DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Criteria for a Recommended Standard

Occupational Exposure 

to Heat and Hot Environments
Cover photo by Thinkstock©
Criteria for a Recommended Standard

Occupational Exposure to 

Heat and Hot Environments

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Institute for Occupational Safety and Health

Revised Criteria 2016

Brenda Jacklitsch, MS; W. Jon Williams, PhD; 

Kristin Musolin, DO, MS; Aitor Coca, PhD; 

Jung-Hyun Kim, PhD; Nina Turner, PhD
ii Occupational Exposure to Heat and Hot Environments

This document is in the public domain and may be freely copied or reprinted.

Disclaimer

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Occupational Safety and Health (NIOSH). In addition, citations of websites external to NIOSH do 

not constitute NIOSH endorsement of the sponsoring organizations or their programs or products. 

Furthermore, NIOSH is not responsible for the content of these websites.

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Suggested Citation

NIOSH [2016]. NIOSH criteria for a recommended standard: occupational exposure to heat and hot 

environments. By Jacklitsch B, Williams WJ, Musolin K, Coca A, Kim J-H, Turner N. Cincinnati, 

OH: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 2016-106.

DHHS (NIOSH) Publication No. 2016-106

February 2016
Occupational Exposure to Heat and Hot Environments iii

Foreword

When the U.S. Congress passed the Occupational Safety and Health Act of 1970 (Public Law 

91-596), it established the National Institute for Occupational Safety and Health (NIOSH). Through 

the Act, Congress charged NIOSH with recommending occupational safety and health standards 

and describing exposure levels that are safe for various periods of employment, including but not 

limited to the exposures at which no worker will suffer diminished health, functional capacity, or life 

expectancy because of his or her work experience.

Criteria documents contain a critical review of the scientific and technical information about the 

prevalence of hazards, the existence of safety and health risks, and the adequacy of control methods. 

By means of criteria documents, NIOSH communicates these recommended standards to regulatory agencies, including the Occupational Safety and Health Administration (OSHA) and the Mine 

Safety and Health Administration (MSHA), health professionals in academic institutions, industry, 

organized labor, public interest groups, and others in the occupational safety and health community.

A criteria document, Criteria for a Recommended Standard: Occupational Exposure to Hot 

Environments, was prepared in 1972 and first revised in 1986. The revision presented here takes 

into account the large amount of new scientific information on working in heat and hot environments. This revision includes updated information on heat-related illnesses, risk factors affecting 

heat-related illness, physiological responses to heat, effects of clothing on heat exchange, and recommendations for control and prevention.

Occupational exposure to heat can result in injuries, disease, death, and reduced productivity. 

Workers may be at risk for heat stress when exposed to hot environments. Exposure to hot environments and extreme heat can result in illnesses, including heat stroke, heat exhaustion, heat syncope, 

heat cramps, and heat rashes, or death. Heat also increases the risk of workplace injuries, such as 

those caused by sweaty palms, fogged-up safety glasses, and dizziness.

NIOSH urges employers to use and disseminate this information to workers. NIOSH also requests 

that professional associations and labor organizations inform their members about the hazards of 

occupational exposure to heat and hot environments.

NIOSH appreciates the time and effort taken by the expert peer, stakeholder, and public reviewers, 

whose comments strengthened this document.

John Howard, MD

Director, National Institute for 

 Occupational Safety and Health

Centers for Disease Control and Prevention
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Occupational Exposure to Heat and Hot Environments v

Executive Summary

Occupational exposure to heat can result in injuries, disease, reduced productivity, and death. 

To address this hazard, the National Institute for Occupational Safety and Health (NIOSH) has 

evaluated the scientific data on heat stress and hot environments and has updated the Criteria 

for a Recommended Standard: Occupational Exposure to Hot Environments [NIOSH 1986a]. This 

document was last updated in 1986, and in recent years, including during the Deepwater Horizon 

oil spill response of 2010, questions were raised regarding the need for revision to reflect recent 

research and findings. In addition, there is evidence that heat stress is an increasing problem for 

many workers, particularly those located in densely populated areas closer to the equator where 

temperatures are expected to rise in relation to the changing climate [Lucas et al. 2014]. This revision includes additional information about the physiological changes that result from heat stress; 

updated information from relevant studies, such as those on caffeine use; evidence to redefine heat 

stroke and associated symptoms; and updated information on physiological monitoring and personal protective equipment and clothing that can be used to control heat stress.

Workers who are exposed to extreme heat or work in hot environments indoors or outdoors, or 

even those engaged in strenuous physical activities may be at risk for heat stress. Exposure to 

extreme heat can result in occupational illnesses caused by heat stress, including heat stroke, heat 

exhaustion, heat syncope, heat cramps, heat rashes, or death. Heat can also increase workers’ risk of 

injuries, as it may result in sweaty palms, fogged-up safety glasses, dizziness, and may reduce brain 

function responsible for reasoning ability, creating additional hazards. Other heat injuries, such as 

burns, may occur as a result of contact with hot surfaces, steam, or fire. Those at risk of heat stress 

include outdoor workers and workers in hot environments, such as fire fighters, bakery workers, 

farmers, construction workers, miners (particularly surface miners), boiler room workers, and factory workers.

In 2011, NIOSH published with the Occupational Safety and Health Administration (OSHA) a 

co-branded infosheet on heat illness. Through this combined effort, many recommendations were 

updated, including those on water consumption. In addition, factors that increase risk and symptoms of heat-related illnesses were more thoroughly defined. In 2013, NIOSH published “Preventing 

Heat-related Illness or Death of Outdoor Workers”. Outdoor workers are exposed to a great deal of 

exertional and environmental heat stress.

Chapters on basic knowledge of heat balance and heat exchange largely remain unchanged, although 

clothing insulation factors have been updated to reflect current International Organization for 

Standardization (ISO) recommendations. Additional information on the biological effects of heat 

has become available in recent studies, specifically increasing the understanding of the central nervous system, circulatory regulation, the sweating mechanism, water and electrolyte balance, and 

dietary factors. New knowledge has been established about risk factors that can increase a worker’s 

risk of heat-related illness. Those over the age of 60 are at additional risk for suffering from heat 

disorders [Kenny et al. 2010]. Additional studies have examined sex-related differences regarding 
Executive Summary

vi Occupational Exposure to Heat and Hot Environments

sweat-induced electrolyte loss and whole-body sweat response, as well as how pregnancy affects heat 

stress tolerance [Meyer et al. 1992; Navy Environmental Health Center 2007; Gagnon and Kenny 

2011]. As obesity and the increasingly overweight percentage of the population in the United States 

continue to increase, this is now a major health concern in workers. Heat disorders among the 

obese and overweight occur more frequently than in lean individuals [Henschel 1967; Chung and 

Pin 1996; Kenny et al. 2010]. Another factor affecting heat-related illness is use of drugs, including cocaine, alcohol, prescription drugs, and caffeine. Caffeine use has long been argued against, 

as it has a diuretic effect and may reduce fluid volume, leading to cardiovascular strain during heat 

exposure [Serafin 1996]. However, more recent studies have found that the effect of caffeine on heat 

tolerance may be much less than previously suspected [Roti et al. 2006; Armstrong et al. 2007a; 

Ely et al. 2011].

The definition of heat stroke has also changed in recent years. Heat stroke is now classified as 

either classic heat stroke or exertional heat stroke which is more common in workplace settings. 

Characteristics of the individual (e.g., age and health status), type of activity (e.g., sedentary versus 

strenuous exertion), and symptoms (e.g., sweating versus dry skin) vary between these two classifications [DOD 2003]. Re-education is needed in the workplace especially about symptoms. Many 

workers have incorrectly been taught that as long as they were still sweating they were not in danger 

of heat stroke.

Measurements of heat stress are largely unchanged since the last revision, although additional 

information has been added about bimetallic thermometers and the psychrometric chart. The latter 

is a useful graphic representation of the relationships among dry bulb temperature, wet bulb temperature, relative humidity, vapor pressure, and dew point temperature. Such charts are especially 

valuable for assessing the indoor thermal environment. In addition, many modern computers and 

mathematical models can be used to calculate heat stress indices, based on weather station data.

Heat stress can be reduced by modifying metabolic heat production or heat exchange by convection, 

radiation, or evaporation. In a controlled environment, these last three can be modified through 

engineering controls, including increasing ventilation, bringing in cooler outside air, reducing the 

hot temperature of a radiant heat source, shielding the worker, and using air conditioning equipment. Heat stress can also be administratively controlled through limiting the exposure time or 

temperature (e.g., work/rest schedules), reducing metabolic heat load, and enhancing heat tolerance (e.g., acclimatization). Although most healthy workers will be able to acclimatize over a period 

of time, some workers may be heat intolerant. Heat intolerance may be related to many factors; 

however, a heat tolerance test can be used to evaluate an individual’s tolerance, especially after an 

episode of heat exhaustion or exertional heat stroke [Moran et al. 2007]. Additional preventive 

strategies against heat stress include establishing a heat alert program and providing auxiliary body 

cooling and protective clothing (e.g., water-cooled garments, air-cooled garments, cooling vests, 

and wetted overgarments).

Employers should establish a medical monitoring program to prevent adverse outcomes and for 

early identification of signs that may be related to heat-related illness. This program should include 

preplacement and periodic medical evaluations, as well as a plan for monitoring workers on the job.

Health and safety training is important for employers to provide to workers and their supervisors before they begin working in a hot environment. This training should include information 
Executive Summary

Occupational Exposure to Heat and Hot Environments vii

about recognizing symptoms of heat-related illness; proper hydration (e.g., drinking 1 cup [8 oz.] 

of water or other fluids every 15–20 minutes); care and use of heat-protective clothing and equipment; effects of various factors (e.g., drugs, alcohol, obesity, etc.) on heat tolerance; and importance 

of acclimatization, reporting symptoms, and giving or receiving appropriate first aid. Supervisors 

also should be provided with appropriate training about how to monitor weather reports and 

weather advisories. 

The NIOSH Recommended Alert Limits (RALs) and Recommended Exposure Limits (RELs) were 

evaluated. It was determined that the current RALs for unacclimatized workers and RELs for acclimatized workers are still protective for most workers. No new data were identified to use as the 

basis for updated RALs and RELs. Most healthy workers exposed to environmental and metabolic heat below the appropriate NIOSH RALs or RELs will be protected from developing adverse 

health effects. The Wet Bulb Globe Temperature–based limits for acclimatized workers are similar 

to those of OSHA, the American Conference of Governmental Industrial Hygienists, the American 

Industrial Hygiene Association, and the ISO. In addition, the Universal Thermal Climate Index 

(UTCI), originally developed in 2009, is gaining acceptance as a means of determining environmental heat stress on workers [Blazejczyk et al. 2013].

During the 2014 peer review of the draft criteria document, concerns were expressed about the sufficiency of the scientific data to support the NIOSH ceiling limits for acclimatized and unacclimatized workers. In fact, many acclimatized workers live and work in temperatures above the ceiling 

limits without adverse health effects. Further consideration of the scientific data led to the decision 

to remove the ceiling limit recommendations from the document.

Although research has produced substantial new information since the previous revision of this 

document, the need for additional research continues. Two newer areas of research that will likely 

continue to grow are the effects of climate change on workers and how heat stress affects the toxic 

response to chemicals. It is likely but unclear to what extent global climate change will impact 

known heat-exposure hazards for workers, especially with regard to severity, prevalence, and distribution [Schulte and Chun 2009; Schulte et al. 2015]. Toxicological research has shown that heat 

exposure can affect the absorption of chemicals into the body. Most of what is known on this subject comes from animal studies, so a better understanding of the mechanisms and role of ambient 

environment with regard to human health is still needed [Gordon 2003; Gordon and Leon 2005]. 

With changes in the climate, the need for a better understanding will become increasingly important [Leon 2008].

In addition to the updated research, this criteria document includes more resources for worker and 

employer training. Information about the use of urine color charts, including a chart and additional 

information, is in Appendix B. The National Weather Service Heat Index is in Appendix C, along 

with the OSHA-modified corresponding worksite protective measures and associated risk levels.

NIOSH recommends that employers implement measures to protect the health of workers exposed 

to heat and hot environments. Employers need to ensure that unacclimatized and acclimatized 

workers are not exposed to combinations of metabolic and environmental heat greater than the 

applicable RALs/RELs (see Figures 8-1 and 8-2). Employers need to monitor environmental heat 

and determine the metabolic heat produced by workers (e.g., light, moderate, or heavy work). 

Additional modifications (e.g., worker health interventions, clothing, and personal protective 
Executive Summary

viii Occupational Exposure to Heat and Hot Environments

equipment) may be necessary to protect workers from heat stress, on the basis of increases in risk. 

In hot conditions, medical screening and physiological monitoring are recommended. Employers, 

supervisors, and workers need to be trained on recognizing symptoms of heat-related illness; 

proper hydration; care and use of heat-protective clothing and equipment; effects of various risk 

factors affecting heat tolerance (e.g., drugs, alcohol, obesity, etc.); importance of acclimatization; 

importance of reporting symptoms; and appropriate first aid. 

Employers should have an acclimatization plan for new and returning workers, because lack of 

acclimatization has been shown to be a major factor associated with worker heat-related illness 

and death. NIOSH recommends that employers provide the means for appropriate hydration and 

encourage their workers to hydrate themselves with potable water <15°C (59°F) made accessible 

near the work area. Workers in heat <2 hours and involved in moderate work activities should 

drink 1 cup (8 oz.) of water every 15–20 minutes, but during prolonged sweating lasting several 

hours, they should drink sports drinks containing balanced electrolytes. In addition, employers 

should implement a work/rest schedule and provide a cool area (e.g., air-conditioned or shaded) 

for workers to rest and recover. These elements are intended to protect the health of workers from 

heat stress in a variety of hot environments.
Occupational Exposure to Heat and Hot Environments ix

Contents

Foreword . 

Executive Summary 

Abbreviations . 

Glossary .  

Symbols . 

Acknowledgments .  

1 Recommendations for an Occupational Standard 

for Workers Exposed to Heat and Hot Environments 

1.1 Workplace Limits and Surveillance 

1.1.1 Recommended Limits 

1.1.2 Determination of Environmental Heat 

1.1.3 Determination of Metabolic Heat 

1.1.4 Physiologic Monitoring  

1.2 Medical Monitoring  

1.2.1 General 

1.2.2 Preplacement Medical Evaluations  

1.2.3 Periodic Medical Evaluations  

1.2.4 Emergency Medical Care 

1.2.5 Information to Be Provided to the Responsible Healthcare Provider  

1.2.6 Responsible Healthcare Provider’s Written Report of Medical Findings  

1.3 Surveillance of Heat-related Sentinel Health Events  

1.3.1 Definition 

1.3.2 Requirements  

1.4 Posting of Hazardous Areas 

1.4.1 Dangerous Heat Stress Areas 

1.4.2 Emergency Situations 

1.4.3 Additional Requirements for Warning Signs 

1.5 Protective Clothing and Equipment  

1.6 Worker Information and Training .

1.6.1 Information Requirements 

1.6.2 Training Programs 

1.6.3 Heat Stress Safety Data Sheet 

1.7 Control of Heat Stress  

1.7.1 General Requirements .

1.7.2 Engineering Controls 

1.7.3 Work and Hygienic Practices 

1.7.4 Heat Alert Program .
Contents

x Occupational Exposure to Heat and Hot Environments

1.8 Recordkeeping  .

1.8.1 Environmental and Metabolic Heat Surveillance . 

1.8.2 Medical Surveillance  

1.8.3 Surveillance of Heat-related Sentinel Health Events .

1.8.4 Heat-related Illness Surveillance .

1.8.5 Heat Stress Tolerance Augmentation   

2 Introduction .

3 Heat Balance and Heat Exchange .

3.1 Heat Balance Equation .

3.2 Modes of Heat Exchange .

3.2.1 Convection (C)   

3.2.2 Radiation (R) .

3.2.3 Evaporation (E)   

3.2.4 Conduction (K) .

3.3 Effects of Clothing on Heat Exchange   

3.3.1 Clothing Insulation and Non-evaporative Heat loss .

3.3.2 Clothing Permeability and Evaporative Heat Loss .

3.3.3 Physiologic Problems of Clothing .

4 Biologic Effects of Heat  .

4.1 Physiologic Responses to Heat  .

4.1.1 The Central Nervous System .

4.1.2 Muscular Activity and Work Capacity . 

4.1.3 Circulatory Regulation .

4.1.4 The Sweating Mechanism .

4.1.5 Acclimatization to Heat .

4.1.6 Other Related Factors .

4.1.7 Heat-Related Illnesses and Work . 

4.2 Acute Heat Disorders .

4.2.1 Heat Stroke .

4.2.2 Heat Exhaustion .

4.2.3 Heat Cramps .

4.2.4 Heat Syncope .

4.2.5 Heat Rashes . 

4.3 Chronic Heat Disorders .

5 Measurement of Heat Stress .

5.1 Environmental Factors .

5.1.1 Air (Dry Bulb) Temperature .

5.1.2 Humidity .

5.1.3 Air Velocity .

5.1.4 Heat Radiation .
Contents

Occupational Exposure to Heat and Hot Environments xi

5.1.5 Psychrometric Chart   

5.2 Prediction of Meteorological Factors from the National Weather Service Data .

5.3 Metabolic Heat  .

5.3.1 Measurements of Metabolic Heat . 

5.3.2 Estimation of Metabolic Heat  .

6 Control of Heat Stress . 

6.1 Engineering Controls .

6.1.1 Convective Heat Control .

6.1.2 Radiant Heat Control .

6.1.3 Evaporative Heat Control .

6.2 Administrative Controls .

6.2.1 Limiting Exposure Timeand/or Temperature  .

6.2.2 Reducing Metabolic Heat Load   

6.2.3 Enhancing Tolerance to Heat .

6.2.4 Health and Safety Training .

6.2.5 Screening for Heat Intolerance .

6.2.6 Heat Alert Program .

6.3 Personal Protective Clothing and Auxiliary Body Cooling .

6.3.1 Water-cooled Garments .

6.3.2 Air-cooled Garments .

6.3.3 Cooling Vests  .

6.3.4. Wetted Overgarments . 

6.3.5 Practical Applicationsand Limitations . 

6.3.6 Performance Degradation   

7 Medical Monitoring .

7.1 Worker Participation .

7.2 Program Oversight .

7.3 Medical Monitoring Program Elements .

7.3.1 Medical Evaluations .

7.4 Medical Surveillance—Periodic Evaluation of Data  .

7.5 Employer Actions .

7.6 Considerations Regarding Reproduction  .

7.6.1 Pregnancy  

7.6.2 Fertility .

7.6.3 Teratogenicity  .

8 Basis for the Recommended Standard . 

8.1 The NIOSH RALs and RELs . 

8.2 Estimation of Risks .

8.3 Correlation between Exposure and Effects .

8.4 Physiologic Monitoring of Heat Strain .

8.5 Recommendations of U.S. Organizations and Agencies .  
Contents

xii Occupational Exposure to Heat and Hot Environments

8.5.1 The American Conference of Governmental Industrial Hygienists (ACGIH)  . 

8.5.2 Occupational Safety and Health Administration (OSHA)   .

8.5.3 American Industrial Hygiene Association (AIHA)  . 

8.5.4 The Armed Services . 

8.5.5 American College of Sports Medicine (ACSM)   .

8.5.6 Washington State Department of Labor and Industries . 

8.5.7 Mine Safety and Health Administration (MSHA) . 

8.6 International and Foreign Standards and Recommendations . 

8.6.1 The International Organization for Standardization (ISO) . 

8.6.2 Canada . 

8.6.3 Japan   .

9 Indices for Assessing Heat Stress and Strain . 

9.1 Direct Indices . 

9.1.1 Dry Bulb Temperature .  

9.1.2 Wet Bulb Temperature .  

9.2 Rational Indices . 

9.2.1 Operative Temperature . 

9.2.2 Belding-Hatch Heat Stress Index .  

9.2.3 Skin Wettedness (% SWA)   .

9.3 Empirical Indices . 

9.3.1 The Effective Temperature (ET, CET, ET*, and P4SR) . 

9.3.2 The Wet Bulb Globe Temperature (WBGT) .  

9.3.3 Wet Globe Temperature (WGT) . 

9.3.4 Universal Thermal Climate Index (UTCI)   .

9.3.5 Psychrometric Chart   .

9.4 Physiologic Monitoring .  

9.4.1 Work and Recovery Heart Rate   .

9.4.2 Body Temperature . 

9.4.3 Dehydration . 

10 Research Needs . 

10.1 Exposure Times and Patterns  . 

10.2 Deep Body Temperature . 

10.3 Electrolyte and Water Balance . 

10.4 Effects of Chronic Heat Exposure . 

10.5 Circadian Rhythm of Heat Tolerance . 

10.6 Heat Tolerance and Shift Work  .

10.7 Detection, Treatment, and Prevention of Heat-related Illnesses . 

10.8 The Effects of Global Climate Change . 

10.9 Heat Stress and Toxicology . 

10.10 Other Research Needs . 

References   .
Contents

Occupational Exposure to Heat and Hot Environments xiii

Appendices . 

A: Heat Exchange Equation Utilizing the SI Units  . 

B: Urine Chart . 

C: Heat Index . 
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Occupational Exposure to Heat and Hot Environments xv

Abbreviations

Ab Area, Body Surface

ADu Area, DuBois

Ar Area, Effective Radiating

As Area, Solar Radiation

Aw Area, Wetted

Aw/SWADu × 100 Wettedness, Percent of Skin

ACGIH American Conference of Governmental Industrial Hygienists

ACSM American College of Sports Medicine

ADH Antidiuretic Hormone

AIHA American Industrial Hygiene Association

ATP Adenosine Triphosphate

BLS Bureau of Labor Statistics

BMI Body Mass Index

bpm Beats Per Minute

BUN Blood Urea Nitrogen

°C Degrees Celsius

C Convection

Cal/OSHA California OSHA

CDC Centers for Disease Control and Prevention

CET Corrected Effective Temperature

CK Creatine Kinase

CNS Central Nervous System

CPK Creatine Phosphokinase

CO Cardiac Output

CSF Cerebrospinal Fluid

DIC Disseminated Intravascular Coagulation

DOD Department of Defense

-E Evaporative Heat Loss

E Evaporation

ECG Electrocardiogram
Abbreviations

xvi Occupational Exposure to Heat and Hot Environments

Emax Maximum Water Uptake

Ereq Amount of Sweat Evaporated to Maintain Body Heat Balance

EHE Extreme Heat Event

EMS Emergency Medical Services

ET Effective Temperature

°F Degrees Fahrenheit

Fcl Reduction Factor for Heat Exchange due to Clothing

g Gram

GI Gastrointestinal

h Hour

H Body Heat Content

hc Convective Heat Transfer Coefficient

he Evaporative Heat Transfer Coefficient

hr Radiative Heat Transfer Coefficient

hr+c Radiative + Convective Heat Transfer Coefficient

HAP Heat Alert Program

HHE Health Hazard Evaluation

HR Heart Rate

HSI Heat Stress Index

HTT Heat Tolerance Test

Ia Thermal Insulation, Still Air

Icl Thermal Insulation, Clothing

Icl+Ia Thermal Insulation, Effective

im Moisture Permeability Index of Clothing

im/clo Permeability Index-Insulation Ratio

ISO International Organization for Standardization 

K Conduction

kcal Kilocalories

kg Kilogram

kJ Kilojoule

L Liter

M Metabolism

Met Unit of metabolism

min Minute
Abbreviations

Occupational Exposure to Heat and Hot Environments xvii

mL milliliter

mmHg Millimeters of Mercury

MR Metabolic Rate

MRT Mean Radiant Temperature

ms-1 Meters per Second

MSHA Mine Safety and Health Administration

Na Sodium

NIOSH National Institute for Occupational Safety and Health

NFPA National Fire Protection Association

NOAA National Oceanic and Atmospheric Administration

NSAIDs Nonsteroidal Anti-inflammatory Drugs 

OEL Occupational Exposure Limit

OSHA Occupational Safety and Health Administration

Pa Pressure, atmospheric

Psk Pressure, Wetted Skin

Psk,s Pressure, Skin Temperature

PCr Creatine Phosphate

PHEL Physiological Heat Exposure Limit

PPE Personal Protective Equipment

R Radiation

RAL Recommended Alert Limit

RAAS Renin-Angiotensin-Aldosterone System

REL Recommended Exposure Limit

RER Respiratory Exchange Ratio

RH Relative Humidity

S Body Heat Storage

SACHS Standards Advisory Committee on Heat Stress

SCBA Self-contained Breathing Apparatus

SR Sweat Produced Per Unit Time

SV Stroke Volume

SWA Area of Skin Wet with Sweat

t a Temperature, Ambient

t adb Temperature, Adjusted Dry Bulb

t b Temperature, Mean Body
Abbreviations

xviii Occupational Exposure to Heat and Hot Environments

tcr Temperature, Core body

tdp Temperature, Dew-point

tg Temperature, Globe

tnwb Temperature, Natural Wet Bulb

to Temperature, Operative

tor Temperature, Oral

tr Temperature, Radiant

r Temperature, Mean Radiant

tre Temperature, Rectal

tsk Temperature, Skin

t sk Temperature, Mean Skin

tty Temperature, Tympanic

twb Temperature, Psychrometric Wet Bulb

TLV® Threshold Limit Value

TWA Time Weighted Average

ULPZ Upper Limit of the Prescriptive Zone

UTCI Universal Thermal Climate Index

Va Air Velocity

V

.

E Minute Ventilation

V

.

O2 max Maximum Oxygen Consumption

w Wettedness, Skin

W Work

WBGT Wet Bulb Globe Temperature

WGT Wet Globe Temperature

WHO World Health Organization

t
Occupational Exposure to Heat and Hot Environments xix

Glossary

Acclimatization: The physiological changes that occur in response to a succession of days of 

exposure to environmental heat stress and reduce the strain caused by the heat stress of the 

environment; and enable a person to work with greater effectiveness and with less chance of 

heat injury.

Area, DuBois (ADu ): Total nude body surface area in square meters (m2), calculated from the 

DuBois formula, and based on body weight (kg) and height (m).

Area, Effective Radiating (Ar): Surface area of the body in square meters (m2) that exchanges 

radiant energy with a radiant source.

Area, Solar Radiation (As ): Surface area of the body in square meters (m2) that is projected 

normal to the sun.

Area, Wetted (Aw ): Square meters (m2) of skin area covered by sweat.

Body Heat Balance: Steady-state equilibrium between body heat production and heat loss to 

the environment.

Body Heat Balance Equation: Mathematical expression of relation between heat gain and heat 

loss, expressed as S = (M – W) ± C ± R ± K – E

Body Heat Storage (S): The change in heat content (either + or −) of the body.

Circadian Rhythm: Synchronized, rhythmic biological phenomena that occur on approximately a 

24-hour cycle.

clo: A unit expression of the insulation value of clothing, 1 clo = 5.55 kcal·m2·h-1·°C-1. A clo of 1 is 

equal to the insulation required to keep a sedentary person comfortable at 21°C (~70°F). It is also 

sometimes expressed as 1 clo = 0.155 m2·°C·W-1.

Conductive Heat Transfer (K ): The net heat exchange involving the direct transfer of heat via 

direct contact between two mediums (solid, liquid, or gas) that have a temperature differential.

Conductive Heat Transfer Coefficient (hk ): The rate of heat transfer between two mediums (solid, 

liquid, or gas) that have a temperature differential, expressed as W·m-2·°C-1.

Convective Heat Transfer (C): The net heat exchange by convection between an individual and 

the environment.

Convective Heat Transfer Coefficient (hc ): The rate of heat transfer between the body surface and 

the ambient air per square meters (m2) of skin surface, expressed as W·m-2·°C-1.

Evaporative Heat Loss (-E): Body heat loss by evaporation of water (sweat) from the skin, 

expressed as kcal or W.

Evaporative Heat Transfer (E): Rate of heat loss by evaporation of water from the skin or gain 

from condensation of water on the skin, expressed as kcal·h-1, W·m-2, or W.
Glossary

xx Occupational Exposure to Heat and Hot Environments

Evaporative Heat Transfer Coefficient (he ): The rate of heat exchange by evaporation between the 

body surface and the ambient air, as a function of the vapor pressure difference between the two 

and air velocity.

Heat Capacity: Mass multiplied by specific heat of a body.

Heat Content of Body: The product of mean body temperature (tb) and body heat capacity (body 

mass × tissue specific heat), with the latter being constant for any given body composition.

Heat Cramp: A heat-related illness characterized by spastic contractions of the voluntary muscles 

(mainly arms, hands, legs, and feet), usually associated with restricted salt intake and profuse 

sweating without significant body dehydration.

Heat Exhaustion: A heat-related illness characterized by elevation of core body temperature 

above 38°C (100.4°F) and abnormal performance of one or more organ systems, without injury to 

the central nervous system. Heat exhaustion may signal impending heat stroke.

Heat Strain: The physiological response to the heat load (external or internal) experienced by a 

person, in which the body attempts to increase heat loss to the environment in order to maintain 

a stable body temperature. 

Heat Stress: The net heat load to which a worker is exposed from the combined contributions 

of metabolic heat, environmental factors, and clothing worn which results in an increase in heat 

storage in the body. 

Heat Stroke: An acute medical emergency caused by exposure to heat from an excessive rise in 

body temperature [above 41.1°C (106°F] and failure of the temperature-regulating mechanism. 

Injury occurs to the central nervous system characterized by a sudden and sustained loss of 

consciousness preceded by vertigo, nausea, headache, cerebral dysfunction, bizarre behavior, and 

excessive body temperature. 

Heat Syncope: Collapse and/or loss of consciousness during heat exposure without an increase 

in body temperature or cessation of sweating, similar to vasovagal fainting except that it is 

heat induced.

Heat Tolerance: The physiological ability to endure heat and regulate body temperature at an 

average or better rate than others, often affected by the individual’s level of acclimatization and 

physical conditioning.

Humidity, Relative (RH): The ratio of the water vapor present in the ambient air to the water 

vapor present in saturated air at the same temperature and pressure.

Hyperpyrexia: A body core temperature exceeding 40°C (104°F).

Hyperthermia: A condition where the core temperature of an individual is higher than 37.2°C 

(99°F). Hyperthermia can be classified as mild (37.2–38.5°C; 99–101.3°F), moderate (i.e., heat 

exhaustion [38.5–39.5°C; 101.3–103.1°F]), profound (>39.5°C; 103.1°F), or profound clinical hyperthermia (i.e., heat stroke [>40.5°C; 104.9°F]), and death can occur without treatment 

(>45°C; 113°F).

Maximum Oxygen Consumption (V.O2 max): The maximum amount of oxygen that can be used 

by the body.
Glossary

Occupational Exposure to Heat and Hot Environments xxi

Metabolic Rate (MR): Amount of chemical energy transferred into free energy per unit time.

Metabolism (M): Transformation of chemical energy into free energy that is used to perform 

work and produce heat.

Prescriptive Zone: The range of environmental temperatures where exercise at a given intensity 

results in thermal equilibrium, i.e., no change in core body temperature.

Pressure, Atmospheric(Pa ): Pressure exerted by the weight of the air, which averages 760 mmHg 

at sea level and decreases with altitude.

Pressure, Water Vapor (Pa ): The pressure exerted by the water vapor in the air.

Qualified Health Care Professional: An individual qualified by education, training, and licensure/regulation and/or facility privileges (when applicable) who performs a professional service 

within his or her scope of practice in an allied health care discipline, and independently reports 

that professional service.

Radiant Heat Exchange (R): The net rate of heat exchange by radiation between two radiant surfaces of different temperatures.

Radiative Heat Transfer Coefficient (hr ): Rate of heat transfer between two black surfaces per 

unit temperature difference, expressed as W·m-2·°C-1.

Recommended Alert Limit (RAL): The NIOSH-recommended heat stress alert limits for unacclimatized workers.

Recommended Exposure Limit (REL): The NIOSH-recommended heat stress exposure limits for 

acclimatized workers.

Rhabdomyolysis: A medical condition associated with heat stress and prolonged physical exertion, resulting in the rapid breakdown of muscle and the rupture and necrosis of the 

affected muscles.

Standard Man: A representative human with a body weight of 70 kg (154 lb) and a body surface 

area of 1.8 m2 (19.4 ft2).

Sweating, Thermal: Response of the sweat glands to thermal stimuli.

Temperature, Adjusted Dry Bulb (tadb ): The dry bulb temperature is the temperature of the air 

measured by a thermometer that is shielded from direct radiation and convection.

Temperature, Ambient (ta ): The temperature of the air surrounding a body. Also called air temperature or dry bulb temperature.

Temperature, Ambient, Mean ( ta ): The mean value of several dry bulb temperature readings 

taken at various locations or at various times.

Temperature, Core Body (tcr ): Temperature of the tissues and organs of the body. Also called 

Core Temperature.

Temperature, Dew-point (tdp ): The temperature at which the water vapor in the air first starts 

to condense.
Glossary

xxii Occupational Exposure to Heat and Hot Environments

Temperature, Effective (ET): Index for estimating the effect of temperature, humidity, and air 

movement on the subjective sensation of warmth.

Temperature, Globe (tg ): The temperature inside a blackened, hollow, thin copper globe measured by a thermometer whose sensing element is in the center of the sphere.

Temperature, Mean Body (t b ): The mean value of temperature at several sites within the body 

and on the skin surface. It can be approximated from skin and core temperatures. 

Temperature, Mean Radiant (tr): The mean surface temperature of the material and objects surrounding the individual.

Temperature, Mean Skin (t sk ): The mean of temperatures taken at several locations on the skin, 

weighted for skin area. 

Temperature, Natural Wet Bulb (tnwb ): The wet bulb temperature under conditions of the prevailing air movement.

Temperature, Operative (to ): The temperature of a uniform black enclosure within which an 

individual would exchange heat by convection and radiation at the same rate as in a nonuniform 

environment being evaluated.

Temperature, Oral (tor ): Temperature measured by placing the sensing element under the tongue 

for 3 to 5 minutes.

Temperature, Psychrometric Wet Bulb (twb ): The lowest temperature to which the ambient air 

can be cooled by evaporation of water from the wet temperature-sensing element with forced 

air movement.

Temperature, Radiant (tr ): The point temperature of the surface of a material or object, calculated from the following: MRT = Tg + (1.8 Va 0.5)(Tg - Ta), where MRT = Mean Radiant 

Temperature (°C), Tg = black globe temperature (°C), Ta = air temperature (°C), and Va = air 

velocity (m·s-1).

Temperature, Rectal (tre ): Temperature measured 10 centimeters (cm) into the rectal canal.

Temperature, Skin (tsk ): Temperature measured by placing the sensing element on the skin.

Temperature, Tympanic (tty ): True tympanic temperature is measured by placing the sensing 

element directly onto the tympanic membrane and recording the temperature. Estimates of tympanic temperature are usually obtained by placing a device into the ear canal close to the tympanic membrane. 

Temperature Regulation: The maintenance of body temperature within a restricted range under 

conditions of positive heat loads (environmental and metabolic) by physiologic and behavioral mechanisms.

Thermal Insulation, Clothing: The insulation value of a clothing ensemble.

Thermal Insulation, Effective: The insulation value of the clothing plus the still air layer.

Thermal Strain: The sum of physiologic responses of the individual to thermal stress.

Thermal Stress: The sum of the environmental and metabolic heat load imposed on 

the individual.
Glossary

Occupational Exposure to Heat and Hot Environments xxiii

Total Heat Load: The total heat exposure of environmental plus metabolic heat.

