Heat Stress in Construction

Hazard Alert

Heat is a serious hazard in construction. Your body builds up heat when you work and sweats to

get rid of extra heat. But sometimes your body may not cool off fast enough. This can happen,

say, if you are up on a roof pouring hot asphalt or you are lifting heavy loads.

Too much heat can make you tired, hurt your job performance, and increase your chance of

injury. You can get skin rash. You can also get:

• Dehydration. When your body loses water, you can’t cool off fast enough. You feel

thirsty and weak.

• Cramps. You can get muscle cramps from the heat even after you leave work.

• Heat exhaustion. You feel tired, nauseous, headachy, and giddy (dizzy and silly). Your

skin is damp and looks muddy or flushed. You may faint.

• Heat stroke. You may have hot dry skin and a high temperature. Or you may feel

confused. You may have convulsions or become unconscious. Heat stroke can kill you

unless you get emergency medical help.

The Risk of Heat Stress

Your risk of heat stress depends on many things. These include:

• Your physical condition

• The weather (temperature, humidity)

• How much clothing you have on

• How fast you must move or how much weight you must lift

• If you are near a fan or there is a breeze

• If you are in the sun.

If there is an industrial hygienist on your work site, ask the hygienist about the Wet-Bulb Globe

Temperature Index. It is a more precise way to estimate the risk of heat stress.

Protect Yourself

Try to do these things:

• Drink when you are thirsty.

• Keep taking rest breaks. Rest in a cool, shady spot. Use fans.

• Wear light-colored clothing made of cotton.

• Do the heaviest work in the coolest time of the day.

• Work in the shade.

• For heavy work in hot areas, take turns with other workers, so some can rest.

• If you travel to a warm area for a new job, you need time for your body to get used to the

heat. Be extra careful the first 2 weeks on the job.

• If you work in protective clothing, you need more rest breaks. You may also need to check

your temperature and heart rate. On a Superfund site where the temperature is 70 degrees or

more, the U.S. Environmental Protection Agency (EPA) writes that while on the job you should

be monitored for heat-stress related symptoms, including changes in body temperature and heart

rate.

• If you think someone has heat stroke, call emergency services (or 911). Immediately move

the victim to the shade. Loosen his/her clothes. Wipe or spray his/her skin with cool water and

fan him/her. You can use a piece of cardboard or other material as a fan.

OSHA does not have a special rule for heat. But because heat stress is known as a serious

hazard, workers are protected under the General Duty Clause of the Occupational Safety and

Health Act. The clause says employers must provide “employment free from recognized

hazards…causing or…likely to cause physical harm.”

For more information, call your local union, the Center to Protect Workers’ Rights (CPWR)

(301-578-8500 or www.cpwr.com ), the National Institute for Occupational Safety and Health

(1-800-35-NIOSH or www.cdc.gov/niosh), or OSHA (1-800-321-OSHA or www.osha.gov). Or

check the website www.elcosh.org

© 2005, The Center to Protect Workers’ Rights. All rights reserved. CPWR is a research, training, and service arm of the

Building and Construction Trades Dept., AFL-CIO: CPWR, Suite 1000, 8484 Georgia Ave., Silver Spring, MD 20910. (Edward

C. Sullivan is president of the Building and Construction Trades Dept. and of CPWR and Sean McGarvey is secretary treasurer.)

Production of this card was supported by grants CCU317202 and 1 U54 OH008307 from the National Institute for Occupational

Safety and Health and grants U45-ES09764 and U45-ES06185 from the National Institute of Environmental Health Sciences.

The contents are solely the responsibility of the authors and do not necessarily represent the official views of NIOSH or NIEHS.

Heat stress – December 15, 2005

Heat Aware Fluid Intake Chart

When properly displayed, the Fluid Intake Chart serves as a quick reminder for workers to stay properly hydrated and to take periodic breaks while working in high heat conditions. Suggested rest times and fluid intake amounts are based on different work levels and may be affected by the predisposition of a worker’s health condition as well as other environmental factors.

Pricing

• 1-12
  $18.00/ea
• 13.25
  $16.00/ea
• 25-50
  $15.00/ea
• 50+
  $14.00+/ea

Heat Aware Fluid Intake Chart

When properly displayed, the Fluid Intake Chart serves as a quick reminder for workers to stay properly hydrated and to take periodic breaks while working in high heat conditions. Suggested rest times and fluid intake amounts are based on different work levels and may be affected by the predisposition of a worker’s health condition as well as other environmental factors.

Pricing

• 1-12
  $18.00/ea
• 13.25
  $16.00/ea
• 25-50
  $15.00/ea
• 50+
  $14.00+/ea

DOL-OSHA Announces Heat Stress Campaign

Written by Bruce Goldstein Wednesday, 27 April 2011 08:36

Secretary of Labor Hilda Solis announced today an educational campaign to prevent occupational illness and death from heat, which is a signficant risk in agriculture.  Below is the press release from DOL.  Secretary Solis’s op-ed on the topic is in the Arizona Daily Star, entitled, “Heat-related illnesses on job are preventable with common-sense practices,”

News Release:  US Labor Department launches national outreach campaign to protect workers from heat-related illnesses

WASHINGTON — Secretary of Labor Hilda L. Solis today will announce, from Anaheim, Calif., a national outreach initiative by the U.S. Department of Labor’s Occupational Safety and Health Administration to educate workers and their employers about the hazards of working outdoors in the heat and steps needed to prevent heat-related illnesses.

