the relationship between temperature (on the left) and humidity (as vapor pressure, related to relative humidity, at the chart top and bottom). This is one of the foundational studies on the combined effects heat and humidity on human survivability.

Elizabeth Schickele’s “Heat Death Line”. The boxes “A” and “B”, above the heat death line, show a range of temperature-humidity conditions when recruit heat illness and heat death were documented. 

Wet Bulb Globe Temperature (WBGT) – The Occupational Standard

Using only temperature and humidity, wet bulb temperature fails to account for several other environmental factors that also influence heat stress. Primary among these is solar radiation. To address the need for a temperature index relevant for work under the sun, researchers developed the wet bulb globe temperature (WBGT) index.

The WBGT index has roots in U.S. military training. In the mid-1950s, military doctors and researchers conducted studies on the environmental conditions resulting in Army and Marine Corps recruit heat illness (and, occasionally, heat stroke deaths). Since much military training is conducted outdoors, it became clear a new thermal index was needed to identify conditions when training new, unacclimatized recruits is dangerous. Employing epidemiological techniques, researchers quantified when the interaction of temperature, humidity, wind, and solar radiation corresponded to higher incidences of heat illness during training.

The result was the WBGT index, used today by the U.S. military and many occupational organizations. A WBGT value is a weighted average of three thermometers, each measuring different environmental conditions. The greatest weight (70% of the value) is provided by the T_wb. As discussed, the T_wb is a useful proxy for a sweating human- perfect to include in a thermal index designed for military training. The ambient temperature, taken from a shaded thermometer, provides just 10% of the WBGT index value. The small portion assigned to ambient temperature reflects that air temperature alone is not a good indicator of human thermal stress.

The third instrument is known as a “black globe”. This is a thermometer inside a hollow, black metal sphere. The black sphere absorbs solar radiation, increasing the temperature of the thermometer inside. At 20% of the total WBGT value, the black globe reflects the added heat burden of being outdoors in the sun.

Today, the U.S. military uses a series of values for activity modification at various levels of thermal stress measured by WBGT index. The Army’s heat stress manual, Technical Bulletin (medical) 507, recommends water intake, work-rest cycles, and continuous work durations for tasks ranging from “easy” to “very heavy” work.

Military WBGT guidelines for intermittent training activities in hot-humid environments. Temperature values are provided by the WBGT index, not ambient temperature.

WBGT guidelines for activities with breaks between intense efforts, such as soccer or football games. Chart from the National Weather Service based on American College of Sports Medicine guidelines. A different chart (available here) provides recommendations for continuous activities like running, recommending activity cancellation if conditions exceed WBGT values between 82.1-86oF.

Designed for use outdoors, the WBGT index is one of the most common thermal indices for evaluating occupational heat stress. Organizations using WBGT are as diverse as the Occupational Safety and Health Administration, American College of Sports Medicine, and the Korey Stringer Institute. Even some school districts use the WBGT index to assess heat stress risk for young kids playing outdoors.  

Heat Index (HI)– More Common, Less Useful

The Heat Index is similar to T_wb because it relies on temperature and humidity to calculate a “feels like” temperature accounting for sweat’s ability to cool a person. Aside from ambient temperature, heat index is perhaps the most common thermal index used in the United States.

The heat index is based on temperature, humidity, and human thermoregulation and assumptions developed back in 1979. However, in very hot conditions, many researchers argue the heat index gives “unphysical results”. When conditions are very hot and/or humid, heat index values are extrapolated using mathematical techniques. By one estimate, about 1% of hot-humid summer days at a study location in Oklahoma are “undefined” by current heat index calculation (i.e., a heat index value can still be provided but it may no longer be an accurate “feels like” temperature). As both temperature and relative humidity continue to increase, the frequency of periods with “undefined conditions” will only increase.

Uncertainty around calculating heat index in hot-humid conditions mean it is used less often in scientific research. However, it is sometimes included in studies. One group of researchers, after developing a method to extend heat index values for extreme hot-humid conditions, estimated a heat index value of 161^oF as the limit at which healthy humans can survive without some form of active cooling. 

(As an aside, the researchers included my favorite lines from any thermoregulatory article.  Scientific reports are generally boring, but the researchers make clear their assumptions include an “ideal human” with “no modesty”, willing to get completely naked to help maximize thermoregulation. I suppose in extreme, life-or-death heat, taking all your clothes off to cool down is appropriate!)

Despite scientific misgivings, the heat index is extensively used in the U.S. to provide excessive heat warnings. The chart below shows the relationship between temperature and humidity, showing the key role humidity has on human thermal comfort. For example, a 100^oF day in Tucson, where relative humidity can be less than 15%, may “feel like” the mid-90s. As humidity increases and less sweat evaporation is possible, heat index values exceed actual ambient temperature.

The heat index “feels like” temperature at different ambient temperature and humidity levels. Source- National Weather Service

Other indices

Other thermal indices exist, developed to address limitations of the WBGT and Twb index. For example, the Universal Thermal Comfort Index (UTCI) was developed to more accurately account for individual heat stress. This index includes advanced physiological and clothing models along with environmental variables like temperature and humidity. Unfortunately, such indices are less user friendly, since they requires inputting values unique to individual clothing or safety gear or estimating how much of the skin surface is uncovered.