Universal Thermal Climate Index (UTCI): This index takes into account the human thermophysiological significance across the entire range of heat exchange and the applicability of wholebody calculations including local skin cooling; it is valid in all climates and seasons.

Wet Bulb Globe Temperature (WBGT): This is an environmental temperature arrived at by 

measuring dry air temperature, humidity, and radiant energy (i.e., usually direct sunlight being 

absorbed by clothing), used to calculate a thermal load on the person.

Wettedness, Skin (w): The amount of skin that is wet with sweat.

Wettedness, Percent of Skin: The percentage of the total body skin surface that is covered 

with sweat.

Work: Physical efforts performed using energy from the metabolic rate of the body. 
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Occupational Exposure to Heat and Hot Environments xxv

Symbols

Symbol Term Units

Ab Body surface area m2

ADu Body surface area, DuBois m2

Ar Skin area exposed to radiation m2

Aw Wetted area of skin m2

C Heat exchange by convection W, W·m-2

CO Cardiac output of blood per minute L·min-1

Emax Maximum water vapor uptake by the air at 

prevailing meteorological conditions

kg·h-1

Ereq Amount of sweat that must be evaporated to 

maintain body heat balance

kg·h-1

Fcl Reduction factor for loss of convective heat 

exchange due to clothing

dimensionless

H Body heat content W

hc Convection heat transfer coefficient W·m-2·°C-1; kcal·h-1·m-2·°C-1

he Evaporative heat transfer coefficient W·m-2·kPa-1

HR Heart rate bpm

hr Radiative heat transfer coefficient W·m-2·°C-1; kcal·h-1·m-2·°C-1

hr+c Radiative + convective heat transfer coefficient W·m-2·°C-1; kcal·h-1·m-2·°C-1

Ia Thermal insulation of still air layer clo

Icl Thermal insulation of clothing layer clo

im Moisture permeability index of clothing dimensionless

im/clo Permeability index–insulation ratio dimensionless

K Heat exchanged by conduction W, W·m-2

kcal Kilocalories kcal·h-1
Symbols

xxvi Occupational Exposure to Heat and Hot Environments

Symbol Term Units

M Metabolism met

Met Unit of metabolism; 1 met = 50 kcal·m-2·h-1 met

mmHg Pressure in millimeters of mercury mmHg

m·s-1 Meters per second m·sec-1

Pa Water vapor pressure of ambient air mmHg, kPa

Psk Water vapor pressure of wetted skin mmHg, kPa

psk,s Water vapor pressure at skin temperature mmHg, kPa

RH Relative humidity percent

R Radiant Heat exchange W, W·m-2

S Sweat produced L

SR Sweat produced per unit time g·min-1, g·h-1, kg·min-1, kg·h-1

SV Stroke volume, or amount of blood pumped 

by the heart per beat

mL

SWA Area of skin wet with sweat m2

%SWA SWA/ADu × 100 = % of body surface wet 

with sweat

percent

T Absolute temperature (t + 273) °K

Ta Ambient air dry-bulb temperature °C, °F

tadb Ambient dry-bulb temperature, adjusted 

for solar radiation

°C, °F

tcr Body core temperature °C, °F

Tdp Dew point temperature °C, °F

Tg Black globe temperature °C, °F

Tnwb Natural wet-bulb temperature °C, °F

to Operative temperature °C, °F

tr Radiant temperature °C, °F

 r Mean radiant temperature °C, °F

tre Rectal temperature °C, °F

t
Symbols

Occupational Exposure to Heat and Hot Environments xxvii

Symbol Term Units

tsk Skin temperature °C, °F

t sk Mean skin temperature °C, °F

Tpwb Psychrometric wet-bulb temperature °C, °F

 w Mean radiant temperature of the surroundings °C,°F

twg Wet globe temperature °C, °F

Va Air velocity m·s-1, fpm

V

.

O2 max Maximum oxygen consumption mL·kg-1·min-1 (a measure 

of aerobic fitness) , or 

L·h-1 (a measure of total 

O2 consumed at peak or 

maximal effort)

W Work kcal·h-1

μ Mechanical efficiency of work %, percent

Skin wettedness dimensionless

σ Stefan-Bolzmann constant W·m-2·K-4

ε Emittance coefficient dimensionless

t

w
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Occupational Exposure to Heat and Hot Environments xxix

Acknowledgments

For contributions to the technical content and review of this document, the authors acknowledge 

the following NIOSH contributors:

Education and Information Division

Paul Schulte, PhD (Director)

Thomas Lentz, PhD, MPH

Kathleen MacMahon, DVM, MS

Lauralynn Taylor McKernan, ScD, CIH

HeeKyoung Chun, DSc

Barbara Dames

Sherry Fendinger

Ralph Zumwalde

Division of Surveillance, 

Hazard Evaluations, and Field Studies

Gregory Burr, CIH

Judith Eisenberg, MD, MS

Melody Kawamoto, MD, MS

Mark Methner, PhD

Doug Trout, MD, MHS

Division of Safety Research

Larry Jackson, PhD

Emergency Preparedness 

and Response Office

Joseph Little, MSPH

Health Effects Laboratory Division

Dan Sharp, MD, PhD

Office of Mine Safety and 

Health Research

Christopher Pritchard, MS, PE

National Personal Protective 

Technology Laboratory

Christopher Coffey, PhD

Western States Division

Yvonne Boudreau, MD, MSPH

Office of the Director

John Decker, MS, RPh, CIH

John Piacentino, MD

Paul Middendorf, PhD, CIH

Nura Sadeghpour, MPH
Acknowledgments

xxx Occupational Exposure to Heat and Hot Environments

Seleen Collins provided editorial support, and Vanessa Williams, Nikki Romero, and Gino Fazio 

contributed to the design and layout of this document. 

Finally, the authors express special appreciation to the following individuals for serving as independent external reviewers and providing comments that contributed to the development of 

this document:

Thomas Bernard, PhD, MS

College of Public Health

University of South Florida

Phillip Bishop, EdD, MSEd

Human Performance Laboratory

University of Alabama

Tord Kjellstrom, MD, PhD, MEng

Environmental Health Consultant, Professor 

Health and Environment International Trust

Mapua, New Zealand

John Muller, MD, MPH, FACOEM

Occupational and Environmental Medicine

Navy Marine Corps Public Health Center

Suzanne Schneider, PhD

Department of Health Exercise and Sports Sciences

University of New Mexico

Rosemary Sokas, MD, MOH

Professor and Chair

Department of Human Science

School of Nursing and Health Studies

Georgetown University 

Comments on the external review draft of this document were also submitted to the NIOSH 

docket and regulations.gov by interested stakeholders and other members of the public. All comments were considered in preparing this final version of the document.
Occupational Exposure to Heat and Hot Environments 1

1 Recommendations for an Occupational 

Standard for Workers Exposed to Heat

and Hot Environments

The National Institute for Occupational Safety 

and Health (NIOSH) recommends that worker 

exposure to heat stress in the workplace be 

controlled by complying with all sections of the 

recommended standard found in this document. Compliance with this recommended 

standard should prevent or greatly reduce the 

risk of adverse health effects to exposed workers. 

Heat-related occupational illnesses, injuries, 

and reduced productivity occur in situations 

in which the total heat load (environmental 

plus metabolic heat) exceeds the capacities of 

the body to maintain normal body functions. 

The reduction of adverse health effects can 

be accomplished by the proper application of 

engineering and work practice controls, worker 

training and acclimatization, measurements 

and assessment of heat stress, medical monitoring, and proper use of heat-protective clothing 

and personal protective equipment (PPE). 

In this criteria document, total heat stress is 

considered to be the sum of the heat generated in the body (metabolic heat), plus the 

heat gained from the environment (environmental heat), minus the heat lost from the 

body to the environment. Environmental 

and/or metabolic heat stress results in physiological responses (heat strain) to promote the 

transfer of heat from the body back to the environment to maintain core body temperature 

[Parsons 2003]. Many of the bodily responses 

to heat exposure are desirable and beneficial. 

However, at some level of heat stress, a worker’s compensatory mechanisms are no longer 

capable of maintaining body temperature at a 

level required for normal body functions. As 

a result, the risk of heat-related illnesses, disorders, and other hazards increases. The level 

of heat stress at which excessive heat strain will 

result depends on the heat tolerance capabilities 

of the worker. However, even though there is a 

wide range of heat tolerance between workers, 

each worker has an upper limit for heat stress, 

beyond which the resulting heat strain can 

cause the worker to become a heat casualty. In 

most workers, appropriate repeated exposure to 

elevated heat stress causes a series of physiologic 

adaptations called acclimatization, whereby the 

body becomes more efficient in coping with the 

heat stress. Such an acclimatized worker can tolerate a greater heat stress before a harmful level 

of heat strain occurs.

The occurrence of heat-related illnesses among 

a group of workers in a hot environment, or 

the recurrence of such illnesses in individual 

workers, represents “sentinel health events”, 

which indicate that heat control measures, 

medical screening, or environmental monitoring measures may not be adequate [Rutstein et 

al. 1983]. One occurrence of heat-related illness 

in a particular worker indicates the need for 

medical inquiry about appropriate workplace 

protections. The recommendations in this document are intended to provide limits of heat 

stress so that workers’ risks of incurring heatrelated illnesses and disorders are reduced. 

Almost all healthy workers who are not acclimatized to working in hot environments and who 

are exposed to combinations of environmental and metabolic heat less than the applicable 

NIOSH Recommended Alert Limits (RALs; 
2 Occupational Exposure to Heat and Hot Environments

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

Figure 8-1) should be able to tolerate the heat 

stress (i.e., the sum of metabolic heat plus environmental heat, minus the heat lost from the 

body to the environment) without a substantial 

increase in their risk of incurring acute adverse 

health effects. Almost all healthy workers who 

are heat-acclimatized to working in hot environments and who are exposed to combinations 

of environmental and metabolic heat less than 

the applicable NIOSH Recommended Exposure 

Limits (RELs; Figure 8-2) should be able to tolerate the heat stress without incurring adverse 

effects. The estimates of both environmental 

and metabolic heat are expressed as 1-hour 

time-weighted averages (TWAs), as described 

by the American Conference of Governmental 

Industrial Hygienists (ACGIH) [ACGIH 2014]. 

In this criteria document, when not otherwise 

qualified, the term “healthy workers” refers to 

those who are physically and medically fit and 

do not require additional protection, modifications in acclimatization procedures, or 

additional physiological monitoring beyond 

the normal recommendations for the amount 

of heat exposure.

The medical monitoring program should be 

designed and implemented to minimize the 

risk to workers’ health and safety from any heat 

hazards in the workplace (see Chapters 4, 5, and 

6). The medical monitoring program should 

provide both preplacement medical evaluations 

for those persons who are candidates for hot 

jobs and periodic medical evaluations for those 

workers who are currently working in hot jobs.

1.1 Workplace Limits 

and Surveillance

1.1.1 Recommended Limits

Unacclimatized workers

Total heat exposure to workers should be 

controlled so that unprotected (i.e., those not 

wearing PPE that would provide protection 

against heat) healthy workers who are not acclimatized to working in hot environments are 

not exposed to combinations of metabolic and 

environmental heat greater than the applicable 

RALs, given in Figure 8-1.

Acclimatized workers

Total heat exposure to workers should be controlled so that unprotected healthy workers 

who are acclimatized to working in hot environments are not exposed to combinations of 

metabolic and environmental heat greater than 

the applicable RELs, given in Figure 8-2. For 

additional information on acclimatization, see 

4.1.5 Acclimatization to Heat.

Effect of Clothing

The recommended limits given in Figures 8-1 

and 8-2 are for healthy workers who are physically and medically fit for the level of activity 

required by their jobs and who are wearing the 

conventional one-layer work clothing ensemble consisting of not more than long-sleeved 

work shirts and trousers (or equivalent). The 

RAL and REL values given in Figures 8-1 

and 8-2 may not provide adequate protection 

if workers wear clothing with lower air and 

vapor permeability or insulation values greater 

than those for the conventional one-layer work 

clothing ensemble. In addition, some workers 

at increased risk may need additional modifications to be protected from heat stress. A 

discussion of these modifications to the RALs 

and RELs is given in 3.3 Effects of Clothing on 

Heat Exchange.

1.1.2 Determination of 

Environmental Heat

Measurement methods

In most situations environmental heat exposures should be assessed by the Wet Bulb Globe 

Thermometer (WBGT) method or equivalent 

techniques, such as Effective Temperature 
Occupational Exposure to Heat and Hot Environments 3

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

(ET), Corrected Effective Temperature (CET), 

or Wet Globe Temperature (WGT), which are 

then converted to estimated WBGT values. 

When air- and vapor-impermeable protective 

clothing is worn, the dry bulb temperature (ta) 

or the adjusted dry bulb temperature (tadb) is 

a more appropriate measurement than the 

WBGT, because impermeable clothing does 

not transfer humid heat loss but only dry heat 

loss (e.g., radiation, convection, and conduction) [Åstrand et al. 2003]. These temperature 

readings may be used to determine the degree 

of heat stress the worker is experiencing in the 

work environment and allow a qualified safety 

and health professional to determine how to 

mitigate that heat stress to prevent heat injury.

Measurement requirements

Environmental heat measurements should be 

made at or as close as feasible to the work area 

where the worker is exposed and represents the 

environmental heat conditions at the worker’s 

position. When a worker is not continuously 

exposed in a single hot area but moves between 

two or more areas with differing levels of environmental heat or when the environmental 

heat substantially varies at the single hot area, 

the environmental heat exposures should be 

measured at each area and during each period 

of constant heat levels where employees are 

exposed. Hourly TWA WBGTs should be calculated for the combination of jobs (tasks), 

including all scheduled and unscheduled 

rest periods.

Modifications of work conditions

Environmental heat measurements should be 

made at least hourly, during the hottest portion 

of each work shift, during the hottest months 

of the year, and when a heat wave occurs or is 

predicted. If two such sequential measurements 

exceed the applicable RAL or REL, then work 

conditions should be modified by use of appropriate engineering controls, work practices, or 

other measures until two sequential measures 

are in compliance with the exposure limits of 

this recommended standard.

Initiation of measurements

A WBGT or individual environmental factors 

profile (e.g., air- and vapor-impermeable protective clothing, etc.) should be established for 

each hot work area, as a guide for determining when engineering controls and/or work 

practices or other control methods should be 

instituted. After the environmental profiles 

have been established, measurements should be 

made as described in this section, during the 

time of year and days when the profile indicates 

that total heat exposures above the applicable 

RALs or RELs may be reasonably anticipated 

or when a heat wave has been forecast by the 

nearest National Weather Service station or 

other competent weather forecasting service.

1.1.3 Determination of Metabolic Heat

The metabolic contribution to the heat load on 

the worker must be estimated or measured to 

ensure a safe working environment. A screening to estimate metabolic heat load should be 

calculated for each worker who is performing 

light, moderate, or heavy work. The metabolic heat rate should be determined in order 

to determine whether the total heat exposure 

exceeds the applicable RAL or REL. 

Whenever the combination of measured 

environmental heat (WBGT) and screening 

estimated metabolic heat exceeds the applicable RAL or REL (Figures 8-1 and 8-2), the 

metabolic heat production should be measured 

using indirect calorimetry (see Chapter 5) or 

an equivalent method. Although performing 

indirect calorimetry in the field or on-site may 

not be feasible, indirect calorimetry can be performed on subjects performing at similar work 

levels in a laboratory setting. This information 

could provide an estimate of the metabolic heat 
4 Occupational Exposure to Heat and Hot Environments

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

production at that workload and thus support 

decisions regarding RAL or REL. Alternatively, 

the responsible individual (i.e., qualified safety 

and health professional) may refer to the 

Compendium of Physical Activities for information on metabolic responses to various 

types of work in order to determine RALs and 

RELs [Ainsworth et al. 2011]. For a short list 

of activities and the associated metabolic heat 

rate, see Table 1-1. 

Metabolic heat rates should be expressed as 

kilocalories per hour ( kcal·h-1) or as watts (W) 

for a 1-hour TWA task basis that includes all 

activities engaged in during each period of 

analysis and all scheduled and nonscheduled 

rest periods (1 kcal·h-1 = 1.16 W).

1.1.4 Physiologic Monitoring 

Physiologic monitoring may be used as an alternative to determining the required estimates 

If a moderate-workload task was 

performed by an acclimatized 70-kg 

(154-lb) worker for the entire 60 

minutes of each hour, then the 

screening estimate for the 1-hour 

TWA metabolic heat would be about 

300 kcal·h-1 (348.9 W). In Figure 

8-2, a vertical line at 300 kcal·h-1 

(348.9 W) intersects the 60 min·h-1

REL curve at a WBGT of 27.8°C 

(82°F). Then, if the measured WBGT 

exceeds 27.8°C (82°F), the worker’s 

metabolic heat could be measured 

by the indirect open-circuit method 

or an equivalent procedure.

If the 70-kg worker was unacclimatized, Figure 8-1 indicates 

that metabolic heat measurement 

of the worker would be required 

above a WBGT of 25°C (77°F).

EXAMPLE

and measurements described in the preceding parts of this section. The total heat stress 

shall be considered to exceed the applicable 

RAL or REL when the physiological functions 

exceed the values given in 9.4 Physiological 

Monitoring. Heart rate, core body temperature, 

and body water loss can be assessed as measures of physiologic response to heat. More 

advanced methods and new tools are also 

available for physiologic monitoring (see 8.4 

Physiologic Monitoring of Heat Strain and 9.4 

Physiologic Monitoring).

1.2 Medical Monitoring

1.2.1 General

(1) The employer should institute a medical 

monitoring program for all workers who 

are or may be exposed to heat stress above 

the RAL, whether they are acclimatized 

or not. A medical monitoring program is 

essential to assess and monitor workers’ 

health and physical well-being both prior 

to and while working in hot environments; 

to provide emergency medical care or other 

treatment as needed and gather medical 

information (e.g., identify changes in health 

status, identify training needs for prevention efforts). More information is available 

in Chapter 7 Medical Monitoring.

(2) The employer should ensure that all medical 

evaluations and procedures are performed 

by or under the direction of the responsible 

healthcare provider (e.g., licensed physician or other licensed and/or credentialed 

healthcare professional).

(3) The employer should provide the required 

medical monitoring without cost to the 

workers, without loss of pay, and at a reasonable time and place.
Occupational Exposure to Heat and Hot Environments 5

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

1.2.2 Preplacement Medical 

Evaluations

For the purposes of the preplacement medical 

evaluation, all workers should be considered 

to be unacclimatized to hot environments. At 

a minimum, the preplacement medical evaluation of each prospective worker for a hot job 

should include the following elements:

(1) A comprehensive work and medical history. 

The medical history should include a comprehensive review of all body systems 

as would be standard for a preplacement physical examination, along with 

specific questions regarding previous episodes of diagnosed heat-related illness, 

rhabdomyolysis, and questions aimed at 

determining acclimatization to the new 

employment environment. 

(2) A comprehensive physical examination 

should be conducted. At the discretion of 

the responsible healthcare provider, candidates who anticipate increased stress of 

physical activity of the job in a hot environment, those over 50 years of age or 

those younger than 50 years of age with 

underlying cardiac risk factors may need 

to have additional testing (e.g., electrocardiogram (ECG) with interpretation by 

a cardiologist). 

(3) An assessment of the use of therapeutic drugs, over-the-counter medications, 

supplements, alcohol, or caffeine that may 

increase the risk of heat injury or illness 

(see Chapter 7).

(4) An assessment of obesity, defined as a body 

mass index (BMI) ≥ 30. Measure height 

and weight to calculate body mass index 

according to the following formula:

BMI = weight (in pounds) × 703 / [height 

(in inches)]2

(5) An assessment of the worker’s ability to 

wear and use any protective clothing and 

equipment, especially respirators, that is or 

may be required to be worn or used.

(6) Other factors and examination details 

included in 7.3.1.1 Preplacement Physical 

Examination.

1.2.3 Periodic Medical Evaluations

Periodic medical evaluations should be made 

available at least annually to all workers who 

may be exposed at the worksite to heat stress 

exceeding the RAL. At minimum, the employer 

should provide the evaluations specified above. 

If circumstances warrant (e.g., an increase in 

job-related heat stress or changes in health 

status), the medical evaluation should be 

offered at more frequent intervals at the discretion of the responsible healthcare provider.

1.2.4 Emergency Medical Care

If the worker develops signs or symptoms of 

heat stroke or heat exhaustion, the employer 

should provide immediate emergency medical 

treatment (e.g., call 911 and cool down the 

worker). Other non-life-threatening heatrelated illnesses may be treated with appropriate 

first aid procedures (see Table 4-3).

1.2.5 Information to Be Provided to the 

Responsible Healthcare Provider

The employer should provide the following 

information to the responsible healthcare provider performing or responsible for the medical 

monitoring program:

(1) A copy of this recommended standard.

(2) A description of the affected worker’s 

duties and activities (e.g., shift schedules, 

work locations) as they relate to the 

worker’s environmental and metabolic 

heat exposure.

(3) An estimate of the worker’s potential exposure to workplace heat (both environmental 
6 Occupational Exposure to Heat and Hot Environments

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

and metabolic), including any available 

workplace measurements or estimates.

(4) A description of any protective equipment 

or clothing the worker uses or may be 

required to use.

(5) Relevant information from previous 

medical evaluations of the affected worker 

that is not readily available to the responsible healthcare provider.

1.2.6 Responsible Healthcare 

Provider’s Written Report of 

Medical Findings

The employer should obtain a written opinion 

from the responsible healthcare provider, which 

should include the following elements:

(1) Occupationally pertinent results of the 

medical evaluation.

(2) A medical opinion as to whether the worker 

has any medical conditions that would 

increase the health risk of exposure to heat 

in the work environment.

(3) An estimate of the individual’s tolerance to 

withstand hot working conditions (see 6.2.3 

Enhancing Tolerance to Heat and 6.2.5 

Screening for Heat Intolerance).

(4) An opinion as to whether the worker can 

perform the work required by the job (i.e., 

physical fitness for the job).

(5) Recommendations for reducing the worker’s risk for heat-related illness, which may 

include use of cooling measures, accommodations or limitations related to work/rest 

schedules and/or workload, or reassignment to another job, as warranted. 

(6) A statement that the worker has been 

informed by the responsible healthcare 

provider of the results of the medical evaluation and any medical conditions that 

require further explanation or treatment. 

The worker is cleared to work in the hot 

environment so long as no adverse health 

effects occur. Specific findings, test results, 

or diagnoses that have no bearing on the 

worker’s ability to work in heat or a hot 

environment should not be included in 

the report to the employer. Safeguards to 

protect the confidentiality of the worker’s 

medical records should be enforced in 

accordance with all applicable federal and 

state privacy regulations and guidelines. 

1.3 Surveillance of 

Heat-related Sentinel 

Health Events

1.3.1 Definition

Surveillance of heat-related sentinel health 

events is defined as the systematic collection 

and analysis of data concerning the occurrence and distribution of adverse health effects 

in defined populations at risk for heat injury 

or illness.

1.3.2 Requirements

In order to evaluate and improve prevention 

and control measures for heat-related effects 

(including the need for exposure assessment), 

the following should be obtained and analyzed 

for each workplace: (a) workplace modifications, (b) identification of highly susceptible 

workers, (c) data on the occurrence or recurrence in the same worker, (d) distribution in 

time, place, and person of heat-related adverse 

effects, and (e) environmental or physiologic 

measurements related to heat.

1.4 Posting of Hazardous 

Areas

1.4.1 Dangerous Heat Stress Areas

In work areas and at entrances to work areas or 

building enclosures where there is a reasonable 

likelihood of the combination(s) of environmental and metabolic heat exceeding the RAL/
Occupational Exposure to Heat and Hot Environments 7

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

REL, readily visible warning signs should be 

posted. These signs should contain information on the required protective clothing or 

equipment, hazardous effects of heat stress on 

human health, and information on emergency 

measures for heat injury or illness. This information should be arranged as follows:

DANGEROUS 

HEAT STRESS AREA

HEAT STRESS–PROTECTIVE 

CLOTHING OR EQUIPMENT REQUIRED

HEAT STROKE OR OTHER 

HEAT-RELATED ILLNESS MAY OCCUR

1.4.2 Emergency Situations

In any area where there is a likelihood of heat 

stress emergency situations occurring, the 

warning signs required in this section should 

be supplemented with signs giving emergency 

and first aid instructions, as well as emergency 

contact information.

1.4.3 Additional Requirements for 

Warning Signs

All hazard warning signs should be printed 

in English and, where appropriate, in the predominant language of workers unable to read 

English. Workers unable to read the signs 

should be informed of the warning printed 

on the signs and the extent of the hazardous 

area(s). All warning signs should be kept clean 

and legible at all times.

1.5 Protective Clothing 

and Equipment

Engineering controls and safe work practices 

should be used to ensure that workers’ exposure to heat stress is maintained at or below the 

applicable RAL or REL specified. In addition, 

protective clothing and equipment (e.g., watercooled garments, air-cooled garments, 

ice-packet vests, wetted overgarments, and 

heat-reflective aprons or suits) should be provided by the employer to the workers when the 

total heat stress exceeds the RAL or REL (see 

6.3 Personal Protective Clothing and Auxiliary 

Body Clothing).

1.6 Worker Information 

and Training

1.6.1 Information Requirements

All new and current workers who work in areas 

where there is reasonable likelihood of heat 

injury or illness, and their supervisors, should 

be kept informed, through continuing education programs, of the following: 

(1) Heat stress hazards.

(2) Predisposing factors.

(3) Relevant signs and symptoms of heat injury 

and illness.

(4) Potential health effects of excessive heat

stress.

(5) General first aid as well as worksite-specific 

first aid procedures.

(6) Proper precautions for work in heat stress 

areas.

(7) Workers’ responsibilities for following 

proper work practices and control procedures to help protect the health and provide 

for the safety of themselves and their 

fellow workers, including instructions to 

immediately report to the supervisor the 

development of signs or symptoms of heatrelated illnesses.

(8) The effects of therapeutic drugs, over-thecounter medications, alcohol, or caffeine 

that may increase the risk of heat injury 

or illness by reducing heat tolerance (see 

Chapter 7).
8 Occupational Exposure to Heat and Hot Environments

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

(9) The purposes for and descriptions of the 

environmental and medical monitoring programs and the advantages to the 

worker of participating in these surveillance programs. 

(10) If necessary, proper use of protective 

clothing and equipment.

(11) Cultural attitude toward heat stress. A 

misperception may exist that someone can 

be “hardened” against the requirement for 

fluids when exposed to heat by deliberately becoming dehydrated before work 

on a regular basis. This misperception 

is dangerous and must be counteracted 

through educational efforts. 

1.6.2 Training Programs

(1) The employer should institute a training 

program, conducted by persons qualified 

by experience or training in occupational 

safety and health, to ensure that all workers 

potentially exposed to heat stress and 

their supervisors have current knowledge of at least the information specified 

in this section. For each affected worker, 

the instructional program should include 

adequate verbal and/or written communication of the specified information. The 

employer should develop a written plan of 

the training program that includes a record 

of all instructional materials.

(2) The employer should inform all affected 

workers of the location of written training 

materials and should make these materials readily available, without cost to the 

affected workers.

1.6.3 Heat Stress Safety Data Sheet

(1) The information specified in this section 

should be recorded on a heat stress 

safety data sheet or on a form specified 

by the Occupational Safety and Health 

Administration (OSHA).

(2) In addition, the safety data sheet should 

contain:

(a) Emergency and first aid procedures, including site-specific contact information.

(b) Notes to the responsible healthcare provider regarding classification, medical 

aspects, and prevention of heat injury 

and illness. These notes should include 

information on the category and clinical 

features of each injury and illness, predisposing factors, underlying physiologic 

disturbance, treatment, and prevention procedures.

1.7 Control of Heat Stress

1.7.1 General Requirements

(1) The employer should establish and 

implement a written program to reduce 

exposures to or below the applicable RAL 

or REL by means of engineering and work 

practice controls.

(2) Where engineering and work practice controls are not sufficient to reduce exposures 

to or below the applicable RAL or REL, they 

should be used to reduce exposures to the 

lowest level achievable by these controls 

and should be supplemented by the use of 

heat-protective clothing or equipment. In 

addition, a heat alert program should be 

implemented as specified in this section.

1.7.2 Engineering Controls

(1) The type and extent of engineering controls 

required to bring the environmental heat 

below the applicable RAL or REL can be 

calculated with the basic heat exchange 

formulae (see Chapters 4 and 5). When the 

environmental heat exceeds the applicable 

RAL or REL, the following control requirements should be used. 
Occupational Exposure to Heat and Hot Environments 9

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

(a) When the air temperature exceeds the 

skin temperature, convective heat gain 

should be reduced by decreasing air 

temperature and/or decreasing the 

air velocity if it exceeds 1.5 meters per 

second (m·sec-1) (300 ft·min-1). When 

air temperature is lower than skin temperature, convective heat loss should 

be increased by increasing air velocity. 

The type, amount, and characteristics 

of clothing will influence heat exchange 

between the body and the environment.

(b) When the temperature of the surrounding solid objects exceeds skin 

temperature, radiative heat gain should 

be reduced by placing shielding or 

barriers that are radiant-reflecting or 

heat-absorbing between the heat source 

and the worker; by isolating the source of 

radiant heat; by increasing the distance 

to the heat source; or by modifying the 

hot process or operation.

(c) When necessary, evaporative heat loss 

should be increased by increasing air 

movement over the worker, by reducing 

the influx of moisture from steam leaks 

or from water on the workplace floors, 

or by reducing the water vapor content 

(humidity) of the air. The air and water 

vapor permeability of the clothing worn 

by the worker will influence the rate of 

heat exchange by evaporation.

1.7.3 Work and Hygienic Practices

(1) Work modifications and hygienic practices should be introduced to reduce both 

environmental and metabolic heat when 

engineering controls are not adequate or 

are not feasible. The most effective preventive work and hygienic practices for 

reducing heat stress include, but are not 

limited to the following:

(a) Limiting the time the worker spends 

each day in the hot environment by 

decreasing exposure time in the hot 

environment and/or increasing recovery 

time spent in a cool environment.

(b) Reducing the metabolic demands of the 

job by such procedures as mechanization, 

the use of special tools, or an increase in 

the number of workers per task.

(c) Increasing heat tolerance by instituting 

a heat acclimatization plan (see Table 

4-1 Acclimatization in workers) and by 

increasing physical fitness.

(d) Training supervisors and workers to recognize early signs and symptoms of heat 

illnesses and to administer relevant first 

aid procedures.

(e) Implementing a buddy system in which 

workers are responsible for observing fellow workers for early signs and 

symptoms of heat intolerance, such as 

weakness, unsteady gait, irritability, disorientation, changes in skin color, or 

general malaise.

(f) Some situations may require workers 

to conduct self-monitoring, and a 

workgroup (i.e., workers, responsible 

healthcare provider, and safety manager) 

should be developed to make decisions 

on self-monitoring options and standard 

operating procedures.

(g) Providing adequate amounts of cool (i.e., 

less than 15°C [59°F]), potable water 

near the work area and encouraging all 

workers that have been in the heat for 

up to 2 hours and involved in moderate 

work activities to drink a cup of water 

(about 8 oz.) every 15 to 20 minutes. 

Individual, not communal, drinking 

cups should be provided. During prolonged sweating lasting more than 2 
10 Occupational Exposure to Heat and Hot Environments

1 . Recommendations for an Occupational Standard for Workers Exposed to Heat and Hot Environments

hours, workers should be provided with 

sports drinks that contain balanced 

electrolytes to replace those lost during 

sweating, as long as the concentration 

of electrolytes/carbohydrates does not 

exceed 8% by volume.

1.7.4 Heat Alert Program

A written Heat Alert Program should be developed and implemented whenever the National 

Weather Service or other competent weather 

service forecasts that a heat wave is likely to 

occur the following day or days. A heat wave is 

indicated when the daily maximum temperature exceeds 35°C (95°F) or when the daily 

maximum temperature exceeds 32°C (90°F) 

and is 5°C (9°F) or more above the maximum 

reached on the preceding days. More details are 

described in 6.2.6 Heat Alert Program.

1.8 Recordkeeping

1.8.1 Environmental and Metabolic 

Heat Surveillance

(1) The employer should establish and maintain an accurate record of all measurements 

made to determine environmental and 

metabolic heat exposures to workers, 

as required in this recommended standard (see 1.1.2 Determination of 

Environmental Heat).

(2) Where the employer has determined that no 

metabolic heat measurements are required 

as specified in this recommended standard, 

the employer should maintain a record of 

the screening estimates relied upon to reach 

the determination (see 1.1.3 Determination 

of Metabolic Heat).

1.8.2 Medical Surveillance

The employer should establish and maintain 

an accurate record for each worker subject to 

medical monitoring, as specified in this recommended standard (see 1.2 Medical Monitoring).

1.8.3 Surveillance of Heat-related 

Sentinel Health Events

The employer should establish and maintain 

an accurate record of the data and analyses specified in this recommended standard 

(see 1.3 Surveillance of Heat-related Sentinel 

Health Events).

1.8.4 Heat-related Illness Surveillance

The employer should establish and maintain an 

accurate record of any heat illness or injury and 

the environmental and work conditions at the 

time of the illness or injury (see 7.4 Medical 

Surveillance—Periodic Evaluation of Data).

1.8.5 Heat Stress Tolerance 

Augmentation

The employer should establish and maintain an 

accurate record of all heat stress tolerance augmentation for workers by heat acclimatization 

procedures (see 4.1.5 Acclimatization to Heat) 

and/or physical fitness enhancement.
Occupational Exposure to Heat and Hot Environments 11

2 Introduction

Criteria documents are developed by the 

National Institute for Occupational Safety 

and Health (NIOSH) under the authority of 

section 20(a) (3) of the Occupational Safety 

and Health Act of 1970. Through the Act, 

Congress charged NIOSH with recommending occupational safety and health standards 

and describing exposure limits that are safe for 

various periods of employment. These limits 

include, but are not limited to, the exposures at 

which no worker will suffer diminished health, 

functional capacity, or life expectancy as a result 

of his or her work experience. By means of criteria documents, NIOSH communicates these 

recommended standards to regulatory agencies 

(including the Occupational Safety and Health 

Administration [OSHA] and the Mine Safety 

and Health Administration [MSHA]), health 

professionals in academic institutions, industry, organized labor, public interest groups, and 

others in the occupational safety and health 

community. Criteria documents contain a 

critical review of the scientific and technical 

information about the prevalence of hazards, 

the existence of safety and health risks, and the 

adequacy of control methods.

In 1972 NIOSH published the Criteria for a 

Recommended Standard: Occupational Exposure 

to Hot Environments [NIOSH 1972], and in 

1986 it published a revised criteria document 

[NIOSH 1986a] and a companion pamphlet, 

“Working in Hot Environments, Revised 1986” 

[NIOSH 1986b]. These publications presented 

the NIOSH assessment of the potential safety 

and health hazards encountered in hot environments, regardless of the workplace, and 

recommended a standard to protect workers 

from those hazards. 

Heat-related occupational illnesses and injuries 

occur in situations where the total heat load 

(environmental and metabolic) exceeds the 

capacities of the body to maintain homeostasis. 

In the 1986 documents, NIOSH recommended 

sliding scale limits based on environmental and 

metabolic heat loads. These recommendations 

were based on the relevant scientific data and 

industry experience at that time. This criteria document reflects the most recent NIOSH 

evaluation of the scientific literature and supersedes the previous NIOSH criteria documents. 

This document presents the updated criteria 

and methods for recognition, evaluation, and 

control of occupational heat stress by engineering and preventive work practices. It also 

addresses the recognition, treatment, and prevention of heat-related illnesses by providing 

guidance for medical supervision, hygienic 

practices, and training programs.