“If you’re working outdoors, you’re at risk for heat-related illnesses that can cause serious medical problems and even death,” said Secretary Solis. “But heat illness can be prevented. This Labor Department campaign will reach across the country with a very simple message — water, rest and shade.”

Each year, thousands of outdoor workers experience heat illness, which often manifests as heat exhaustion. If not quickly addressed, heat exhaustion can become heat stroke, which killed more than 30 workers last year.

“As we move into the summer months, it is very important for workers and employers to take the steps necessary to stay safe in extreme heat,” said OSHA Assistant Secretary Dr. David Michaels. “Drinking water often, taking breaks and limiting time in the heat are simple, effective ways to prevent heat illness.”

Heat can be a real danger for workers in jobs ranging from agriculture and landscaping to construction, road repair, airport baggage handling and even car sales.

OSHA has developed heat illness educational materials in English and Spanish, as well as a curriculum to be used for workplace training. Additionally, a new Web page provides information and resources on heat illness — including how to prevent it and what to do in case of an emergency — for workers and employers. The page is available at http://www.osha.gov/SLTC/heatillness/index.html.

Federal OSHA has worked closely with the California Occupational Safety and Health Administration to adapt materials from that state’s successful outreach campaign on heat illness for use in this national effort. In addition, OSHA is now partnering with the National Oceanic and Atmospheric Administration on weather service alerts that will incorporate worker safety precautions when heat alerts are issued across the U.S. NOAA also will include pertinent worker safety information on its Heat Watch Web page at http://www.noaawatch.gov/themes/heat.php.

OSHA will leverage relationships with other state and local partners, employers, trade organizations, unions, community groups, educational institutions and health care professionals to disseminate training materials, and educate workers and employers, on the hazards of working in the heat and how to prevent heat-related illnesses.

 

Workplace Fairness®

 

The information contained in this page was extracted from MSHA Safety Manual Number 6. For the purpose of providing you with a document better suited for presentation on the internet we have excluded some of the graphics found in the manual.

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PREFACE

   Historically, the engineer has made mining possible for human beings. Today the proper object is to provide conditions in which the miner is a contented individual operating at maximum efficiency.

P.R. Davis and A. A. Knight
Ergonomics of Mining, Medicine in the Mining Industries
J. M. Rogan, ed., F. A. Davis Company, 1972

  This is one of a series of manuals prepared by the technical staff of the Mine Safety and Health Administration (MSHA) to acquaint the reader with a specific area of mining. This manual deals with the nature of heat stress and strain and the mechanism of heat control in the human body. The signs of heat-related physiological disorders and the first aid for heat stress victims are described, and the stress control measures in mines are noted.

   This is a revised version of a manual originally published under the same title in 1976. Since then, no new heat stress studies have been made in mining.

   A list of references (Bibliography) is also included for those interested in additional information on the topics discussed in this pamphlet.

INTRODUCTION
There is the right of every red-blooded man to be assured that his work will be a daily satisfaction to himself; that it is a work which is contributing to the welfare and advance of his country; and that it will build for him a position of dignity and consequence among his fellows.

Herbert C. Hoover,
Principles of Mining ,
McGraw-Hill Book Co., Inc. 1909

  Enforcement is a good principle which can only be effective when accompanied with a good education and training effort, an effort to educate the miners in the health hazards, an effort to train miners on the importance of maintaining a healthy environment, an effort to motivate the miners to know and play a major role to improve the environment in mines.

Robert E. Barrett, Administrator,
Mining Enforcement and Safety Administration,
Annual Meeting of the American Public Health
Association, 1976

 

   Deep mines and mines sunk in hot countries are hot work sites. Some underground mines in moderate geographic zones are hot because of the unusually high heat flow from the earth. Many mines in the southwestern United States are located along a high heat zone. The thrust for the development of new sources of minerals calls for an expansion of underground mining in deeper, and therefore, hotter levels of the earth crust.

   In mining, as in other industries, the exposure of workers to very hot conditions is unhealthy and unproductive. Persons working in hot, humid work sites tend to be inefficient; quite often workers prefer to stay away from work or ignore unsafe working situations. Studies in South African gold mines have shown that high temperatures reduce the work output of miners.

   Dexterity and coordination, ability to observe irregular, faint optical signs, ability to remain alert during lengthy and monotonous tasks, and the ability to make quick decisions are adversely affected by the heat strain. For example, in a 3hour drilling operation performed under varying room temperatures, the best results were obtained at 84º F; the performance was reduced to 75 percent capacity at a room temperature of 91º F, to 50 percent at 96, and to 25 percent at 99º F. For a new employee., 77º F room temperature is the upper limit for best performance. Heat strain can also show itself in the form of irritation, anger, and other emotions leading to rash acts by persons performing hazardous jobs. The lowest accident rates have been related to men working at temperatures below 70º F, the highest to temperatures of 80º F and over.

   There is some evidence that older persons have a lower tolerance for heat. They start to sweat later than do younger individuals. It takes longer for body temperature to return to normal levels in older individuals exposed to heat stress. In one study the majority of all the individuals who fell victim to heatstroke were over 60 years of age.

    Warm-blooded animals can function regularly in almost all types of weather and climate because they can maintain their body temperature within a narrow range. Carbohydrates, fats and proteins in our food provide energy for our daily activities. The human body, like mechanical devices and machines, is not 100 percent efficient. At best, only 25 percent of the energy generated by the body’s metabolism is converted into mechanical work. Thus, at least 75 percent of the energy produced by metabolism is converted into heat which is needed to support the metabolic process. However, too much heat will interfere with the metabolism and cause health problems such as heatstroke, fainting, exhaustion, cramps and water deficiency.

