Industrial Hygiene

Fort Detrick
Industrial Hygiene & Environmental Health Office

Heat Injury Definition

Heat Can Kill - PosterA heat injury occurs when an individual engages in physical activity to the extent where the heat production within the body exceeds its ability to lose heat adequately. This results in a rise in inner body (body core) temperature to the levels at which normal body functions are interfered with. This may lead to temporary or permanent disturbances in bodily functions.

All heat injuries are preventable, but in order to prevent heat injuries, it is important to understand them.

Heat stress is caused by the interaction of three main variables: the mission / task, the environment, and the soldier / individual.

Each has several variables of their own; together, they can set the stage for causing or preventing a heat injury.

Failing to consider the variables while planning, performing a risk assessment, or determining risk management steps will result in heat injuries.

When a heat injury occurs, it is an indication of failure in one or more components of the prevention system. This makes prevention at command/supervisor level even more important. A good understanding of how to prevent heat injuries will go a long way towards the reduction of the incidence of heat injuries.

Three Types of Heat Injuries

Heat cramps - are the result of excessive salt and water losses due to profuse sweating in individuals whose bodies are attempting to rapidly lose heat. It presents as intermittent muscle cramps, which usually occur on the legs (calves and thighs).

Heat exhaustion - is a more severe form of heat injury. It implies a significant loss of water from the body. The signs and symptoms are weakness, exhaustion, headaches, dizziness and profuse sweating with an elevated body temperature.

Heat stroke - is the most serious form of heat injuries. It manifests with a body core temperature of 105.8°F and above. Individuals may present with confusion, aggressive behavior and may progress into a comatosed state. It is a medical emergency!

What Causes Heat Injuries?

The human body gains heat continuously through various channels. This gain is even more significant when individuals exert themselves physically in a hot and humid environment. Factors such as acclimatization, hydration and rest play a very important role. Lack of acclimatization, poor physical fitness, obesity, illness and a lack of instinct to drink water adequately are major risk factors for heat injuries.

Heat Gain and Heat Loss

HEAT GAIN by the body is due to:

  • Heat generated within the body by muscle activity and other body functions
  • Direct radiation from the sun's rays
  • Heat transfer from the air
  • High humidity which hinders the cooling of the body through the evaporation of sweat

HEAT LOSS is achieved by:

  • Evaporation of sweat
  • Radiation of heat outwards from the body surface
  • Transfer of heat from the skin to the surrounding air (convection)
  • Breathing
  • Urination

Body Heat Regulation - Staying Safe

  • In order for the body to lose heat adequately, regular rest in a cool or shady environment is also required. This allows the heat loss process to keep up with and "overtake" the heat gain process. Concurrent hydration is critical (see the Army's Fluid Replacement and Work/Rest Guide)
  • High air temperature, high relative humidity and exposure to the sun make it difficult for individuals to regulate their body temperature.
  • Excessive clothing will prevent heat from being lost to the environment.
  • When the environmental conditions and/ or clothing prevent the heat generated within the body from being dissipated, the body temperature will rise significantly. If this cycle is not stopped, heat injuries will occur.
  • The heat injuries are commonly associated with hard work in hot weather. However, they can also occur in relatively cool conditions when individuals are dressed in heavy protective clothing.
  • The same principles apply at night as well. If there is inadequate cooling of the body during physical exertion at night, heat injuries can occur.

What is Acclimatization?

Acclimatization is the ability of the body to undergo physiological adaptations so that the individual is able to cope better with the environmental and physiological heat stress.


  • Heat acclimatization increases sweating (by 50-100%) and this enhances the evaporative cooling capacity of the body. Increased sweating, however, can lead to dehydration. As such, individuals can adapt to heat (i.e. they can acclimatize), but not to dehydration.
  • Physically fit individuals acclimatize more rapidly than the less fit.

What is Dehydration?

Dehydration refers to the reduction of body water content to that below the normal physiological (and safe) level. Some degree of dehydration is inevitable when working in a hot and humid environment. This is due to water loss through sweating.


  • An individual under stress in a hot and humid environment may not sense dehydration at the early stages.
  • Individuals may maintain themselves at about 1.5 quarts below their ideal hydration status without any sense of thirst, thus exhibiting "voluntary dehydration".
  • The body may suffer dehydration of 1-2% of body weight and perform less effectively before the feeling of thirst is even noticed.
  • Caffeine and alcohol beverages have diuretic properties, which increase the risk of dehydration through increased urination.
  • Heat, wind and dry air increase the body's water requirements through loss of body water as sweat.
  • At high altitudes (>6,500 feet above sea level), there is an increase in the rate of breathing due to reduced oxygen levels. The dryness of the atmosphere also increases water loss through breathing. However, the feeling of thirst becomes less felt, and the desire to drink is suppressed at an altitude.

Physical Indicators of Dehydration

  • Less elastic
  • On pinch test, the skin regains its shape slowly
  • Reduced in volume and frequency
  • Concentrated and darker
  • Higher sweat rate
  • If sweat production suddenly stops despite continued heat exposure, dehydration has reached a severe level
Physical Work Capacity
  • Reduced endurance
  • Accelerated onset of fatigue
Heart Rate
  • Faster heart rate
  • Work seems increasingly more tiring and increases the heart rate rapidly
  • Suppressed appetite
  • Food intake is reduced during water deprivation, and water intake reduced during starvation
Mental Indicators
  • Less Alert
  • Increased lethargy
  • Difficulty in concentrating
  • Confusion
  • Irrational behavior

Urine Guide

If the water in the body is balanced, the urine will be a pale straw or lemonade color. When water loss from the body exceeds water intake, the kidneys need to conserve water, making the urine much more concentrated with waste products and subsequently darker in color.