The recommended criteria were developed to 

ensure that adherence to them will (1) protect 

against the risk of heat-related illnesses and 

heat-related reduction in safety performance, 

(2) be achievable by techniques that are valid 

and reproducible and (3) be attainable by 

means of existing techniques. This recommended standard is also designed to prevent 

harmful effects from interactions between heat 

and toxic chemical and physical agents. The 

recommended environmental limits for various 

intensities of physical work, as indicated in 

Figures 8-1 and 8-2, are not upper tolerance 
12 Occupational Exposure to Heat and Hot Environments

2 . Introduction

limits for heat exposure for all workers but, 

rather, levels at which engineering controls, 

preventive work and hygienic practices, and 

administrative or other control procedures 

should be implemented in order to reduce the 

risk of heat-related illnesses, even in the least 

heat tolerant workers.

Despite efforts to prevent heat-related deaths 

and illnesses, they continue. A 2008 Centers for 

Disease Control and Prevention (CDC) report 

identified 423 worker deaths among U.S. agricultural industries (16% were crop workers) 

and nonagricultural industries during 1992–

2006. The heat-related average annual death 

rate for the crop workers was 0.39 per 100,000 

workers, compared with 0.02 for all U.S. civilian workers [Luginbuhl et al. 2008]. Even with 

heat-specific workplace regulations in place in 

California, heat-related illnesses and deaths 

still occur particularily in agricultural workers 

who are at additional risk (e.g., extreme conditions, lack of knowledge, poverty, seasonality, 

low level of education, and other vulnerabilities 

related to migratory status) [Stoecklin-Marois 

et al. 2013].

In 2010, 4,190 injury or illness cases arising 

from exposure to environmental heat among 

private industry and state and local government workers resulted in one or more days of 

lost work [Bureau of Labor Statistics 2011]. 

Eighty-six percent of the heat-affected workers 

were aged 16–54 years. In that same year, 40 

workers died from exposure to environmental 

heat. The largest number of workers (18) died 

in the construction industry, followed by 6 

deaths in natural resources (including agriculture) and mining, 6 deaths in professional and 

business services (including waste management 

and remediation), and 3 deaths in manufacturing. Eighty percent of the deaths occurred 

among workers 25–54 years of age. Because 

heat-related illnesses are often not recognized, 

and only illnesses involving days away from 

work are reported, the actual number of occupational heat-related illnesses and deaths is not 

known. Additionally, estimates of the number 

of workers exposed to heat are not available.

A study of OSHA citations issued between 

2012 and 2013 revealed 20 cases of heatrelated illness or death of workers [Arbury et 

al. 2014]. In most of these cases, employers 

had no program to prevent heat illness, or programs were deficient; and acclimatization was 

the program element most commonly missing 

and most clearly associated with worker death. 
Occupational Exposure to Heat and Hot Environments 13

3 Heat Balance and Heat Exchange

An essential requirement for continued normal 

body function is that the deep body core temperature be maintained within the range of 

about 37°C (98.6°F) ± 1°C (1.8°F). Achieving 

this body temperature equilibrium requires a 

constant exchange of heat between the body and 

the environment. The rate and amount of the 

heat exchange are governed by the fundamental laws of thermodynamics of heat exchange 

between objects. The amount of heat that must 

be exchanged is a function of (1) the total heat 

produced by the body (metabolic heat), which 

typically range from about 1 kcal per kilogram 

(kg) of body weight per hour (1.16 W) at rest to 

5 kcal·kg-1 body weight·h-1 (7 W) for moderately 

hard industrial work, and (2) the heat gained, 

if any, from the environment. The rate of heat 

exchange with the environment is a function 

of air temperature and humidity, skin temperature, air velocity, evaporation of sweat, radiant 

temperature, and type, amount, and characteristics of the clothing worn. Respiratory heat 

loss is generally of minor consequence except 

during hard work in very dry environments. 

The following is a simple version of the heat 

balance equation.

3.1 Heat Balance Equation

The basic heat balance equation is

S = (M−W) ± C ± R ± K−E

where

S = change in body heat content

(M−W) = total metabolism minus 

external work performed

C = convective heat exchange

R = radiative heat exchange

K = conductive heat exchange

E = evaporative heat loss

To solve the equation, measurement of metabolic heat production, air temperature, 

water-vapor pressure, wind velocity, and mean 

radiant temperature are required [Belding 1971; 

Ramsey 1975; Lind 1977; Grayson and Kuehn 

1979; Goldman 1981; Nishi 1981; ISO 1982b; 

ACGIH 1985; DiBenedetto and Worobec 1985; 

Goldman 1985a,b; Horvath 1985; Havenith 

1999; Malchaire et al. 2001].

3.2 Modes of Heat Exchange

The major modes of heat exchange between 

humans and the environment are convection, 

radiation, and evaporation. Conduction usually 

plays a minor role in workplace heat stress, 

other than for brief periods of body contact 

with hot tools, equipment, floors, or other items 

in the work environment, or for people working 

in water or in supine positions [Havenith 1999]. 

The equations for calculating heat exchange 

by convection, radiation, and evaporation are 

available in Standard International (SI) units, 

metric units, and English units. In SI units, heat 

exchange is in watts per square meter of body 

surface (W·m-2). The heat-exchange equations 

are available in metric and English units for 

both the seminude individual and the worker 

wearing a conventional long-sleeved work shirt 

and trousers. The values are in kcal·h-1 for the 

“standard man,” defined as one who weighs 
14 Occupational Exposure to Heat and Hot Environments

3 . Heat Balance and Heat Exchange

70 kg (154 lb) and has a body surface area of 

1.8 m2 (19.4 ft2). For the purpose of this discussion, only SI or metric units will be used. 

For workers who are smaller or larger than the 

standard man, appropriate correction factors 

must be applied [Belding 1971]. The equations 

utilizing the SI units for heat exchange by C, R, 

and E are presented in Appendix A.

3.2.1 Convection (C)

The rate of convective heat exchange between 

the skin of a person and the ambient air immediately surrounding the skin is a function of the 

difference in temperature between the ambient 

air (ta) and the mean weighted skin temperature (t sk) and the rate of air movement over 

the skin (Va). This relationship is stated algebraically for the “standard man” wearing the 

conventional one-layer work clothing ensemble 

as [Belding 1971]:

C = 7.0 Va 0.6(ta − t sk)

where 

C = convective heat exchange, kcal·h-1

Va = air velocity in meters per second 

(m·sec-1)

ta = ambient air temperature °C 

t sk = mean weighted skin temperature, 

usually assumed to be 35°C

When ta >35°C, there will be a gain in body 

heat from the ambient air by convection;

when ta <35°C, heat will be lost from the body 

to the ambient air by convection.

This basic convective heat-exchange equation 

in English units has been revised for the “standard man” wearing the conventional one-layer 

work clothing ensemble:

C = 0.65 Va 0.6(ta − t sk)

where 

C = convective heat exchange in Btu·h-1

Va = air velocity in feet per minute (fpm)

ta = ambient air temperature °C (°F)

t sk = mean weighted skin temperature, 

usually assumed to be 35°C (95°F)

3.2.2 Radiation (R)

The radiative heat exchange is primarily a function of the temperature gradient between the 

mean radiant temperature of the surroundings 

(t w) and the mean weighted skin temperature 

(t sk). Radiant heat exchange is a function of the 

fourth power of the absolute temperature of the 

solid surroundings, less the skin temperature 

(Tw−Tsk)4, but an acceptable approximation for 

the conventional one-layer clothed individual 

is this [Belding 1971]:

R = 6.6 (t w − t sk)

R = radiant heat exchange, kcal·h-1 or 

W·m-2 

t w = mean radiant temperature of the solid 

surrounding surface, °C (°F)

t sk = mean weighted skin temperature

For the conventional one-layer clothed individual and English units, the equation becomes

R = 15.0 (t w − t sk)

R = radiant heat exchange, Btu·h-1

t w = mean radiant temperature, 

°C (°F)

t sk = mean weighted skin temperature

3.2.3 Evaporation (E)

The evaporation of water (sweat) from the skin 

surface results in a heat loss from the body. The 

maximum evaporative capacity (and heat loss) 
Occupational Exposure to Heat and Hot Environments 15

3 . Heat Balance and Heat Exchange

is a function of air motion (Va) and the water 

vapor pressure difference between the ambient 

air (Pa) and the wetted skin at skin temperature (Psk). The equation for this relationship is 

for the conventional one-layer clothed worker 

[Belding 1971]:

E = 14Va 0.6 (Psk − Pa)

E = Evaporative heat loss, kcal·h-1

Va = air speed, m·s-1

Pa = water vapor pressure of ambient air, 

mmHg

Psk = vapor pressure of water on skin, 

assumed to be 42 mmHg (5.6 kPa) at 

a 35°C (95°F) skin temperature

This translates in English units for the conventional one-layer clothed worker to:

E = 2.4Va 0.6 (Psk− Pa)

E = Evaporative heat loss, Btu·h-1

Va = air velocity, f·min-1

Pa = water vapor pressure air, mmHg

Psk = water vapor pressure on the skin, 

assumed to be 42 mmHg (5.6 kPa) at 

a 95°F (35°C) skin temperature

3.2.4 Conduction (K)

This type of heat exchange involves the direct 

transfer of heat via direct contact between 

two mediums (solid, liquid, or gas) that have 

a temperature differential. Therefore, for the 

purposes of this document, rate of heat transfer 

depends on the temperature gradient between 

the skin surface and the surrounding surfaces 

(hot metal surface, ice vest against the skin) and 

the thermal qualities of the surfaces (e.g., water 

absorbs heat at a much greater rate than air or 

stone) [McArdle et al. 2010a]. Under most circumstances, conduction is small in comparison 

with radiation, convection, and evaporative 

losses. However, under special circumstances 

such as wearing ice vests or liquid circulating 

personal cooling systems, conduction becomes 

more important. The equation for this theoretical relationship is as follows:

hk =KA (T1 − T2) / l

hk = Heat exchange via conduction

K = thermal “conductivity” determined by 

the physical properties of the objects

A = area for conduction of heat, m2

l = distance between points at T1 and T2

for the conduction for heat, m

T1 = temperature of the warmer 

object, °C

T2 = temperature of the cooler 

object, °C

This equation could be rewritten to represent 

thermal conduction to or from skin, as follows:

hk = KA (Tskin − Tobject) / l

where 

K = conductive heat exchange in kcal·h-1

3.3 Effects of Clothing on 

Heat Exchange

Clothing serves as a barrier between the skin 

and the environment to protect against normal 

environmental elements of heat, cold, moisture, and abrasion. Special clothing has been 

developed to add further protection against hazardous chemical, physical, and biologic agents. 

A clothing ensemble will, of necessity, alter the 

rate and amount of heat exchange between the 

skin and the ambient air by convection, conduction, radiation, and sweat evaporation. 

Therefore, to calculate heat exchange by each 

of these modes, it is necessary to apply correction factors that reflect the type, amount, and 

characteristics of the clothing being worn when 
16 Occupational Exposure to Heat and Hot Environments

3 . Heat Balance and Heat Exchange

the clothing differs substantially from the conventional one-layer work clothing (i.e., more 

than one layer and/or greater air and vapor 

impermeability). This clothing efficiency factor 

(Fcl) for dry heat exchange is nondimensional 

[Goldman 1978; McCullough et al. 1982; Vogt 

et al. 1982]. In general, the thicker and greater 

the air and vapor impermeability of the clothing barrier layer or layers, the more it interferes 

with convective, radiative, and evaporative 

heat exchange.

Corrections of the RAL and REL have been 

suggested to reflect the Fcl based on heat transfer calculation for a variety of environmental 

and metabolic heat loads and three clothing 

ensembles [Heat Stress Management Program 

for the Nuclear Power Industry: Interim Report

1986]. The conventional one-layer clothing 

ensemble was used as the basis for comparisons with the other clothing ensembles. When 

a two-layer clothing ensemble is worn, the RAL 

and REL should be lowered by 2°C (3.8°F). 

When a partially air and/or vapor impermeable ensemble or heat reflective or protective 

aprons, leggings, gauntlets, etc., are worn, the 

RAL and REL should be lowered 4°C (7.2°F). 

These suggested corrections of the RAL or REL 

are scientific judgments that have not been substantiated by controlled laboratory studies or 

long-term workplace experience.

In those workplaces where a vapor and air 

impermeable encapsulating ensemble must be 

worn, the WBGT is not the appropriate measurement of environmental heat stress. In these 

instances, the adjusted dry bulb air temperature 

(tadb) must be measured and used instead of the 

WBGT. Where the tadb exceeds approximately 

20°C (68°F), physiologic monitoring (core body 

temperature and/or pulse rate) is required. The 

method used for measuring core body temperature and pulse rate will be determined by 

the circumstances, and proper advance planning (attaching the physiological monitoring 

system) must take place prior to donning of 

the impermeable PPE. This physiologic monitoring must be conducted on a time schedule 

based upon metabolic heat production and tadb. 

The suggested frequency of physiologic monitoring for moderate work varies from once 

every two hours at tadb of 24°C (75°F) to every 

15 minutes for moderate work at tadb of 32°C 

(90°F) [NIOSH 1985].

3.3.1 Clothing Insulation and 

Non-evaporative Heat loss

Even without any clothing, a thin layer of still 

air (the boundary layer) is trapped next to the 

skin. This external still air film acts as a layer of 

insulation against heat exchange between the 

skin and the ambient environment. Typically, 

without body or air motion, this air layer (la) 

provides about 0.8 clo units of insulation. One 

clo unit of clothing insulation is defined as 

allowing 5.55 kcal·m2·h-1 of heat exchange by 

radiation and convection (HR+C) for each °C of 

temperature difference between the skin (at a 

mean skin temperature t sk) and adjusted dry 

bulb temperature tadb = (ta+t r)/2. The mean 

skin temperature is the average of temperatures 

taken at several locations on the skin, weighted 

for skin area. The “weighting” system takes into 

consideration the amount of heat transfer over 

these various areas of skin. The areas of skin are 

typically chest, bicep, forearm, thigh, calf, and 

subscapular region. The weighting system used 

by Ramanathan [1964] is generally considered 

an accurate manner of determining mean skin 

temperature from these data. For the standard 

man with 1.8 m2 of surface area, the hourly heat 

exchange by radiation and convection (HR+C) 

can be estimated as 

HR+C = (10/clo) (t sk − tadb)

Thus, the 0.8 clo still air layer limits the heat 

exchange by radiation and convection for the 

nude standard individual to about 12.5 kcal·h-1

(14.5 W) for each °C of difference between skin 

temperature and air temperature. A resting 
Occupational Exposure to Heat and Hot Environments 17

3 . Heat Balance and Heat Exchange

individual in still air producing 90 kcal·h-1

(104.7 W) of metabolic heat will lose about 

11 kcal·h-1 (12.8 W or 12%) by respiration and 

about the same by evaporation of the body 

water diffusing through the skin. The worker 

will then have to sweat and lose heat by evaporation to eliminate some of the remaining 68 

kcal·h-1 (79.1 W) of metabolic heat if the tadb is 

less than 5.5°C below tsk [Goldman 1981].

The still air layer is reduced by increasing air 

motion, reaching a minimal value of approximately 0.2 clo at air speeds above 4.5 m·sec-1

(890 f·min-1 or 10 mph). At this wind speed, 68 

kcal·h-1 (79.1 W) can be eliminated from the 

skin without sweating at an air temperature 

only 1.4°C below skin temperature, that is, 

68/(10/0.2) = 1.4°C.

Studies of clothing materials have led to the conclusion that the insulation provided by clothing 

is generally a linear function of its thickness. 

Differences in fibers or fabric weave have only 

very minor effects on insulation, unless these 

directly affect the thickness or the vapor or air 

permeability of the fabric. The function of the 

fibers is to maintain a given thickness of still 

air in the fabric and block heat exchange. The 

fibers are more conductive than insulating; 

increasing fiber density (as when trying to fit 

two socks into a boot that has been sized to fit 

properly with one sock) can actually reduce the 

insulation provided [Goldman 1981].

The typical value for clothing insulation is 1.57 

clo·cm-1 of thickness (4 clo·inch-1). It is difficult 

to extend this generalization to very thin fabric 

layers or to garments like underwear, which 

may simply occupy an existing still air layer 

of not more than 0.5 cm thickness. These thin 

layers show little contribution to the intrinsic 

insulation of the clothing, unless there is (a) 

“pumping action” of the clothing layers by body 

motion (circulation of air through and between 

layers of clothing due to body movement); (b) 

compression of the clothing by pressure from 

other clothing, by objects in contact with the 

body, or by external wind; or (c) penetration 

of some of the wind (as a function of the air 

permeability of the outer covering fabric) 

into the trapped air layer [ASHRAE 1981b; 

Goldman 1981; McCullough et al. 1982]. Table 

3-1 lists the intrinsic insulation contributed by 

adding each of the listed items of typical clothing ensembles. The total intrinsic insulation is 

not the sum of the individual items, but 80% 

of their total insulation value; this allows for 

an average loss of 20% of the sum of the individual items to account for the compression of 

one layer on the next. This average 20% reduction is a rough approximation, which is highly 

dependent on such factors as the nature of the 

fiber, the weave, the weight of the fabric, the 

use of foam or other nonfibrous layers, and the 

clothing fit and cut.

In summary, insulation is generally a function 

of the thickness of the clothing ensemble, and 

this, in turn, is usually a function of the number 

of clothing layers. Thus, each added layer of 

clothing, if not compressed, will increase the 

total insulation. That is why most two-layer 

protective clothing ensembles exhibit quite 

similar insulation characteristics and most 

three-layer systems are comparable, regardless of some rather major differences in fiber or 

fabric type [Goldman 1981].

Because clothing can significantly insulate the 

wearer from the external environment and trap 

body heat because of the insulation or low permeability, the thermal influence of the clothing 

has led to the development of clothing adjustment factors that can be used to determine the 

overall thermal stress of the wearer. An example 

of adjustments for the thermal properties of 

clothing appears in Table 3-2.

As can be seen from Table 3-2, adjustments 

to the total thermal stress can be made for 

a variety of clothing and PPE. This table is 

useful for determining the total heat stress 
18 Occupational Exposure to Heat and Hot Environments

3 . Heat Balance and Heat Exchange

that is imposed on the worker wearing these 

or similar types of clothing. More comprehensive lists are available in the literature or in 

ACGIH guidelines.

3.3.2 Clothing Permeability and 

Evaporative Heat Loss

Evaporative heat transfer through clothing 

tends to be affected linearly by the thickness of 

the ensemble. The moisture permeability index 

(im) is a dimensionless unit with a theoretical 

lower limit value of 0 for a vapor- and airimpermeable layer and an upper value of 1 if 

all the moisture that the ambient environment 

can take up (as a function of the ambient air 

vapor pressure and fabric permeability) can 

pass through the fabric. Since moisture vapor 

transfer is a diffusion process limited by the 

characteristic value for diffusion of moisture 

through still air, values of im approaching 1 

should be found only with high wind and thin 

clothing. A typical im value for most clothing 

Table 3-1. Clo insulation values for typical clothing ensembles

Icl

Clothing ensemble Clo M2 °C W-1

Underpants, coveralls, socks, shoes 0.70 0.11

Underpants, shirt, pants, socks, shoes 0.75 0.115

Underpants, shirt, coveralls, socks, shoes 0.80 0.125

Underpants, shirt, pants, light jacket, socks, shoes 0.85 0.135

Underpants, shirts, pants, smock, socks, shoes 0.90 0.14

Underpants, short-sleeve undershirt, shirt, pants, light jacket, 

socks, shoes

1.0 0.155

Underpants, short-sleeve undershirt, shirt, pants, coveralls, 

socks, shoes

1.1 0.17

Long underwear shirt and bottoms, heavy jacket, pants, 

socks, shoes

1.2 0.185

Underpants, short-sleeve undershirt, shirt, pants, light jacket, 

heavy jacket, socks, shoes

1.25 0.19

Underpants, short-sleeve undershirt, coveralls, heavy jacket 

and pants, socks, shoes

1.40 0.22

Underpants, short-sleeve undershirt, pants, light jacket, 

heavy jacket and pants, socks, shoes

1.55 0.225

Underpants, short-sleeve undershirt, pants, light jacket, 

heavy quilted outer jacket and overalls, socks, shoes

1.85 0.285

Underpants, short-sleeve undershirt, pants, jacket, heavy 

quilted outer jacket and overalls, socks, shoes, cap, gloves

2.0 0.31

Long underwear shirt and bottoms, heavy jacket and pants, 

parka with heavy quilting, overalls with heavy quilting, 

socks, shoes, cap, gloves

2.55 0.395

Adapted from the International Organization for Standardization (ISO) [2007]. 
Occupational Exposure to Heat and Hot Environments 19

3 . Heat Balance and Heat Exchange

materials in still air is less than 0.5 (e.g., im

will range from 0.45 to 0.48). Water repellent 

treatment, very tight weaves, and chemical protective impregnations can reduce the im value 

significantly. However, even impermeable 

layers seldom reduce the im value to zero, since 

an internal evaporation-condensation cycle is 

set up between the skin surface and the inner 

surface of the impermeable layer, which effectively transfers some heat from the skin to the 

vapor barrier. This shunting, by passing heat 

across the intervening insulation layers, can be 

reflected as an im value of about 0.08, even for a 

totally impermeable overgarment.

Very few fiber treatments have been found 

to improve the im index value of fabric layers; 

surfactants, which increase the number of free 

hydroxyl (OH) radicals on the fiber surface 

or which somehow improve wicking, appear 

to increase the im value of a fabric. However, 

the ultimate evaporative heat transferred from 

the skin through the clothing and external 

air layers to the environment is not simply a 

function of the im but a function of the permeability index–insulation ratio (im/clo). The 

maximum evaporative heat exchange with the 

environment can be estimated for the HR+C of a 

“standard man” with 1.8 m2 of surface area, as:

HEmax=10im/clo × 2.2(Psk−Pa)

The constant 2.2 is the Lewis number; Psk is the 

water vapor pressure of sweat (water) at skin 

temperature (tsk); and Pa is the water vapor pressure of the ambient air at air temperature, ta. 

Thus, the maximum evaporative transfer tends 

to be a linear, inverse function of insulation, 

if not further degraded by various protective 

treatments, which range from total impermeability to water repellent treatments [Goldman 

1973, 1981, 1985a]. The Lewis number is 

derived from fraction of skin wettedness, latent 

heat, and evaporative heat transfer (he).

3.3.3 Physiologic Problems of Clothing

The percent of sweat-wetted skin surface area 

(w ) needed to eliminate the required amount 

of heat from the body by evaporation can be 

estimated simply as the ratio of the required 

evaporative cooling (Ereq) and the maximum 

water vapor uptake capacity of the ambient air 

(Emax). A totally wetted skin = 100%. 

w = Ereq / Emax

Having some sweat-wetted skin is not uncomfortable; in fact, some sweating during exercise 

 Table 3-2. Clothing adjustment factors for types of clothing

Clothing adjustment factors (°C−WBGT)

Clothing Previous 2006

Work clothing (baseline) 0 0

Cloth coveralls 3.5 0

Double-layer cloth clothing 5 3

Spunbound melt-blown synthetic (SMS) coveralls – 0.5

Polyolefin coveralls – 1

Limited-use vapor-barrier coveralls – 11

Adapted from Bernard TE, Threshold Limit Values for Physical Agents Committee, ACGIH [2014].
20 Occupational Exposure to Heat and Hot Environments

3 . Heat Balance and Heat Exchange

in heat increases comfort. As the extent of 

skin wetted with sweat approaches 20%, the 

sensation of discomfort begins to be noted. 

Discomfort is marked and performance decrements can appear with between 20% and 

40% wetting of the body surface; performance 

decrements become increasingly noted as w

approaches 60%. Sweat begins to be wasted, 

dripping rather than evaporating at 70%; 

physiologic strain becomes marked between 

60% and 80% w . Increases of w above 80% 

result in limited tolerance, even for physically fit, heat-acclimatized young workers. The 

above arguments indicate that any protective 

work clothing will pose some limitations on 

tolerance, since with Ia plus Iclo rarely below 

2.5 clo, their im/clo ratios are rarely above 0.20 

[Goldman 1985b].

The physiologic strain that arises with wearing 

clothing, heat transfer, and work can be estimated from equations that describe the 

competition for the blood pumped by the 

heart. The cardiac output (CO) is the stroke 

volume (SV) (or volume of blood pumped per 

beat) multiplied by heart rate (HR) in beats per 

minute (CO = SV × HR). The cardiac output 

increases essentially linearly with increasing 

work; the rate-limiting process for metabolism 

is the maximum rate of delivery of oxygen to 

the working muscle via the blood supply. It is 

expressed in liters per minute (L·min-1). In heat 

stress, this total blood supply must be divided 

between the working muscles and the skin 

where the heat exchange occurs. In this consideration, it is important to realize that not all 

of the CO is diverted to muscle and skin. Other 

organ systems must receive oxygenated blood 

during exercise as well, especially the brain, 

heart, and viscera [McArdle et al. 2010a].

SV rapidly reaches a constant value for a given 

intensity of work. Thus, the work intensity (i.e., 

the rate of oxygen delivered to the working 

muscles) is essentially indicated by HR; the 

individual worker’s maximum HR limits the 

ability to continue work. Conditions that 

impair the return of blood from the peripheral 

circulation to fill the heart between beats will 

affect work capacity. The maximum achievable 

HR is a function of age and can be roughly 

estimated by this relationship: 220 beats per 

minute (bpm) minus age in years [Hellon and 

Lind 1958; Drinkwater and Horvath 1979]. 

Given equivalent HR at rest (e.g., 60 bpm), 

a 20-year-old worker’s HR has the capacity 

to increase by 140 bpm, i.e., (220−20)−60, 

while a 60-year-old worker can increase his 

HR only 100 bpm, i.e., (220−60)−60. Because 

the demands of a specific task will be roughly 

the same for 20- and 60-year-old individuals 

who weigh the same and do the same amount 

of physical work, the decline in maximum 

achievable HR with age increases both the perceived and the actual relative physiologic strain 

of work on the older worker. Given the above, 

it is preferable to use a percent of the person’s 

HRmax rather than refer to a specific absolute 

HR as a target for work.

The ability to transfer the heat produced by 

muscle activity from the core of the body to 

the skin is also a function of the CO. Blood 

passing through core body tissues is warmed 

by heat from metabolism during rest and work. 

The basic requirement is that skin temperature 

(tsk) must be maintained at least 1°C (1.8°F) 

below deep body temperature (tre) if blood that 

reaches the skin is to be cooled before returning to the body core. The heat transferred to 

the skin is limited, ultimately, by the CO and 

by the extent to which tsk can be maintained 

below tre. 

A worker’s tre is a function of metabolic heat 

production (M) (tre = 36.7 + 0.004M), as 

long as there are no restrictions on evaporative and convective heat loss by clothing, 

high ambient vapor pressures, or very low air 

motion; for example, at rest, if M = 105W, tre

is about 37.1°C (98.8°F). Normally, under the 

same conditions of unlimited evaporation, 
Occupational Exposure to Heat and Hot Environments 21

3 . Heat Balance and Heat Exchange

skin temperatures are below tre by about 3.3°C 

+ (0.006M); thus, at rest, when tre is 37°C, the 

corresponding tsk is about 33°C, that is, 37− 

(3.3 + 0.6). This 3° C–4°C difference between 

tre and tsk indicates that, at rest, each liter of 

blood flowing from the deep body to the skin 

can transfer approximately 4.6 watts or 4 kcal 

of heat to the skin. Since tre increases and tsk

decreases from the evaporation of sweat with 

increasing M, it normally becomes easier to 

eliminate body heat with increasing work, 

since the difference between tre and tsk increases 

by about 1°C (1.8°F) per 100 watts (86 kcal) of 

increase in M (i.e., tre up 0.4°C (0.7°F), and tsk

down 0.6°C (1.1°F) per 100 watts of M). Thus, 

at sustainable hard work (M = 500 watts or 

430 kcal·h-1), each liter of blood flowing from 

core to skin can transfer 9 kcal (10.5 W) to the 

skin, which is 2.5 times that at rest [Goldman 

1973, 1985a].

Work under a heat stress condition sets up a 

competition for CO, particularly as the blood 

vessels in the skin dilate to their maximum and 

less blood is returned to the central circulation. 

Gradually, less blood is available in the venous 

return to fully fill the heart between beats, 

causing the SV to decrease; therefore, HR 

must increase to maintain the same CO. For fit, 

young workers, the average work HR should 

be limited to about 110 bpm if an 8-hour work 

shift is to be completed; an average HR of 140 

bpm should be maintained for no more than 4 

hours, and an average HR of 160 bpm should be 

maintained for no more than 2 hours [Brouha 

1960]. If the intensity of work results in a HR 

in excess of these values, then the intensity of 

work should be reduced. Thus, heat added to 

the demands of work rapidly results in problems, even in healthy, young workers. These 

problems are amplified if circulating blood 

volume is reduced as a result of inadequate 

water intake to replace sweat losses, which can 

average one liter an hour over an 8-hour work 

shift, or by vomiting, diarrhea, or diuresis.

The crisis point, heat exhaustion and collapse, is a manifestation of inadequate blood 

supply to the brain; this occurs when CO 

becomes inadequate because of insufficient 

return of blood from the periphery to fill the 

heart for each beat or because of inadequate 

time between beats to fill the heart as HR 

approaches its maximum. 

Clothing interferes with heat loss from the 

skin, and skin temperature rises predictably 

with increased clothing. Because of the insulation-induced rise in tsk and the resultant 

limited ability to dissipate heat that has been 

transferred from the core to the skin, core temperature (tre) also rises when clothing is worn. 

Another type of interference with heat loss 

from the skin arises when sweat evaporation is 

required for body cooling (i.e., when M + HR 

+ C > O) but is limited by high ambient water 

vapor pressure, low wind, or low clothing permeability index (im/clo).

As Ereq approaches Emax, skin temperature 

increases dramatically and deep body temperature begins to increase rapidly. Deep body 

temperatures above 38.0°C (100.4°F) are considered undesirable for an average worker. 

The risk of heat-exhaustion collapse is about 

25% at a deep body temperature of 39.2°C 

(102.6°F), associated with a skin temperature 

of 38°C (100.4°F) (i.e., tsk converging toward 

tre and approaching the 1°C [1.8°F] limiting 

difference where one liter of blood can transfer only 1 or 2 kcal·h-1 [1.16 W or 2.33 W] to 

the skin). At a similarly elevated tsk where tre

is 39.5°C (103.1°F), there is an even greater 

risk of heat-exhaustion collapse, and as tre 

approaches 40°C (104°F), with elevated skin 

temperatures, most individuals are in imminent danger of heat-related illness. Finally, tre

levels above 41°C (105.8°F) are associated with 

heat stroke, a life-threatening major medical 
22 Occupational Exposure to Heat and Hot Environments

3 . Heat Balance and Heat Exchange

emergency. The competition for CO is exacerbated by dehydration (limited SV), age (limited 

maximum HR), and reduced physical fitness 

(compromised CO). These work-limiting and 

potentially serious deep body temperatures are 

reached more rapidly when combinations of 

these three factors are involved.

As indicated in the above statements, 

maximum work output may be substantially 

degraded by almost any protective clothing 

worn during either heavy work in moderately 

cool environments or low work intensities in 

hot conditions, because of the clothing interfering with heat elimination. Heat stress is also 

likely to be increased with any two-layer protective ensembles or any effective single-layer 

vapor barrier system for protection against 

toxic products, unless some form of auxiliary 

cooling is provided [Goldman 1973, 1985a].
Occupational Exposure to Heat and Hot Environments 23

4 Biologic Effects of Heat

4.1 Physiologic Responses 

to Heat

4.1.1 The Central Nervous System

The central nervous system is responsible for 

the integrated organization of thermoregulation. The hypothalamus is thought to be the 

central nervous system’s primary seat of control. 

Historically, in general terms, the anterior 

hypothalamus has been considered to function 

as an integrator and “thermostat,” whereas the 

posterior hypothalamus provides a “set point” 

of the core or deep-body temperature and initiates the appropriate physiologic responses to 

keep the body temperature at that set point if 

the core temperature changes.

According to this model, the anterior hypothalamus receives the information from receptors 

sensitive to changes in temperature in the 

skin, muscle, stomach, other central nervous 

system tissues, and elsewhere. In addition, the 

anterior hypothalamus itself contains neurons 

that are responsive to changes in temperature 

of the arterial blood serving the region. The 

neurons responsible for the transmission of 

the temperature information use monoamines, 

among other neurotransmitters; this has been 

demonstrated in animals [Cooper et al. 1982]. 

These monoamine transmitters are important 

in the passage of appropriate information to 

the posterior hypothalamus. It is known that 

the set point in the posterior hypothalamus 

is regulated by ionic exchanges. However, the 

set-point hypothesis has generated considerable controversy [Greenleaf 1979]. The problem 

with the notion of a set point is that (1) a neuroanatomical region controlling the set point has 

never been identified and (2) the physiological 

responses to heat cannot be explained by the 

notion of a set point. At present, it appears that 

the hypothalamic region does integrate neural 

traffic from thermoreceptors and integrates a 

physiological response to an increase in temperature. However, the current data suggest 

that the hypothalamus controls temperature 

within a so-called inter-thermal threshold 

(range of temperatures around a mean with 

which no physiological response occurs). The 

physiological responses occur only when the 

temperature moves beyond the “thresholds” 

to elicit either sweating or thermogenesis and 

the appropriate vasomotor response (i.e., vasoconstriction or vasodilation) [Mekjavic and 

Eiken 2006]. The ratio of sodium to calcium 

ions is also important in thermoregulation. 

The sodium ion concentration in the blood and 

other tissues can be readily altered by exercise 

and by exposure to heat.

When a train of neural traffic is activated from 

the anterior to the posterior hypothalamus, 

it is reasonable to suppose that once a “hot” 

pathway is activated, it will inhibit the function 

of the “cold” pathway and vice versa. However, 

there is a multiplicity of neural inputs at all 

levels in the central nervous system, and many 

complicated neural “loops” undoubtedly exist.

Current research suggests that, instead of the 

historical notion of a set point, neural input into 

the hypothalamus is integrated into a response 

that can be described as “cross inhibitory.” In 
24 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

other words, when neural inputs from warm 

thermoreceptors in the skin are dominant, 

the integrated response results in an increase 

in sweating and cutaneous vasodilation, 

while simultaneously inhibiting thermogenesis and vice versa [Mekjavic and Eiken 2006]. 

Paradoxically, when appropriate, the core body 

temperature will increase and be regulated at 

a higher level in order to maintain an increase 

in heat loss, by maintaining a thermal gradient between the core body temperature and 

the skin temperature for the transfer of heat to 

the environment [Taylor et al. 2008]. Although 

this discussion focuses on the role of the anterior hypothalamus in heat stress and injury, 

there are many other factors that influence 

the anterior hypothalamus and the control of 

heat balance. These factors include thyroid and 

reproductive hormones, cerebrospinal fluid 

(CSF) ions and osmolality, glucose, and input 

from other brain regions related to arousal, 

circadian variation, and menstrual function 

[Kandel and Schwartz 2013].

A question that must be addressed is the difference between a physiologically raised body 

temperature and a fever; it is considered that 

the set point is elevated, as determined by 

the posterior hypothalamus. At the onset of 

a fever, the body invokes heat-conservation 

mechanisms (such as shivering and cutaneous 

vasoconstriction) in order to raise the body 

temperature to its new, regulated stable temperature [Cooper et al. 1982]. In contrast, during 

exercise in heat, which may result in an increase 

in body temperature, body temperature rises to 

a new stable level where it is regulated by the 

hypothalamus, and only heat-dissipation mechanisms are invoked. Once a fever is induced, 

the elevated body temperature appears to be 

normally controlled by the usual physiologic 

processes around its new and higher regulated 

level [Taylor et al. 2008].