The human body may be thought of as having a core and a shell; this assumption will make it easier to understand the heat control in the body. The core contains the deep muscles and tissues, including the heart, lungs, abdominal organs, and brain. The shell contains the skin, tissues forming the skin base, and the muscles close to the skin; the hands and feet are also part of the body shell.

    Rectal temperatures are a measure of the core temperature. At rest, the body core temperature remains almost uniform. Under extreme conditions-from sleeping in a cold environment to doing hard work in a hot work site — the core temperature varies from 95 to 104ºF.

    The health of a person at work and at rest depends upon the stability of the core body temperature. The core cannot store an excessive amount of heat without upsetting its delicate temperature balance. oral temperatures are a measure of core and the shell temperatures; the body core can dissipate its heat only through the shell. Blood serves as an effective vehicle for heat transfer between the body core and the shell.

   

Dissipation of Heat From Human Body

Two or more bodies having different temperatures can exchange their heat in several ways:

 

• convection is a mechanism of heat exchange through a medium such as air or water
• conduction is the transfer of heat between two bodies in contact
• radiation is a heat loss due to the emission of heat rays from hotter to cooler surfaces

  Conduction is not a direct route of heat loss to the environment for the human body — most of the body is in contact with the clothing. Heat rays (radiation) do not need a medium like air or water for their transfer. Thus, hot walls in a mine will not only warm the mine air through convection, but also will emit heat directly through radiation and affect persons working in the area.

    Muscular work can increase the heat production in the body 10 to 20 times that at rest. The excess heat generated in the body is dissipated through convection, radiation and evaporation. When the body is at rest, the combination of convection and radiation accounts for about threefourths of the dissipated body heat; and the remaining one-fourth is lost through evaporation. Heat loss through evaporation can take place in the following ways:

• sweat evaporation through almost two million sweat glands in the skin
• saturation of the inhaled air in the lungs with water vapor
• invisible loss of water through the skin without involvement of the sweat glands

   Any increase in physical activity calls for more intense evaporation from the skin because heat losses from the lungs and through convection and radiation are not enough to keep the body temperature steady. 
   

    The human body, when exposed to a wide range of increasing heat loads, can mobilize its resources and restore a balance between heat gain and heat loss. This leads to a new steady core temperature at a somewhat higher level. Heat stress refers to the total heat-related load on the individual from both environmental and metabolic sources.

    An increasing environmental heat stress causes changes in sweat rate, heartbeat rate, and body core temperature of the affected individual.

    Heat strain refers to the adjustments made by the individual in response to the heat stress. These adjustments include biochemical, physiological, and psychological processes.

    Healthy and physically fit persons are able to work under heat strain as long as sweat evaporation takes place; by contrast, persons with health problems have a limited capacity for heat strain endurance.

    An increase in the sweat rate is the first sign of the heat strain. The steady rise of the sweat rate causes an excessive wetting of the skin. Extended exposure to heat will cause a decline in sweat rate. The sweat gland fatigue and consequent reduction in sweat production indicates a very high level of heat strain.

    Some people have no sweat glands at all; such a condition should disqualify them from working in hot environments.

    Individuals working in hot areas under emergency conditions (mine rescue workers) and highly motivated individuals working in nonemergency conditions may overstrain themselves. To prevent this kind of strain, the workloads may be reduced, more recovery time may be allowed, or cool rest areas may be provided.
   

What Constitutes a “Hot” Work Site

    High or moderately high levels of work produce a great deal of heat in the body. Moreover, the following factors add to the discomfort of a hot work site:

 

• high air temperatures
• high surface temperatures
• high atmospheric humidity
• relatively low air movement

Here is one definition of hot working place:

Hot work site means any combination of air temperature, humidity, radiation and. wind speed that exceeds a wet bulb globe temperature of 79º F.

   The wet bulb globe temperature is measured with the help of a cluster of thermometers adapted to measure various work .site temperatures:

• dry bulb thermometer is used to measure the temperature of mine air to get an estimate of convective heat exchange in the mine work area
• wet-bulb thermometer reading and its comparison with the dry-bulb thermometer reading is used as an estimate of evaporative heat exchange
• globe-thermometer reading indicates the radiative heat coming from the surfaces around the work site
• the temperatures indicated by three thermometers are used in a formula which produces the wet bulb globe temperature (WBGT). If the WBGT exceeds 79ºF, the work site is “hot” according to the above definition

Proper Work Practices

   Generally speaking, work site temperatures; humidity, and air movement can be controlled to lower the heat load and to provide tolerable working conditions in hot mines. At a point, however, these control measures fail to prevent the temperature rise in a worker’s body core and proper work practices may be the only heat-stress control measure. The objective of a good work practice in a hot work site is to prevent the body core temperature from rising above 100º F (38ºC). The excessive heat gain must be offset by adequate periods of heat loss. Desirable work practices include the following:

• increasing workers’ heat tolerance by a heat acclimatization, and by increasing their physical fitness
• a work-rest regimen–frequent breaks and reasonably short work periods
• pacing a task
• performing heavy tasks in cooler areas or at cooler times
• rotating personnel on hot jobs providing readily accessible cooler rest areas, cool–50 degrees to 60 degrees F (10 degrees to 15 degrees C)–drinking water, and encouraging all workers to drink a cup of water every 15 to 20 minutes
• for persons not on a restricted salt diet by physician’s orders, a heavier use of salt at meals and drinking slightly salted water (about one level tablespoon salt to fifteen quarts of water)

Long-Term Adjustment of Individuals to
Heat Stress

 

 

    Repeated exposure to heat stress may increase the body’s tolerance to heat. Acclimatization is a long-term adjustment of an individual to a stress. An acclimatized person can perform many tasks in a hot and humid work site where a non-acclimatized person cannot work.

