Heat Acclimatization: Our Natural Defense Against Heat
Heat Acclimatization
This summer, every state in the country is expected to experience warmer-than-average conditions. Already, an early-season heatwave has broken records in the South, sending temperatures well above 100oF in “one of the hottest May heatwaves of all time”. Early arrival of such intense heat is particularly dangerous. The first heatwave of a year increases mortality rates by over 5%, much higher than the 2.65% increase in fatalities typically attributed to late summer heatwaves.
June, July, & August Seasonal Temperature Outlook for 2025. Source: NOAA
Early season heatwaves are dangerous because people in mid-latitudes have not had time to acclimatize to the heat. (Aside from Hawaii and Alaska, the U.S. is in the mid-latitudes).
The most common recommendation during early season heat is to stay indoors in air conditioning. Of course, for many, staying indoors on a hot day isn’t an option even during record-breaking temperature. This article is written for those of us who can’t let extreme heat slow them down, no matter when it happens. Aside from active thermal management like air conditioning or Qore Performance products, the best defense against extreme heat is acclimatization. We’ll discusses heat acclimatization in two parts:
(1) Physiological changes and benefits from heat acclimatization, and
(2) How achieve heat acclimatization.
Heat Acclimatization 101: Physiological Changes and Benefits
Heat acclimatization is the body’s natural defense against heat illness. Through dedicated training and exposure over two weeks, most of us can build a degree of tolerance towards heat (although there are physiological limits to how far such tolerance extends). This acclimatization comes from physiological adaptations that occur during repeated and sustained exposure when working or training in hot environments.
Physiologically, we temperature regulate in three main ways: (1) controlling the rate of metabolic heat production (for example, slowing down when running in hot weather), (2) blood flow redirection and increasing blood flow towards the skin through vasodilation (i.e., expanding blood vessels to direct more flow near the skin surface), and (3) sweating. Heat acclimatization generates changes to each method, making them more efficient at cooling us.
Controlling metabolic heat production.
It’s no surprise that extreme heat makes it harder to sustain a hard working effort. Reducing metabolic heat production is one of the fundamental ways humans respond to heat stress. The causal link is clear- if we produce more metabolic heat than we shed, we must reduce the amount of heat production or run the risk of heat stroke.
Compared to non-acclimatized subjects, heat acclimatization has been found to reduce metabolic heat production even when the working effort doesn’t change. After heat acclimation, people produce less metabolic heat for the same amount of physical work, leading to a slower rate of core temperature rise. Some studies find this is because muscles become more efficient, producing less waste heat and less lactate (sometimes called “lactic acid”) compared to pre-acclimatization. Additionally, heat acclimatized individuals can have slightly lower resting core temperature than their non-acclimatized counterparts.
This idea that heat acclimatization increases muscle efficiency, thereby reducing metabolic heat, is occasionally disputed. As humans, we typically modify metabolic heat production in a more direct manner- by reducing our exertions. Athletes reduce metabolic production in hot temperatures, even if involuntarily, resulting in slower times and decreased performance. One study of recreational athletes during a 10 kilometer race found a 27% reduction in time pace (from 52 minutes to 66 minutes) for races run at 95oF compared with 77oF. Another study found skin temperature directly correlates to how hard an athlete feels they are working, even though actual working effort in hot conditions may be 6-7% lower. These conclusions extend to anyone, not just athletes, working in hot conditions.
Vasodilation.
Blood flow shuttles heat from our core to the skin surface similar to how a car’s coolant shuttles heat away from the engine block to radiator. At skin surface, excess heat is released to the environment through convection, radiation, or sweat evaporation. Heat acclimatization adapts our cardiovascular system, making it more efficient at moving blood and, by extension, shuttling heat. Full acclimatization can increase our total body water mass by up to 7% (around 3 liters) and our plasma volume by up to 27% (although values less than 15% are more common), creating more “coolant” in our body.
This increased plasma volume (essentially, an increase of the amount of blood in our veins) is a critical physiological adaptation. As our body redirects more blood to the skin surface to offload heat, this thermoregulatory response competes with other body functions that require blood flow, such as sustaining muscles and organs like our brain. The greater blood volume induced by heat acclimatization helps us avoid potentially dangerous low blood pressure if there is not enough blood volume to meet all our body’s competing demands.
Increased plasma volume triggered by heat acclimatization is accompanied by more red blood cells, indirectly aiding working effort in the heat by boosting oxygen delivery to our muscles. Compared to a control group training in cold conditions, one study found a 3% increase in hemoglobin mass (i.e., the protein in our blood that transports oxygen) in athletes who underwent heat acclimatization training.
Sweating
Sweat plays such a vital and uniquely human role in thermoregulation that it’s been said “there is no one human attribute of more importance than the ability to sweat skillfully”. Despite a modern distain for sweat, it’s fundamental to what makes us human. Our ability to sweat is widely believed to be a key evolutionary adaptation that made it possible for humans to run, hunt, and become the dominant species on the planet.
Since sweating is the most important thermoregulatory tool we have, it’s unsurprising that heat acclimatization modifies both how we sweat and sweat itself. Heat acclimatized individuals start sweating sooner and at a lower core temperature. Both modifications mean the body’s primary defense against heat illness in hot conditions kick in faster and more robustly.