4.1.2 Muscular Activity and 

Work Capacity

The muscles are by far the largest single group 

of tissues in the body, representing some 45% of 

body weight. The bony skeleton, on which the 

muscles operate to generate their forces, represents a further 15% of body weight. The bony 

skeleton is relatively inert in terms of metabolic heat production. Even at rest, the muscles 

produce about 20% to 25% of the body’s total 

heat production [Rowell 1993]. The amount of 

metabolic heat produced at rest is quite similar 

for all individuals when it is expressed per 

unit of surface area or of lean or fat-free body 

weight. On the other hand, the heat produced 

by the muscles during exercise can be much 

higher and must be dissipated if a heat balance 

is to be maintained. The heat load from metabolism is therefore widely variable, and working 

in hot environments (which imposes its own 

heat load or restricts heat dissipation) poses the 

greatest challenge to normal thermoregulation 

[Parsons 2003].

The proportion of maximal aerobic capacity (V

.

O2 max) needed to do a specific job is 

important for several reasons. First, the cardiovascular system must respond with an 

increased CO, which, at levels of work up to 

about 40% V.O2 max, is brought about by an 

increase in both SV and HR. When maximum 

SV is reached, additional increases in CO can be 

achieved solely by increased HR until maximal 

HR is reached [McArdle et al. 1996b; Taylor et 

al. 2008]. These changes in the cardiovascular 

response to exercise are responsible for providing sufficient blood flow to muscle to allow for 

the increase in muscular work [McArdle et al. 

1996b]. Further complexities arise when high 

work intensities are sustained for long periods, 

particularly when work is carried out in hot 

surroundings [Åstrand et al. 2003]. Second, 

muscular activity is associated with an increase 

in muscle temperature, which then is associated with an increase in core temperature, with 
Occupational Exposure to Heat and Hot Environments 25

4 . Biologic Effects of Heat

attendant influences on the thermoregulatory 

controls. Third, at high levels of exercise, even 

in a temperate environment, the oxygen supply 

to the tissues may be insufficient to completely 

meet the oxygen needs of the working muscles 

[Taylor et al. 2008]. 

In warmer conditions, an adequate supply of 

oxygen to the tissues may become a problem 

even at moderate work intensities because of 

competition for blood distribution between the 

working muscle and the skin [Rowell 1993]. 

Because of the lack of oxygen, the working 

muscles must begin to draw on their anaerobic 

reserves, deriving energy from the oxidation of 

glycogen in the muscles [McArdle et al. 1996b]. 

That leads to the accumulation of lactic acid, 

which may be associated with the development of muscular fatigue. As the proportion 

of V.O2 max used increases further, anaerobic 

metabolism assumes a relatively greater proportion of the total muscular metabolism. An 

oxygen “debt” occurs when oxygen is required 

to metabolize the lactic acid that accumulates 

in the muscles. This debt must be repaid during 

the rest period. In hot environments, the recovery period is prolonged as both the heat and the 

lactic acid stored in the body must be eliminated 

and water loss must be replenished. Generally, 

a euhydrated (normal hydration) state can be 

reached readily with normal ad libitum consumption of nonalcoholic beverages [Montain 

and Cheuvront 2008]. However, if a person 

consumes alcoholic beverages after a long day 

of hot work, the diuretic properties of alcohol 

may cause dehydration [Schuckit 2011]. This 

may delay the return to a euhydrated condition 

until the following day. There are also so-called 

alactate (absence of lactic acid accumulation) 

components to the oxygen debt and recovery 

process. Some of these alactate components 

involve blood returning to the lungs after distribution to the muscle, release of oxygen bound 

to myoglobin, residual effects of thermogenic 

hormones (epinephrine, norepinephrine, 

thyroxine, and glucocorticoids), and replenishment of adenosine triphosphate (ATP) and 

creatine phosphate (PCr) within the muscle 

cells [McArdle et al. 2010a].

It is well established that in a wide range of 

cool to warm environments, 5°C to 29°C 

(41°F–84.2°F), the deep body temperature rises 

during exercise to a similar equilibrium value 

in subjects exercising at the same proportion of 

V

.

O2 max [Lind 1976, 1977]. However, two individuals doing the same job and working at the 

same absolute load level who have widely different V.

O2 max values will have quite different 

core temperatures. Current recommendations 

for an acceptable proportion of V.O2 max for 

daily industrial work vary from 30% to 40% of 

the V.O2 max, which, in comfortably cool surroundings [Åstrand et al. 2003], is associated 

with rectal temperatures of, respectively, 37.4°C 

to 37.7°C (99.3°F–99.9°F), whereas work at 50% 

V

.

O2 max yields a rectal temperature of 38°C 

(100.4°F) in the absence of heat stress.

In addition to sex- and age-related variability, 

the interindividual variability of V.O2 max is 

high; in a diverse worker population the range 

of V.O2 max that includes 95 of every 100 individuals is ±20% of the mean V.

O2 max value. 

Differences in body weight (particularly the 

muscle mass) can account for about half that 

variability, but the source of the remaining variation has not been identified. Age is associated 

with a reduction in V.O2 max after its peak at 

about 20 years of age; in healthy individuals, the 

V

.

O2 max falls by nearly 10% each decade after 

age 30. The decrease with age is less in individuals who have maintained a higher degree 

of physical fitness. Women’s V.O2 max average 

about 70% of those for men in the same age 

group, because of lower absolute muscle mass, 

higher body fat content, and lower hemoglobin concentration [Åstrand and Rodahl 1977; 

Åstrand et al. 2003]. Many factors affect deep 

body temperature when men and women of 
26 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

varying body weights, ages, and work capacities do the same job.

Other sources of variability when individuals 

work in hot environments are differences in 

circulatory system capacity, sweat production, 

and ability to regulate electrolyte balance, each 

of which may be large.

Work capacity is reduced to a limited extent 

in hot surroundings if body temperature is 

elevated. That reduction becomes greater as 

body temperature increases. The V.O2 max is 

not reduced by dehydration itself (except for 

severe dehydration), so its reduction in hot 

environments seems to be principally a function of body temperature. Core temperature 

must be above 38°C (100.4°F) before a reduction is noticeable; however, a rectal temperature 

of about 39°C (102.2°F) may result in some 

reduction of V.O2 max.

The capacity for prolonged exercise of moderate intensity in hot environments is adversely 

affected by dehydration, which may be associated with a reduction of sweat production and 

a concomitant rise in rectal temperature and 

HR. If the total heat load and the sweat rate are 

high, it is increasingly more difficult to replace 

the water lost in the sweat (750–1,000 mL·h-1). 

The thirst mechanism is usually not strong 

enough to drive one to drink the large quantities of water needed to replace the water lost in 

the sweat [DOD 2003]. Evidence shows that as 

body temperature increases in a hot working 

environment, endurance decreases.

Cognitive function of individuals exposed to 

physical activity in a hot environment may 

increase, decrease, or change very little [O’Neal 

and Bishop 2010]. If cognitive function is 

impaired as the environmental heat stress 

increases, psychomotor, vigilance, and other 

experimental psychological tasks may show 

decrements in performance [Givoni and Rim 

1962; Ramsey and Morrissey 1978; Hancock 

1981, 1982; Marg 1983]. The decrement in 

performance may be at least partly related to 

increases in core temperature and dehydration. In some cases of heat exhaustion the rectal 

temperature is raised to the range of 38.5°C to 

39.0°C (101.3°F–102.2°F), and disorganized 

central nervous system activity, as evidenced 

by poor motor function, confusion, increased 

irritability, blurring of vision, and changes in 

personality are present. These observations 

have prompted the unproven suggestion that 

reduced oxygen supply to the brain, called cerebral anoxia may be responsible [Macpherson 

1960; Leithead and Lind 1964; Hancock 1982]. 

4.1.3 Circulatory Regulation

The autonomic nervous system and endocrine 

system control the allocation of blood flow 

among competing organ systems. The circulatory system delivers oxygen and nutrients to all 

tissues and for transports unwanted metabolites 

and heat from the tissues. However, as work in 

heat continues, the heart reaches a point where 

it cannot provide enough CO to meet both the 

peak needs of all of the body’s organ systems 

and the need for dissipation of body heat. 

During exercise, widespread, sympathetic 

circulatory vasoconstriction occurs initially 

throughout the body, even in the cutaneous 

bed. The increase in blood supply to the active 

muscles is ensured by the action of locally 

produced vasodilator substances, which also 

inhibit (in the blood vessels supplying the active 

muscles) the increased sympathetic vasoconstrictor activity. In inactive vascular beds, there 

is a progressive vasoconstriction with the severity of the exercise. This is particularly important 

in the large vascular bed in the digestive organs, 

where venoconstriction also permits the return 

of blood sequestered in its large venous bed, 

allowing up to one liter of blood to be added 

to the circulating volume [Rowell 1977, 1993].
Occupational Exposure to Heat and Hot Environments 27

4 . Biologic Effects of Heat

If the need to dissipate heat arises, the 

autonomic nervous system reduces the vasoconstrictor tone of the cutaneous vascular bed, 

followed by “active” dilation by an unknown 

mechanism. The sweating mechanism and an 

unknown critical factor that causes the importantly large dilation of the peripheral blood 

vessels in the skin are mutually responsible for 

humans’ large thermoregulatory capacity in 

the heat.

When individuals are exposed to continuous 

work at high proportions of V.O2 max or to 

continuous work at lower intensities in hot surroundings, the cardiac filling pressure remains 

relatively constant, but the central venous blood 

volume decreases as the cutaneous vessels 

dilate. The SV falls gradually and the HR must 

increase to maintain the CO. The effective circulatory volume also decreases, partly because 

of dehydration from sweating, and partly as 

the thermoregulatory system tries to maintain 

adequate circulation to the exercising muscles 

and the skin [Rowell 1977].

One of the most important roles of the cardiovascular system in thermoregulation is the 

circulation of warm blood from the body core 

to the skin for heat transfer to the environment. 

When the body is at rest, in the absence of heat 

strain, the skin blood flow is approximately 

200 to 500 mL·min-1, but it can increase up to 

7–8 L·min-1 when under high heat strain. The 

skin blood flow responds to changes in body 

core temperature and is required to mobilize 

warm blood from the body core to the periphery to transfer heat to the environment [Taylor 

et al. 2008]. However, the redistribution of 

blood flow to the skin results in a corresponding decrease in splanchnic, renal, and muscle 

blood flow. During exercise in the heat, blood 

is simultaneously required to supply oxygen to 

working muscle and to carry heat from the body 

core to the periphery (skin). However, muscle 

blood flow decreases by about 25% under heat 

strain, from approximately 2.4 mL·g-1·min-1

down to 2.1 mL·g-1·min-1, because of increased 

cutaneous demands [Taylor et al. 2008]. This 

is readily accomplished in a well-hydrated 

individual under compensable heat stress 

[González-Alonzo et al. 2008; Taylor et al. 

2008]. In the dehydrated individual working 

in the heat, increased core body temperature 

imposes stress on the cardiovascular system, 

as well as the thermoregulatory system in the 

hypothalamus [Taylor et al. 2008]. The mechanism of redistribution of blood to muscle and 

to the cutaneous circulation, under conditions 

of dehydration secondary to sweating, leads 

to an effective contraction of plasma volume 

[González-Alonzo et al. 2008]. A decrease in 

an effective plasma volume can result in an 

increase in heart rate and myocardial oxygen 

demand [Parsons 2003]. During heat stress at 

both rest and exercise, the HR, CO, and SV all 

increase to a greater extent at a given workload 

than would normally be observed under thermoneutral conditions [Rowell 1993]. However, 

this cannot be sustained indefinitely or when 

the body is dehydrated from sweating or substantial blood flow is redistributed to the 

cutaneous circulation because they effectively 

reduce pressure volume and, therefore, SV and 

CO [Taylor et al. 2008].

4.1.4 The Sweating Mechanism

In a hot environment, where heat transfer by 

radiation is not possible, the primary means 

for the transfer of heat to the environment is 

evaporative heat loss through the vaporization 

of sweat from the skin. The sweat glands are 

found in abundance in the outer layers of the 

skin. They are stimulated by cholinergic sympathetic nerves and secrete a hypotonic watery 

solution onto the surface of the skin. Several 

other mechanisms for heat transfer to the 

environment include convection, conduction, 

and behavioral (e.g., leave the area, put on or 

take off clothes, drink water, or modify environmental controls) [Taylor et al. 2008]. In a 
28 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

hot environment that has an ambient wet-bulb 

temperature of 35°C (95°F) the body at rest can 

sweat at a rate that results in a body fluid loss 

of 0.8 to 1.0 L·h-1. For every liter of water that 

evaporates, 2,436 kJ (580 kcal) are extracted 

from the body and transferred to the environment [McArdle et al. 1996a]. The enormous 

capacity for heat loss through evaporation 

is generally more than adequate to dissipate 

metabolic heat generated by a subject at rest 

(~315 kJ·h-1for a 75-kg man) and at high levels 

of activity. The mean sweat rate in endurance 

athletes ranges from 1.5 to 2.0 L·h-1, which provides an evaporative heat loss capacity of 3,654 

to 4,872 kJ that is about 11.6–15.5 times the 

amount of heat produced at rest [Gisolfi 2000]. 

This is generally more than adequate to remove 

heat from the body, even at extreme levels of 

metabolic heat production.

However, in environments with high humidity, even though sweating continues (increasing 

the level of dehydration), the evaporation of 

sweat is inhibited, heat transfer from the body 

is reduced, and the internal body temperature 

increases. Thus, when the heat index is greater 

than 35°C (95°F), largely due to high relative 

humidity (RH) (or with a WBGT of 33°C to 

25°C [91.4°F to 77°F], depending on the workload), the evaporative heat loss is virtually 

nonexistent. Consequently, even if the ambient 

dry temperature is within a comfortable range 

(e.g., 23°C [73.4°F]), the high humidity could 

result in an “apparent temperature” or heat 

index high enough to cause heat stress for 

the worker and possible heat injury [Taylor et 

al. 2008].

Sweating results in significant dehydration, 

which leads to thermal and cardiovascular 

strain. People who are acclimatized to the heat 

lose water at a peak rate of 3 L·h-1 through 

sweating and may lose up to 12 L·h-1 during 

intense exercise in hot environments [McArdle 

et al. 1996a]. Thus, a major issue resulting 

from high heat stress is the need for adequate 

rehydration in order to replace the water lost to 

the environment from sweating and to reduce 

the risk of hyperthermia. 

A rule of thumb is that a 0.45-kg (1.0-lb) 

decrease in body weight represents a 450-mL 

(15.2-oz) decrease in body water in extracellular and intracellular compartments that 

needs to be replaced by consumption of water. 

Another source of body water loss is the respiratory tract [McArdle et al. 1996a]. Average 

water loss from the respiratory tract at rest is 

about 350 mL per day under mild conditions 

of heat and humidity. Respiratory water loss 

will also contribute to the dehydration of the 

worker, and fluid loss increases with activity.

An important constituent of sweat is salt, or 

sodium chloride. In most circumstances in 

the United States, a salt deficit does not readily 

occur because the normal American diet provides 4 grams per day (174 mEq per day) of 

sodium [Food and Nutrition Board, Institute 

of Medicine 2004]. However, the sodium 

content of sweat in unacclimatized individuals 

may range from 10 to 70 mEq per day sweat 

(0.23–1.62 g·L-1) [Montain and Cheuvront 

2008], whereas for the acclimatized individual, sodium lost to sweat may be reduced to

23 mEq·L-1 (0.530 g·L-1), less than 50% of that 

of the unacclimatized individual. It is possible 

for a heat-unacclimatized individual who consumes a restricted salt diet to develop a negative 

salt balance. In theory, a prolonged negative 

salt balance with a large fluid intake may result 

in a need for moderate supplementation of 

dietary salt. If there is a continuing negative 

salt balance, acclimatization to heat is diminished. However, salt supplementation of the 

normal diet is rarely required, except possibly 

for heat-unacclimatized individuals during the 

first 2 or 3 days of heat exposure [Lind 1976; 

DOD 2003]. By the end of the third day of heat 

exposure, a significant amount of heat acclimatization will have occurred, and salt loss in the 

sweat and urine and the need for salt in the diet 
Occupational Exposure to Heat and Hot Environments 29

4 . Biologic Effects of Heat

are decreased. In view of the high incidence of 

elevated blood pressure in the U.S. worker population and the relatively high salt content of 

the average U.S. diet, even for those who watch 

salt intake, recommending an increase in salt 

intake is probably not warranted. Salt tablets 

can irritate the stomach and should not be 

used [DOD 1980; 2003]. Heavier use of salt at 

meals has been suggested for heat-unacclimatized workers during the first 2 or 3 days of heat 

exposure if they are not on a restricted salt diet 

by order of the responsible healthcare provider 

[DOD 2003]. Sodium can also be replenished 

by drinking fluids containing approximately 20 

mEq·L-1 sodium, which is an amount found in 

many sports drinks [Montain and Cheuvront 

2008]. In general, simply adding salt to the 

diet will adequately restore electrolyte balance. 

Moreover, carefully induced heat acclimatization reduces or eliminates the need for salt 

supplementation of the normal diet. 

Because potassium is lost in sweat, it can be 

substantially depleted when unacclimatized 

workers suddenly have to work hard in hot climates; marked depletion of potassium can lead 

to serious physiologic consequences, including 

the development of heat stroke [Leithead and 

Lind 1964]. High table salt intake may increase 

potassium loss. However, potassium loss is 

usually not a problem, except for individuals 

taking diuretics, because potassium is present 

in most foods, particularly meats and fruits 

[Greenleaf and Harrison 1986]. Since some 

diuretics cause potassium loss, workers taking 

such medication while working in a hot environment should seek medical advice, telling 

their doctor that they work in the heat and 

asking how medications may affect how their 

body responds to the hot environment. Salt and 

potassium supplements, if recommended by 

the responsible healthcare provider, may only 

be needed during the acclimatization period or 

under extraordinary circumstances.

4.1.4.1 Water and Electrolyte Balance and 

the Influence of Endocrines

It is imperative to replace the water lost in the 

sweat. It is not uncommon for workers to lose 

6 to 8 L of sweat during a working shift in hot 

industries. If the lost water is not replaced, then 

body water levels will progressively decrease, 

which shrinks the extracellular space, interstitial and plasma volumes, and water in the cells. 

Evidence supports that the amount of sweat 

production depends on the state of hydration [Leithead and Lind 1964; Henschel 1971; 

Greenleaf and Harrison 1986], so that progressive dehydration results in a lower sweat 

production and a corresponding increase in 

body temperature, which can be a dangerous situation.

Water lost in large quantities of sweat is often 

difficult to replace completely as the day’s work 

proceeds, and it is not uncommon for individuals to register a water deficit of 2% to 3% or 

greater of their body weight. During exercise 

in either cool or hot environments, a correlation has been reported between the elevation 

of rectal temperature and the percentage of 

water deficit in excess of 3% of body weight 

[Kerslake 1972 (p. 316)]. Because the normal 

thirst mechanism is not sensitive enough to 

ensure a sufficient water intake [Greenleaf and 

Harrison 1986; DOD 2003], every effort should 

be made to encourage individuals to drink 

water or other fluids (e.g., sports drinks). The 

fluid should be as palatable as possible, at less 

than 15°C (59°F). Small quantities taken at frequent intervals is a more effective regimen for 

practical fluid replacement than the intake of 

large amounts of fluids per hour [McArdle et 

al. 2010b]. Individual, not communal, drinking 

cups should be provided. Individuals are seldom 

aware of just how much sweat they produce or 

how much water is needed to replace that lost 

in the sweat; 1 L·h-1 is a common rate of water 

loss. With suitable instruction on how much to 

drink, most individuals will comply. A general 
30 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

rule of thumb for those exercising in the heat 

for 1 to 2 hours is to drink plain, cool water. 

Sweat is hypotonic to the plasma, and one 

does not lose a significant amount of sodium 

in the first hour or two of exercise [McArdle 

et al. 1996b]. Therefore, one does not require 

fluids containing electrolytes for this exposure. 

However, during prolonged sweating lasting 

several hours, it is advisable to consume a 

sports drink that contains balanced electrolytes 

to replace those lost during sweating, as long as 

the concentration of electrolytes/carbohydrates 

does not exceed 8% by volume. Exceeding the 

8% limit will slow absorption of fluids from the 

gastrointestinal (GI) tract [Parsons 2003]. Since 

thirst is a poor indicator of hydration status, 

fluids should be consumed at regular intervals 

to replace water lost from sweating [McArdle 

et al. 1996b].

Plasma electrolytes (primarily sodium, potassium, and chlorine) are very strongly regulated 

by physiologic mechanisms because of their 

high importance in cellular volume and function. The control is accomplished by the kidney 

and is influenced by several hormonal pathways, 

including the renin-angiotensin-aldosterone 

system (RAAS) as well as antidiuretic hormone 

(ADH). Normal plasma sodium (Na+) concentration falls within the range of 135 to 145 

mmol·L-1. Thus, hyponatremia is defined as a 

Na+ concentration <135 mmol·L-1 (mild) and 

severe hyponatremia as a plasma Na+ concentration of 120 mmol·L-1. Hyponatremia (low 

plasma sodium) is regarded as the consequence 

of a poor fluid-replacement strategy before, 

during, and after prolonged exercise and may 

become a potentially life-threatening condition. However, consumption of large quantities 

of water alone is not enough to cause hyponatremia. The condition is accompanied by 

significant loss of Na+ from sweating [Montain 

et al. 2006].

Hyponatremia usually occurs in high-endurance athletes participating in marathons, 

ultra-marathons, and so-called Iron Man triathlons, but it can also occur in people working 

for long periods of time in hot environments 

[Rosner and Kirven 2007]. The incidence of 

hyponatremia in high-endurance athletes 

ranges from 13% to 18%. Symptoms of hyponatremia can range from none to minimal (~70% 

of cases) to severe, including encephalopathy, 

respiratory distress, and death. During these 

events or work periods, hyponatremia develops 

when the athlete or worker consumes too much 

plain water in an attempt to rehydrate after 

copious sweating. The excess water results in a 

dilution of plasma Na+, which, in turn, causes 

an osmotic disequilibrium that can lead to cerebral edema (brain swelling) and pulmonary 

edema. These conditions can be fatal in a small 

number of patients [Rosner and Kirven 2007].

Risk factors for the development of hyponatremia are an exercise duration of >4 hours, 

gender (females are more likely to develop 

hyponatremia), low body mass, excessive consumption of water (>1.5 L·h-1), pre-exercise 

hydration, consumption of nonsteroidal antiinflammatory medications (although not all 

studies have shown this), and extreme environmental temperature [Rosner and Kirven 

2007]. Consumption of large quantities of water 

is highly correlated with the development of 

hyponatremia [Almond et al. 2005], the majority of athletes or workers in hot environments 

proceed to develop hyponatremia; however, not 

all those with hyponatremia become symptomatic [Rosner and Kirven 2007]. Therefore, the 

presence of mild hyponatremia may not be 

harmful. Although hyponatremia can have devastating consequences, such as death, for the 

sufferer and the pathophysiology of the condition involves multiple factors including the 

environmental as well as individual predisposition, the condition can be treated effectively 

to reduce the level of morbidity and mortality 

[Rosner and Kirven 2007]. Prevention of this 
Occupational Exposure to Heat and Hot Environments 31

4 . Biologic Effects of Heat

condition involves appropriate use of fluidreplacement strategies.

Two hormones are important in thermoregulation: ADH and aldosterone. A variety of stimuli 

encourage the synthesis and release of those 

hormones, such as changes in plasma volume 

and plasma concentration of sodium chloride. 

ADH is released by the pituitary gland, which 

has direct neural connections with the hypothalamus but may receive neural input from 

other sources. Its function is to reduce water 

loss by the kidney, but it has no effect on the 

water loss through sweat glands. Body water, 

including the plasma volume in the vascular compartment, is also controlled by the 

RAAS. Changes in fluid volume or electrolyte (sodium) concentration will activate the 

RAAS to conserve fluids and electrolytes at the 

level of the kidney and sweat glands through 

the action of aldosterone [Jackson 2006]. The 

control of fluid (especially plasma) volume is 

also important in the maintenance of blood 

pressure and organ perfusion. Another product 

of the RAAS is angiotensin II, a powerful vasoconstrictor that helps maintain blood pressure 

and overall cardiovascular function in the presence of significant fluid loss from the vascular 

compartment, as well as a significant stimulator 

of the release of aldosterone from the adrenal 

glands [Williams et al. 2003].

4.1.4.2 Dietary Factors

A well-balanced diet in temperate environments 

usually suffices for hot climates. A very high 

protein diet might increase the urine output 

needed for nitrogen removal and increase water 

intake requirements [Greenleaf 1979; Greenleaf 

and Harrison 1986]. The importance of water 

and salt balance has been emphasized in the 

previous discussion, and the possibility that 

it might be desirable to supplement the diet 

with potassium has also been discussed. In 

some countries where the normal diet is low 

or deficient in vitamin C, supplements may 

enhance heat acclimatization and thermoregulatory function [Strydom et al. 1976]. Sodium 

supplements can be abused in the interest of 

increasing fluid retention. However, an excess 

of dietary sodium may actually decrease plasma 

volume, even with a controlled fluid intake. 

Therefore, supplemental dietary sodium must 

be used judiciously to prevent further dehydration and electrolyte depletion in workers 

[McArdle et al. 1996a; Williams et al. 2003]. 

In addition, certain dietary supplements may 

impair or alter cellular and systemic adaptions 

associated with both thermotolerance and heat 

acclimation in exercising humans [Kuennen 

et al. 2011].

4.1.4.3 Gastrointestinal Factors

Exertional hyperthermia may result in changes 

in GI permeability (“leaky gut”) that can result 

in the release of paracellular endotoxins [Zuhl 

et al. 2014]. The release of these endotoxins 

can trigger the release of leukocytes, which can 

result in the further release of proinflammatory cytokines [Leon 2007]. This inflammatory 

cascade results in further damage to the organ 

systems. Certain factors increase the susceptibility to increased GI permeability and 

endotoxin damage, including age; dehydration; 

loss of electrolytes from sweating; cardiovascular disease; use of certain medications (e.g., 

diuretics, anticholinergics, and NSAIDs); and 

alcoholism. In addition, certain foods containing quercetin (e.g., capers, red onions, etc.) can 

increase hyperthermia-induced GI permeability and aggravate the effects of hyperthermia 

[Kuennen et al. 2011; Zuhl et al. 2014].

4.1.5 Acclimatization to Heat

When workers are exposed to hot work environments, they readily show signs of distress 

and discomfort, such as increased core temperatures and heart rates, headache or nausea, and 

other symptoms of heat exhaustion [Leithead 

and Lind 1964; WHO 1969; Kerslake 1972 
32 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

(p. 316); Wyndham 1973; Knochel 1974; 

Hancock 1982; Spaul and Greenleaf 1984; DOD 

2003]. On repeated exposure to a hot environment, there is a marked adaptation in which 

the principal physiologic benefit appears to 

result from an increased sweating efficiency 

evidenced by earlier onset of sweating, greater 

sweat production, and lower electrolyte concentration) and a concomitant stabilization of the 

circulation. As a result, after daily heat exposure for 7 to 14 days, most individuals perform 

the work with a much lower core temperature 

and HR and a higher sweat rate (i.e., a reduced 

thermoregulatory strain) and with none of 

the symptoms that were experienced initially 

[Moseley 1994; Armstrong and Stoppani 2002; 

DOD 2003; Navy Environmental Health Center 

2007; Casa et al. 2009; ACGIH 2014]. During 

that period, the plasma volume rapidly expands, 

so that even though the blood is concentrated 

throughout the exposure to heat, the plasma 

volume at the end of the heat exposure in acclimatized individuals is often equal to or greater 

than before the first day of heat exposure.

Acclimatization to heat is an example of physiologic adaptation, which is well demonstrated 

in laboratory experiments and field experience 

[Lind and Bass 1963; WHO 1969]. However, 

acclimatization does not necessarily mean that 

individuals can work above the Prescriptive 

Zone (see Glossary) as effectively as below it 

[Lind 1977].

Full heat acclimatization occurs with relatively 

brief daily exposures to working in the heat. It 

does not require exposure to heat at work and 

rest for the entire 24 hours; in fact, such excessive exposures may be deleterious because it is 

difficult for individuals without heat acclimatization experience to replace all of the water 

lost in sweat. The minimum exposure time for 

achieving heat acclimatization is at least 2 hours 

per day, which may be broken into 1-hour 

exposures [DOD 2003]. Some daily period of 

relief from exposure to heat, in air-conditioned 

surroundings, is beneficial to the well-being 

of the individuals, if for no other reason than 

that they find it difficult to rest effectively in a 

hot environment [Kerslake 1972 (p. 316)]. The 

level of acclimatization depends on the initial 

level of individual physical fitness and the total 

heat stress experienced by the individual [DOD 

2003]. Thus, a worker who does only light work 

indoors in a hot climate will not achieve the 

level of acclimatization needed to work outdoors with the additional heat load from the 

sun or to do harder physical work in the same 

hot environment indoors. Increased aerobic 

fitness confers at least partial acclimatization 

to the heat because of the increased metabolic 

heat production that occurs during exercise. 

Physically fit individuals have a reduced incidence of heat injury or illness during exposure 

to hot environments [Tipton et al. 2008].

Failure to replace the water lost in sweat will 

slow or even prevent the development of 

the physiologic adaptations described. It is 

important to understand that heat acclimatization increases the sweating rate; therefore, 

workers will have an increased water requirement [DOD 2003; Navy Environmental Health 

Center 2007]. In spite of the fact that acclimatization will be reasonably well maintained for 

a few days of no heat exposure, absence from 

work in the heat for a week or more results 

in a significant loss in beneficial adaptations. 

However, heat acclimatization can usually be 

regained in 2 to 3 days upon return to a hot job 

[Lind and Bass 1963; Wyndham 1973]. Heat 

acclimatization appears to be better maintained 

by individuals who are physically fit [Pandolf et 

al. 1977].

The total sweat production increases with acclimatization, and sweating begins at a lower core 

temperature [DOD 2003]. Cutaneous circulation and circulatory conductance decrease with 

acclimatization, reflecting the reduction in the 

proportion of CO that must be allocated for 

thermoregulation because of the more efficient 
Occupational Exposure to Heat and Hot Environments 33

4 . Biologic Effects of Heat

sweating mechanism. Also during acclimatization, cardiovascular stability is improved, heart 

rate is lowered, stroke volume is increased, and 

myocardial compliance is improved [DOD 

2003]. It is clear, however, that during exercise 

in heat, the production of aldosterone increases 

to conserve salt from both the kidney and the 

sweat glands, whereas an increase in ADH conserves the amount of water lost through the 

kidneys. The increase in levels of aldosterone 

results in a lower concentration of sodium in 

the sweat and thus serves to limit sodium and 

fluid loss from the plasma during exercise in 

the heat [Taylor et al. 2008].

It is clear from the foregoing descriptions that 

sudden seasonal shifts and sudden increases in 

environmental temperature may result in thermoregulatory difficulties for exposed workers. 

At such times, cases of heat disorder may occur, 

even for acclimatized workers. 

Acclimatization to work in hot, humid environments provides adaptive benefits that also 

apply in desert environments, and vice versa; 

the qualifying factor appears to be the total 

heat load experienced by the individual [DOD 

2003]. An acclimatization plan should be 

implemented at all workplaces where workers 

are exposed to heat. A recent study presented 

20 cases of heat-related illness or death among 

workers [Arbury et al. 2014]. In most of these 

cases, employers had no program to prevent 

heat illness, or the program was deficient. 

Acclimatization was the program element most 

commonly missing and most clearly associated 

with worker death.

For a summary on acclimatization, see 

Table 4-1.

4.1.6 Other Related Factors

Many factors can increase a worker’s risk of 

heat-related illness. Some of the factors are 

environmental, such as direct sun exposure, 

high temperatures, and humidity. Indoor 

radiant heat sources, like ovens and furnaces, 

also can increase the amount of heat in the 

environment. An indoor work environment 

can become a heat hazard if air conditioning is 

unavailable or ventilation is insufficient [Chen 

et al. 2003]. Other factors may be related to 

characteristics of each individual worker or 

an individual’s current status of health at the 

time of exposure to heat stress in a hot environment. Heat-related illness factors are presented 

in Figure 4-1.

4.1.6.1 Age

The aging process results in a more sluggish 

response of the sweat glands, which leads to a 

less effective control of body temperature in the 

sedentary individual [Taylor et al. 2008]. Aging 

also results in a decreased level of skin blood 

flow associated with exposure to heat. The cause 

remains undetermined, but the decrease in skin 

blood flow implies an impaired thermoregulatory mechanism, possibly related to reduced 

efficiency of the sympathetic nervous system 

[Hellon and Lind 1958; Lind 1977; Drinkwater 

and Horvath 1979]. For women, it has been 

found that the skin temperature increases 

with age in moderate and high heat loads but 

not in low heat loads [Hellon and Lind 1958; 

Drinkwater and Horvath 1979]. When two 

groups of male coal miners of average age 47 

and 27 years, respectively, worked in several 

comfortable or cool environments, they showed 

little difference in their responses to heat near 

the REL with light work, but in hotter environments the older men showed a substantially 

greater thermoregulatory strain than their 

younger counterparts; the older men also 

had lower aerobic work capacities [Lind et al. 

1970]. In analyzing the distribution of 5 years’ 

accumulation of data on heat stroke in South 

African gold mines, Strydom [1971] found a 

marked increase in heat stroke with increasing 

age of the workers. Thus, men over 40 years of 

age represented less than 10% of the mining 
34 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

Table 4-1. Acclimatization in workers

Topics Additional information

Disadvantages of 

being unacclimatized

■ Readily show signs of heat stress when exposed to hot environments.

■ Difficulty replacing all of the water lost in sweat.

■ Failure to replace the water lost will slow or prevent acclimatization.

Benefits of acclimatization ■ Increased sweating efficiency (earlier onset of sweating, greater sweat 

production, and reduced electrolyte loss in sweat).

■ Stabilization of the circulation.

■ Work is performed with lower core temperature and heart rate.

■ Increased skin blood flow at a given core temperature.

Acclimatization plan ■ Gradually increase exposure time in hot environmental conditions 

over a period of 7 to 14 days.

■ For new workers, the schedule should be no more than 20% of the 

usual duration of work in the hot environment on day 1 and a no more 

than 20% increase on each additional day.

■ For workers who have had previous experience with the job, the 

acclimatization regimen should be no more than 50% of the usual 

duration of work in the hot environment on day 1, 60% on day 2, 80% 

on day 3, and 100% on day 4.

■ The time required for non–physically fit individuals to develop 

acclimatization is about 50% greater than for the physically fit.

Level of acclimatization ■ Relative to the initial level of physical fitness and the total heat stress 

experienced by the individual.

Maintaining acclimatization ■ Can be maintained for a few days of non-heat exposure.

■ Absence from work in the heat for a week or more results in a 

significant loss in the beneficial adaptations leading to an increased 

likelihood of acute dehydration, illness, or fatigue.

■ Can be regained in 2 to 3 days upon return to a hot job.

■ Appears to be better maintained by those who are physically fit.

■ Seasonal shifts in temperatures may result in difficulties.

■ Working in hot, humid environments provides adaptive benefits that 

also apply in hot, desert environments, and vice versa.