    A person should be given enough time for adjustment to a hot work site where the wet bulb globe temperature (WBGT) exceeds 79ºF. A recommended six-day acclimatization schedule calls for the miner to work in the hot work site for 60 percent of the time on the first work day, and an additional 10 percent of the time on the days that follow the first day:

First day Second day Third day Fourth day Fifth day Sixth day

50 percent exposure 60 percent exposure 70 percent exposure 80 percent exposure 90 percent exposure 100 percent exposure

   Acclimatized workers who return after nine or more consecutive calendar days of leave, should undergo a four-day acclimatization schedule:

First day Second day Third day Fourth day

50 percent exposure 60 percent exposure 90 percent exposure 100 percent exposure

The mining industry in South Africa has made a good deal of progress in reducing the incidence of heatstroke through selection and acclimatization of miners.

    A mine supervisor who is knowledgeable about the symptoms of heat disorders can recognize heat stress areas and take corrective action for workers who display heat-related symptoms. Underground miners tend to work in a self-paced manner due to the nature of mining; even’ so, supervisors with an understanding of heat stress should look out for miners who may be straining themselves.
    A person subject to heat stress may not be able to recognize the strain symptoms. This is why all miners working in hot areas should know the following:

• what are the signs of imminent heat illness
• how to administer first aid to heat stress victims
• how to reduce the heat stress

   Acclimatization and education of new employees to heat stress should be conducted at the same time. New miners should be warned against trying to keep up with active acclimatized miners during the initial stage of acclimatization.

    Medical surveillance can be set up for miners who may be working in hot work sites. This will require medical examination by a physician for all personnel who are to be assigned to hot jobs for the first time. The physician will examine the condition of the heart, blood vessels, kidneys, liver, glands of internal secretion, respiratory system, and the skin.

    The incidence of heat stroke, heat exhaustion, and heat cramps should be reported. Such reports will be useful for future heat stress studies.

    Mine planning, ventilation and air conditioning may reduce the heat stress to acceptable levels. Proper mine planning will provide for conveniently available cool rest areas and allow the worker to take the needed rest where he can cool off. When natural cooler air is not available, air conditioning becomes necessary.

   The sources of heat in surface mines and mills include the sun, machines, dryers, and the kilns. The solar heat load can be reduced by the use of a canopy in most cases. The radiant heat from dryers and kilns can be controlled by shielding. The heat gain by the operator’s body is partially lost through evaporation; therefore dry air moving past the body can be helpful.

    Wall-rock heat is the major source of heat in many underground mines. Ventilation is the best method of lessening the effect of wall-rock heat on miners. Sometimes air cooling or conditioning is necessary.

    All powered equipment-diesel engines, electric motors, compressed air equipment-contributes to the heat load of an underground mine. Generally, most lighting systems convert energy to heat. Excess heat can be reduced by using more efficient equipment and providing efficient ventilation in the mine.

    Ground water flowing through hot rock formations becomes hot. The transfer of heat from the ground water to the underground mine air can be controlled by using covered ditches or insulated piping for a speedy transfer of hot waters to the surface.

    From 50 to 100 percent of the energy set free in blasting shows up in the form of heat. Efficient blasting procedures will reduce the unwanted release of heat into the mine air.

    A part of the miner’s body metabolic energy is released to the mine air. In a crowded work site metabolic heat can be a problem. Automation and remote control of mining will serve as a control measure where metabolic heat is a problem.

• hot dry skin of red, spotted, or bluish or purplish coloration
• rising, high body temperature
• brain disorders — mental confusion, delirium, fainting, convulsions, and coma

   Unless promptly and properly treated, heat stroke may be fatal. The victim may suffer permanent brain injury and complications such as kidney, liver, and blood circulation disorders. Survival and complete recovery after undergoing an initially high body temperature is possible if prompt and effective cooling is provided. The victim must be moved to a cool area; further, soaking of the victim’s clothing with cold water and fanning will cool the body.

    Heat stroke results from the failure of the heat regulatory system in the body. The failure of sweating leads to the loss of evaporative cooling of the skin and an uncontrolled rapid rising of the body temperature. In milder cases of heat disorders, sweating may still be evident in spite of high body temperature.

    Heat fainting is the most common form of heat disability. It happens when the individual is in a standing position, the return of venous blood to the heart is not sufficient and the brain suffers from a temporary shortage of blood supply. The victim’s. prostration helps to restore the normal blood circulation. The victim should be removed to a cooler area. A prompt and complete recovery usually follows the prostration.

Heat Exhaustion and Heat Cramps

    The signs of heat exhaustion include weakness, lowered blood pressure, signs of fainting, tiredness, nausea and headache, even between work activities. Clammy and moist skin and a pale, muddy, or flushed look are also present. If sitting, the victim may faint on getting up. Heat cramps are painful contractions of skeletal muscles in arms, legs, or the abdominal area of the body.

 

    Heat exhaustion and heat cramps are the results of salt deficiency in- the body. The loss of salt through sweating and the urine may exceed the salt intake. Drinking large volumes of water without replacing the lost salt will allow the water to enter the muscles and cause spasms. Workers not on a restricted salt diet by physician’s orders should use more salt at meal time to make up for the loss of salt. Salt tablets irritate the stomach and should not be used. Painful spasms of muscles can be promptly relieved by intravenous infusion of salted liquid. In cases of high salt deficiency, restoration of body salt balance takes several days.

