Heat Acclimation/Acclimatization
Topics of health and fitness are surrounded by myth and ill founded claims and heat acclimation is no exception. Over 15 years in the fire service I have heard everything from “we don’t have air conditioning in our engine because the heat helps acclimate you on the way to the fire” to “heat acclimation is a myth, just stay in shape and drink lots of water”, while both of those quotes have their truths neither is entirely accurate. In this article I am hoping to dispel some of the more common myths and present my current understanding of heat acclimation and a strategy for achieving and maintaining it. To that end I will start with some basic physics and physiology.
Heat exchange in the body occurs in a few different ways, namely convection, conduction, radiation, and evaporation. A person standing in the open is being heated by radiation from the sun, convection from his surroundings, and conduction from objects from his environment with which he is in contact (e.g., the ground); in short he is being heated from all angles.
Body temperature is normally regulated within a narrow rage through two parallel processes: behavioral temperature regulations and physiological temperature regulation. Behavioral thermoregulation includes seeking shade, slowing down or discontinuing exercise or work, or removing clothing/equipment. On the fire ground, behavioral thermoregulation drives are often overridden by the need for PPE and motivation to complete the mission. Physiological temperature regulation operates through heat loss responses (sweating and increased skin blood flow), which are proportional to the elevated core temperature and modified skin temperature (warm skin enhances heat loss response). Body heat loss by conduction, convection, and radiation is mediated by altering skin blood flow. Body heat loss by evaporation is primarily by secreting sweat. If the body stores heat, skin and or core temperature will increase. In response, the body initiates heat loss responses (sweating and increased blood flow). Unless the heat stress exceedsthe thermoregulatory system’s capacity to dissipate heat, heat loss response will increase until they restore heat balance, so core temperature stops increasing. If climate or clothing limits heat loss below the rate of heat production then sweating and increased blood flow will not restore heat balance but will only increase physiological strain.
Maintaining a high skin blood flow strains the cardiovascular system during physical work in the heat. High skin blood flow is associated with pooling of blood in the compliant skin and subcutaneous vascular beds. This pooling reduces cardiac filling and stroke volume, thus requiring a higher heart rate to maintain cardiac output. For these conditions, the primary cardiovascular challenge is to have sufficient cardiac output to simultaneously support high skin blood flow for heat dissipation and high muscle blood flow for metabolism and work out put. To help compensate for reduced filling, sympathetic activity is increased to elevate heart rate and divert blood flow from the core to the skin and muscle. The reduction in visceral blood flow, if excessive, can contribute to the development of heat injury.
So with that understanding of how heat acts on the body and how the body responds, what exactly is heat acclimation or acclimatization?
Heat acclimatization refers to biological adaptations that reduce physiologic strain (e.g., heart rate and body temperature), improve physical work capabilities, improve comfort and protect vital organs (brain, liver, kidneys, muscles) from heat injury. The most important biological adaptation from heat acclimatization is an earlier and greater sweating response, and for this response to improve it needs to be invoked.
How do you become heat acclimatized?
Heat acclimatization is the body adapting to heat and work stressors; it does this by increasing blood volume, reducing heart rate, redirecting blood to the skin, prioritizing water use toward sweat, and reducing salt excretion. This all starts to occur with repeated heat exposures that are sufficiently stressful to elevate body temperature and provoke perfuse sweating. Resting in the heat, with physical activity limited to that required for existence, results in only partial acclimatization. Physical exercise in the heat is required to achieve optimal heat acclimatization for that exercise intensity in a given hot environment.
Generally, about two weeks of daily heat exposure is needed to induce heat acclimatization. Heat acclimatization requires a minimum daily heat exposure of about two hours, which can be broken into two 1-hour exposures, combined with physical exercise that requires cardiovascular endurance, (e.g., hiking, running, or high intensity interval training) rather than strength training or low intensity drills. Gradually increase the exercise/drill intensity and/or duration each day, working up to an appropriate physical training schedule adapted to the high temperature environment.
The benefits of heat acclimatization will be retained for ~1 week and then decay with about 75 percent lost by ~3 weeks, once heat exposure ends. A day or two of intervening cool weather will not interfere with acclimatization but drinking alcohol in conjunction with reduced heat exposure can negate the acclimatization benefits in as little as 3 days.
What are the best heat acclimatization strategies?
Maximize physical fitness and heat acclimatization prior to the onset of hotweather.
Integrate physical training and heat acclimatization. PT in the coolest part of the day and acclimatize in the heat of the day with drills in full gear. Start slowly by reducing training intensity and duration (compared to what you could achieve intemperate climates). Increase training and heat exposure volume as your heat tolerance permits. Use interval training (work /rest cycles) to modify your activity level.
If the new climate is much hotter than what you are accustomed to, recreational type activities may be appropriate for the first two days with periods of run / walk. By the third day, you should be able to integratePT and skill training (20 to 40 minutes) at a reduced pace.
Consume sufficient water to replace sweat losses. Sweating rates of >1 quart per hour are common. Heat acclimatization increases the sweating rate, and therefore increases water requirements. As a result, heat acclimatized firefighters will dehydrate faster if they do not consume fluids. Dehydration negates many of the thermoregulatory advantages conferred by heat acclimatization and high physical fitness.
Strategy / Suggestions for ImplementationStart early / 1. Start at least 1 month prior to Summer
2. Be flexible and patient: performance benefits take longer than the physiological benefits
Mimic the training environment climate / 1. In warm climates, acclimatize in the heat of day.
2. In temperate climates workout in a warm room wearing sweats.
Ensure adequate heat stress / 1. Induce sweating.
2. Work up to 100 minutes of continuous physical exercise in the heat. Be patient. The first few days, you may not be able to go 100 minutes without resting.
3. Once you can comfortably exercise for 100 minutes in the heat, continue for at least 7-14 days with added exercise intensity (loads, or training runs).
Teach yourself to drink and eat / 1. Your thirst mechanism will improve as you become heat acclimatized, but you will still under-drink if relying on thirst sensation.
2. Heat acclimatization will increase your water requirements.
3. Dehydration will negate most benefits of physical fitness and heat acclimatization.
4. You will sweat out more electrolytes when not acclimatized, so add salt to your food, or drink electrolyte solutions during the first week of heat acclimatization.
5. A convenient way to learn how much water your body needs to replace is to weigh yourself before and after the 100 minutes of exercise in the heat. For each pound lost, you should drink about one-half quart of fluid.
6. Do not skip meals, as this is when your body replaces most of its water and salt losses.
The Myths
Myth – Age affects your ability to tolerate and acclimate to heat. Not true; research that controls for body size and composition, aerobic fitness, hydration, degree of acclimatization, and chronological age shows little or no age-related difference in one’s ability to manage or acclimatize to extreme temperatures.
Myth – Those who are physically fit do not need to be heat acclimatized. Not true; even physically active individuals cannot be fully acclimatized without exposure to environmental heat stress, although they will probably acclimatize to heat faster than less fit people.
Myth - Women were also thought to need longer acclimatization, since they are more vulnerable to heat illness. Not true; women and men, in fact, show equivalent reactions to heat during exercise when controlled for levels of fitness and acclimatization.
Myth – Spending time in air conditioned areas (barracks, engines) reduces heat tolerance. Not true; in fact the opposite is true, pre-cooling can extend the work time of firefighters who are heat acclimatized by allowing them to start the work with a lower core temperature.
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