90 Preventing Hyperthermia

Whole body hyperthermia is a method to raise a patient’s body temperature for the treatment of advanced cancer. This technique is based on laboratory studies that show cancer cells are more sensitive to heat injury than normal cells. Physicians induce hyperthermia using a high-flow water suit controlled by a microprocessor, a machine which closely monitors body temperature. The patient’s body temperature is raised by the insulated build-up of metabolic (body) heat, plus by the heat delivered by the warm-water suit. Image Credit: Hyperthermia Patient by Mike Mitchell (photographer) [Public domain], via Wikimedia Commons


Hyperthermia, as opposed to hypothermia, occurs when body temperature increases as thermal energy builds up the body because heat is not transferred out of the body fast enough to keep up with the body’s thermal power. We can try to avoid such a situation by minimizing our work output to reduce overall thermal power (remember, the body has low efficiency so doing work means generating thermal energy). We can also use our understanding of the conduction, convection, and thermal radiation to ensure maximum heat transfer away from the body. For example, we can minimize the thickness of clothing to increase conduction, wear light colored clothing to reduce radiation absorbed from the sun, and encourage air circulation (convection).

The clothing warn by the people in this image was designed to minimize energy absorbed from the sun (light colors), not hinder conduction (thin), and allow convection (open, breathable). Image credit: OpenStax University Physics.



In some cases our thermal power outpaces the rate at which we exhaust heat by conduction, convection and radiation. Our strategy to deal with this situation is sweating. When we sweat some of the water on our skin evaporates into a water vapor. Only the molecules with the most kinetic energy are able to escape the attraction of their fellow water molecules and enter the air. Therefore the evaporating molecules remove more than a fair share of the thermal energy (thermal energy is just molecular kinetic energy remember).  The remaining liquid water molecules then have less thermal energy on average, so they are at a lower temperature and must absorb more energy from your body as they come to thermal equilibrium with your body again. This evaporation process allows the body to dump thermal energy even when the environment is too warm for significant heat loss by conduction, convection, and radiation. The amount of energy removed by evaporation is quantified by the latent heat of vaporization (Lv). For water Lv = 2,260 kJ/kg, which means that for every kilogram of sweat evaporated, 2260 kiloJoules of energy is transferred away from the skin.

The liquid temperature is determined by the average of the kinetic energy of atoms and molecules. At any moment the molecules will have a range of individual kinetic energies, some will have greater energy than the average and some less. (a) Those molecules with sufficiently large kinetic energy can break away to the vapor phase even at temperatures below the ordinary boiling point. (b) If the container is sealed, evaporation will continue until the space above the water reaches 100% Relative Humidity, meaning there is so many water molecules in the vapor phase that they re-enter the liquid phase just as often as they evaporate.  At 100% humidity evaporation will no longer provide a net cooling effect. Image Credit: OpenStax, Humidity, Evaporation, and Boiling


Everyday Example

A person working in an environment that happens to be very close to body temperature (about 100 °F) would not be able to get rid of thermal energy by conduction, convection, or radiation. If the person was working hard and generating about 250 W of thermal power (similar to the thermal power while shivering) then how much sweat would need to be evaporated each hour to keep their body temperature from rising?

In order to keep the body temperature from rising the person needs to get rid of 250 of thermal energy, that’s 250 J/s. Let’s convert that to Joules per hour:

(1)   \begin{equation*} (250 \,\bold{J/s}) = (250 \,\bold{J/s})(60 \,\bold{s/min})(60 \,\bold{min/hr}) = 900,000 \,\bold{J/hr} \end{equation*}

Each kilogram of water that evaporates removes 2,260,000 J of energy, so only a fraction of a kilogram will need to be evaporated every hour:

(2)   \begin{equation*} \frac{mass}{second}= \frac{900,000\,\bold{J/hr}}{2,260,000\,\bold{J/kg}} = 0.4 \,\bold{kg/hr} \end{equation*}

The body would need to evaporate 0.4 kg per hour. Water has a density of about 1 kg/L, so that would be a volume of 0.4 L/hr, or roughly 1.7 cups/hr, or 13.5 fluid oz/hr.



Heat Index

The rate at which water will evaporate depends on the liquid temperature and the relative humidity of the surrounding air. The relative humidity compares how many water molecules are in the vapor phase relative to the maximum number that could possibly be in the vapor phase at the current temperature. A relative humidity of 100% means that no more water molecules can be added to the vapor phase.  If the humidity is high, then evaporation will be slow and may not provide sufficient cooling. The heat index takes into account both air temperature and the relative humidity to determine how difficult it will be for your body to exhaust heat. Specifically, the heat index provides the theoretical air temperature that would be required at 20% humidity to create the same difficulty in exhausting heat as the actual temperature and humidity. Heat index values were devised for shady conditions with a light wind. Exposure to full sun or stagnant air can increase feel-like values by up to 15 degrees!

A table with rows labeled by relative humidity from 40% in the first row to 100% in the last row. The columns are labeled with temperatures in degrees Fahrenheit from 80 on the left to 110 on the right. The cells of the table show the heat index due to the humility of that row and the temperature of that column. The heat index values range from 80 in the upper left cell to 136 in the upper right cell, 87 in the bottom left cell to 132 in the bottom cell of the 90 F column. The bottom right 1/3 of the chart is empty as those situations are non likely to occur.
The Heat Index is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature. To find the Heat Index temperature, look at the Heat Index Chart above or check our Heat Index Calculator. As an example, if the air temperature is 96°F and the relative humidity is 65%, the heat index–how hot it feels–is 121°F. The red area without numbers indicates extreme danger. Image Credit: “Heat Index” by National Weather ServiceNOAA is in the Public Domain


Everyday Examples: Sweating, Dew, and Rain

When we sweat to exhaust thermal energy by evaporation we aren’t actively grabbing the hottest water molecules, pulling then away from their neighbors, and throwing them into the gas phase. The evaporation happens spontaneously because thermal energy stored in water molecules that are stuck together is relatively concentrated compared to thermal energy stored in water molecules zipping around in the air and free to disperse. The transfer of thermal energy to the environment by evaporation is a spontaneous process because it increases the dispersion of energy throughout the system made up of you, the sweat, and the surrounding air.