    Some individuals feel that by restricting their water intake in hot jobs they reduce the amount of sweat dripping from their faces and into their eyes. They should be convinced that they are trading safety for comfort and that a voluntary restriction of their water intake may lead to water?deficiency heat exhaustion and even heat stroke. The risk of dehydration is greater if the major part of the daily meals is dry or dehydrated. The victim of water deficiency and heat exhaustion is thirsty. In mild cases, rest in a cool area and the taking of water results in a speedy recovery.

    Prickly heat or heat rash is in the form of tiny red blisters in the affected skin area. Affected areas of the skin are treated with mild drying lotions; cooled sleeping quarters allow for the drying of the skin between heat exposures. Prickly heat is related to the wasting away (maceration) of the skin by the continuous presence of unevaporated sweat.

Certain areas of the earth’s crust release more heat than others; mines located along high heat zones are normally hot. This map shows the zones of high heat in the Western United States.

    Some deep mining operations, located along high heat zones in western United States, represent hot work sites. Workers performing hazardous jobs and exposed to temperatures in excess of 80º F are known to have high accident rates. Intensive physical activity in hot work sites results in high sweat and heartbeat rates. Long-term exposure of non-acclimatized persons to heat stress is unhealthy and unproductive. Selection, acclimatization, and education of employees for working in hot work sites, coupled with an effective engineering control of heat at the work site, will provide a healthy work environment in mines and mills.

1. Astrand, P.O., and K. Rodahl. Textbook of Work Physiology. Third Edition. McGraw-Hill Book Co., New York, 1986.

2. Brouha, L. Physiology in Industry. Second Edition, Pergamon Press, New York, 1967.

3. Leithead, C.S. , and A.R. Lind. Heat Stress and Heat Disorders. Akademiai Kiado, Budapest, Distrib. In U.S. by Intercontinental Medical Book Corp., New York, 1969.

4. Misaqi, F.L., J.G. Inderberg, P.D. Blumenstem, and Ted Naiman. Heat Stress in Hot U. S. Mines and Criteria for Standards for Mining in Hot Environments. MESA IR 1048, 1976.

5. Rogan, J.M.. (ed.). Medicine in the Mining Industries. F. A. Davis Co., Philadelphia, 1972.

6. U.S. Department of Health, Education, and Welfare,
NIOSH. Criteria for Recommended Standards . . . Occupational Exposure to Hot Environments. Revised Criteria. Publication No. HSM 86?113, 1986.

7. World Health Organization (WHO). Health Factors Involved in Working Under Conditions of Heat Stress. WHO Technical Report Series, No. 412, Geneva, 1969.

Carbohydrates – Organic compounds, such as sugar, starches, celluloses, which form the supporting tissues of plants and are important food for animals.

Dehydration – Loss of water or body fluids.

Metabolism – Chemical changes in living cells by which energy is provided for vital processes and activities and new material assimilated to repair the waste.

Wall Rock – The country rock immediately adjoining mineral deposits.

 

Heat Stroke

HEAT ACCLIMATIZATION

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Lawrence E. Armstrong, Ph.D.
Department of Sport, Leisure, and Exercise Science
University of Connecticut
Storrs, CT 06269-111
USA

 

Armstrong, L.E. (1998). Heat acclimatization. In: Encyclopedia of Sports Medicine and Science, T.D.Fahey (Editor). Internet Society for Sport Science: http://sportsci.org. 10 March 1998.

Physiological Responses
Heat Illness
Factors Affecting Acclimatization
Loss of Acclimatization
References

Subsequent to repeated bouts of exercise in a hot environment, there is a marked improvement in the physiologic responses of healthy humans. This improved tolerance to exercise in heat is known as heat acclimatization. When accomplished in an artificially controlled environmental chamber, this process is known as heat acclimation. The primary benefit of heat acclimatization is improved tolerance of exercise in the heat, evident as a reduction of the incidence or severity of symptoms of heat illness, and increased work output concurrent with reduced cardiovascular, thermal, and metabolic strain.

Physiological Responses

Heat acclimatization is specific to the stress imposed on the human body. For example, passive exposure to heat induces some responses, notably an improved ability to dissipate heat. In contrast, physical training in a cool-dry environment results in metabolic, biochemical, hematologic, and cardiovascular adaptations. Heat acclimatization via strenuous exercise induces responses attributed to both passive heat exposure and training in cool environments. Table 1 illustrates these relationships.

 

Table 1: The effects of 14 days of passive and strenuous exercise protocols in cool and hot conditions on selected physiological responses (Armstrong and Maresh, 1991). Symbols: O = minimal effect; + = moderate effect; ++ = major effect.
Physiological responses	No exercise hot conditions	Exercise cool conditions	Exercise hot conditions
Lower core temperature at the onset of sweating	++	+	++
Increased heat loss via radiation & convection (skin blood flow)	++	++	++
Increased plasma volume	+	+	++
Decreased heart rate	O	++	++
Decreased core body temperature	++	+	++
Decreased skin temperature	+	+	+
Altered metabolic fuel utilization	O	++	++
Increased sympathetic nervous system outflow (efferent)	+	++	++
Increased oxygen consumption	O	++	++
Improved exercise economy	O	O	+
Adaptation to exercise in a cool environment	O	++	++
Adaptation to exercise in a hot environment	+	+	++

