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
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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
exceeds the 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.
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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 hot
weather.
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 in
temperate 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 integrate PT 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.
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Strategy
Start early
Suggestions for Implementation
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 underdrink 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 onehalf quart of fluid.
6. Do not skip meals, as this is when your body
replaces most of its water and salt losses.
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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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