■ Air conditioning will not affect acclimatization.

Adapted from [Moseley 1994; Armstrong and Stoppani 2002; DOD 2003; Casa et al. 2009; ACGIH 2014; OSHA-NIOSH 2011].
Occupational Exposure to Heat and Hot Environments 35

4 . Biologic Effects of Heat

population, but they accounted for 50% of the 

fatal cases and 25% of the nonfatal cases of 

heat stroke. The incidence of cases per 100,000 

workers was 10 or more times greater for men 

over 40 years than for men under 25 years of 

age. In all the experimental and epidemiologic 

studies described above, the workers had been 

medically examined and were considered free 

of disease. Chronic hypohydration increases 

with age, which may be a factor in the observed 

higher incidence of fatal and nonfatal heat 

stroke in the older group. The reasons for hypohydration in older adults appear to be related 

to a decreased thirst drive resulting in, among 

Figure 4-1. Examples of heat-related illness risk factors
36 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

other things, a suboptimal plasma volume. A 

reduced plasma volume (and most likely total 

body water, given the normal transfer of fluids 

between compartments) can impair thermoregulatory dynamics [McArdle et al. 2010a]. 

Another study suggests that age-related impairments in heat loss may not be evident during 

durations of <15 minutes of exercise; therefore, 

older workers may be safe from thermal strain 

and dehydration if they work for shorter intervals [Wright et al. 2014]. 

Older people are more susceptible to the effects 

of heat, and a significant fraction of those who 

suffer from heat disorders are older than 60 

[Kenny et al. 2010]. The age-related susceptibility to heat is multifactorial and may be related 

to decreases in sweating and cutaneous blood 

flow, changes in cardiovascular function, and 

decreases in overall fitness [Kenney et al. 1990; 

Minson et al. 1998; Inoue et al. 1999]. Decreased 

sweating may be due to reduced sweat production rather than a reduced number of sweat 

glands [Inbar et al. 2004]. Thus, while acclimatization to heat can occur in the elderly, the rate 

of acclimatization is reduced [Armstrong and 

Kenney 1993; Inoue et al. 1999].

4.1.6.2 Sex

Although not all aspects of heat tolerance 

in women have been fully examined, their 

thermoregulatory capacities have been. An 

average-sized woman has a lower aerobic 

capacity than an average-sized man. When they 

work at similar proportions of their O2max, 

women perform either similarly or only slightly 

less well than men [Drinkwater et al. 1976; 

Avellini et al. 1980a; Avellini et al. 1980b; Frye 

and Kamon 1981]. 

A study examining sweat electrolyte loss during 

exercise in the heat found that sweat concentrations of Na+ and Cl-

 in men were higher than 

in women [Meyer et al. 1992]. The study was 

unable to explain why there appeared to be a 

difference between the sexes, although sweating 

rate and the effects of hormonal variations may 

play a part. 

A recent study investigated whether there was 

an effect of sex on whole-body sudometer (a 

device to measure total sweat loss from the 

individual) activity during exercise in the heat 

[Gagnon and Kenny 2011]. Gagnon and Kenny 

found that females have a lower whole-body 

sweat response during exercise in the heat, 

which results in a greater increase in body temperature. The study concludes that the “results 

were not due to differences in physical characteristics, as both sexes were matched for body 

mass and surface area.”

According to Nunneley [1978], there seemed 

to be little change in thermoregulatory 

capacities at different times during women’s 

menstrual cycles. 

Pregnancy is a consideration for women in 

coping with heat stress, and as it progresses 

tolerance of heat stress is reduced [Navy 

Environmental Health Center 2007]. Pregnancy 

naturally elevates the body’s temperature, 

which subjects women to heat exhaustion more 

rapidly during periods of extreme temperatures. 

The fetus acts as a source of metabolic heat and 

also increases the weight of the mother. Also, 

because the pregnant woman is caring for more 

than one body, more fluids and energy are 

needed to cool her core temperature. Increased 

heat loads during early pregnancy increase the 

fetus’ risk of teratogenic effects [Tillett 2011]. 

However, earlier studies suggested that the 

basal core temperature of pregnant women participating in submaximal exercise (i.e., bicycle 

test) studies over the course of their pregnancy 

decreased slightly postpartum [Lindqvist et al. 

2003]. This result agrees with an even earlier 

study which reported that the magnitude of any 

first trimester exercise-associated thermal stress 

for the embryo or fetus is reduced by the maternal physiologic adaptions during pregnancy 
Occupational Exposure to Heat and Hot Environments 37

4 . Biologic Effects of Heat

[Clapp 1991]. Moderate thermal stress (21.1°C

[70°F] for 20 minutes) did not induce regular 

uterine contractility, nor was it harmful to the 

fetus when it was exposed during late pregnancy [Vaha-Eskeli and Erkkola 1991].

4.1.6.3 Body Fat

It is well established that obesity predisposes 

individuals to heat disorders [Leithead and 

Lind 1964]. In fact, heat disorders occur 3.5 

times more frequently in the obese than in 

lean individuals [Henschel 1967; Chung and 

Pin 1996; Kenny et al. 2010]. The acquisition 

of fat means that additional weight must be 

carried, thereby calling for a greater expenditure of energy to perform a given task and use 

of a greater proportion of the V.O2 max on an 

overall per-weight basis. However, there is no 

difference in V.O2 max between obese and lean 

subjects if measured on an oxygen consumption per lean body weight basis [Vroman et al. 

1983]. In addition, the ratio of body surface 

area to body weight (m2 to kg) in the obese 

individual becomes less favorable for heat dissipation. Probably more important is the lower 

physical fitness and decreased maximum work 

capacity and cardiovascular capacity frequently 

associated with obesity. The increased layer of 

subcutaneous fat provides an insulation barrier 

between the skin and the deep-lying tissues. 

The fat layer theoretically reduces the direct 

transfer of heat from the muscles to the skin 

[Wells and Buskirk 1971].

The limited number of studies in the area of 

heat stress and obesity has shown that obese 

individuals have a lower forearm blood flow 

during exercise in the heat, which is thought 

to reduce the cutaneous exchange of heat with 

the environment [Vroman et al. 1983; Kenny et 

al. 2010]. The reason for this occurrence is not 

completely clear, but it may be due to changes 

in sympathetic control over the vasculature or 

reduced stroke volume that regulate the relative 

blood flow to muscle (to perform work) and 

the blood flow to the cutaneous vasculature for 

the purpose of heat exchange. Some evidence 

exists that obese individuals suffer from asymptomatic small-fiber neuropathies, which lower 

thermal sensitivity [Herman et al. 2007]. 

It has also been proposed that the increase in 

thermal load in the obese is due simply to the 

reduced specific heat capacity of adipose tissue, 

which contains a lower amount of water per 

gram, in comparison with lean mass. Thus, for 

a given thermal load, the obese individual will 

store thermal energy at a greater rate than the 

lean individual, resulting in a greater average 

tissue temperature [Henschel 1967; Kenny et 

al. 2010].

Finally, the extra weight carried by the obese 

versus lean individual results in an increase 

in metabolic energy for any given task. The 

increase in metabolic energy produced in the 

form of muscular work results in an increase in 

body temperature that must be exchanged, in 

comparison with a lean individual performing 

the same task in the same environment [Bar-Or 

et al. 1969; Kenny et al. 2010]. Therefore, 

because it is apparent that obesity places the 

individual at a significantly higher risk of suffering a heat-related illness at any given workload 

or environmental temperature than the risk for 

a lean individual, additional accommodations 

may be necessary.

4.1.6.4 Drugs

(1) Therapeutic Drugs

Many drugs prescribed for therapeutic purposes can interfere with thermoregulation 

[Khagali and Hayes 1983]. Some of these drugs 

are anticholinergic in nature or involve inhibition of monoamine oxidative reactions, but 

almost any drug that affects central nervous 

system activity, cardiovascular reserve (e.g., 

beta blockers), or body hydration could potentially affect heat tolerance. Cardioselective beta 

blockers (e.g., atenolol, betaxolol, metoprolol, 
38 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

and acebutolol) do not allow the small blood 

vessels in the skin to dilate, thereby reducing 

blood flow, which impedes sweat production 

and causes body temperature to rise. Thus, 

beta blockers predispose those working in heat 

to heat-related emergencies. A worker who 

requires therapeutic medications should be 

under the supervision of the responsible healthcare provider who understands the potential 

ramifications of drugs on heat tolerance. In 

such instances, a worker taking therapeutic 

medications who is exposed only intermittently or occasionally to a hot environment 

should seek the guidance from the responsible 

healthcare provider. See Table 4-2 for additional 

information on proposed mechanisms of action 

of drugs implicated in intolerance of heat.

(2) Alcohol and Caffeine

It is hard to separate drugs used therapeutically 

from those used socially. Alcohol use (combined with heat stress) commonly has been 

associated with the occurrence of heat stroke 

[Leithead and Lind 1964]. It is a drug which 

interferes with central and peripheral nervous 

function and is associated with dehydration by 

suppressing ADH production. The ingestion 

of alcohol prior to or during work in the heat 

should not be permitted because it reduces heat 

tolerance and increases the risk of heat-related 

illnesses. 

There are many drugs other than alcohol that 

are used on social occasions and have been 

implicated in cases of heat disorder, sometimes 

leading to death [Khagali and Hayes 1983]. 

Caffeine may be considered a socially accepted 

drug found in common beverages and foods 

(eg., coffee, tea, soft drinks, energy drinks, 

cocoa, chocolate) and in some over-the-counter analgesics that are consumed worldwide to 

enhance alertness, reduce fatigue, enhance athletic performance, augment the effects of mild 

analgesics and for simple enjoyment [Undem 

2006; Taylor et al. 2008]. Coffee is one of the 

most widely consumed beverages in the world, 

and contains caffeine, which has a mild diuretic 

effect and should not be provided to workers to 

replace fluids lost to sweating. Moreover, coffee 

is generally consumed as a hot beverage and 

has the potential to exacerbate heat stress. In 

the past, caffeine was considered to contribute 

to heat stress by reducing fluid volume and 

result in cardiovascular strain during exposure 

to the heat [Serafin 1996]. 

Recent studies present evidence that caffeine 

may have less effect on heat tolerance than previously suspected [Roti et al. 2006; Armstrong 

et al. 2007a; Ely et al. 2011]. Armstrong et al. 

[2007a] propose that caffeine consumption 

does not result in water-electrolyte imbalances 

and does not reduce exercise-heat tolerance. 

They also suggest that “caffeinated fluids contribute to the daily human water requirement 

in a manner that is similar to pure water.” Ely 

et al. [2011] similarly found that a caffeine 

dose of 9 mg·kg-1 did not substantially alter 

heat balance during work in a hot environment. Caffeine appeared not to interfere with 

dry heat gains or evaporative heat losses, and 

total caffeine levels of 9 mg·kg-1 or less (approximately the amount found in one cup of coffee) 

could be considered safe in hot, dry environments. Roti et al. [2006] concluded that there 

was no evidence that dehydration or impaired 

thermoregulation resulted from chronic caffeine ingestion prior to or during exercise in 

the heat.

Although these studies present evidence that 

caffeine use may be harmless and acceptable 

for those exerting themselves in a hot environment, water is still the preferred hydrating 

beverage for before, during, and after work. 

However, virtually any nonalcoholic beverage 

is better than drinking nothing during heat 

exposure. Further research is needed on the 

effects of large doses of caffeine consumed at 

one time and the differences between modes 

of caffeine delivery (i.e., capsules, various 
Occupational Exposure to Heat and Hot Environments 39

4 . Biologic Effects of Heat

beverages, and solid food) [Armstrong et al. 

2007a]. Caffeine-containing fluids are now 

marketed to the public as energy drinks. These 

drinks contain higher than normal doses of 

caffeine (more than what is found in a cup of 

coffee or soft drink) and have been used extensively among competitive athletes prior to 

participation in athletic events [Burke 2008]. 

The lethal oral dose for caffeine in humans 

has been reported to range between 18 and 50 

grams [HSDB 2011]. Other than the diuretic 

effects, this dose of caffeine is capable of inducing cardiac arrhythmias [Undem 2006], which 

could be potentiated by heat stress (i.e., as a 

result of already existing cardiovascular strain). 

It seems that there would also be a tendency 

Table 4-2. Drugs implicated in intolerance to heat

Drug or drug class Proposed mechanism of action

Anticholinergics 

(e.g., benzotropine, trihexyphenidyl)

■ Impaired sweating

Antihistamines ■ Impaired sweating

Phenothiazines ■ Impaired sweating, (possibly) disturbed hypothalamic 

temperature regulation

Tricyclic antidepressants (e.g., imipramine, 

amitriptyline, protriptyline)

■ Impaired sweating, increased motor activity and 

heat production

Amphetamines, cocaine, ecstasy ■ Increased psychomotor activity, activated 

vascular endothelium

Analgesics (e.g., acetaminophen, aspirin) ■ Liver or kidney damage

Ergogenic stimulants 

(e.g., ephedrine/ephedra)

■ Increased heat production

Lithium ■ Nephrogenic diabetes insipidus and water loss

Diuretics ■ Salt depletion and dehydration

Calcium channel blockers 

(e.g., amlodipine, verapamil)

■ Reduced skin blood flow and reduced blood pressure

Ethanol ■ Diuresis, possible effects on intestinal permeability

Barbiturates ■ Reduced blood pressure

Antispasmodics ■ Impaired sweating

Haloperidol ■ Tachycardia, altered central temperature regulation, 

and hyponatremia

Laxatives ■ Dehydration

Beta blockers (atenolol, betaxolol) ■ Reduced skin blood flow, reduced blood pressure, and 

impaired sweating

Narcotics ■ Excessive sweating, salt depletion and dehydration

Levothyroxine ■ Excessive sweating, salt depletion and dehydration

Adapted from Heat Stress Control and Heat Casualty Management [DOD 2003].
40 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

to drink several energy drinks just to alleviate thirst as they are often available in smaller 

containers with less liquid, thus inadvertently 

overdosing on caffeine.

4.1.6.5 Non-heat Disorders

It has long been recognized that individuals suffering from degenerative diseases of 

the cardiovascular system and other diseases, 

such as diabetes or simple malnutrition, are at 

increased risk when they are exposed to heat 

and when stress is imposed on the cardiovascular system. The outcome is readily seen during 

sudden or prolonged heat waves in urban areas 

where there is a sudden increase in mortality, 

especially among older individuals, who supposedly have age-related reduced physiologic 

reserves [Leithead and Lind 1964; Henschel et 

al. 1969; Ellis 1972; Kilbourne et al. 1982]. In 

prolonged heat waves, the mortality is higher 

in the early phase of the heat wave [Henschel 

et al. 1969; Ellis 1972]. While acclimatization 

may play a part in the decrease in mortality 

during the latter part of a prolonged heat wave, 

the increased death rate in the early days of a 

heat wave may reflect an “accelerated mortality,” with the most vulnerable more likely to 

succumb at that time rather than more gradually as a result of degenerative diseases.

Any sicknesses or diseases in which GI permeability is compromised could result in greater 

susceptibility to leakage of endotoxin and 

the cascade of immune reactions and death 

that occur with heat stroke. For example, 

quercetin ingestion during heat acclimation 

prevented many of the benefits of heat acclimation, whereas glutamine ingestion improved 

GI barrier function and prevented endotoxin 

absorption and GI distress [Zuhl et al. 2014]. 

4.1.6.6 Individual Variation

In all experimental studies of the responses of 

humans to hot environmental conditions, a 

wide variation in responses has been observed. 

These variations are seen not only between 

individuals but also, to some extent, in the 

same individual exposed to high environmental stress on different occasions. Such variations 

are not totally understood. It has been shown 

[Wyndham 1973] that the influence of body 

size and its relationship to aerobic capacity 

in tolerance to heat could account for about 

half of the variability, leaving the remainder 

to be determined. Changes in hydration and 

salt balance might be responsible for some of 

the remaining variability [Buskirk and Bass 

1980]. However, the degree of variability in 

tolerance to hot environments is poorly understood. Nevertheless, some individuals exhibit 

poor tolerance to heat and, of this group, some 

may have hormonal issues that disrupt their 

ability to control fluids and electrolytes which 

increases their susceptibility to heat injury 

[Rosner and Kirven 2007].

4.1.7 Heat-Related Illnesses and Work

The incidence of occupational heat-related 

illness in the United States is not well-documented by an existing surveillance system 

specific to occupational injury and illness. 

According to BLS data from 2010, 4,190 cases 

of illness and injury from exposure to environmental heat, resulting in one or more days 

of lost work, occurred among private industry and state and local workers [Bureau of 

Labor Statistics 2011]. In that same year, 40 

workers died from exposure to environmental heat. Eighteen of these workers died in the 

construction industry; 6 workers died in the 

natural resources field (including agriculture) 

and mining; 6 died in professional and business services (including waste management 

and remediation); and 3 died in manufacturing. In addition, the National Fire Protection 

Association (NFPA) reported 2,890 cases of 

“thermal stress” in 2008, but this category 

includes both frostbite and heat exhaustion as 

per their definition [NIOSH 2010].
Occupational Exposure to Heat and Hot Environments 41

4 . Biologic Effects of Heat

In the following sections, various worksites that 

may have heat exposure are evaluated and presented in short case study reports.

4.1.7.1 Health Hazard Evaluation 

(HHE) Reports

National Park

(The full report is available at http://www.

cdc.gov/niosh/hhe/reports/pdfs/2013-

0109-3214.pdf.)

Park management requested NIOSH to evaluate employees working in extreme heat (e.g., 

employees involved in maintenance of asphalt 

parking lots, housing repairs, grounds maintenance, or archaeological surveys), to review 

current and proposed heat stress management 

policies, and to recommend ways of preventing heat-related illnesses [NIOSH 2014b]. July 

temperatures in the park ranged from 31.1°C to 

46.7°C (88°F to 116°F). Employees were asked 

to participate in work and medical history 

questionnaires, which also asked about health 

symptoms. Core body temperatures and heart 

rate were measured. Blood was analyzed daily 

for markers of muscle breakdown and dehydration during 4 workdays and the following 3 rest 

days. Estimates of how hard the workers were 

working were noted and daily temperatures and 

humidity were measured. The park’s heat stress 

policies and records of work-related injuries 

and illnesses were reviewed.

One worker was found to have a core body 

temperature above the ACGIH defined heat 

stress criteria (>38.5°C [101.3°F]); however, no 

workers were determined to be dehydrated or 

experiencing significant muscle breakdown. 

Several workers had sustained maximum heart 

rates. The heat stress policy was found to lack 

appropriate work/rest scheduling, and workers 

were not consistent in following the policy (i.e., 

failing to observe the buddy system rule). 

The NIOSH HHE made the following recommendations for managers and workers:

Managers

■ Avoid moderate to heavy outdoor tasks 

during summer months, or if necessary, 

work at night.

■ Reduce the amount of time workers work 

in extremely hot weather.

■ Revise the park’s heat stress policy to 

include work/rest schedules based on 

WBGT and workload.

■ Require workers to conduct selfmonitoring.

■ Develop a workgroup (i.e., workers, 

responsible healthcare provider, and 

safety manger) to make decisions on 

self-monitoring options and standard 

operating procedures.

Workers

■ Follow the heat stress policy.

■ Carry a radio at all times.

■ Avoid working alone (i.e., buddy system).

■ Learn the signs and symptoms of heatrelated illnesses.

■ Self-monitor and document signs and 

symptoms of heat-related illnesses.

■ Tell your supervisor if you have symptoms 

or if you note symptoms in a coworker.

■ Drink plenty of fluids, and take rest 

breaks as needed.

■ Volunteer to be on the work group to 

develop self-monitoring guidance for 

working in the heat.

Aluminum Smelter Potrooms

(The full report is available at http://www.

cdc.gov/niosh/hhe/reports/pdfs/2006-0307-

3139.pdf.)

NIOSH assessed workers’ exposure to heat 

while working in the potrooms at an aluminum 
42 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

smelter [NIOSH 2006a]. In the smelting 

process, alumina is reduced to nearly pure 

aluminum at an operating temperature of 

approximately 982.2°C (1,800°F). Workers 

were interviewed and completed questionnaires about their medical history, work 

history, and symptoms experienced during 

the shift on which they were monitored. 

Monitoring included core body temperature 

and heart rate. In addition, urine specific 

gravity and blood electrolytes were measured 

before and after shifts. WBGTs were monitored 

in several locations inside the potrooms, and 

outdoor weather conditions were monitored. 

The mean outdoor temperature for the 5 days 

of evaluation was 25.6°C (78°F). The WBGT 

measurements ranged from 28.3° C to 48.9°C 

(83°F to 120°F), with dry bulb air temperatures 

reaching 56.7°C (134°F) and radiant temperatures reaching 86.7°C (188°F). High radiant 

heat means that the workers were absorbing 

rather than radiating heat, unless proper shielding was provided. Metabolic rates of employees 

were estimated to be light to moderate (115–

360 watts). Except for the crane operator, 

portions of all tasks were found to exceed the 

NIOSH recommended ceiling limit (NOTE: 

the NIOSH recommended ceiling limit has 

been discontinued) and the ACGIH threshold 

limit value (TLV®) for working in a hot environment. Common symptoms reported during 

shifts included racing heartbeat or palpitations, 

headache, muscle cramps, and lightheadedness 

or dizziness. Post-shift values for blood bicarbonate, blood urea nitrogen (BUN), creatinine, 

and urine specific gravity increased significantly. Plasma volume depletion was suggested 

by the substantial decrease over the shift of the 

BUN to creatinine ratio and potassium level, 

and may have been caused by excessive sweating. Many of the workers were found to not 

be sufficiently hydrated. In addition, several 

participating workers had evidence of acute 

kidney injury, which may have been a result 

of or affected by volume depletion rhabdomyolysis caused by excess heat stress exposure or 

extreme physical activity.

The NIOSH HHE made the following recommendations for managers and workers:

Managers

■ Reduce the physical demands on workers 

working in the potrooms.

■ Require the use of heat-reflective personal protective equipment.

■ Install cooling recovery areas in the 

potrooms.

■ Do not use outdoor air to cool workers 

when it is over 35°C (95°F) outside.

■ Follow the heat stress management 

program.

■ Stop 8-hour overtime shifts during 

extremely hot weather.

Workers

■ Use reflective personal protective equipment.

■ Use the cooling recovery areas when 

on breaks.

■ Take time to work safely.

Automobile Parts Manufacturing Facility

(The full report is available at http://www.

cdc.gov/niosh/hhe/reports/pdfs/2003-0268-

3065.pdf.)

The painting department of an automobile 

parts manufacturing facility was assessed 

in part for workers subjected to high heat 

[NIOSH 2003a]. WBGT monitors were placed 

in the loading and unloading area among the 

workers and in the cafeteria (for comparison) 

for the entire work shift. Heat stress was measured for six workers over 2 days, through 

the use of wireless devices that are swallowed 

and monitor core body temperature. In addition, heart rate and skin temperature were 
Occupational Exposure to Heat and Hot Environments 43

4 . Biologic Effects of Heat

monitored with other devices. Pre- and postshift body weights were measured on both days 

to determine degree of dehydration. 

Four of the six participating workers exceeded 

the ACGIH core body temperature’s lower limit 

(38°C [100.4°F]) six times, and one worker 

exceeded its upper limit (38.5°C [101.3°F]) 

once. Of the 13 measurements taken over the 

2 days in participating workers, nine showed 

signs of dehydration (post-weight was less 

than pre-weight). Three of these measures met 

or exceeded the 1.5% guideline for adequate 

hydration. The dry bulb temperatures ranged 

from 26.9°C to 30.1°C (80.5°F to 86.2°F) in 

the loading and unloading areas and 21.2°C 

to 21.5°C (70.2°F to 70.7°F) in the cafeteria. 

Inadequate ventilation was suspected by the 

steadily increasing temperature in the loading 

and unloading areas.

The NIOSH HHE made the following recommendations for managers:

■ Allow workers to rest during the rest 

portion of the work/rest regiment, and 

do not assign any duties during this time.

■ Position fans above workstations, not 

directly in front of the workers.

■ Hire a consultant familiar with ventilation in hot processes to reduce heat.

Glass Bottle Manufacturer

(The full report is available at http://www.

cdc.gov/niosh/hhe/reports/pdfs/2003-0311-

3052.pdf.)

A manufacturer of glass containers for the beer, 

spirit, juice, and tea industries was assessed by 

NIOSH because of concern regarding heatrelated illnesses among employees exposed 

to hot working environments in the hot end 

of the plant, including the forming department [NIOSH 2003b]. In this department, raw 

materials are melted together in a furnace at 

temperatures of 1260°C to 1537.8°C (2,300°F to 

2,800°F). The manufacturer used various controls such as fans that supply cooler air from 

the basement, evaporative cooling fans, sports 

drinks, two 25-minute worker rest breaks (plus 

additional breaks at management’s discretion), 

and a review of heat safety during safety meetings and through displayed posters. WBGT 

measurements were collected in the forming 

department, metabolic rates of workers were 

estimated, and employees were interviewed. 

The highest WBGT reading was 30.7°C 

(87.2°F), with a dry bulb temperature of 

30.6°C (87.0°F) and a globe temperature of 

46.5°C (115.7°F). These results indicated that 

most surfaces in the department were at an 

elevated temperature and acted as radiant heat 

sources. The nearby break room’s WBGT was 

21.3°C (70.3°F), with a dry bulb temperature 

of 24.2°C (75.5°F). NIOSH guidelines were 

used to estimate the metabolic heat produced 

by the workers (186 kcal·h-1), resulting in a 

light workload rate. WBGTs and metabolic 

rates were then compared to those listed in the 

NIOSH RELs and ACGIH TLVs®, and both 

recommended a continuous work schedule in 

similar environments. Eighteen workers were 

interviewed, with two having experienced heatrelated symptoms on a hot day a few months 

earlier (i.e. heart racing, lack of sweating, persistent headache). Other workers mentioned 

symptoms in previous years, including those 

related to heat exhaustion, cramping, and 

nausea. Some workers mentioned that new 

workers typically start work in June and are not 

given enough time to acclimatize, resulting in 

some quitting. In addition, workers noted that 

the fans were useful (particularly the evaporative coolers) but they were not well maintained 

and some were not functional.

The NIOSH HHE made the following recommendations for managers:

■ Place the fans that supply cooler air from 

the basement and the evaporative cooling 
44 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

fans on a preventative maintenance 

schedule to ensure they are operational 

throughout the summer months.

■ Develop a heat acclimatization program to 

decrease the risk of heat-related illnesses.

■ Develop continuing education programs 

to ensure that all workers potentially 

exposed to hot environments and physically demanding job activities stay 

current on heat stress and heat stress prevention information.

■ Monitor environmental heat exposures 

during the hottest months with a WBGT 

monitor at (or as close as possible to) the 

area where the workers are exposed.

■ Establish criteria for the declaration of a 

heat alert.

■ Develop a heat-related illness surveillance 

program, which includes establishing and 

maintaining accurate records of any heatrelated disorder events and noting the 

environmental and work conditions at 

the time of disorder.

■ Ensure that workers stay hydrated and 

do not lose more than 1.5% body weight 

during their shift.

■ Create a buddy system so that workers 

can monitor each other for symptoms of 

heat disorders.

■ Allow workers to take unscheduled breaks 

if they report feeling weak, nauseated, 

excessively fatigued, confused, and/or 

irritable during work in hot temperatures.

Truck Drivers 

(The full report is available at http://www.

cdc.gov/niosh/hhe/reports/pdfs/2011-

0131-3221.pdf.)

An airline catering facility that included truck 

drivers was evaluated for ergonomic risk 

factors, heat and cold exposure, and job stress 

[NIOSH 2014a]. Although heat stress conditions in the food delivery trucks were not 

specifically evaluated, several employees and 

management representatives discussed temperatures in the trucks. Employees reported that 

the trucks were poorly maintained (e.g., some 

trucks had no air conditioning or had windows 

that did not open or close). Employees reported 

that the heating system in some of the trucks 

could not be turned off so it operated throughout the year. 

Managers reported that a typical delivery job 

took about 2 hours and included loading the 

truck at the dock, driving approximately 25 

minutes to the airport, going through airport 

security, driving to the plane parked on the 

tarmac, unloading carts from the truck to 

the plane, and then driving back to the dock. 

The delivery could take longer if the plane 

was not ready to receive the carts. Employees 

were expected to do three or four deliveries during their shift. Employees could take a 

break between deliveries in an air conditioned 

room at the airline catering facility. However, 

managers stated that on busy days employees sometimes skipped these breaks to keep 

up with job demands. Some employees also 

reported that they skipped breaks to shorten 

their shift so they could leave work early. 

Drivers, loaders, and sanitation (i.e., autoclave operators, dishwashers) employees were 

potentially exposed to hot temperatures. Heat 

measurements in the autoclave area suggested 

a continuous work schedule is acceptable for 

acclimatized employees with moderate workloads when outdoor temperatures are in the 

mid-70s. For the temperatures measured in 

these work areas, acclimatization for most 

people occurred in 4 days by exposing them 

to progressively longer periods in a hot work 

environment. The company had heat-related 

training materials available. The training 

included sources of heat, symptoms of heatrelated disorders, and recommendations for 

preventing heat stress, such as taking breaks 

in a cool area and drinking one glass of water 
Occupational Exposure to Heat and Hot Environments 45

4 . Biologic Effects of Heat

every 20 minutes. However, breaks were limited 

to two per 8-hour shift, and beverages were 

prohibited in most work areas.

The NIOSH HHE made the following heat 

stress-related recommendations for managers 

and workers:

Managers

■ Train workers on the health effects of 

exposure to hot temperatures and ways 

to be more comfortable at work. Inform 

employees and supervisors of OSHA 

heat safety tools found at http://www.

osha.gov/SLTC/heatillness/heat_index/

heat_app.html. The website provides 

information on protective measures they 

can take on the basis of the heat index at 

the worksite.

■ Develop and implement a heat stress prevention program. Establish mandatory 

breaks and access to fluids for workers 

exposed to heat.

■ Ensure new trucks have air conditioning 

and repair systems in existing vehicles.

Workers

■ Drink plenty of fluids when exposed 

to heat.

■ Take regular breaks to recover from 

extreme temperatures.

■ Take part in safety committees.

■ Report symptoms to supervisors and 

medical staff as soon as they occur.

4.1.7.2 Case Studies

Landscaping Case Study

A 30-year-old male landscape mowing assistant 

collapsed and died of heat stroke after a day of 

caring for residential lawns [NIOSH 2002]. 

Two hours before his death he had complained 

of feeling light-headed and short of breath, 

but he refused assistance offered to him by his 

partner. The worker was on medication that 

had a warning about exposure to extreme heat, 

and this might have interfered with body temperature regulation. The landscape worker had 

been wearing two pairs of work pants on the 

day he died, but his partner did not notice any 

profuse sweating or flushed or extremely dry 

skin. Upon collapse, the victim was treated by 

emergency medical services (EMS) personnel 

at the site and then transported to the hospital. 

There he was pronounced dead, with an internal temperature of 42°C (107.6°F). On the day 

of the incident, the maximum air temperature 

was 17.2°C (81°F).

The following recommendations were made 

after the incident:

■ Employers should ensure that supervisors/managers monitor workers during 

periods of high heat stress.

■ Identify workers with risk factors 

that would predispose them to heatrelated illnesses.

■ Train workers about heat stress, heat 

strain, and heat-related illnesses.

■ Ensure all workers are able to recognize 

the signs and symptoms of heat-related 

illnesses in themselves and in others.

■ Stress the importance of drinking nonalcoholic beverages before, during, and 

after working in hot conditions.

■ Periodically remind workers of the signs 

of heat-related illness and encourage 

them to drink copious amounts of water 

during hot conditions.

Migrant Farm Worker Case Studies

A male Hispanic worker aged 56 years died of 

heat stroke after hand-harvesting ripe tobacco 

leaves for 3 days on a North Carolina farm 

[CDC 2008]. On the third day, the man started 

working at 6:00 a.m. and took a short midmorning break and a 90-minute lunch break. 

Mid-afternoon, a supervisor observed the man 

working slowly and reportedly instructed him 
46 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

to rest, but the man continued working. An 

hour later, the man appeared confused and 

coworkers carried him to the shade and tried to 

get him to drink water. The man was taken by 

ambulance to an emergency department, where 

his core temperature was recorded as 42.2°C 

(108°F) and, despite treatment, he died. On the 

day of the incident, the local temperature was 

approximately 33.9°C (93°F) with 44% RH and 

clear skies. The heat index (a measurement of 

how hot it feels when both actual temperature 

and RH are considered) was in the range of 

30°C to 44.4°C (86°F to 112°F) that day.

In an additional, similar case study, a male 

Hispanic migrant worker aged 44 years died 

of heat stroke while working at another North 

Carolina tobacco farm [NIOSH 2006b]. He 

had been working in the fields for about the 

last week of July. On August 1, the heat index 

was between 37.8°C and 43.3°C (100°F and 

110°F). Around 3 p.m. the worker complained 

to the crew leader that he was not feeling well. 

He drank some water and was driven to the 

workers’ housing and left alone. He was found 

unconscious approximately 45 minutes later. 

Emergency medical personnel responded 

within 5 minutes and the worker was taken to 

the hospital and pronounced dead. His core 

body temperature was recorded at 42.2°C 

(108°F).

The following recommendations were made 

after the incident:

■ Agricultural employers should develop, 

implement, and enforce a comprehensive 

safety and health program that includes 

standard operating procedures for prevention of heat-related illnesses.

■ Train supervisors and workers on how to 

prevent, recognize, and treat heat illness, 

using a language and literacy level that 

workers can understand.

■ Establish a hydration program that provides adequate potable water (or other 

appropriate hydrating fluid) for each 

worker and which encourages workers to 

drink at regular intervals.

■ Monitor environmental conditions and 

develop work/rest schedules to accommodate high heat and humidity.

■ Provide an appropriate acclimatization 

program for new workers to a hot environment, workers who have not been on 

the job for a period of time, and experienced workers during a rapid change in 

excessively hot weather.

■ Provide prompt medical attention 

to workers who show signs of heatrelated illness.

Construction Case Study

A 41-year-old male construction laborer was 

sawing boards to make concrete forms that 

were to be part of an addition to a factory 

[NIOSH 2004]. At 5 p.m. the worker collapsed 

in the parking lot on the way to his vehicle. He 

was found 30 minutes later by a factory worker, 

who then returned to the factory and reported 

the situation to a supervisor. The receptionist 

was instructed to call EMS while the supervisor 

administered emergency care to the collapsed 

worker. The worker’s body temperature was 

recorded as 41.7°C (107°F) by the EMS and as 

42.2°C (108°F) when admitted to the hospital. 

The worker died the next day from heat stroke.

The following recommendations were made 

after the incident:

■ Train supervisors and workers to recognize symptoms of heat exhaustion/stroke 

when working in high heat index and/or 

humid conditions.

■ To avoid dehydration and heat exhaustion/stroke, workers should be given 

frequent breaks and be provided drinking 

water and other hydrating drinks when 

working in humid or hot conditions.
Occupational Exposure to Heat and Hot Environments 47

4 . Biologic Effects of Heat

■ Work hours should be adjusted to 

accommodate environmental work conditions such as a high heat index and/or 

high humidity.