 

Complete heat acclimatization requires up to 14 days, but the systems of the body adapt to heat exposure at varying rates. The early adaptations (initial 1-5 days) involve an improved control of cardiovascular function, including expanded plasma volume, reduced heart rate, and autonomic nervous system habituation which redirects cardiac output to skin capillary beds and active muscle. Plasma volume expansion resulting from increased plasma proteins and increased sodium chloride retention, ranges from +3 to +27%, and is accompanied by a 15-25% decrease in heart rate. This reduction of cardiovascular strain reduces rating of perceived exertion, which is proportional to central cardiorespiratory stress, also decreases during the first five days of exercise-heat exposure. Plasma volume expansion is a temporary phenomenon, which decays during the 8th to 14th days of heat acclimatization (as do fluid-regulatory hormone responses, see below), and then is replaced by a longer-lasting reduction in skin blood flow that serves to increase central blood volume.

The regulation of body temperature during exercise in the heat is critical, because of the great potential for lethal hyperthermia. Thermoregulatory adaptations (i.e., increased sweat rate, earlier onset of sweat production), coupled with cardiovascular adjustments, result in a decreased central body temperature. This response is maximized after 5 to 8 days of heat acclimatization. However, the adaptations of eccrine sweat glands are different during humid and dry heat exposures. Heat acclimatization performed in a hot-humid condition stimulates a greater sweat rate than heat acclimatization in a hot-dry environment. Also, the absolute rate of sweating influences thermoregulation. If hourly sweat rate is small (<400-600 ml), a peripheral adaptation of whole body sweat rate may not occur.

Conservation of sodium chloride (NaCl) also occurs during heat acclimatization. The NaCl losses in sweat and urine decrease during days 3-9 of heat acclimatization, resulting in an expanded extracellular fluid volume. Subsequently, NaCl losses in sweat and urine increase toward pre-acclimatization levels, once physiologic strain (i.e., cardiovascular, thermal) moderates. Francesconi and colleagues (1993) recently demonstrated that NaCl losses, during a strenuous 10-day heat acclimatization protocol, were related to plasma renin (PR) and plasma aldosterone (A) concentrations. When subjects consumed a low salt diet (4g NaCl per day) and a moderate salt diet (8g NaCl per day), both PR and A increased during the first four days of heat acclimatization, but decreased during the remaining 6 days of heat acclimatization. The improved cardiovascular stability, which occurred on days 1-4 of heat acclimatization, allowed equivalent exercise performance with both diets and apparently reduced the stimulation and need for large elevations of PR and A. No change in plasma levels of arginine vasopressin (AVP) were observed across time, in either dietary group, possibly because hourly water intake matched the fluid lost in sweat. Usually, AVP synthesis is stimulated by an increase in plasma osmolarity or alterations in blood pressure, plasma volume, and renal or hepatic blood blow. Therefore, it is unlikely that the ability to successfully sustain exercise, during the latter days of the heat acclimatization process, is specifically related to the action of hormones that regulate fluid-electrolyte balance. This is particularly true when salt balance has been achieved.

Excess dietary water and electrolytes do not speed the process of heat acclimatization. When dehydration or salt deficits exist, however, cardiovascular and thermoregulatory responses may be negatively affected, and the theoretical risk of heat illness increases. Consistent daily monitoring of body weight will allow athletes to recognize water deficits which require consumption of fluid (-2 to -3% of body weight), reduction of training duration/intensity (-4 to -6%), or consultation with an experienced physician (in excess of -7%).

Plasma cortisol concentration generally indicates the strain experienced by the body. Heat-acclimated, well-hydrated humans exhibit no change in plasma cortisol when exercise in a hot environment is mild. Under the same conditions, the lack of heat acclimatization and dehydration can result in large plasma cortisol increases. When exercise is intense and core body temperature rises markedly, the plasma cortisol concentration increases during the initial days of heat acclimatization, but returns to control levels after 8 days of heat acclimatization, reflecting the reduction in total body strain.

Physical training in a cool environment may or may not improve exercise economy. Metabolism can be affected by heat acclimatization, in that oxygen uptake during submaximal exercise is reduced. Large effects have been reported for stair stepping; treadmill and cycle-ergometer exercise produce smaller, but statistically significant, changes. The physiologic mechanism has not been defined exactly, but three theories exist: (a) blood flow to the skin increases, thus reducing central blood volume, venous return to the heart, and cardiac output; (b) the portion of cardiac output perfusing muscle decreases; and (c) the recruitment of muscle fibers shifts from predominantly oxidative to glycolytic fibers. Heat acclimatization reduces muscle glycogen utilization and post-exercise muscle lactate concentration.

Heat Illness

Heat acclimatization is of interest to physicians as well as athletes, because it reduces the incidence of heat illness and the intensity of symptoms. The most common heat illnesses among athletes are heat cramps, heat syncope, and heat exhaustion.

Heat cramps are usually unheralded and occur in the voluntary muscles of the legs, arms, and abdomen, after several hours of strenuous exercise in individuals who have lost a large volume of sweat, have drunk a large volume of hypotonic fluid, and who have excreted a small volume of urine. Sodium depletion probably causes heat cramps. Heat acclimatization decreases the risk of experiencing heat cramps.