  • All personnel should monitor hydration status by noting the color and volume of urine.
  • Even dehydrated personnel will continue to produce urine, called "obligatory urine". When dehydration is inevitable for operational reasons, obligatory water loss in urine can be reduced by avoiding diuretics like coffee and tea.
Urine Color Test

Dark yellow urine is a sure indicator that the individual is dehydrated and that the fluid consumption must be increased.

The aim is to produce urine no darker than color 3 of the Urine Color Chart. Desire to urinate less than twice per day and/ or producing urine darker than color 3 in the chart indicate severe dehydration; the individual must start drinking immediately.

6 Do's for Prevention of Heat Injuries

  • Do drink water until you are no longer thirsty and then a little more
  • Do rest well before and in-between strenuous exercises
  • Do loosen your clothing while resting
  • Do report sick if you are not feeling well before, after or during strenuous exercises
  • Do avoid exercises if medical leave is granted
  • Do remember the 7R's of the first aid for heat injuries

7 R Heat Management


Recognize Symptoms
Recognize early signs and symptoms of heat illness: weakness, tiredness, nausea, vomiting headache, dizzy, disorientation, confusion, inability to work, confused look

Rest Casualty
Rest the heat injury stricken individual in the shade to cool down

Remove Clothing
Remove all clothing

Resuscitate with cardiopulmonary resuscitation (CPR) if the individual has collapsed - heart beat and/or breathing stopped (if unsure about correct CPR techniques, get someone who does know)

Reduce Temperature
Reduce the body temperature as fast as possible by applying wet towels or pouring water on the body

If the individual is conscious, rehydrate by giving him water (slowly)

Rush the individual to the nearest medical treatment facility

Protect Your Skin

If all sunburns were reported, they would probably be the most common heat injury. The simple fact is that sunburn is just that - a burn, and really no different than any other thermal burn. In the worst cases, you can experience severe blistering. If a large area is burned, you can get fever, infections, and wind up in shock. An individual with a minor burn could have serious consequences, especially if the burn made it hard to use or wear required equipment. Let's face it, trying to carry equipment or even a shoulder bag with a burn on the shoulders and back isn't really going to be much fun.


Sunburn is caused by exposure to ultraviolet (UV) light. The UV light from the sun that reaches the earth is either UVA or UVB. UVA has a longer wavelength than UVB; UVB is the more dangerous of the two. The amount of UVB that reaches the surface of the earth is variable and depends on many factors; for example, exposure increases with higher altitude and being closer to the equator. Sand and snow reflect light, thus increasing exposure.

On the other hand, ordinary window glass filters out almost all UVB, as will smoke and smog. For the tanning enthusiast, sunlamps and tanning beds are mainly UVA, but there is still some UVB, so they are not completely safe.

It's no surprise, but the lighter your skin, the more likely you are to burn. If you have naturally blonde or red hair, you are more at risk. Some medications may also increase your risk, as can some colognes, perfumes, and soaps.

Aside from sunburns, you can run into other problems. Long-term exposure to sunlight ages the skin prematurely and can lead to wrinkled, mottled, or discolored skin. Actinic keratoses, which are pre-cancerous, are much more common. Every year, there are about 400,000 new cases of basal cell skin cancer, 80,000 new cases of squamous skin cell cancer, and 25,000 new cases of malignant melanoma. Although basal cell is usually cured by removing the cancer, squamous cell and malignant melanoma can spread - there are around 6,000 deaths each year from malignant melanoma alone.

Obviously, the best way to avoid sunburn is to avoid exposure. No, that doesn't mean you can't go outside. Like anything else, you can take some preventive steps.

  • Cover up. If you can prevent the sun's rays from getting to your skin, it is harder to get burned. Be careful, it is possible to get burned through some light fabrics.
  • Limit exposure. Hitting the beach for 8 hours a day after a winter at Fort Drum probably isn't a real good idea. Work up gradually. Start with not more than 30 minutes and work up from there. If you do go outside in most of the U.S. and similar latitudes, UVB is increased between 10 AM and 3 PM. Don't think you're safe in a pool. Often the water just cools the skin enough so that you don't notice you are getting a burn. And don't let an overcast day fool you - clouds aren't much protection either.
  • Use sunscreen. All sunscreens are not created equal and all suntan lotions aren't sunscreens - you have to read the labels. Look for 5 percent paraaminobenzoic acid (PABA). Sunscreens with PABA should be put on 30 to 60 minutes before going into the sun so that it will have time to bind to the skin, so it doesn't wash off with swimming or through perspiration. Look for a sunscreen with an SPF of at least 15, though some common sense is in order. Remember, the higher the SPF number, the better the protection. An SPF 15 is probably fine to use if you are mowing your lawn on a cloudy day, but not near enough for a full day of volleyball at the beach. If you are allergic to PABA, there are other sunscreens; look for anthranilate or cinnamate; another is benzophenone, but this works better for UVA. Zinc or titanium oxide creams can also be used.

If you do get a burn, get out of the sun! Cold water compresses are one of the best first-aid measures to relieve pain. Over-the-counter steroid creams aren't really useful, but analgesic ointments or sprays may be. Be careful, these can sometimes cause allergic reactions.

There are a lot of folk-remedies for sunburn, whether they work or not is a topic for debate. One thing about them is certain, however, if the skin is broken or if there is blistering, don't put things you don't know about on the burn! When in doubt, check with the medical folks.

Sunburn is preventable.
You only have one skin, and it has to last a lifetime - protect it!

Heat Prevention Posters and Cards

Heat injury prevention products from the Army Public Health Command are available on the USAPHC Website.

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