Fire Fighter Case Study

During the construction of a fire line during 

a small wildland fire, a 21-year-old fire fighter 

died from heat stroke, another was overcome 

by heat stroke and survived, and two others 

suffered heat exhaustion [NIOSH 1997]. The 

crew had initially started their day by exercising 1 to 1.5 hours as part of their physical 

training regimen. The 21-year-old had been 

sick with a suspected viral or bacterial infection in recent days. After physical training, 

the crew practiced constructing a fire line for 

about 1 hour. At 11:45 a.m., the crew arrived 

at the actual fire that had been reported and 

constructed a fire line as a precaution in an area 

with no fire. Crews carried canteens and could 

drink when desired. Around 1:45 p.m., the 

crew took a 15-minute break and drank water 

or Gatorade. At 2 p.m. they resumed the fire 

line, and 15 minutes later one member fell from 

heat exhaustion and broke his shoulder. He 

was administered first aid and transported to 

the hospital. Prior to this, a member of another 

crew suffered heat exhaustion, was treated by 

paramedics, and was returned to his base of 

operation. Around 2:30 p.m., the 21-year-old 

moved off the line as though he was going to 

relieve himself. However, 5 minutes later he 

was found on the ground thrashing. The crew 

leader found that he was semiconscious and 

suffering from heat stroke. His clothing was 

removed, water was dumped on his skin, and 

chemical cold packs were applied to his body. 

At 2:50 p.m. paramedics arrived and continued to administer aid. At 3 p.m. another 

crew member experienced symptoms of heat 

stroke. An ambulance transported the 21-yearold to the hospital at 3:20 p.m., but he died 

early the following morning. The maximum 

measured temperature during the incident was 

36.7°C (98°F).

The following recommendations were made 

after the incident:

■ Require supervisors to regularly medically monitor fire fighters, using generally 

accepted techniques, during periods of 

high heat stress.

■ Ensure fire fighters’ workloads are appropriate for their level of acclimatization.

■ Ensure fire fighters’ workloads are appropriate for the ambient weather conditions 

and clothing.

4.2 Acute Heat Disorders

Although heat disorders are interrelated and 

seldom occur as discrete entities, each has 

unique clinical characteristics [Minard and 

Copman 1963 (p. 253); Leithead and Lind 1964; 

Minard 1973; Lind 1977; Dinman and Horvath 

1984; Springer 1985]. These disorders range 

from simple postural heat syncope (fainting) 

to the complexities of heat stroke. A common 

feature in all the heat-related disorders (except 

simple postural heat syncope) is some degree of 

elevated body temperature, which may be complicated by deficits of body water. The prognosis 

depends on the absolute level of the elevated 

body temperature, the promptness of treatment 

to lower the body temperature, and the extent 

of deficiency or imbalance of fluids or electrolytes. A summary of classification, clinical 

features, prevention, and first-aid treatment of 

heat-related illnesses is presented in Table 4-3. 

4.2.1 Heat Stroke

Heat stroke can occur as either classic or exertional. Classic heat stroke includes (1) a major 

disruption of central nervous system function (unconsciousness or convulsions); (2) a 

lack of sweating; and (3) a rectal temperature 
48 Occupational Exposure to Heat and Hot Environments

See footnotes at end of table. (Continued)

4 . Biologic Effects of Heat

Table 4-3. Classification, medical aspects, and first aid of heat-related illness

Signs and symptoms

Examples of 

predisposing factors

Underlying physiologic 

disturbance First aid

(1) Temperature regulation

Heat stroke

■ Confusion, altered mental status, 

slurred speech

■ Loss of consciousness (coma)■ Hot, dry skin or profuse sweating■ Seizures■ Very high body temperature■ Fatal if treatment delayed

■ Sustained exertion in heat■ Obesity and lack of 

physical fitness

■ Recent alcohol intake■ Dehydration■ Individual susceptibility■ Chronic cardiovascular disease

■ Failure of the central drive 

for sweating, leading to loss of evaporative cooling and an uncontrolled accelerating rise in temperature

■ A medical emergency: call 911 

for emergency medical care

■ Someone should stay with 

worker until emergency medical services arrive

■ Move the worker to a shaded, 

cool area and remove outer clothing

■ Cool the worker quickly with 

a cold water or ice bath if possible; wet the skin, place cold wet cloths on skin, or soak clothing with cool water

■ Circulate the air around the 

worker to speed cooling

■ Place cold wet cloths or ice 

on head, neck, armpits, and groin; or soak the clothing with cool water

(2) Circulatory hypostasis

Heat syncope

■ Fainting (short duration)■ Dizziness■ Light-headedness during prolonged 

standing or suddenly rising from a sitting or lying position 

■ Dehydration■ Lack of acclimatization

■ Pooling of blood in dilated 

vessels of skin and lower parts of body

■ Sit or lie down in a cool place ■ Slowly drink water, clear juice, 

or a sports drink 
Occupational Exposure to Heat and Hot Environments 49

See footnotes at end of table. (Continued)

4 

. Biologic Effects of Heat

Signs and symptoms

Examples of 

predisposing factors

Underlying physiologic 

disturbance First aid

(3) Water and/or salt depletion

Heat exhaustion

■ Headache■ Nausea■ Dizziness■ Weakness■ Irritability■ Thirst■ Heavy sweating■ Elevated body temperature■ Decreased urine output

■ Sustained exertion in heat■ Lack of acclimatization■ Failure to replace water lost 

in sweat

■ Dehydration■ Depletion of circulating 

blood volume

■ Circulatory strain from 

competing demands for blood flow to skin and to active muscles

■ Take worker to a clinic or 

emergency room for medical evaluation and treatment

■ If medical care is unavailable, 

call 911

■ Someone should stay with 

worker until help arrives

■ Remove worker from hot area 

and give liquids to drink

■ Remove unnecessary clothing, 

including shoes and socks

■ Cool the worker with cold 

compresses or have the worker wash head, face, and neck with cold water

■ Encourage frequent sips of 

cool water

Table 4-3 (Continued). Classification, medical aspects, and first aid of heat-related illness
50 Occupational Exposure to Heat and Hot Environments

See footnotes at end of table. (Continued)

4 . Biologic Effects of Heat

Table 4-3 (Continued). Classification, medical aspects, and first aid of heat-related illness

Signs and symptoms

Examples of 

predisposing Factors

Underlying physiologic 

disturbance First aid

Heat cramps

■ Muscle cramps, pain, or 

spasms in the abdomen, arms, or legs

■ Heavy sweating during hot work■ Drinking large volumes of water 

without replacing salt loss

■ Loss of electrolytes in sweat■ Water intake dilutes electrolytes■ Muscle spasm

■ Drink water and have a snack and/or 

carbohydrate-electrolyte replacement liquid (e.g., sports drinks) every 15 to 20 minutes

■ Avoid salt tablets■ Get medical help if the worker has 

heart problems, is on a low sodium diet, or if cramps do not subside within 1 hour

Hyponatremia

■ Symptoms range from 

none, to minimal, to severe, including encephalopathy, cerebral and pulmonary edema, respiratory distress, and death

■ Heavy sweating during hot work■ Drinking large volumes of water 

without replacing salt loss

■ Loss of electrolytes in sweat■ Low plasma sodium■ Osmotic disequilibrium

■ Drink water and have a snack and/or 

carbohydrate-electrolyte replacement liquid (e.g., sports drinks)

■ Avoid salt tablets■ Get medical help if the worker 

has heart problems or is on a lowsodium diet, or if cramps do not subside within 1 hour

(4) Skin eruptions

Heat rash (miliaria rubra, “prickly heat,” “sweat rash”)

■ Looks like red cluster of 

pimples or small blisters that usually appears on the neck, upper chest, groin, under the breasts, and in elbow creases

■ Unrelieved exposure to humid 

heat with skin continuously wet with unevaporated sweat

■ Plugging of sweat gland ducts 

with retention of sweat and inflammatory reaction

■ When possible, a cooler, less humid 

work environment is best treatment

■ Keep rash area dry■ Powder may be applied to 

increase comfort

■ Ointments and creams should not 

be used
Occupational Exposure to Heat and Hot Environments 51

4 . Biologic Effects of Heat

Signs and symptoms

Examples of 

predisposing Factors

Underlying physiologic 

disturbance First aid

Heat rash (miliaria profunda, “wildfire”)

■ Extensive areas of skin 

which do not sweat on heat exposure, but present gooseflesh appearance, which subsides with cool environments

■ Associated with 

incapacitation in heat

■ Weeks or months of constant 

exposure to heat with previous history of extensive heat rash and sunburn

■ Skin trauma (heat rash; 

sunburn) causes sweat retention deep in skin, reduced evaporative cooling causes heat intolerance

■ No effective treatment■ Recovery of sweating occurs 

gradually on return to cooler climate

(5) Muscle tissue injury

Rhabdomyolysis

■ Muscle cramps/pain■ Abnormally dark (tea or 

cola colored) urine

■ Weakness■ Exercise intolerance■ Asymptomatic

■ End result of any process that 

damages skeletal muscle, such as the following:

■ Prolonged, intense 

physical exertion

■ Elevated body temperature 

(associated with heat stroke)

■ Use of certain prescription and 

over-the-counter medications

■ Use of certain dietary 

supplements like creatine and caffeine

■ Use of illicit drugs that can 

reduce blood flow to muscle tissue, such as cocaine and methamphetamine

■ Direct injury to the muscle (i.e., 

trauma, burns) or infections

■ Leakage of muscle cell 

contents into the bloodstream, which may result in seizures, abnormal heart rhythms, nausea, vomiting, fatigue, and kidney damage

■ Injured muscles located in 

muscle fascial compartments may swell and cut off blood supply to entire muscle group, which may result in loss of function and permanent disability 

■ Stop activity■ Increase oral hydration 

(water preferred)

■ Seek immediate care at the nearest 

medical facility

■ Ask to be checked for 

rhabdomyolysis (i.e., blood sample analyzed for creatine kinase)

Adapted from Minard 1973; DOD 2003; Cervellin et al. 2010; OSHA-NIOSH 2011.

Table 4-3 (Continued). Classification, medical aspects, and first aid of heat-related illness
52 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

in excess of 41°C (105.8°F) [Minard and 

Copman 1963 (p. 253); Leithead and Lind 

1964; Shibolet et al. 1976; Khagali and Hayes 

1983]. The 41°C (105.8°F) rectal temperature 

is an arbitrary value for hyperpyrexia because 

observations are made only after the admission of patients to hospitals, which may occur 

from about 30 minutes to several hours after 

the event. Exertional heat stroke occurs in 

physically active individuals who will often 

continue sweating [DOD 2003; Armstrong et 

al. 2007b; Navy Environmental Health Center 

2007]. With exertional heat stroke, the skeletal muscle often rapidly breaks down, which 

is called acute rhabdomyolysis (see 4.2.1.1), 

and also results in renal failure [DOD 2003]. 

The risk of renal failure is about 25% for 

those suffering from exertional heat stroke 

[Navy Environmental Health Center 2007]. 

For additional comparisons between classic 

and exertional heat stroke see Table 4-4. The 

metabolic and environmental heat loads that 

give rise to heat stroke are highly variable and 

are often difficult or impossible to accurately 

reconstruct. Also, the medical outcomes vary 

among patients, depending on the caregiver’s 

knowledge, understanding, skill, and available facilities.

Heat stroke is a medical emergency, and 

rapidly cooling the affected worker is imperative. If possible, cool the worker quickly with 

an ice water bath. Placing the affected worker 

in a shady area, removing outer clothing and 

wetting or applying ice to the head, neck, 

armpits, and groin areas, and increasing air 

movement to enhance evaporative cooling 

are all important activities to perform while 

waiting for or transporting to medical care. 

A worker experiencing heat exhaustion or 

heat stroke should not be sent home or be left 

unattended without a specific order from a 

responsible healthcare provider.

Frequently, by the time a worker is admitted 

to a hospital, the disorder has progressed to a 

multisystem emergency affecting virtually all 

tissues and organs [Dukes-Dobos 1981]. In 

the typical clinical presentation, the central 

nervous system is disorganized and there is 

commonly evidence of fragility of small blood 

vessels, possibly coupled with the loss of integrity of cellular membranes in many tissues. The 

blood-clotting mechanism is often severely 

disturbed, as are liver and kidney functions. 

It is not clear, however, whether these events 

are present at the onset of the disorder or 

whether they develop over time. Postmortem 

evaluation indicates that few tissues escape 

pathological involvement. Early recognition 

of the disorder or its impending onset, when 

combined with appropriate treatment, considerably reduces the death rate and the extent 

of organ and tissue involvement [DOD 2003; 

Navy Environmental Health Center 2007]. 

4.2.1.1 Rhabdomyolysis

Rhabdomyolysis is a medical condition associated with heat stress and prolonged physical 

exertion, resulting in the rapid breakdown of 

muscle and the rupture and necrosis of the 

affected muscles. When muscle tissue dies, 

electrolytes and large proteins that formed the 

muscle’s contractile mechanism are released 

into the bloodstream [Khan 2009; Cervellin 

et al. 2010]. Potassium is the main electrolyte 

released into the blood by the death of muscle 

tissue, and high levels can cause irregular and 

dangerous heart rhythms and seizures. In addition, the large muscle proteins can damage the 

delicate filtration system of the kidneys. 

Classic symptoms of rhabdomyolysis are 

muscle pain, cramping, swelling, weakness, 

and decreased range of motion of joints. One 

of the signs of rhabdomyolysis is dark or teacolored urine [Brudvig and Fitzgerald 2007; 

Khan 2009; Cervellin et al. 2010]. However, 

symptoms can vary between individuals, 

with some experiencing nonspecific symptoms such as fatigue, exercise intolerance, 
Occupational Exposure to Heat and Hot Environments 53

4 . Biologic Effects of Heat

abdominal pain, back pain, nausea or vomiting, and confusion, while others might have 

no symptoms [Huerta-Alardin et al. 2005; 

Brudvig and Fitzgerald 2007]. In one study, 

only half of patients with confirmed rhabdomyolysis reported muscle pain or weakness 

[Cervellin et al. 2010]. Because muscle cramps 

and dark urine after prolonged exertion may 

be the only symptom and sign, rhabdomyolysis may be mistaken for another heat-related 

illness and dehydration. This was confirmed by 

a study of cases in which rhabdomyolysis was 

initially misdiagnosed; it found that the most 

common diagnoses given were heat stress and 

dehydration [Gardner and Kark 1994]. Delays 

in recognizing rhabdomyolysis due to misdiagnosis are problematic. The more serious 

the case is and the longer the delay in making 

the correct diagnosis and initiating treatment, 

the greater the risk for increased complications, some of which may be permanent such 

as reduced or loss of kidney function due to 

compartment syndrome.

Rhabdomyolysis is diagnosed by measurement of creatine kinase (CK), also known as 

Table 4-4. Comparison of classic and exertional heat stroke

Patient characteristics Classic Exertional

Age Young children or elderly Typically 15–45 years

Health Chronic illness or debilitation 

common

Usually healthy

Prevailing weather Frequent in prolonged 

heat waves

Variable

Drug use Diuretics, antidepressants, anticholinergics, phenothiazinesUsually none; sometimes ergogenic 

stimulants or cocaine

Activity Sedentary Strenuous exercise

Sweating Usually absent Often present

History of febrile illness Unusual Common

Acid–base disturbances Respiratory alkalosis Lactic acidosis

Acute renal failure Fairly rare Common

Rhabdomyolysis Seldom severe Common; may be severe

Hyperuricemia Modest Marked

Creatinine: blood urea nitrogen ratio 1:10 Elevated

Creatine kinase (CK), aldolase Mildly elevated Markedly elevated

Hyperkalemia Usually absent Often present

Hypocalcemia Uncommon Common

Disseminated

intravascular coagulation (DIC)

Mild May be marked

Hypoglycemia Uncommon Common

Adapted from Heat Stress Control and Heat Casualty Management [DOD 2003].
54 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

creatine phosphokinase (CPK), in the blood by 

a licensed health care professional. The severity of rhabdomyolysis depends upon damage 

to other organ systems and the peak CK level. 

Long-term health consequences of rhabdomyolysis vary widely and largely depend on 

speed of recognition and treatment [Line 

and Rust 1995]. Mild rhabdomyolysis can be 

treated by drinking lots of fluids [George et 

al. 2010]. Severe cases require hospitalization 

and aggressive treatment with intravenous 

fluids to dilute the proteins and thus minimize 

their damage to the kidney; monitoring of the 

heart for dangerous rhythm changes from the 

surge of electrolytes; and monitoring of kidney 

function [Sauret et al. 2002]. In severe cases, 

the kidneys may fail and immediate dialysis is 

needed to mechanically remove proteins and 

electrolytes from the blood [Bosch et al. 2009]. 

The drop in kidney function may be temporary, 

but in some cases kidney function does not 

recover, leaving a formerly healthy individual 

facing a lifetime of dialysis or possibly a kidney 

transplant. Up to 8% of cases of rhabdomyolysis are fatal [Cervellin et al. 2010]. In the past, 

rhabdomyolysis screening involved a urine test 

that was positive if it contained myoglobin but 

no red blood cells. However, because myoglobin is quickly excreted and levels may return 

to normal within 6 hours of muscle injury, it is 

not considered a reliable test for the condition. 

One study showed that only 19% of patients 

diagnosed with rhabdomyolysis had elevated 

urine myoglobin levels at the time of diagnosis 

[Counselman and Lo 2011].

Risk factors for rhabdomyolysis include 

elevated core body temperature from environmental heat, from heat generated by 

physical exertion, or from medical conditions 

that raise the body’s temperature (e.g., malignant hyperthermia); dehydration; prescription 

medications (e.g., cholesterol-lowering statins 

and antidepressants); over-the-counter medications (e.g., antihistamines, non-steroidal 

anti-inflammatory medications, omeprazole); 

excessive caffeine intake; use of dietary supplements (e.g., creatine and HydroxycutTM); 

use of medications, alcohol, or amphetamines; underlying medical conditions (e.g., 

sickle cell trait or lupus); and concurrent 

bacterial or viral infections (e.g., influenza, 

Epstein-Barr, or Legionella) [Wrenn and 

Oschner 1989; Line and Rust 1995; HuertaAlardin et al. 2005; Melli et al. 2005; Do et 

al. 2007; Makaryus et al. 2007; Dehoney and 

Wellein 2009; Nauss et al. 2009; Chatzizisis 

et al. 2010; George et al. 2010; de Carvalho 

et al. 2011]. 

Differentiation between the types of heat 

exposure that led to the rhabdomyolysis is 

important because they have different signs, 

symptoms, and potential sequelae. Victims of 

classic heat stroke usually do not experience 

rhabdomyolysis as severe as those with exertional heat stroke. Individuals with exertional 

heat stroke with rhabdomyolysis often have 

higher levels of CK, presence of lactic acidosis, 

significant drop in blood calcium levels (while 

showing a dangerously high spike in blood 

potassium levels that can cause heart rhythm 

irregularities), increased incidence of kidney 

failure (20%–30% vs <5%), and increased risk 

for a potentially fatal coagulation disorder 

called disseminated intravascular coagulation 

[DOD 2003]. 

Some jobs intrinsically have risk factors for 

heat related illness and rhabdomyolysis, such 

as fire fighting. Fire fighters are at increased 

risk because they routinely encounter several 

heat sources as they do their jobs: heat from 

the fire, ambient heat when working during 

hot weather, and heat generated by prolonged 

physical exertion during fire-suppression 

efforts. Physical exertion is often increased 

by the effort of carrying additional weight. 

Structural fire fighters may carry an additional 

40 to 60 pounds of turnout gear, self-contained 

breathing apparatus (SCBA) air tanks, and 
Occupational Exposure to Heat and Hot Environments 55

4 . Biologic Effects of Heat

various equipment (e.g., ventilation hooks 

can weigh 20–80 lb). Although wildland fire 

fighters do not wear turnout gear because of 

the remoteness of the fires they fight, they must 

be able to be self-sufficient once they are at the 

fire location, which can mean carrying 40- to 

110-lb packs while hiking over steep terrain 

for prolonged periods. In a 2011 investigation 

into the heat stroke death of a wildland fire 

fighter belonging to an interagency “hotshot 

crew.” NIOSH reviewed agency records and 

found that they had documented 255 nonfatal cases of heat-related illness in the previous 

12 years [NIOSH 2012]. NIOSH investigated 

the exertional heat stroke death of a 26-yearold structural fire fighter who collapsed at the 

end of a 4.4-mile jog during his cadet training 

course in 22.8°C (73°F) ambient temperature. 

On arrival at the emergency department he 

had a core temperature of 40.7°C (105.3°F) and 

was found to have rhabdomyolysis. He subsequently developed acute kidney failure and 

disseminated intravascular coagulation before 

he died 5 days later. 

The incidence of symptomatic exertional 

rhabdomyolysis in the general population is 

unknown [Alpers and Jones 2010]. Individuals 

in occupations that require prolonged, intense 

physical activity, such as military personnel/

recruits, law enforcement, and athletes, are at 

increased risk for developing rhabdomyolysis 

[CDC 1990; Gardner and Kark 1994; Walsh 

and Page 2006; O’Connor and Duester 2011]. 

Most cases of acute exertional rhabdomyolysis 

among military trainees occur during the first 

week of training [Olerud et al. 1976]. More 

recent data, from the Armed Forces Health 

Surveillance Center, reported 378 cases of 

“rhabdomyolysis likely due to physical exertion or heat stress” in 2013 alone, representing 

a 33% increase in the annual incidence rate 

between 2009 and 2013. It was noted that 

71% of these cases occurred between May and 

September [Armed Forces Health Surveillance 

Center 2014]. It is a recurrent issue during 

summer football practice, as this involves 

intense physical training regimens often in full 

protective gear during hot weather. 

Repetitive exhaustive exercise regimens 

including “incentive training” with highintensity repetitive exercises such as push-ups 

are more likely to lead to rhabdomyolysis than 

prolonged submaximal exercise regimens 

[Olerud et al. 1976]. Although it is not uncommon for individuals in the general population 

who engage in exertional activities higher than 

their baseline level of fitness to develop exertional rhabdomyolysis, it also occurs in highly 

conditioned individuals who may engage in 

supramaximal (i.e., high intensity, usually 

short duration) exercise or who have other risk 

factors concurrent with an exertional activity 

[Walsh and Page 2006].

Aside from addressing the cause of rhabdomyolysis (i.e., cool the body down if temperature 

is high), the common theme of treatment of 

rhabdomyolysis is fluids. Mild cases of rhabdomyolysis can be treated with oral rehydration 

fluids, which serve to flush the harmful proteins out of the blood before they can harm 

the kidneys while restoring the volume depletion that often accompanies rhabdomyolysis. 

Treatment of more severe cases of rhabdomyolysis require fluid intake rates for durations 

that cannot be accomplished by drinking, 

which is why inpatient treatment with intravenous fluids is necessary. The treatment goal 

is to obtain urine output of 200 to 300 mL per 

hour while monitoring serum CK, potassium, 

and kidney function markers. Patients with 

severe rhabdomyolysis should be placed on 

cardiac monitors, because the elevated levels of 

potassium can produce potentially fatal heartrhythm abnormalities. Low blood calcium 

levels are common with exertional rhabdomyolysis and can result in seizures. Consultation 

with a nephrologist may be needed for emergency hemodialysis if the kidney function 
56 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

declines. Kidney function may or may not 

recover, and if not, the patient will require 

regular dialysis for the rest of her or his life. 

Another potential serious complication of 

rhabdomyolysis is compartment syndrome, 

which can occur if the damaged muscle is 

inside a fibrous sheath with other muscles. 

Most of these sheaths are found in the arms 

and legs [Cervellin et al. 2010]. All muscles 

within the same sheath are considered to be in 

the same compartment. During rhabdomyolysis, the injured muscle becomes inflamed and 

swollen. Because the fibrous sheath of the compartment cannot expand, the pressure from 

this swelling can become severe enough to 

prevent blood from entering the sheath, endangering all muscles inside that compartment. 

Unless the sheath is cut open by a surgeon to 

relieve the pressure and restore blood flow, all 

the muscles in that compartment could die, 

resulting in permanent loss of function of the 

affected limb(s) [Walsh and Page 2006]. 

Patients with compartment syndrome may 

present with some of the classic “5 P’s” of vascular insufficiency: pain, pallor, pulselessness, 

paresthesias (i.e., sensation of tingling, numbness, or burning, usually felt in the hands, feet, 

arms, or legs), and paralysis. Pain is the most 

common of these symptoms and is often severe 

to the point of being difficult to control even 

with narcotic analgesics. This pain is worsened with passive extension, and onset may be 

delayed for hours to days after the initial injury 

because it can take time for the swelling of the 

damaged muscle to become severe enough to 

block the blood flow into the compartment. On 

examination, the affected limb is swollen, tense, 

and very tender to applied pressure (although 

compartment syndrome may occur in the 

abdomen and neck, it most often occurs in 

the lower extremities). Swelling of the affected 

area may also be evaluated by measurement of 

that extremity’s circumference and comparison with that of the unaffected side. However, 

if compartment syndrome is suspected, then 

immediate surgical consultation is indicated, 

along with measurement of the pressure within 

the compartment. Compartment pressures 

should not exceed 30 mmHg (4 kPa); when 

they do, it is an indication for immediate transfer to surgery for fasciotomy. Prompt treatment 

is critical because permanent damage to the 

muscle occurs when its blood flow is blocked 

for more than 8 hours [Haller 2011].

4.2.2 Heat Exhaustion

Heat exhaustion is often considered a precursor 

to the more serious heat stroke. This disorder 

has been encountered frequently in experimental assessment of heat tolerance. Usually, it 

is accompanied by a slightly elevated core body 

temperature (38°C–39°C, or 100.4°F–102.2°F). 

The symptoms of headache, nausea, vertigo, 

weakness, thirst, heavy sweating, irritability, 

and a decreased urine output are common to 

both heat exhaustion and the early stage of heat 

stroke. There is wide variation in the ability to 

tolerate an increased mean body temperature; 

some individuals cannot tolerate rectal temperatures of 38°C–39°C (100.4°F‒102.2°F) and 

others continue to perform well at even higher 

rectal temperatures [Joy and Goldman 1968].

Failure to replace water predisposes the individual to one or more of the heat disorders, 

especially heat exhaustion and heat stroke. 

Data suggest that cases of heat exhaustion can 

be expected to occur some 10 times more frequently than cases of heat stroke [Khagali and 

Hayes 1983].

4.2.3 Heat Cramps

Heat cramps are not uncommon in individuals who work hard in the heat. The exact cause 

or causes have not been determined, but heat 

cramps may be attributed to or associated with 

a continued loss of salt in the sweat, accompanied by a copious intake of water without 
Occupational Exposure to Heat and Hot Environments 57

4 . Biologic Effects of Heat

appropriate replacement of salt. Other electrolytes, such as magnesium, calcium, and 

potassium, may also be involved. Cramps 

often occur in the muscles principally used 

during work and can be readily alleviated by 

rest, the ingestion of water, and the correction 

of any body fluid electrolyte imbalance (e.g., 

with sports drinks containing carbohydrates 

and electrolytes [serum sodium concentration <136 mEq·L-1]). Salt tablets should not be 

taken. Salt losses are best replaced by the ingestion of normal salted foods or fluids over many 

hours [DOD 2003].

4.2.4 Heat Syncope

Heat syncope (fainting) usually occurs with 

prolonged standing or sudden rising from a 

sitting or supine position; in such instances, 

temporary circulatory failure, due to pooling 

of blood in the peripheral veins, results in a 

decrease in diastolic filling of the heart [DOD 

2003]. Symptoms of heat syncope include 

light-headedness, dizziness, and fainting. 

Factors that may contribute to heat syncope 

include dehydration and lack of acclimatization. Workers who have fainted will usually 

recover rapidly if they sit or lay down; however, 

complete recovery of stable blood pressure and 

HR may take an hour or two [DOD 2003].

4.2.5 Heat Rashes

The most common heat rash is prickly heat 

(miliaria rubra), which appears as red papules, 

usually in areas where clothing is restrictive, 

and gives rise to a prickling sensation, particularly as sweating increases. It occurs in skin 

that is persistently wetted by unevaporated 

sweat, apparently because the keratinous layers 

of the skin absorb water, swell, and mechanically obstruct the sweat ducts [Pandolf et al. 

1980b, 1980a; DiBenedetto and Worobec 

1985]. If untreated, the papules may become 

infected and develop secondary staphylococcal infections [DOD 2003]. Another skin 

disorder (miliaria crystallina) appears with the 

onset of sweating in skin previously injured at 

the surface, commonly in sunburned areas. 

The damage prevents the escape of sweat and 

results in the formation of small to large watery 

vesicles, which rapidly subside once sweating 

stops; the problem ceases to exist once the 

damaged skin is sloughed.

Miliaria profunda occurs when sweat ducts are 

blocked below the skin surface. This rash also 

occurs following a sunburn injury but has been 

reported to occur without clear evidence of 

previous skin injury. Discrete, pale elevations 

of the skin, resembling gooseflesh, are present.

In most cases, these rashes disappear with 

return to a cool environment. When a substantial part of the day is spent in cool and/

or dry areas so that the skin surface can dry, 

the rashes are less likely to occur or occur with 

diminished frequency.

Although heat rashes are not dangerous in 

themselves, each can impair areas of skin and 

reduce sweating that reduces evaporative heat 

loss and impacts thermoregulation. Wet and/

or damaged skin can also absorb toxic chemicals more readily than dry, unbroken skin. In 

experimentally induced miliaria rubra, sweating capacity recovered within 3 to 4 weeks 

[Pandolf et al. 1980b, 1980a]. 

4.3 Chronic Heat Disorders

Some long-term effects from heat stress (noted 

from anecdotal, historical, epidemiologic, and 

experimental evidence) have been suggested. 

Severe heat-related illness may cause permanent damage to a person’s organs, such as the 

heart, kidneys, and liver, which may result in a 

chronic disorder. 

One study compared a cohort of U.S. Army 

personnel hospitalized for heat-related illness 

with those that had appendicitis. Those with 
58 Occupational Exposure to Heat and Hot Environments

4 . Biologic Effects of Heat

heat-related illness had a 40% higher risk 

of all-cause mortality than the appendicitis 

patients [Wallace et al. 2007]. Furthermore, it 

was found that males with heat-related illness 

were at an increased rate of death from cardiovascular disease and ischemic heart disease, 

compared to the appendicitis cases.

More studies are needed to increase our understanding of long-term effects of heat-related 

illness. The severity of illness, the duration 

of exposures, and the etiology and prevention of heat cramps are just a few of many 

factors that may have an effect on a worker’s 

chronic condition.
Occupational Exposure to Heat and Hot Environments 59

5 Measurement of Heat Stress

Heat stress is the net heat load to which a 

worker is exposed from the combined contributions of metabolic heat, environmental 

factors, and clothing worn which results in 

an increase in heat storage in the body. The 

heat load experienced by the worker provokes 

a physiological response (heat strain) which 

functions to increase heat loss from the body 

in order to maintain a stable body temperature. 

This is not always successful and, when unsuccessful, may result in heat injury and death.

The environmental factors of heat stress are 

temperature and movement of air, water vapor 

pressure, and radiant heat. Physical work contributes to total heat stress of a job by producing metabolic heat in the body in proportion 

to the work intensity. The amount, thermal 

characteristics, and type of clothing worn also 

contribute by altering the rate of heat exchange 

between the skin and the air [OSHA 1999].

Heat stress may be assessed by measuring the 

climatic and physical factors of the environment and then evaluating their effects on the 

human body via an appropriate heat stress 

index. This chapter presents information on 

(1) measurement of environmental factors, 

(2) prediction of meteorological factors from 

National Weather Service data, and (3) measurement of metabolic heat.

5.1 Environmental Factors

The environmental factors of concern in occupational heat stress are (1) air (dry bulb) temperature; (2) humidity or, more precisely, water 

vapor pressure; (3) air velocity; and (4) radiation (solar and infrared). One can measure 

“heat exposure,” which is the amount of heat 

experienced by the worker. Heat stress is the 

thermal environment, including radiant temperature, air temperature, humidity, activity, 

and other factors imposed on a person that can 

increase body heat storage. Heat strain is the 

physiological response to the heat stress that 

attempts to transfer the stored heat back to the 

environment [Parsons 2003].

5.1.1 Air (Dry Bulb) Temperature

The dry bulb temperature (ta) is the simplest to 

measure of the climatic factors. It is the temperature of the ambient air, measured with a 

thermometer. Temperature units are in degrees 

Celsius [°C = (°F − 32) × 5/9]. The primary types 

of thermometers used for measuring dry bulb 

temperature are (a) liquid-in-glass thermometers, (b) thermocouples, and (c) resistance 

thermometers (thermistors). These thermometers differ in the nature, properties, characteristics, and materials of their sensing elements.

General precautions must be considered 

in using any thermometer [Ramsey and 

Beshir 2003]:

■ The temperature to be measured must 

be within the measuring range of 

the thermometer.

■ The time allowed for measurement must 

be greater than the time required for thermometer stabilization.
60 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

■ The sensing element must be in contact 

with or as close as possible to the area of 

thermal interest.

■ Under radiant conditions (i.e., in sunlight 

or where the temperature of the surrounding surfaces is different from the air 

temperature), the sensing element should 

be shielded. 

5.1.1.1 Liquid-in-Glass Thermometers

Although a thermometer is any instrument for 

measuring temperature, this term is commonly 

identified with the liquid-in-glass thermometer, which is the simplest, most familiar, and 

most widely used. Historically, mercury and 

alcohol are the more commonly used liquids. 

Mercury-in-glass thermometers are preferred 

under hot conditions, whereas alcohol-inglass thermometers are preferred under cold 

conditions, since the freezing point of mercury 

is -40°C (-40°F) and that of alcohol is -114°C 

(-173.6°F). Because of the environmental hazards of mercury, these thermometers are not 

generally available to the public (but are available for research). Therefore, other liquids such 

as alcohol are used in most generally available 

thermometers. Thermometers used for measuring dry bulb temperature must be total 

immersion types. These thermometers are 

calibrated by total immersion in a thermostatically controlled medium, and their calibration 

scale depends on the coefficients of expansion 

of both the glass and the liquid. Only thermometers with the graduations marked on the 

stem should be used. These thermometers are 

simple to use but may be fragile and can be 

affected by radiation.

5.1.1.2 Thermocouples

A thermocouple consists of two wires of different metals connected together at both ends by 

soldering, welding, or merely twisting to form 

a pair of junctions. One junction is kept at a 

constant reference temperature (e.g., usually 

at 0°C, or 32°F) by immersion in an ice bath. 

The second junction is exposed to the measured temperature. Due to the difference in 

electrochemical properties of the two metals, 

an electromotive force (emf), or voltage, is created, whose potential is a function of the temperature difference between the two junctions. 

With use of a millivolt meter or a potentiometer to measure the existing emf or the induced 

electric current, respectively, the temperature 

of the second junction can be determined 

from an appropriate calibration table or curve. 

Copper (a metal) and constantan (a coppernickel alloy) are the most commonly used to 

form the thermocouple. Thermocouples are 

less affected by radiation, are highly accurate, have a fast response, and may be used for 

remote measurements.

5.1.1.3 Resistance Thermometers

A resistance thermometer, or thermistor, uses 

a metal wire (i.e., a resistor) as its sensing element; the resistance of the sensing element 

increases as the temperature increases. The 

resistance of the sensor element is determined 

with a Wheatstone bridge and/or a galvanometer, enabling the temperature to be determined 

from an appropriate calibration table or curve. 

In some cases, the thermistors are calibrated to 

give a direct temperature reading. Thermistors 

are also less sensitive to radiation, but probes 

may require individual calibration.

5.1.1.4 Bimetallic Thermometers

Bimetallic thermometers are composed of two 

strips of different metals connected to each 

other at one end. They operate according to 

each metal’s coefficient of expansion; when the 

strips are heated or cooled, they change length 

by a differing amount, which produces a movement in an indicator calibrated to temperature. 