Heat syncope (e.g., fainting) occurs most commonly during the first 3-5 days of heat exposure. This illness is related to the shunting of blood through dilated cutaneous vessels, postural pooling of blood, diminished venous return to the heart, reduction of cardiac output, and cerebral ischemia. Heat syncope typically occurs when the ambient temperature or humidity rises suddenly, or when a non-acclimatized individual performs exercise in a hot environment. Heat acclimatization reduces the incidence of heat syncope to nearly zero, after 3-5 days of exercise-heat exposure. This period corresponds with cardiovascular stabilization, early in the course of heat acclimatization (see above). Heat syncope is a syndrome distinct from heat exhaustion, because water and salt depletion do not always contribute to heat syncope.

Heat exhaustion is the most commonly diagnosed form of heat illness among athletes, despite the fact that its symptoms are often vague and differ greatly from one situation to another. Clinical descriptions include various combinations of headache, dizziness, fatigue, hyperirritability, tachycardia, hyperventilation, diarrhea, piloerection, hypotension, nausea, vomiting, syncope, heat cramps, as well as “heat sensations” in the head and upper torso. This explains why heat exhaustion is defined as the inability to continue exercise in a hot environment, and involves a diagnosis of exclusion. Heat acclimatization significantly reduces the signs and symptoms of heat exhaustion, after eight days of strenuous, intermittent running.

The three aforementioned heat illnesses all involve either fluid-electrolyte balance, extracellular volume and tonicity, or cardiovascular adaptation. This emphasizes the importance of (a) ample dietary intake of NaCl and fluids, and (b) fluid-electrolyte hormone regulation during heat acclimatization.

Factors Affecting Acclimatization

It is believed that host factors may influence the capacity to acclimatize to exercise in a hot environment. For example, older persons were previously thought to be less heat tolerant than their younger counterparts. Middle aged men (>45 yr) were shown to have higher heart rates, higher rectal temperatures, and lower sweat rates than young men, during exercise in the heat, both before and during exercise in the heat, both before and after heat acclimatization. Similarly, studies conducted in the late 1960s suggested that women were less tolerant of exercise in a hot environment than men. However, recent research has qualified and/or reversed these viewpoints. It is now recognized that few gender-related differences exist, when female and male subjects are matched for pertinent physical and morphological characteristics. It is also recognized that differences between older and younger subjects are not necessarily due to aging per se, but may be due to other factors such as decreased training volume and lower maximal aerobic power (VO2max)

Most experts agree that intense physical training in a cool environment improves physiologic responses and speeds the process of heat acclimatization. During training in cool conditions, optimal physiologic adaptations may be achieved if strenuous interval training or continuous exercise, at an intensity above 50% of VO2max, is performed for 8-12 weeks. Maintenance of an elevated core body temperature appears to be the most important physiologic stimulus.

Irrespective of physical training, VO2max generally influences physiologic responses during the development of heat acclimatization. Individuals with a high VO2max (>60 ml.kg^-1.min^-1) exhibit superior heart rate and rectal temperature responses, and usually reach a stable heat acclimatization state faster, when compared to those with a low VO2max (<40 ml.kg^-1.min^-1). However, maximal aerobic power per se may not be as important in conferring heat tolerance as the underlying physiologic adaptations (i.e. altered blood volume, vasodilation/vasoconstriction, and muscle metabolism) which result in VO2max differences between individuals. A recent publication by Pandolf et al. (1988) demonstrates this concept well. They exposed nine young men (21 y) and nine middle-aged men (46 y) to a 10-day heat acclimatization protocol (100 min treadmill walking per day, 49°C air temperature). The results of testing on Day 1 indicated that middle-aged men were able to exercise longer, had lower heart rates and rectal temperatures, and exhibited greater whole-body sweat rates than young men. The differences persisted for the first few days of heat acclimatization, but were absent by day 10 of heat acclimatization. Both groups were closely matched for body mass, surface area, percent body fat, and maximal aerobic power (51 versus 53 ml.kg^-1.min^-1, respectively). The factor that distinguished these two groups was their level of regular weekly physical training: middle-aged men ran an average of 39 km per week, whereas young men averaged only 8 km per week.

The phrase “heat intolerance” has been used in a wide variety of contexts. Interestingly, heat intolerance has been defined by some experts as an inability to develop normal physiologic adaptations, during repeated days of exercise in a hot environment. Some humans do not show the classic decreases in heart rate and rectal temperature that exemplify successful heat acclimatization. This has been of particular concern among persons with cardiovascular disease and prior heat stroke patients. One recent publication (Armstrong et al., 1990), however, reported that 9 out of 10 prior heat stroke patients exhibited normal heat acclimatization responses (90 minutes treadmill walking per day, 7 days, 40°C air temperature), 61 days after experiencing heatstroke.

Loss of Acclimatization

The physiologic adaptations to exercise training in a cool environment are lost after several weeks or months of inactivity. In contrast, heat acclimatization adaptations may vanish after only a few days or weeks of inactivity (i.e., 18-28 days). The first adaptations to decay are those that develop first: heart rate and other cardiovascular variables. The rate of decay of adaptations is affected by the number of heat exposures per week, the number and format of training sessions, and the degree to which core body temperature is elevated. Athletes with high VO2max usually will lose heat acclimatization adaptations slower than individuals with low VO2max.

References

Armstrong, L E, J P De Luca, and R W Hubbard. Time course of recovery and heat acclimation ability of prior exertional heatstroke patients. Med. Sci. Sports Exerc. 22: 36-48, 1990.

Armstrong, L E and C M Maresh. The induction and decay of heat acclimatization in trained athletes. Sports Med. 12: 302-312, 1991.