The composition of sweat itself also changes in heat acclimatized individuals. Sweat always contains some amount of sodium which is lost by sweating and must be replaced. Hyponatremia- a lack of sufficient sodium in the blood- can result from excessive sweating and manifests as cramps and fatigue or, in severe cases, seizures and coma.
The concentration of sodium in the sweat of heat acclimatize people can be more than 80% lower than before heat acclimatization. During acclimatization, sweat glands learn to reabsorb more sodium, slowing our sodium loss rate to help prevent hyponatremia. In addition, less-salty sweat more easily vaporizes, meaning heat is more efficiently transferred away from the skin surface.
Table adapted from work by Swaka et al and Periard et al
Improvements in these areas (heat production, blood flow, and sweating) result in four classical markers of acclimatization to a hot environment: lower heart rate, lower core temperature, higher sweat rate, and improved aerobic fitness. For completeness, we’ll note there are additional benefits of heat acclimation we haven’t identified; these deserve a later article to explore comprehensively (for a summary, including adaptations like muscle glycogen sparing and increased levels of heat shock proteins, see this work by Sawka et al.).
Achieving Heat Acclimatization
The relationship between time in the heat and acclimatization is of special interest to the military, especially elite, rapid response units that may deploy to hot, unforgiving parts of the world with little notice. For example, one US Army company in the 82nd Airborne Division modified their daily physical training schedule for hot parts of the day to prepare for an upcoming operation in the tropical Pacific, with great success at reducing heat casualties.
The need for heat acclimatization is limited to elite military operators though; in June, 2003, during Operation Iraqi Freedom, over 300 British soldiers were admitted to a military hospital in the Middle East with heat illness. 90% had arrived in country within 10 days. The military’s heat stress manual notes that before heat acclimatization, about 90% of service members can’t patrol for 100 continuous minutes in 120oF conditions. After heat acclimatization, about 90% of service members can.
Athletes also have special interest in heat acclimatization, especially because physiological benefits gained by training in the heat extend to competition in cooler climates.
Regardless who is seeking heat acclimatization, achieving heat acclimatization requires about two hours a day of hard aerobic exercise under hot conditions over about two weeks. The effort should be increased slightly each day, but it is important to balance heat stress and cooling off cycles, especially in early days of a heat acclimatization program. There is no added benefit from any effort that results in heat illness (if fact, it is counterproductive!). However, work or exercise must be rigorous, at least to the point of sweating. Absent a formal program, acclimatization can also be partially obtained passively by living and working continuously in the heat, although it may take longer and be less robustly developed.
Not all physiological processes improve at the same time. Cardiovascular benefits develop quickly, starting in 4 or 5 days, with all efficiency achieved around day seven of a heat acclimatization program. Also related to the cardiovascular system, plasma volume increases begin early, about 3-4 days after first heat exposure.
Changes to sweating take slightly longer. Sweating starts sooner, and at a lower core temperature, around day five of an acclimatization program and becomes fully developed around day fourteen. Sweat sodium concentration reductions start about day seven and are also fully developed by about day fourteen.
Lastly, lower body core temperature during exercise takes about two weeks to fully develop. Changes to physical tolerance, often indicated by a reduced rate of perceived exertion when working in the heat (i.e., work feeling easier), take fourteen days or longer to fully develop.
Unfortunately, acclimatization is lost if there is not sustained heat exposure. Around 3% of the benefits achieved through acclimatization are lost for each day without heat stress, and acclimatization is fully gone after about three weeks. Periodic heat stress can prevent a full loss of acclimatization benefits, so even a few sessions of heat stress each week can help maintain some level of acclimatization.
Heat Acclimatization and Active Cooling
If heat acclimatization naturally protects us from heat stress, why bother with products like those manufactured by Qore Performance? There are three good reasons why acclimatization should be supplemented with active cooling products like ICEPLATE® and products that assist natural physiological processes like ICEVENTS®.
First, and most importantly, heat acclimatization increases human thermoregulatory efficiency only so much. Heat acclimatization mitigates heat illness, but we are still subject to physiological human limitations regardless of how acclimatized we get- and each benefit has an upper limit. Work hard enough, or in a hot enough environment, and metabolic heat production overwhelms even the most acclimatized physiology.
Qore Performance products provide the competitive edge to keep us working harder and longer in conditions when even the most acclimatized among us can’t keep going unaided. (See our articles on cooling vests and performance or cooling vests and dehydration for the science). This is especially relevant now, as heat extremes occur more frequently and continue increasing in intensity and duration as our planet warms.
Second, heat acclimatization takes time. Not every worker has the ability to slowly ramp up effort in their first two weeks on the job. This makes Qore Performance especially useful for newer employees and for anyone working during the first heatwave of the year. When heat acclimatization isn’t or can’t be developed, active cooling products like ICEPLATE EXO® Gen 3 and ICEPLATE® SLK Gen 3 provide mitigation against heat illness.
Finally, heat acclimatation is hard. Studies referenced in this article focused mostly on athletes and soldiers with training plans who devote time and effort to achieving heat acclimatization. While some acclimatization develops naturally by living and working in hot conditions, most of us don’t have the time or energy for the strict heat acclimatization protocol needed to fully develop physiological benefits. ICEPLATE® and ICEFLASK products help extend what natural acclimatization we can develop to keep us moving and working longer.
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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).