These thermometers are frequently used in 

thermostats and appliances.
Occupational Exposure to Heat and Hot Environments 61

5 . Measurement of Heat Stress

5.1.2 Humidity

Humidity, the amount of water vapor within 

a given space, is commonly measured as the 

relative humidity (RH), that is, the percentage 

of moisture in the air relative to the amount it 

could hold if saturated at the same temperature. Humidity is important as a temperaturedependent expression of the actual water vapor 

pressure, which is the key climatic factor affecting heat exchange between the body and the 

environment by evaporation. The higher the 

water vapor pressure, the lower the evaporative 

heat loss will be.

A hygrometer, or psychrometer, measures 

humidity; however, the term is commonly 

used for those instruments which yield a 

direct reading of RH. Hygrometers utilizing hair or other organic material are rugged, 

simple, and inexpensive, but they have low 

sensitivity, especially at temperatures above 

50°C (122°F) and RH below 20%.

5.1.2.1 Water Vapor Pressure

Vapor pressure (Pa) is the pressure at which a 

vapor can accumulate above its liquid if the 

vapor is kept in confinement and the temperature is held constant. Vapor pressure is 

generally expressed in millimeters of mercury 

(mmHg). However, International System of 

Units (SI) units for water vapor pressure are 

kilopascals (kPa). To convert mmHg to kPa, 

multiply by 7.5 (760/101.325). To calculate 

heat loss by evaporation of sweat, the ambient 

water vapor pressure must be used. The lower 

the ambient water vapor pressure, the higher 

the rate of evaporative heat loss will be. 

Water vapor pressure is most commonly determined from a psychrometric chart. The psychrometric chart graphically represents the 

relationships among the dry bulb temperature 

(ta), wet bulb temperature (twb), dew point temperature (tdp), RH, and vapor pressure (Pa). By 

knowing any two of these five climatic factors, 

one can obtain the other three from the psychrometric chart.

5.1.2.2 Natural Wet Bulb Temperature

The natural wet bulb temperature (tnwb) is the 

temperature measured by a thermometer in 

which the sensor is covered by a wetted cotton 

wick and is exposed only to the natural prevailing air movement.

The tnwb should be measured only with a liquidin-glass partial immersion thermometer, which 

is calibrated by immersing only its bulb in a 

thermostatically controlled medium. If a total 

immersion thermometer is used, the measurements must be corrected by applying a 

correction factor [Benedict 1977]. Accurate 

measurements of tnwb require using a clean wick, 

distilled water, and proper shielding to prevent 

radiant heat gain.

5.1.2.3 Psychrometric Wet

Bulb Temperature

The psychrometric wet bulb temperature (tpwb) 

is obtained when the wetted wick covering the 

sensor is exposed to a high forced-air movement. The tpwb is commonly measured with a 

psychrometer, which consists of two mercuryin-glass thermometers mounted alongside 

each other on the frame of the psychrometer. 

One thermometer is used to measure the tpwb

by covering its bulb with a clean cotton wick 

wetted with water and the second measures the 

dry bulb temperature (ta). The air movement is 

obtained manually with a sling psychrometer or 

mechanically with a motor-driven psychrometer. The sling psychrometer is usually whirled 

by a handle, which is jointed to the frame, for 

about 1 minute. A motor-driven psychrometer uses a battery or spring-operated fan to 

pull air across the wick. When no temperature 

change occurs between two repeated readings, 

measurement of tpwb is taken. Psychrometers 

are simple, more precise, and faster responding than hygrometers; however, they cannot 
62 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

be used under temperatures near or below 

the freezing point of water (humidity is usually 100% and water vapor pressure is about 3 

mmHg [0.4 kPa]).

5.1.2.4 Dew Point Temperature

Dew point temperature (tdp) is the temperature 

at which the condensation of water vapor in air 

begins for a given state of humidity and pressure as the vapor temperature is reduced. The 

dew point hygrometer measures the dew point 

temperature by means of cooling a highly polished surface exposed to the atmosphere and 

observing the temperature at which condensation starts. Dew point hygrometers are more 

precise than other hygrometers and are useful 

in laboratory measurements; however, they are 

more expensive and less rugged than the other 

humidity-measuring instruments and generally require an electric power source.

5.1.3 Air Velocity

Wind, whether generated by body movements 

or air movement (Va), is the rate in feet per 

minute (fpm) or meters per second (m·s-1) at 

which the air moves. Wind is important in heat 

exchange between the human body and the 

environment because of its role in convective 

and evaporative heat transfer.

Wind velocity is measured with an anemometer. The two major types are vane anemometers 

(swinging and rotating) and thermoanemometers. Accurate wind velocity contour maps in a 

work area are very difficult to develop because 

of the large variability in air movement over 

time and within space. In this case, the thermoanemometers are quite reliable and are sensitive to 0.05 m·s-1 (10 fpm) but are not very 

sensitive to wind direction.

If an anemometer is not available for accurate air velocity measurement, then air velocity can be estimated as follows [Ramsey and 

Beshir 2003]:

Air movement Vam·s-1 Vafpm

No sensation of 

air movement 

(e.g., closed room 

without any air source)

Va < 0.2 39

Sensing light breezes 

(e.g., slight perception 

of air movement)

0.2 < Va < 1.0 39–197

Sensing moderate 

breezes 

(e.g., few meters away 

from a fan; definite 

perception of air 

movement; air 

causing movement 

of hair and paper)

1.0 < Va < 1.5 197–235

Sensing heavy breezes 

(e.g., located close to a 

fan; air causing marked 

movement of clothing)

Va > 1.5 > 235

5.1.3.1 Vane Anemometers (swing and cup)

Vane anemometers are often used for weather 

forecasting and reporting wind speed. The 

two major types are the propeller (or rotating) vane and the deflecting (or swinging) 

vane anemometers. The propeller (or rotating) 

vane anemometer consists of a light, rotating, 

wind-driven wheel enclosed in a ring. It indicates the number of revolutions of the wheel or 

the linear distance in meters or feet. Another 

type of rotating anemometer consists of three 

or four hemispherical cups mounted radially 

from a vertical shaft. Wind from any direction causes the cups to rotate the shaft, and 

wind speed is determined from the shaft speed 

[ASHRAE 1981a]. The swinging anemometer 

consists of a vane enclosed in a case, which has 

an inlet and an outlet air opening. The vane is 

placed in the pathway of the air and the movement of the air causes the vane to deflect. This 

deflection can be translated to a direct readout 

of the wind velocity by means of a gear train. 
Occupational Exposure to Heat and Hot Environments 63

5 . Measurement of Heat Stress

Rotating vane anemometers are more accurate 

than swinging vane anemometers.

5.1.3.2 Thermoanemometers

Air velocity is determined with thermoanemometers by measuring the cooling effect of 

air movement on a heated element. Two types 

are hot-wire anemometers, which use resistance thermometers, and heated thermocouple 

anemometers. Two measurement techniques 

are used: (1) bring the resistance (voltage) of 

a hot-wire anemometer or the electromotive 

force (emf) of a heated thermocouple to a 

specified value, measure the current required 

to maintain this value, and then determine the 

wind velocity from a calibration chart; or (2) 

heat the thermometer (usually by applying a 

specific electric current) and then determine 

the air velocity from a direct reading or a calibration chart relating air velocity to the wire 

resistance of the hot-wire anemometer or to the 

emf of the heated thermocouple anemometer. 

5.1.4 Heat Radiation

Radiant heat sources can be classified as artificial (e.g., infrared radiation in the iron and steel 

industries, the glass industry, and foundries) 

or natural (i.e., solar radiation). Instruments 

that are used for measuring radiation (black 

globe thermometers or radiometers) have different characteristics from pyrheliometers or 

pyranometers, which are used to measure solar 

radiation. However, the black globe thermometer is the most commonly used instrument for 

measuring the thermal load of solar and infrared radiation on man.

5.1.4.1 Artificial (Non-solar or 

Occupational) Radiation

(1) Black Globe Thermometers

In 1932, Vernon developed the black globe 

thermometer to measure radiant heat. The thermometer consists of a 15-centimeter (6-inch) 

hollow copper sphere (a globe) painted a matte 

black to absorb the incident infrared radiation 

(0.95 emissivity) and a sensor (thermistor, 

thermocouple, or mercury-in-glass partial 

immersion thermometer), with its sensing 

element placed in the center of the globe. The 

Vernon globe thermometer is the most commonly used device for evaluating occupational 

radiant heat, and it is recommended by NIOSH 

for measuring the black globe temperature (tg) 

[NIOSH 1972]; it is sometimes called the standard 6-inch black globe.

Black globe thermometers exchange heat with 

the environment by radiation and convection. The temperature stabilizes when the heat 

exchange by radiation is equivalent to the heat 

exchange by convection. Both the thermometer stabilization time and the conversion of 

globe temperature to mean radiant temperature are functions of the globe size [Kuehn 

1973]. The standard 6-inch globe requires a 

period of 15 to 20 minutes to stabilize, whereas 

small black globe thermometers of 4.2 centimeters (1.65 inches) in diameter, which are 

commercially available, require about 5 minutes to stabilize [Kuehn and Machattie 1975]. 

While the Vernon 6 inch (150 mm) black globe 

it is still used and is considered the standard of 

the industry (ISO 7243 2008), smaller, handheld, WBGT units are available equipped with 

black globes < 2 inches in diameter. However, 

whether one uses a 6 inch black globe or a 

hand held 2 inch black globe, the principle is 

the same [Parsons 2003; McArdle et al. 2010b].

The tg is used to calculate the Mean Radiant 

Temperature (MRT). The MRT is defined as 

the temperature of a “black enclosure of uniform wall temperature which would provide 

the same radiant heat loss or gain as the nonuniform radiant environment being measured.” 

The MRT for a standard 6-inch black globe can 

be determined from the following equation:

MRT = tg + (1.8 Va 0.5)(tg − ta)
64 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

where

MRT = Mean Radiant Temperature (°C)

tg = black globe temperature (°C)

ta = air temperature (°C)

Va = air velocity (m·s-1)

(2) Radiometers

A radiometer is an instrument for measuring 

infrared radiation. Some radiometers, such as 

infrared pyrometers, use the measured radiant energy to indicate the surface temperature 

of the radiant source by measuring radiation 

emitted from a so-called “black body radiator” 

or object radiating heat. Surface temperatures 

ranging from -30° to 3000°C (-22°F to 5432°F)

can be measured with an infrared pyrometer. 

Modern IR pyrometers are hand-held devices 

that are readily used in an occupational setting 

[Åstrand et al. 2003].

The net radiometer consists of a thermopile 

with the sensitive elements exposed on the two 

opposite faces of a blackened disc. It has been 

used to measure the radiant energy balance of 

human subjects [Cena et al. 1981]. A variety 

of radiometers has been used to measure radiant flux [Gagge 1970]. Radiometers are not 

commonly used in occupational radiant heat 

measurements, however. They are used most 

commonly in laboratories, or for measuring 

surface temperature and direct radiant energy 

from the sun falling on the surface of the earth. 

In addition, radiometers are used to measure 

skin temperature. The radiometer (skin sensor) 

is attached to the skin and records changes in 

skin temperature during exposure to hot environments or during exercise (usually in a laboratory setting) [Åstrand et al. 2003].

5.1.4.2 Natural (Solar) Radiation

Solar radiation can be classified as direct, diffuse, or reflected. Direct solar radiation comes 

from the solid angle of the sun’s disc and 

implies no barrier between the sun and the 

worker. Diffuse solar radiation (sky radiation) 

is the scattered and reflected solar radiation 

coming from the whole hemisphere after shading the solid angle of the sun’s disc. Reflected 

solar radiation is the solar radiation reflected 

from the ground or water. The total solar heat 

load is the sum of direct, diffuse and reflected 

solar radiation, as modified by clothing worn 

and position of the body relative to the solar 

radiation [Roller and Goldman 1967].

(1) Pyrheliometers

Direct solar radiation is measured with a pyrheliometer. A pyrheliometer consists of a tube 

that can be directed at the sun’s disc and a thermal sensor. Generally, a pyrheliometer with a 

thermopile as sensor and a view angle of 5.7° is 

recommended [Allen et al. 1976; Garg 1982]. 

Two different pyrheliometers are widely used: 

the Angstrom compensation pyrheliometer 

and the Smithsonian silver disc pyrheliometer, each of which uses a slightly different 

scale factor.

(2) Pyranometers

Diffuse and total solar radiations can be measured with a pyranometer. For measuring diffuse radiation, the pyranometer is fitted with 

a disc or a shading ring to prevent direct 

solar radiation from reaching the sensor. The 

receiver usually takes a hemispherical dome 

shape to provide a 180° view angle for total 

sun and sky radiation. It is used in an inverted 

position to measure reflected radiation. The 

thermal sensor may be a thermopile, a silicon 

cell, or a bimetallic strip. Pyranometers can be 

used for measuring solar or other radiation 

between 0.35 and 2.5 micrometers (μm), which 

includes the ultraviolet, visible, and infrared 

range. Additional descriptions of solar radiation measurement can be found elsewhere 
Occupational Exposure to Heat and Hot Environments 65

5 . Measurement of Heat Stress

[Duffie and Beckman 1980; Garg 1982; Chang 

and Ge 1983]. 

Pyrheliometers and pyranometers are the 

standard means to measure radiant energy 

(ISO 9060 Solar energy—Specification and 

classification of instruments for measuring 

hemispherical solar and direct solar radiation 

[1990]). However, a more practical and complete way to measure heat stress in an occupational setting is to use a WBGT. Many of the 

WBGT devices are hand held and are sufficiently accurate for most occupational settings. 

Unlike pyrheliometers and pyranometers, 

WBGT takes into account not only the radiant 

heat but also humidity and dry air temperature 

thus providing a more comprehensive measure of all the components of environmental 

heat stress.

5.1.5 Psychrometric Chart

The psychrometric chart is a graphical representation of the relationships among dry bulb 

temperature, wet bulb temperature, RH, vapor 

pressure, and dew point temperature. If any two 

of these variables are known, any of the others 

can be determined from the psychrometric 

chart. Figure 5-1 depicts a standard psychrometric chart [ISO 1993]. Note that when RH 

equals 100%, dry bulb, wet bulb, and dew point 

temperature are equal. Psychrometric charts 

are valuable tools for assessing the thermal 

environment indoors where there is negligible 

solar or radiant heat exposure.

5.2 Prediction of 

Meteorological Factors 

from the National 

Weather Service Data

The National Oceanic and Atmospheric 

Administration’s National Weather Service 

provides daily environmental measurements, 

which can be a useful supplement to the climatic factors measured at a worksite. The 

National Weather Service data include timely 

observations on air temperature, humidity, 

wind speed, dew point, and visibility. These data 

can be used for approximate assessment of the 

worksite environmental heat load for outdoor 

jobs or for some indoor jobs where air conditioning is not in use. Atmospheric pressure 

data can also be used for both indoor and outdoor jobs. The actual and projected data may 

be helpful in predicting local WBGT [Bernard 

and Barrow 2013]. In addition, the National 

Weather Service may issue specific advisories 

during extreme heat, based on the heat index. 

The heat index incorporates temperature with 

RH to estimate the “feels like” temperature 

[Golden et al. 2008]. A recent study found that 

86% of heat injuries were associated with a heat 

index range of 32.2°C to 40°C (90°F to 104°F) 

[Armed Forces Health Surveillance 2011]. For 

additional information on the heat index, see 

Appendix C.

National Weather Service data have also been 

used in studies of mortality due to heat-aggravated illness resulting from heat waves in the 

United States [Semenza et al. 1996; Curriero 

et al. 2002; Knowlton et al. 2007; Golden et al. 

2008]. However, attributing heat waves and 

extreme heat events (EHE) to related health 

impacts can be a difficult task. Heat waves are 

often referred to as silent killers because unlike 

with other natural disasters such as hurricanes, 

they do not leave an obvious trail of destruction [Luber and McGeehin 2008]. Despite 

this, extreme heat is responsible for nearly 

the same or greater number of deaths in the 

United States as lightening, storms, floods, and 

earth movements (e.g., landslides and earthquakes) combined [Berko et al. 2014; Thacker 

et al. 2008]. Heat-related illnesses and deaths 

estimates due to a heat wave are often misclassified, unrecognized, or not reported at all 

[Luber and McGeehin 2008].
66 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

Figure 5-1. The psychrometric chart (adapted from ISO [1993] and Coolerado [2012])

0.40 0.35 0.30 0.25 0.20

60

130

120

110

100

90

80

70

ENTHALPY (KJ PER KILOGRAM OF DRY AIR)

35.030.025.020.010.05.00.0

VAPOR PRESSURE (MM OF MERCURY)

0.450.500.550.600.650.700.750.800.850.900.951.00

SENSIBLE HEAT RATIO = Qs / Qt

-30

-20

-10

0

10

20

30

40

50

PSYCHROMETRIC CHARTNORMAL TEMPERATURESI UnitsSEA LEVELBAROMETRIC PRESSURE: 101.325 kPa

SENSIBLE HEAT

TOTAL HEAT

QsQt =

ENTHALPY

HUMIDITY RATIO

•h

•W =

1.0 1.0

0.8

0.6

0.5

0.4

0.3

0.2

0.1

2.0

4.0

8.0

–8.0

–4.0

–2.0

–1.0

–0.5

–0.3

–0.4

–0.1

–0.2

–4.6

–2.3

0.0

1.2

2.3

3.5

4.7

7.0

11.7

0

 30 25 20 15 10 5 0-5-10

DEW POINT TEMPERATURE (°C)

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55

80

90

100

ENTHALPY (KJ PER KILOGRAM OF DRY AIR)

70

40

50

60

30

10

20

0

-10

-30

-20

10

-25 -20

-15

-10

-5

 0

5

SATURATION TEMPERATURE (°C) 15 20

25

30

DRY BULB TEMPERATURE (°C)

864

2

10

HUMIDITY RATIO (GRAMS MOISTURE PER KILOGRAM DRY AIR)

282624222018161412

5

10

15

-20

-10

-5

20

WET BULB TEMPERATURE (°C)

25

30

10%

6%

4%

2%

8% 

RELATIVE HUMIDITY

20%

15%

30%

25%

40%

50%

70%

60%

90%

80%

100%

 VOLUME (CUBIC METER PER KILOGRAM DRY AIR)

0.82

0.84

0.86

0.74

0.76

0.78

0.80

0.88

0.90

0.92

0.94

SENSIBLE HEAT RATIO = Qs / Qt

DRY BULB TEMPERATURE (°C)RELATIVE HUMIDITYVOLUME (CUBIC METER PER KILOGRAM DRY AIR)HUMIDITY RATIO (GRAMS MOISTURE PER KILOGRAM DRY AIR)WET BULB TEMPERATURE (°C)SATURATION AND DEW POINT TEMPERATURE (°C)
Occupational Exposure to Heat and Hot Environments 67

5 . Measurement of Heat Stress

Continuous monitoring of the environmental factors at the worksite provides information on the level of heat exposure at the time 

the measurements are made. Such data are 

useful for developing heat stress engineering controls. However, in order to have established work practices in place when needed, 

it is desirable to predict the anticipated level 

of heat stress for a day or more in advance. A 

methodology has been developed based on 

the psychrometric wet bulb for calculating 

the WBGT at the worksite from the National 

Weather Service meteorologic data. The data 

upon which the method is based were derived 

from simultaneous measurements of the thermal environment in 15 representative worksites, from outside the worksites, and from 

the closest National Weather Service station. 

The empirical relationships between the inside 

and outside data were established. From these 

empirical relationships, it is possible to predict worksite WBGT, effective temperature 

(ET), or corrected effective temperature (CET) 

values from weather forecasts or local meteorologic measurements. To apply the predictions model, it is first necessary to perform a 

short environmental study at each worksite 

to establish the differences in inside and outside values and to determine the regression 

constants that are unique to each workplace, 

perhaps because of the differences in actual 

worksite air motion compared to the constant 

high air motion associated with the use of the 

ventilated wet bulb thermometer [Mutchler et 

al. 1976]. Another example involves heat stress 

evaluation at an aluminum smelter, where 

targeted environmental measurements were 

estimated based on National Weather Service 

data, a task analysis was performed, and heat 

stress was analyzed on the basis of these elements [Logan and Bernard 1999].

5.3 Metabolic Heat

The total heat load imposed on the human 

body is the aggregate of environmental and 

physical work factors. The energy cost of an 

activity, as measured by the metabolic heat 

(M), is a major element in the heat-exchange 

balance between the human body and the environment. The M value can be measured or estimated. The energy cost of an activity is made 

up of two parts: the energy expended in doing 

the work and the energy transformed into 

heat. On the average, muscles may reach 20% 

efficiency in performing heavy physical work. 

However, unless external physical work is conducted, the body heat load is approximately 

equal to the total metabolic energy turnover. 

For practical purposes, M is equated with total 

energy turnover.

5.3.1 Measurements of Metabolic Heat

5.3.1.1 Measurement of Metabolic Heat by 

Direct Calorimetry

To determine the worker’s heat production by 

direct calorimetry, the subject is placed in a 

calorimeter, an enclosed chamber surrounded 

by circulating water; the increase in the temperature of the circulating water is used to 

determine the amount of heat liberated from 

the human body. The direct procedure has limited practical use in occupational heat stress 

studies, because the procedure is difficult and 

time consuming and the equipment and chambers are expensive [Banister and Brown 1968].

5.3.1.2 Measurements of Metabolic Heat 

by Indirect Calorimetry

Primary methods of measurements of metabolic heat by indirect calorimetry are based 

on measuring oxygen consumption. Each liter 

of oxygen consumed results in the production 

of approximately 4.8 kcal (5.6 W) of metabolic heat. Indirect calorimetry uses either the 

closed circuit or the open circuit procedure. An 
68 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

even more indirect procedure for measuring 

metabolic heat is based on the linear relationship between HR and oxygen consumption. 

The linearity, however, usually holds only at 

submaximal HRs because, on approaching 

the maximum, the pulse rate begins to level 

off while the oxygen intake continues to rise. 

The linearity also holds only on an individual 

basis because of the large inter-individual differences in responses [Karpovich and Sinning 

1971; Berger 1982].

(1) Closed Circuit

In the closed circuit procedure, the subject 

inhales from a spirometer and the expired air 

returns to the spirometer after passing through 

carbon dioxide and water vapor absorbents. 

The depletion in the amount of oxygen in the 

spirometer represents the oxygen consumed by 

the subject. The development of computerized 

techniques, however, has led to revisions of the 

classic procedures so that equipment and the 

evaluation can be controlled by a computer, 

which results in prompt, precise, and simultaneous measurement of the significant variables 

[Stegman 1981].

(2) Open Circuit

Historically, in the open circuit procedure, 

the worker breathes atmospheric air and the 

exhaled air is collected in a large container, that 

is, a Douglas bag or meteorological balloon. 

The volume of the expired air can be accurately measured with a calibrated gasometer. 

The concentration of oxygen in the expired 

air can be measured by chemical or electronic 

methods. The oxygen and carbon dioxide in 

atmospheric air usually average 20.90% and 

0.03%, respectively, or they can be measured 

so that the amount of oxygen consumed and 

the metabolic heat production for the performed activities can be determined.

Another open circuit procedure, the Max 

Planck respiration gasometer, eliminates the 

need for an expired air collection bag and a 

calibrated gasometer [Stegman 1981]. The 

subject breathes atmospheric air and exhales 

into the gasometer, where the volume and 

temperature of the expired air are immediately measured. An aliquot sample of the 

expired air is collected in a rubber bladder for 

later analysis for oxygen and carbon dioxide 

concentrations. Both the Douglas bag and the 

respiration gasometer are portable and thus 

appropriate for collecting expired air of workers at different industrial or laboratory sites 

[Stegman 1981].

Many new technologies for measuring metabolic rates in humans are now available. These 

new techniques involve a metabolic cart that 

samples breathing air on a breath-by-breath 

basis and calculates oxygen consumption (V.O2 

max), carbon dioxide production (V.CO2), 

minute ventilation (V.

E), respiratory exchange 

ratio (RER), etc. Wearable metabolic carts 

also can make these measurements [McArdle 

et al. 2010a]. Nearly every modern exercise 

physiology laboratory employs the metabolic 

cart while conducting its research (ISO 8996: 

Ergonomics of the thermal environment—

Determination of metabolic rate [2004]).

5.3.2 Estimation of Metabolic Heat

The procedures for direct or indirect measurement of metabolic heat are limited to relatively 

short duration activities and require equipment for collecting and measuring the volume 

of the expired air and for measuring the 

oxygen and carbon dioxide concentrations. 

Alternatively, although they are less accurate 

and reproducible, metabolic heat estimates 

on the basis of tables of energy expenditure 

or task analysis can be applied for short and 

long-duration activities and require no special 

equipment. However, the accuracy of the estimates made by a trained observer may vary 

by about ±10% to 15%. A training program 

consisting of supervised practice in using the 
Occupational Exposure to Heat and Hot Environments 69

5 . Measurement of Heat Stress

tables of energy expenditure in a workplace 

situation will usually result in an increase in 

accuracy of the estimates of metabolic heat 

production [AIHA 1971; Garg et al. 1978].

5.3.2.1 Tables of Energy Expenditures

Estimates of metabolic heat for use in assessing muscular workload and human heat regulation are commonly obtained from tabulated 

descriptions of energy cost for typical work 

tasks and activities [Smith and Ramsey 1980; 

ACGIH 2014]. Errors in estimating metabolic rate from energy expenditure tables are 

reported to be as high as 30% [ISO 1990]. The 

ISO [1990] recommends that the metabolic 

rate be estimated by adding the following 

values: (1) basal metabolic rate, (2) metabolic 

rate for body position or body motion, (3) 

metabolic rate for type of work, and (4) metabolic rate related to work speed. The basal 

metabolic rate averages 44 and 41 W·m-2 for 

the “standard” man and woman, respectively. 

Metabolic rate values for body position, and 

body motion, type of work, and those related 

to work speed are provided [ISO 1990].

5.3.2.2 Task Analysis

In order to evaluate the average energy requirements over an extended period of time for a 

task, including both work and rest activities, 

it is necessary to divide the task into its basic 

activities and subactivities. The metabolic heat 

of each activity or subactivity is then measured or estimated and a time-weighted average for the energy required for the task can be 

obtained. It is common in such analyses to estimate the metabolic rate for the different activities by utilizing tabulated energy values from 

tables (see Table 5-1) that specify incremental 

metabolic heat resulting from the movement 

of different body parts (e.g., arm work, leg 

work, standing, and walking) [McArdle et al. 

1996b]. The metabolic heat of the activity can 

then be estimated by summing the component 

M values based on the actual body movements.
70 Occupational Exposure to Heat and Hot Environments

5 . Measurement of Heat Stress

Table 5-1. Comparison of WBGT exposure limits for acclimatized workers

Workload ACGIH AIHA OSHA ISO NIOSHResting 32.2°C (90°F)

100 kcal·h-1(117 W)

33°C (91.4°F)≤100 kcal·h-1(117 W)

Light 30°C (86°F)

100–200 kcal·h-1(117–233 W)

30°C (86°F)200 kcal·h-1(233 W)

30.0°C* (86°F) 32.2°C† (90°F)<200 kcal·h-1(233 W)

30°C (86°F)100–201 kcal·h-1(117–234 W)

30°C (86°F)<200 kcal·h-1(233 W)

Moderate 26.7°C (80°F)

201–350 kcal·h-1(234–407 W)

26.7°C (80°F)300 kcal·h-1(349 W)

27.8°C* (82°F)30.6°C† (87.1°F)201–300 kcal·h-1(234–349 W)

28°C (82.4°F)201–310 kcal·h-1(234–360 W)

28°C (82.4°F)201–300 kcal·h-1(234–349 W)

Heavy 26.1°C* (79°F)

28.9°C† (84°F)>301 kcal·h-1(350 W)

25°C* (77°F)26°C† (78.8°F)

310–403 kcal·h-1

(360–468 W)

26°C (78.8°F)301–400 kcal·h-1(350–465 W)

Very heavy 25°C (77°F)

350–500 kcal·h-1(407–581 W)

23°C* (73.4°F)25°C† (77°F)

>403 kcal·h-1(468 W)

25°C (77°F)401–500 kcal·h-1(466–580 W)

*Low velocity; †high velocity; kcal·h-1 = kilocalories per hour.NOTE: Unacclimatized workers would have greater heat expenditures during the same amount of work and temperature.Adapted from the American Industrial Hygiene Association (AIHA) [2003].
Occupational Exposure to Heat and Hot Environments 71

6 Control of Heat Stress

According to the heat balance equation [S = (M 

− W) ± C ± R ± K – E] described in Section 3.1, 

heat stress can be reduced only by modifying 

one or more of the following factors: metabolic 

heat production, heat exchange by convection, 

heat exchange by radiation, or heat exchange 

by evaporation. Environmental heat load (C, 

R, and E) can be modified by engineering controls (e.g., ventilation, air conditioning, screening, insulation, and modification of process or 

operation) and protective clothing and equipment, whereas metabolic heat production can 

be modified by work practices and application of labor-reducing devices. Each of these 

alternative control strategies will be discussed 

separately. Actions that can be taken to control 

heat stress and strain are listed in Table 6-1 

[Belding 1973].

6.1 Engineering Controls

The environmental factors that can be modified by engineering procedures are those 

involved in convective, radiative, and evaporative heat exchange.

6.1.1 Convective Heat Control

As discussed earlier, the environmental variables concerned with convective heat exchange 

between the worker and the ambient environment are dry bulb air temperature (ta) and the 

speed of air movement (Va). When air temperature is higher than the mean skin temperature 

(tsk of 35°C or 95°F), heat is gained by convection. The rate of heat gain is dependent on temperature differential (ta − tsk) and air velocity 

(Va), where if ta is below tsk, then heat is lost 

from the body; the rate of loss is dependent on 

ta −tsk and air velocity.

Engineering approaches to enhancing convective heat exchange are limited to modifying air 

temperature and air movement. When ta is less 

than tsk, increasing air movement across the skin 

by increasing either general or local ventilation 

will increase the rate of body heat loss. When ta 

exceeds tsk (convective heat gain), ta should be 

reduced by bringing in cooler outside air or by 

evaporative or refrigerative cooling of the air. 

In addition, as long as ta exceeds t sk, air speed 

should be reduced to levels that will still permit 

sweat to evaporate freely but will reduce convective heat gain (see Table 6-1). The effect 

of air speed on convective heat exchange is a 

function of air speed. Spot cooling (ta less than 

t sk) of the individual worker can be an effective approach to controlling convective heat 

exchange, especially in large workshops where 

the cost of cooling the entire space would be 

prohibitive. However, spot coolers or blowers may interfere with the ventilating systems 

required to control toxic chemical agents.

6.1.2 Radiant Heat Control

Radiant heat exchange between the worker and 

hot equipment, processes, and walls that surround the worker is a fourth power function 

of the difference between skin temperature 

(t sk) and the temperature of hot objects that are 

in direct, unobstructed line to the worker (t r ). 

The only engineering approaches to controlling 
72 Occupational Exposure to Heat and Hot Environments

See footnotes at end of table. (Continued)

6 

. Control of Heat Stress

Table 6-1. Checklist for controlling heat stress and strain

Item Actions for consideration

1. Engineering Controls

Body heat production of task (M) ■ Reduce physical demands of the work; use powered assistance for heavy tasksRadiative load (R) ■ Interpose line-of-sight barrier; use furnace wall insulation, metallic reflecting screen, heat reflective clothing; 

cover exposed parts of body

Convective load (C) ■ If air temperature is above 35°C (95°F), reduce air temperature, reduce air speed across skin, wear clothing

■ If air temperature is below 35°C (95°F), increase air speed across skin and reduce clothing

Maximum evaporative cooling by sweating (Emax)

■ Increase cooling by decreasing humidity and/or increasing air speed■ Decrease clothing

2. Administrative Controls

Acclimatization ■ Gradually increase exposure time in hot environmental conditions over 7–14 days

■ For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment 

on day 1 and no more than 20% increase each day

■ For workers with experience with the job, the regimen should be no more than 50% of the usual duration of work 

in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4

Work schedule ■ Shorten duration of each exposure;

more-frequent short exposures are better than fewer long exposures

■ Schedule very hot jobs in cooler parts of day when possible

Hydration ■ Encourage water intake at frequent intervals to prevent dehydration (1 cup every 15–20 minutes)Rest and recovery ■ Shaded and/or air-conditioned space nearbyExtreme heat event (e.g., heat wave)

■ Heat alert program
Occupational Exposure to Heat and Hot Environments 73

6 

. Control of Heat Stress

Item Actions for consideration

Other considerations ■ Worker medical evaluations

■ Fatigue or mild illness not related to the job may temporarily contraindicate exposure (e.g., low-grade infection, 

diarrhea, sleepless night, alcohol ingestion)

3. Personal protective clothing 

and equipment 

■ Cooled air, cooled fluid, or ice-cooled conditioned clothing■ Reflective clothing or aprons

Adapted from Belding [1973] and OSHA-NIOSH [2011].

Table 6-1 (Continued). Checklist for controlling heat stress and strain
74 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

radiant heat gain are to reduce tr or to shield 

the worker from the radiant heat source.

To reduce tr would require (1) lowering the 

process temperature, which is usually not compatible with the temperature requirements of 

the manufacturing processes; (2) relocating, 

insulating, or cooling the heat source; (3) placing line-of-sight radiant reflective shielding 

between the heat source and the worker; or (4) 

changing the emissivity of the hot surface by 

coating the material. Of the alternatives, radiant reflective shielding is generally the easiest to install and the least expensive. Radiant 

reflective shielding can reduce the radiant heat 

load by as much as 80% to 85%. Some ingenuity 

may be required in placing the shielding so that 

it does not interfere with the work. Remotely 

operated tongs, metal chain screens, or air- or 

hydraulically activated doors that are opened 

only as needed are some of the possibilities.

6.1.3 Evaporative Heat Control

Heat is lost from the body when sweat evaporates from the skin surface. The rate and 

amount of evaporation are functions of the 

speed of air movement over the skin and the 

difference between the water vapor pressure 

of the air (pa) at ambient temperature and the 

water vapor pressure of the wetted skin (with 

an assumed skin temperature of 34°C–35°C 

[93.2°F–95°F]). At any air-to-skin vapor pressure gradient, the evaporation increases as a 

0.6 root function of increased air movement. 

Evaporative heat loss at low air velocities can 

be greatly increased by improving ventilation 

(increasing air velocity). At high air velocities 

(2.5 m·s-1 or 500 fpm), an additional increase 

will be ineffective, except when the clothing worn interferes with air movement over 

the skin.

Engineering control of evaporative cooling can 

be accomplished in two ways: (1) increase air 

movement or (2) decrease ambient water vapor 

pressure. Of these, increasing air movement by 

the use of fans or blowers is often the simplest 

and usually the cheapest approach to increasing the rate of evaporative heat loss. Ambient 

water vapor pressure reduction usually requires 

air-conditioning equipment (cooling compressors). In some cases, the installation of air conditioning, particularly spot air conditioning, 

may be less expensive than the installation of 

increased ventilation because of the lower airflow involved. The vapor pressure of the worksite air is usually at least equal to that of the 

outside ambient air, except when all incoming 

and recirculated air is humidity controlled by 

absorbing or condensing the moisture from the 

air (i.e., by air conditioning). In addition to the 

ambient air as a source of water vapor, water 

vapor may be added from the manufacturing 

processes as steam, leaks from steam valves and 

steam lines, and evaporation of water from wet 

floors. Eliminating these additional sources of 

water vapor can help reduce the overall vapor 

pressure in the air and thereby increase evaporative heat loss by facilitating the rate of evaporation of sweat from the skin [Dasler 1977].