Armstrong, L E and K B Pandolf. Physical training, cardiorespiratory physical fitness, and exercise – heat tolerance. In: Human Performance Physiology and Environmental Medicine at Terrestrial Extremes, K.B. Pandolf, M.N. Sawka, and R.R. Gonzalez (Eds.). Indianapolis: Benchmark Press, 1988, pp. 199-226

Francesconi, R P, L E Armstrong, N M Leva, R J Moore, P C Szlyk, W T Matthew, W C Curtis, R W Hubbard, and E W Askew. Endocrinological responses to dietary salt restriction during heat acclimation. In: Nutritional Needs in Hot Environments, B.M. Marriott (Ed.). Washington, D.C.: National Academy Press, 1993, pp. 259-276.

Greenleaf, J E and C J Greenleaf. Human acclimation and acclimatization to heat: A compendium of Research. Moffett Field, CA: Ames Research Center, Technical Memorandum no. TM X-62008, 1970, pp. 1-188.

Hubbard, R W and L E Armstrong. The heat illnesses: biochemical, ultrastructural, and fluid-electrolyte considerations. In: Human Performance Physiology and Environmental Medicine at Terrestrial Extremes, K.B. Pandolf, M.N. Sawka, and R.R. Gonzalez (Eds.). Indianapolis: Benchmark Press, 1988, pp. 305-359.

Pandolf, K B, B S Cadarette, M N Sawka, A J Young, R P Francesconi, and R R Gonzalez. Thermoregulatory responses of matched middle-aged and young men during dry-heat acclimation. J. Appl. Physiol. 65: 65-71, 1988.

Sawka, M N, C B Wenger, A J Young, and K B Pandolf. Physiological responses to exercise in the heat. In: Nutritional Needs in Hot Environments, B.M. Marriott (Ed.). Washington, D.C.: National Academy Press, 1993, pp. 55-74.

Sciaraffa, D, S C Fox, R Stockmann, and J E Greenleaf. Human acclimation and acclimatization to heat: a compendium of research, 1968-1978. Moffett Field, CA: Ames Research Center, National Aeronautics and Space Administration Technical Memorandum no. 81181, 1981, pp. 1-102..

Wenger, C B Human heat acclimatization. In: Human Performance Physiology and Environmental Medicine at Terrestrial Extremes, K.B. Pandolf, M.N. Sawka, and R.R. Gonzalez (Eds.). Indianapolis Benchmark Press, 1988, pp. 153-198.

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The following letter and information was written and sent to us by James Baker from Shell Oil. As you can see from the information below James has been in the petrochemical industry for 40 years and has had the experience of wearing FRC clothing for many years and  shares  some vital information about the proper undergarments to be worn with FRC’s.

 

 

 

Bryan,

 

Below is a document with some general comments about FRC use.

I have been in the petro/chemical business for 40 years. The first 5 years was in construction, then 32 years in process operations, and the last nearly 3 have been as a HEO/Rigger.

During the early/mid 70′s man made material clothing was very popular. There was an explosion and fire on the unit I was working. Every man that had on synthetic clothing had it melted to his body.

Fire is a great motivator. One man jumped from the third floor, hitting different equipment on the way down. It was said that he broke all of the ribs on one side, his arm, and both legs. He then got up on broken legs and ran some 2 or 3 hundred feet before collapsing. He was then carried to the control room to be administered first aid. The sight of him was so terrible that when the board operator saw him, he had a heart attack. The man that jumped did not survive.

The outside operator was found trapped in the elevator. His coveralls were not FRC material. I think they were a cotton/polyester blend. They did not melt to his body, but they were smoldering in different areas.

Through my career PPE has constantly been improved. FRC’s is one of the many. PPE is no longer an option, it is mandatory as it should be.

In my opinion if you are exposed to 2000 degree plus fire, you are going to get burned, period. With FRC’s, at least your clothes should not flame up or melt on you.

 

The purpose of Fire Retardant Clothing (FRC) is to protect personnel against flash fire.

 

Nylon or other synthetic materials can melt to your body when exposed to a flash fire. This is a very ugly sight to see. Melted material on skin is nearly impossible to remove.

  

Normally, FRC shall be worn as the outermost garment.  Specific jobs may preclude this requirement such as jobs working with:

 

• Asbestos

 

• Chemicals/Hydrocarbons that require chemical protective clothing or special personal protective equipment (PPE).

 

In such cases, FRC must still be worn beneath the outermost layer in FRC designated areas.

 

Rain gear is acceptable as an individual’s outermost garment (over FRC) for inclement weather or when testing sprinkler and deluge systems.

 

Duct tape, masking tape, or other types of material should not be used to seal FRC arm cuffs or leg cuffs openings. This material can catch on fire or melt.

 

Mosquito repellent containing DEET has been found to adversely affect the properties of FRC.

 

It is recommended by Dupont (manufacturer of Nomex) that flame-resistant undergarments or undergarments made only of cotton, silk, or

wool worn beneath the FRC to reduce the risk of flash fire burns.

 

Heavily soiled or contaminated garments should not be taken home and handling such garments in a manner that prevents exposure to personnel from contaminated garments. 

 

Such garments should be tagged and placed in dedicated plastic bags and bins at the drop off locations.  A MSDS Summary Sheet of the contaminating material shall also be included.

 

FRC are not to be used in lieu of approved Bunker Gear as PPE for fire fighting activities.  (FRC is designed to offer protection against flash fires, not extended exposures.)

 

If performing spark-producing activities such as welding or grinding, the wearer should verify they are wearing FRC appropriate for such activities (IE. FR treated cotton).