When the relative humidity reaches 100% then evaporation has maximally dispersed the available thermal energy. Any additional evaporation would begin to over-concentrate energy in the air and decrease the overall level of energy dispersion. Therefore, we don’t see evaporation occurring once 100% humidity is reached. In fact if the humidity gets pushed above 100% (by a drop in air [temperature without a loss of water vapor) then energy is over-concentrated in the air and thus increasing dispersion of energy requires that water molecules come out of the vapor phase and condensation occurs spontaneously. When the liquid condenses on surfaces we call it dew, when the liquid condenses on particles in the air and falls to the ground we call it rain.


Everyday Examples: Winter Dry Skin

The Pacific Northwest is famous for its winter rain, fog and general high humidity. However, people in the pacific northwest often suffer from dry skin in winter, but not summer when humidity is often less than 20 %. During winter, humid air is brought in from the outside and warmed by the heating system. That air still contains the same amount of water vapor, but is now at higher temperature, so the relative humidity is significantly reduced, even to the point of causing dry skin.

The Bends

We have learned that evaporation takes place even when a liquid isn’t boiling, so we may be wondering what causes boiling and how is it different from normal evaporation? Water ordinarily contains significant amounts of dissolved air and other impurities, which are observed as small bubbles of air in a glass of water. The bubbles formed within the water so the relative humidity inside the bubbles is 100%, meaning the maximum possible number of water molecules are inside the bubble as vapor. Those molecules collide with the walls of the bubble causing an outward pressure. The speed of the water molecules increases with temperature, so the pressure they exert does as well. At 100 °C the internal pressure exerted by the water vapor is equal to the atmospheric pressure trying to collapse the bubbles, so rather than collapse they will expand and rise, causing boiling.  Once water is boiling, any additional thermal energy input goes into changing liquid water to water vapor, so the water will not increase temperature. Turning up the burner on the stove will not cook the food faster, it will just more quickly boil away (evaporate) the water.

Everyday Examples: The Bends

At high altitude the atmospheric pressure is lower, so molecules of water vapor don’t need to create as much pressure within bubbles to maintain boiling. Therefore, boiling will occur at a lower temperature and cooking foods by boiling will take longer. (Food packaging often gives alternative cook times for high altitude).

The same process is responsible for the bends, which refers to the formation of nitrogen bubbles within the blood upon rapid ascent while SCUBA diving. You might imaging that you could hang out underwater by breathing through a hose, and that would work in very shallow water.  However, the high pressure exerted by water at depths below roughly 2 (6 ft) would prevent the diaphragm and rib cage from expanding to pull air into the lungs. At greater depths you need to breath from a pressurized container which helps to force air into your lungs against the additional hydrostatic pressure. Of course if you breathed from the container at shallow depth then the pressure would be too high and would cause damage to your lungs. A pressure regulator that outputs the appropriate pressure according to the water depth is the core of the SCUBA system.

There is always some gas dissolved in your blood, including carbon dioxide, oxygen, and nitrogen. The amount of dissolved gas is determined by the temperature and the pressure. If temperature is high enough, and pressure is low enough, then boiling will occur. Breathing high pressure air from a SCUBA system while at depth forces these gases to dissolve into your blood in the amounts determined by your body temperature and the high pressure.

When ascending, the pressure drops quickly, but the  body temperature stays constant, so the blood gases can begin to boil, starting with Nitrogen. There is not issue with blood temperature here, blood is still at body temperature, but the bubbles are a problem for the cardiovascular system. To prevent the bends, you must ascend slowly, allowing the gasses to slowly escape from the blood and be expelled in the breath, without forming large bubbles in the blood.

To treat the bends a patient is placed in a  hyperbaric (high pressure) chamber. The high pressure collapses the bubbles and prevents new ones from forming. The pressure is then slowly decreases to allow the blood gasses to escape slowly, simulating a gradual ascent.

Two people sit inside a steel chamber with airtight door, porthole window, and various sensors
Students at the Naval Diving and Salvage Training Center undergo various training scenarios to prepare them for duties involving underwater emergencies and procedures. Image Credit: “Decompression Chamber” by U.S. Navy Mass Communication Specialist 2nd Class Jayme Pastoric, via Wikimedia Commons.


  1. Hyperthermia Patient by Mike Mitchell (photographer) [Public domain], via Wikimedia Commons
  2. OpenStax University Physics, University Physics Volume 2. OpenStax CNX. Feb 6, 2019 http://cnx.org/contents/7a0f9770-1c44-4acd-9920-1cd9a99f2a1e@15.2
  3. OpenStax, Humidity, Evaporation, and Boiling. OpenStax CNX. Sep 9, 2013 http://cnx.org/contents/030347e9-f128-486f-a779-019ac474ff90@5
  4. "Zion National Park Visitor Center" by National Renewable Energy LaboratoryU.S. Department of Energy is in the Public Domain
  5. "Heat Index" by National Weather ServiceNOAA is in the Public Domain
  6. "Decompression Chamber" by U.S. Navy Mass Communication Specialist 2nd Class Jayme Pastoric, is in the Public Domain.


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