6.2 Administrative Controls

The job risk factors for occupational heat stress 

are thermal environment, work demands, and 

clothing requirements. These are reflected in 

occupational exposure limits (OELs) traditionally based on WBGT, such as NIOSH RELs 

and ACGIH TLVs®, and in ISO 7243. Many 

workers spend some part of their working day 

in a hot environment where the temperature is 

above the OELs. Strategies to reduce the effects 

of heat in the workplace include primarily 

eliminating the heat with engineering controls, 

although in some situations administrative 

controls and PPE will be necessary.

In some situations, it may be technologically 

impossible or impractical to completely control 

heat stress by the application of engineering 
Occupational Exposure to Heat and Hot Environments 75

6 . Control of Heat Stress

controls; the level of environmental heat stress 

may be unpredictable and variable (as in seasonal heat waves), and exposure time may 

vary with the task and with unforeseen critical 

events. When applying engineering controls 

for heat stress is not practical or sufficient, 

other solutions must be sought to keep the 

worker’s total heat stress level within limits that 

will not be associated with an increased risk of 

heat-related illnesses.

Administrative controls consist of mainly five 

strategies: (1) limiting or modifying the duration of exposure time; (2) reducing the metabolic component of the total heat load; (3) 

enhancing the heat tolerance of the workers by, 

for example, heat acclimatization and physical 

conditioning; (4) training the workers in safety 

and health procedures for work in hot environments; and (5) medical screening of workers to 

be aware of which individuals have low heat 

tolerance and/or low physical fitness.

6.2.1 Limiting Exposure Time

and/or Temperature

There are several ways to control the daily 

length of time and temperature to which a 

worker is exposed in heat stress conditions 

[OSHA-NIOSH 2011].

■ When possible, schedule hot jobs for the 

cooler part of the day (early morning, late 

afternoon, or night shift) and/or schedule 

hot jobs on alternate rather than successive days.

■ Schedule routine maintenance and repair 

work in hot areas for the cooler seasons 

of the year.

■ Redesign the work to increase distance 

from radiant heat sources.

■ Alter the work/rest schedule to permit 

more rest time (see examples in Tables 

6-2 and 6-3).

■ Provide cool areas (e.g., air-conditioned 

or shaded) for rest and recovery.

■ Add extra personnel to reduce exposure 

time for each member of the crew.

■ Permit work interruption when a worker 

feels heat discomfort.

■ Increase workers’ water intake on the job.

■ Adjust schedule, when possible, so that 

hot operations are not performed at the 

same time and place as other operations 

that require the presence of workers, 

such as maintenance and cleanup while 

tapping a furnace.

■ Utilize the Thermal Work Limit guide 

as the limiting (or maximum) sustainable metabolic rate that well-hydrated, 

acclimatized individuals can maintain in a specific thermal environment, 

within a safe deep body core temperature (< 38.2°C or 100.8°F) and sweat rate 

(< 1.2 kg or 2.6 lb per hour). The index is 

designed for self-paced workers and does 

not rely on estimation of actual metabolic rates, a process that is difficult and 

subject to considerable error. However, 

the maximum metabolic rate that can be 

maintained under these circumstances is 

limited by the workers overall fitness, not 

simply level of hydration and acclimatization to the heat. The Thermal Work Limit 

is therefore of limited use because of this.

6.2.2 Reducing Metabolic Heat Load

In most work situations, metabolic heat is not 

the major part of the total heat load. However, 

because it represents an extra load on the circulatory system, it can be a critical component 

in high heat exposures. Heavy and very heavy 

metabolic rates require substantial rest periods. 

For some examples of work/rest schedules, see 

Tables 6-2 and 6-3. Metabolic heat production 
76 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

Table 6-2. Work/rest schedules for workers wearing normal work clothing*

Adjusted 

temperature (°F)†

Light work

(minutes work/rest)

Moderate work 

(minutes work/rest)

Heavy work 

(minutes work/rest)

90 Normal Normal Normal

91 Normal Normal Normal

92 Normal Normal Normal

93 Normal Normal Normal

94 Normal Normal Normal

95 Normal Normal 45/15

96 Normal Normal 45/15

97 Normal Normal 40/20

98 Normal Normal 35/25

99 Normal Normal 35/25

100 Normal 45/15 30/30

101 Normal 40/20 30/30

102 Normal 35/25 25/35

103 Normal 30/30 20/40

104 Normal 30/30 20/40

105 Normal 25/35 15/45

106 45/15 20/40 Caution‡

107 40/20 15/45 Caution‡

108 35/25 Caution‡ Caution‡

109 30/30 Caution‡ Caution‡

110 15/45 Caution‡ Caution‡

111 Caution‡ Caution‡ Caution‡

112 Caution‡ Caution‡ Caution‡

*

With the assumption that workers are physically fit, well-rested, fully hydrated, under age 40, and have adequate water intake and 

that there is 30% RH and natural ventilation with perceptible air movement.

†

Note: Adjust the temperature reading as follows before going to the temperature column in the table: 

Full sun (no clouds): Add 13°

Partly cloudy/overcast: Add 7°

No shadows visible/work is in the shade or at night: no adjustment

Per relative humidity: 

10%: Subtract 8°

20%: Subtract 4°

30%: No adjustment

40%: Add 3°

50%: Add 6°

60%: Add 9°

‡

High levels of heat stress; consider rescheduling activities.

Adapted from EPA [1993].
Occupational Exposure to Heat and Hot Environments 77

6 

. Control of Heat Stress

Table 6-3. Work/rest schedules for workers wearing chemical-resistant suits*

Light work Moderate work Heavy work

Air Temp (°F) Full sun Partly cloudy No sun† Full sun Partly cloudy No sun† Full sun Partly cloudy No sun†

75 Normal Normal Normal Normal Normal Normal 35/25‡ Normal Normal80 30/30 Normal Normal 20/40 Normal Normal 10/50 40/20 Normal85 15/45 40/20 Normal 10/50 25/35 Normal Caution§ 15/45 40/2090 Caution§ 15/45 40/20 Caution§ Caution§ 25/35 Stop work Caution§ 15/4595 Stop work Stop work 15/45 Stop work Stop work Stop work Stop work Stop work Stop work

*With the assumption that workers are heat-acclimatized, under the age of 40, physically fit, well-rested, fully hydrated, and wearing Tyvek coveralls, gloves, boots, and a respirator. 

Cooling vests may enable workers to work for longer periods. Adjustments must be made when additional protective gear is worn.

†No shadows are visible or work is in the shade or at night.‡35 minutes work and 25 minutes rest each hour.§High levels of heat stress; consider rescheduling activities.Adapted from EPA [1993].
78 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

can be reduced—but usually by not more than 

200 kcal·h-1 (800 Btu·h-1)—by these strategies:

■ Mechanization of the physical components of the job

■ Reduction of work time (reduce work 

day, increase rest time, restrict doubleshifting) and planned times (e.g., U.S. 

Navy Physiological Heat Exposure Limit 

[PHEL] times, Electric Power Research 

Institute [EPRI] action times, University 

of South Florida (USF) WBGT–based 

Safe Exposure Times [Bernard & Ashley 

2009], Predicted Heat Strain [ISO 7933])

■ Increase of the work force.

6.2.3 Enhancing Tolerance to Heat

Stimulating the human heat-adaptive mechanisms can significantly increase the capacity 

to tolerate work in heat. However, the ability 

of people to adapt to heat varies widely, which 

must be kept in mind when considering any 

group of workers.

A properly designed and applied heat-acclimatization program will increase the ability of workers to work at a hot job and will 

decrease the risk for heat-related illnesses and 

unsafe acts. Heat acclimatization can usually be 

induced in 7 to 14 days of exposure at the hot 

job [DOD 2003; Navy Environmental Health 

Center 2007; ACGIH 2014]. For workers who 

have had previous experience with the job, the 

acclimatization regimen should be no more 

than 50% of the usual duration of work in the 

hot environment on day 1, 60% on day 2, 80% 

on day 3, and 100% on day 4. For new workers, the schedule should be no more than 20% 

of the usual duration of work in the hot environment on day 1, increasing by no more than 

20% each day.

Being physically fit for the job will not replace 

heat acclimatization but can enhance heat 

tolerance for both heat-acclimatized and 

nonacclimatized workers [Pandolf et al. 

1977; DOD 2003; Yeargin et al. 2006; Navy 

Environmental Health Center 2007]. The time 

required for non–physically fit individuals to 

develop acclimatization is about 50% greater 

than for the physically fit. For more information on acclimatization, see Table 4-1.

To ensure that water lost in the sweat and urine 

is replaced (at least hourly) during the work 

day, an adequate water supply and intake are 

essential for heat tolerance and prevention of 

heat-related illnesses.

Electrolyte balance in the body fluids must be 

maintained to help prevent heat-related illnesses. For unacclimatized workers who may 

be on a salt-restricted diet, additional salting of the food, with the concurrence of the 

responsible healthcare provider, during the 

first two days of heat exposure, may be needed 

to replace the salt lost in the sweat [Lind 1976; 

DOD 2003]. The heat-acclimatized worker 

loses relatively little salt in sweat and therefore 

usually does not need salt supplementation.

6.2.4 Health and Safety Training

A heat stress training program should be in 

place for all who work in hot environments 

and their supervisors. Workers and supervisors should be trained about the prevention and first aid of heat-related illness before 

they begin work in a hot environment and 

before heat index levels go up. Heat prevention training should be reinforced on hot days. 

Prevention of heat-related illnesses depends on 

early recognition of the signs and symptoms of 

impending heat-related illness and initiation of 

first aid and corrective procedures at the earliest possible moment. 
Occupational Exposure to Heat and Hot Environments 79

6 . Control of Heat Stress

Employers should provide a heat stress training program that effectively trains all workers 

and supervisors about the following:

(1) Recognition of the signs and symptoms of 

the various types of heat-related illnesses—

such as heat cramps, heat exhaustion, heat 

rash, and heat stroke—and in administration of first aid (see Table 4-3).

(2) The causes of heat-related illnesses and the 

personal care procedures that will minimize the risk of their occurrence, such as 

drinking enough water and monitoring 

the color and amount of urine output (see 

Appendix B).

(3) The proper care and use of heat-protective 

clothing and equipment and the added heat 

load caused by exertion, clothing, and personal protective equipment. 

(4) The effects of nonoccupational factors 

(drugs, alcohol, obesity, etc.) on tolerance 

to occupational heat stress.

(5) The importance of acclimatization (see 

Table 4-1).

(6) The importance of immediately reporting 

to the supervisor any symptoms or signs 

of heat-related illness in themselves or in 

their coworkers.

(7) The employer’s procedures for responding 

to symptoms of possible heat-related illness 

and for contacting emergency medical services if needed.

In addition to being trained about each of 

those topics, supervisors should be trained on 

the following:

(1) How to implement appropriate acclimatization.

(2) What procedures to follow when a worker 

has symptoms consistent with heatrelated illness, including emergency 

response procedures.

(3) How to monitor weather reports. 

(4) How to respond to hot weather advisories.

(5) How to monitor and encourage adequate 

fluid intake and rest breaks. 

A buddy system should be initiated, in which 

workers on hot jobs are taught to recognize 

the early signs and symptoms of heat-related 

illness. Each worker and supervisor who has 

received the instructions is assigned the responsibility for observing, at periodic intervals, one 

or more fellow workers to determine whether 

they have any early symptoms of a heat-related 

illness. Any worker who exhibits signs and 

symptoms of an impending heat-related illness should be sent to the first-aid station for 

more complete evaluation and possible initiation of first-aid treatment. Workers on hot jobs 

where the heat stress exceeds the RAL or REL 

(for unacclimatized and acclimatized workers, 

respectively) should be observed by a fellow 

worker or supervisor. 

Additional training and educational materials 

are available from:

■ NIOSH at http://www.cdc.gov/niosh/

topics/heatstress/

■ OSHA at https://www.osha.gov/SLTC/

heatstress/prevention.html#training

■ Cal/OSHA at http://www.dir.ca.gov/

DOSH/HeatIllnessInfo.html

6.2.5 Screening for Heat Intolerance

The ability to tolerate heat stress varies widely, 

even between healthy individuals with similar 

heat exposure experiences [Shvartz and Benor 

1972; Wyndham 1974; Strydom 1975; Khogali 

1997; Moran et al. 2007]. Heat intolerance 

factors in young, active persons may be congenital (e.g., ectodermal dysplasia or chronic 

idiopathic anhidrosis), functional (e.g., low 

physical fitness, lack of acclimatization, low 

work efficiency, or reduced skin area to body 

mass ratio), or acquired (e.g., sweat gland dysfunction, dehydration, infectious disease, x-ray 
80 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

irradiation, previous heat stroke, large scarred 

burns, or drugs) [Epstein et al. 1997; Moran et 

al. 2007]. One way to reduce the risk of heatrelated illnesses and disorders within a heatexposed workforce is to reduce or eliminate 

the exposure of heat-intolerant individuals to 

heat stress. The ability to identify heat-intolerant individuals, without resorting to strenuous, 

time-consuming heat-tolerance tests, is basic 

to any such screening process.

Data from laboratory and field studies indicate 

that individuals with low physical work capacity are more likely to develop higher body 

temperatures than are individuals with high 

physical work capacity when exposed to equally 

hard work in high temperatures. In these studies, none of the individuals with a maximum 

work capacity (V.O2 max) of at least 2.5 liters of 

oxygen per minute (L∙min-1) were heat intolerant, but 63% of those with V.

O2 max below 

2.5 L∙min-1 were. It has also been shown that 

heat-acclimatized individuals with a V.O2 max 

less than 2.5 L∙min-1 had a 5% risk of reaching 

heat stroke levels of body temperature (40°C, 

or 104°F), whereas those with a V.O2 max above 

2.5 L∙min-1 had only a 0.05% risk [Wyndham 

1974a; Strydom 1975].

Medical screening for heat intolerance in otherwise healthy individuals should include 

obtaining a history of any previous incidents 

of heat-related illness. Workers who have experienced a heat-related illness may be less heattolerant [Leithead and Lind 1964; Armstrong 

et al. 1990]. In a study by Moran et al. [2007], 

a heat tolerance test (HTT) was evaluated and 

found to be efficient at differentiating between 

a temporary and permanent state of heat susceptibility, either of which could occur following exertional heat stroke. The test is described 

as a 120 minute exposure to 40°C and 40% 

RH in a climatic chamber while walking on a 

treadmill. The person being tested wears shorts 

and a t-shirt and walks at a pace of 5 km·h-1 (3 

mph) at a 2% elevation. Rectal temperature and 

heart rate are continuously monitored. Sweat 

rate is determined by differences in weight and 

corrected for fluid intake. At the end of the test, 

measurements for heat-tolerant individuals 

will be rectal temperature of 38 ± 0.3°C, heart 

rate of 120 ± 15 bpm, and sweat rate of 780 ± 

160g·h-1. Heat intolerance is determined when 

the rectal temperature is higher than 38.5°C 

(101.3°F) or the heart rate exceeds 145 bpm. 

Larger increases indicate heat intolerance. 

Moran suggests that the HTT be conducted 

within 6 to 8 weeks after a heat exhaustion or 

exertional heat stroke episode and that the test 

may be repeated 4 to 8 weeks later to evaluate 

the diagnosis of heat intolerance. However, it 

should be noted that heart rate varies in populations with differing levels of fitness and age. 

Therefore, heart rate must be used cautiously 

to determine heat tolerance.

6.2.6 Heat Alert Program

When heat-related illnesses and disorders 

occur mainly during heat waves in the summer, 

a Heat Alert Program (HAP) should be established for preventive purposes. Although such 

programs differ in detail from one worksite to 

another, they all use the weather forecast of 

the National Weather Service. If a heat wave is 

predicted for the next day or days, a state of 

Heat Alert is declared to make sure that measures to prevent heat casualties will be strictly 

observed. Although this sounds quite simple 

and straightforward, in practical application, 

it requires the cooperation of the administrative staff, the maintenance and operative workforce, and the medical, industrial hygiene, 

and/or safety departments. An effective HAP 

is described below [Dukes-Dobos 1981]. 

Although this HAP is designed for indoor 

work settings, many aspects can also be used 

or modified for outdoor work settings such as 

in construction or agriculture.

(1) Each year, early in the spring, establish a 

Heat Alert Committee, which may consist 
Occupational Exposure to Heat and Hot Environments 81

6 . Control of Heat Stress

of a responsible healthcare provider, industrial hygienist or qualified safety and health 

professional, safety engineer, operation engineer, and manager. Once established, this 

committee takes care of the following tasks:

(a) Arrange a training course for all 

involved in the HAP that provides procedures to follow in the event a Heat 

Alert is declared; emphasize the prevention and early recognition of heatrelated illnesses and first aid procedures 

when a heat-related illness occurs.

(b) By memorandum, instruct the supervisors to perform these tasks:

(i) Reverse winterization of the site, that 

is, open windows, doors, skylights, 

and vents according to instructions 

and if appropriate (e.g., outside temperatures are cooler) for greatest 

ventilating efficiency at places where 

high air movement is needed.

(ii) Check drinking fountains, fans, and 

air conditioners to make sure that 

they are functional, that the necessary maintenance and repairs are 

performed, that they are regularly 

rechecked, and that workers know 

how to use them.

(c) Ascertain that, in the medical department, as well as at the job sites, all facilities required to give first aid in cases 

of heat-related illness are in a state 

of readiness.

(d) Establish criteria for the declaration of 

a Heat Alert. For instance, a Heat Alert 

would be declared if the area weather 

forecast for the next day predicts a maximum air temperature of at least 35°C 

(95°F) or if a maximum temperature of 

32°C (90°F) is predicted and is 5°C (9°F) 

higher than the temperature reached on 

any of the preceding three days.

(2) Procedures to be followed during the state of 

Heat Alert are as follows:

(a) Postpone tasks that are not urgent (e.g., 

preventive maintenance involving high 

activity or heat exposure) until the heat 

wave is over.

(b) Increase the number of workers on each 

team in order to reduce each worker’s 

heat exposure. Introduce new workers 

gradually to allow acclimatization (follow heat-acclimatization procedure).

(c) Increase rest allowances. Let workers 

recover in air-conditioned rest places.

(d) Turn off heat sources that are not absolutely necessary.

(e) Remind workers to drink water in small 

amounts frequently to prevent excessive 

dehydration, to weigh themselves before 

and after the shift, and to be sure to drink 

enough water to maintain body weight.

(f) Monitor the environmental heat at the 

job sites and resting places.

(g) Check workers’ core temperature during 

their most severe heat-exposure period.

(h) Exercise additional caution on the first 

day of a shift change to make sure that 

workers are not overexposed to heat, 

because they may have lost some of 

their acclimatization over the weekend 

and during days off.

(i) Send workers who show signs of a heat 

disorder, even a minor one, for medical 

evaluation. Permission of the responsible healthcare provider to return to 

work must be given in writing.

(j) Restrict overtime work.

(k) Suspend piece-rate payment mechanisms.
82 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

6.3 Personal Protective 

Clothing and Auxiliary 

Body Cooling

When unacceptable levels of heat stress occur, 

there are generally only four approaches to a 

solution: (1) modify the work; (2) modify the 

environment; (3) modify the worker by heat 

acclimatization; or (4) modify the clothing 

or equipment. Improving human tolerance 

requires that the individuals be fully heat acclimated to the level of heat, be trained in the use 

of and practice in wearing protective clothing, 

be in good physical condition, and be encouraged to drink as much water as necessary (e.g., 

1 cup [8 oz.] of water or other fluids every 

15–20 minutes, or see Table 8-1) to compensate for sweat water loss.

The issues of clothing and its effects on heat 

exchange have been discussed in detail elsewhere in this document (see 3.3 Effects of 

Clothing on Heat Exchange). Wearing impermeable clothing (e.g., PPE) effectively prevents 

heat exchange (i.e., conduction, convection, 

radiation, and sweat evaporation) from the 

body to the external environment. Moreover, 

the additional metabolic energy required 

to wear and work in the clothing produces a 

greater amount of heat than would occur without the impermeable clothing adding to the 

heat burden. Unfortunately, the necessity of 

wearing an impermeable clothing ensemble 

suggests the need to protect against a significant environmental hazard that could injure 

or kill the worker. Since the PPE is required 

to protect against this hazard and yet produces its own heat hazard, there is a need to 

protect against the heat stress induced by the 

impermeable garment. The following sections 

discuss the various personal cooling technology used in conjunction with encapsulating 

and impermeable PPE to limit the heat stress 

imposed by these garments. 

It may be possible to redesign ventilation systems for occupied spaces to avoid interior 

humidity and temperature buildup; however, 

such redesign may not completely solve the 

heat stress problem. When air temperature is 

above 35°C (95°F) with an RH of 75% to 85%, 

or when there is an intense radiant heat source, 

a suitable (and in some ways more functional) 

approach is to modify the clothing to include 

some form of auxiliary body cooling. Even 

mobile individuals afoot can be provided some 

form of auxiliary cooling for limited periods of 

time. A properly designed system will reduce 

heat stress, conserve large amounts of drinking water that would otherwise be required, 

and allow unimpaired performance across a 

wide range of climatic factors. A seated individual will rarely require more than 100 W

(86 kcal∙h-1 or 344 Btu∙h-1) of auxiliary cooling, 

and the most active individuals, not more than 

400 W (345 kcal∙h-1 or 1380 Btu∙h-1), unless 

they work at a level where physical exhaustion would limit the duration of work. Some 

form of heat-protective clothing or equipment 

should be provided for exposures at heat stress 

levels that exceed the RAL or REL (Figures 8-1 

and 8-2).

Auxiliary cooling systems can range from 

simple approaches such as applying frozen 

materials under the clothing to more complex 

systems, such as cooled garments; however, 

cost of logistics and maintenance are considerations of varying magnitude in all of these systems. Four auxiliary cooling approaches have 

been evaluated: (1) water-cooled garments, (2) 

air-cooled garments, (3) cooling vests, and (4) 

wetted overgarments. Each of these auxiliary 

body cooling approaches might be applied in 
Occupational Exposure to Heat and Hot Environments 83

6 . Control of Heat Stress

alleviating risk of severe heat stress in a specific 

workplace setting [Goldman 1973, 1981].

6.3.1 Water-cooled Garments

Water-cooled garments have been designed 

and constructed in various forms with significant improvements on both engineering and 

physiological perspectives. Water-cooled garments provide cooling by means of conductive 

heat exchange between skin and coolant tubing 

sewn inside a garment in which a network 

of tubing is distributed onto either a whole 

body or limited body regions. Water-cooled 

garments also require an external device for 

operation, which may include battery, circulating pump, heat exchanger, fluid container, 

and control pad. The weight and volume of the 

operating device may limit a wearer’s movement and impose an extra weight burden; this 

factor, along with the nature of the work and 

environmental conditions, will determine the 

effective use time of the water-cooled garment. 

In addition, at water temperatures at or below 

the dew point, condensation around the tubes 

may increase heat loss from the skin through 

permeable clothing [Nag et al. 1998].

The range of cooling provided by each of the 

water-cooled garments versus the cooling 

water inlet temperature has been studied. The 

rate of increase in cooling, with decrease in 

cooling water inlet temperature, is 3.1 W·°C-1

for the water-cooled cap with water-cooled 

vest, 17.6 W·°C for the short water-cooled 

undergarment, and 25.8 W·°C-1 for the long 

water-cooled undergarments. A “comfortable” 

cooling water inlet temperature of 20°C (68°F) 

should provide 46 W of cooling with use of the 

water-cooled cap; 66 W with use of the watercooled vest; 112 W with use of the water-cooled 

cap and water-cooled vest together; 264 W 

with use of the short water-cooled undergarment; and 387 W with use of the long watercooled undergarment [Kim et al. 2011].

6.3.2 Air-cooled Garments

Air-cooled garments distribute cooling air 

next to the skin. The total heat exchange from 

completely sweat-wetted skin when cooling 

air is supplied to the air-cooled garment is a 

function of cooling air temperature and cooling airflow rate. Both the total heat exchanges 

and the cooling power increase with cooling 

airflow rate and decrease with increasing cooling air inlet temperature. For an air inlet temperature of 10°C (50°F) at 20% RH and a flow 

rate of 0.28 m3·min-1 (10 ft3·min-1), the total 

heat exchanges over the body surface would be 

233 W in a 29.4°C (84.9°F), 85% RH environment and 180 W in a 51.7°C (125.1°F), 

25% RH environment. Increasing the cooling 

air inlet temperature to 21°C (69.8°F) at 10% 

RH would reduce the total heat exchanges to 

148 W and 211 W, respectively. Either air inlet 

temperature easily provides 100 W of cooling.

The use of a vortex tube as a source of cooled 

air for body cooling is applicable in many hot 

workplace situations. The vortex tube, which 

is attached to the worker, requires a constant 

source of compressed air supplied through an 

air hose. The hose connecting the vortex tube 

to the compressed air source limits the space 

within which the worker can operate. However, 

unless mobility of the worker is required, the 

vortex tube, even though noisy, may be considered as a simple cooled air source.

6.3.3 Cooling Vests

Currently available cooling vests may contain 

as many as seventy-two cooling packs made of 

ice or phase-change materials; cooling packs 

may also vary in weight and size. These cooling 

packs are generally secured to the vest by tape, 

inserted into the vest pockets, or integrated 

with the vest, which requires freezing the whole 

vest before use. The cooling provided by each 

individual cooling pack will vary with time and 

with its contact pressure with the body surface, 
84 Occupational Exposure to Heat and Hot Environments

6 . Control of Heat Stress

plus any heating effect of the clothing and hot 

environment; thus, the environmental conditions have an effect on both the cooling provided and the duration of time this cooling is 

provided. In environments of 29.4°C (84.9°F) 

at 85% RH and 35.0°C (95°F) at 62% RH, a 

cooling vest can still provide some cooling for 

up to four hours of operation (about two to 

three hours of effective cooling is usually the 

case). However, in an environment of 51.7°C 

(125.1°F) at 25% RH, any benefit is negligible 

after about three hours of operation. With 60% 

of the cooling packs in place in the vest, the 

cooling provided may be negligible after two 

hours of operation. Since the cooling vest does 

not provide continuous and regulated cooling over an indefinite time period, exposure 

to a hot environment would require redressing with backup cooling packs every two to 

four hours. Replacing a cooling vest would 

have to be accomplished when an individual 

is not in a work situation. However, the cooling is supplied noise-free and independent of 

any energy source or umbilical cord that would 

limit a worker’s mobility. The greatest potential for the ice packet vest appears to be for 

work where other conditions limit the length 

of exposure, such as short-duration tasks and 

emergency repairs. The cooling vest is also relatively cheaper than other cooling approaches.

6.3.4. Wetted Overgarments

A wetted overgarment is a wetted cotton terry 

cloth coverall or a two-piece cotton cover that 

extends from just above the boots and from the 

wrists to a V-neck. When used with impermeable protective clothing, it can be a simple and 

effective auxiliary cooling garment.

Predicted values can be calculated to determine 

supplementary cooling and the minimal water 

requirements to maintain the cover wettedness 

in various combinations of air temperature, RH, 

and wind speed. Under environmental conditions of low humidity and high temperatures 

where evaporation of moisture from the wet 

cover garment is not restricted, this approach 

to auxiliary cooling can be effective, relatively 

simple, and inexpensive.

6.3.5 Practical Applications

and Limitations

All of these cooling systems have substantial 

limitations within a work setting. For example, 

phase-change systems are cheap, but their temperature cannot be controlled and they often 

do not stay cool long enough to be practical. 

Ice vests have the same limitation. Indeed, 

if the system is too cold, this will result in 

peripheral vasoconstriction, which reduces 

the heat transfer from the body to the environment. Water cooling systems require that the 

person be tethered to a system that circulates 

the cool water, which limits the person’s range 

of operation. Many of the systems are too 

heavy or too cumbersome to be practical in a 

work environment.

However, each system could be used during 

a rest period where the worker is not actively 

engaged in work. For example, fire fighters 

generally take frequent rest breaks because of 

the exhaustion of the air supply in their SCBA. 

During these rest breaks, active or passive 

cooling can be applied to supplement rehydration by removing their ensemble jacket 

and immersing their arms in cool water. This 

reduces body core temperature and allows for 

a more rapid “rehabilitation” during the rest 

break. 

Core body temperature (Tcore) changes relatively slowly, and simply stopping hard work 

will not result in an immediate decrease in Tcore. 

Therefore, increasing the rate of heat removal 

from the body would reduce the chances of 

heat injury. Using auxiliary cooling systems, 

such as those described previously, could 

reduce the time required to lower Tcore, lessen 
Occupational Exposure to Heat and Hot Environments 85

6 . Control of Heat Stress

the chances of heat injury, and enable a safer 

return to work.

6.3.6 Performance Degradation

A variety of options for auxiliary cooling to 

reduce or eliminate the level of heat stress 

under indoor and outdoor environmental 

conditions have been discussed. However, 

wearing protective clothing systems is also 

associated with decreases in workers’ performance. Performance decrements are associated 

with wearing encapsulating protective ensembles, even in the absence of any heat stress [Joy 

and Goldman 1968]. The majority of decrements result from mechanical barriers to sensory inputs to the wearer and from barriers to 

communication between individuals. Overall, 

it is clear that elimination of heat stress allows 

work to continue, but does not eliminate the 

constraints imposed by encapsulating protective clothing systems [Joy and Goldman 1968; 

Nag et al. 1998].
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Occupational Exposure to Heat and Hot Environments 87

7 Medical Monitoring

Employers should establish a medical monitoring program for workers with occupational 

exposure to hot environments. The medical 

monitoring program should include elements 

of medical screening (secondary prevention) 

and surveillance (primary prevention) with the 

goal of early identification of signs or symptoms that may be related to heat-related illness 

and prevention of adverse outcomes. Early 

detection of symptoms using medical screening, subsequent treatment, and workplace 

interventions are intended to minimize the 

adverse health effects of exposure to hot environments. Medical screening data may also be 

used for the purposes of medical surveillance 

to identify work areas, tasks, and processes that 

require additional prevention efforts. 

7.1 Worker Participation

Workers exposed to hot environments who 

should be included in a medical monitoring 

program and could receive the greatest benefit 

include the following:

■ Workers exposed to a hot environment 

above the RAL.

■ Workers with personal risk factors, including medical conditions and/or a history 

of heat-related illness, that put them at a 

higher risk of heat-related illness.

7.2 Program Oversight

The employer should assign responsibility for 

the medical monitoring program to a responsible 

healthcare provider. The responsible healthcare 

provider should be a qualified physician or other 

qualified health care professional (as determined 

by appropriate federal and state laws and regulations) who is informed and knowledgeable about 

the following:

■ Potential workplace exposures to heat 

and hot environments.

■ Administration and management of a 

medical monitoring program for occupational hazards.

■ Identification and management of heatrelated illnesses.

■ Where respiratory protection is being 

used, establishment of a respiratory 

protection program based on an understanding of the requirements of the OSHA 

respiratory protection standard and types 

of respiratory protection devices available 

at the workplace.

7.3 Medical Monitoring 

Program Elements

Recommended elements of a medical monitoring program for workers at risk for heat-related 

illnesses and injuries should include worker 

education, a preplacement medical evaluation, regularly scheduled or periodic follow-up 

medical evaluations, and reports of incidents 

of heat-related illnesses and injuries. The purpose of preplacement and periodic medical 

evaluations of persons working at a particular 

hot job is (1) to determine if the persons can 

meet the demands and stresses of the hot job, 

with reasonable assurance that the safety and 
88 Occupational Exposure to Heat and Hot Environments

7 . Medical Monitoring

health of the individuals and/or fellow workers 

will not be placed at risk; (2) to inquire whether 

persons have already suffered from an adverse 

health effect from heat stress exposure; and (3) 

to work with management to modify the job 

as necessary. Based on the findings from these 

evaluations, more frequent and detailed medical evaluation may be necessary.

7.3.1 Medical Evaluations

7.3.1.1 Preplacement Medical Evaluation

All new workers or workers who are transferring from jobs that do not involve exposure to 

heat should undergo a preplacement medical 

evaluation. Unless demonstrated otherwise, it 

should be assumed that these workers are not 

acclimatized to work in hot environments. In 

addition, greater physical demand and exertion in a hot environment can result in more 

stress on the cardiopulmonary system; therefore screening may need to be more thorough 

[Ramphal-Naley 2012].

(1) The responsible healthcare provider should 

obtain information including:

(a) A medical and surgical history that includes the cardiac, vascular, respiratory, 

neurologic, renal, hematologic, GI, and 

reproductive systems and information 

about dermatologic, endocrine, musculoskeletal, and metabolic conditions that 

might affect heat acclimatization or the 

ability to eliminate heat.

(b) A complete occupational history, including years of work in each job, the physical and chemical hazards encountered, 

the physical demands of these jobs, ability to use personal protective equipment, 

intensity and duration of heat exposure, 

and nonoccupational exposures to heat 

and strenuous activities. This history 

should identify episodes of heat-related 

disorders and evidence of successful 

adaptation to work in heat in previous 

jobs or nonoccupational activities.

(c) A list of all prescribed and over-the-counter medications used by the worker. The 

responsible healthcare provider should 

consider the possible impact of medications that may affect cardiac output, 

electrolyte balance, renal function, 

sweating capacity, or autonomic nervous 

system function. These include diuretics, 

antihypertensive drugs (e.g., atenolol, 

betaxolol), sedatives (e.g., barbiturates), 

antispasmodics, psychotropics, anticholinergics, and drugs that may alter the 

thirst (haloperidol) or sweating mechanism (e.g., phenothiazines, antihistamines, anticholinergics). See Table 4-2 

for additional information on proposed 

mechanisms of action of drugs implicated in intolerance of heat. 

(d) Information about relevant personal 

habits, including the use of substances 

such as alcohol and caffeine which could 

affect a person’s heat tolerance.

(2) The direct medical evaluation of the worker 

should include the following:

(a) Physical examination. At the discretion 

of the responsible healthcare provider, 

candidates who anticipate increased 

stress of physical activity of the job in a 

hot environment, those over 50 years of 

age or those younger than 50 years of age

with underlying cardiac risk factors may 

need to have additional testing (e.g., ECG 

with interpretation by a cardiologist).

(b) Clinical chemistry values, determined 

from urinalysis and any additional clinical chemistry tests deemed appropriate 

by the responsible healthcare provider.

(c) Blood pressure evaluation.
Occupational Exposure to Heat and Hot Environments 89

7 . Medical Monitoring

(d) Assessment of the ability of the worker 

to understand the health and safety hazards of the job, understand the required 

preventive measures, communicate with 

fellow workers, and have mobility and 

orientation capacities to respond properly to emergency situations.

(e) For workers who must wear respiratory 

protection or other PPE, a respiratory 

protection program should be provided. 

The responsible healthcare provider 

should assess the worker’s ability to tolerate the total heat stress of a job, which 

includes the metabolic burdens of wearing and using protective equipment.

(3) More detailed medical evaluation may be 

deemed appropriate on a case-by-case basis 

by the responsible healthcare professional. 

Communication between the program’s 

responsible healthcare provider and the 

worker’s private healthcare provider may 

be appropriate. 

The following are examples of findings 

on preplacement medical evaluation that 

may indicate the need for further medical evaluation:

(a) History of ischemic heart disease, valvular heart disease, cardiac arrhythmias, 

cardiomyopathy, congestive heart failure, congenital heart disease , current use 

of certain antihypertensive medications 

indicating the possibility of reduced 

(a) maximum cardiac output, obstructive or 

restrictive pulmonary disease, diabetes 

insipidus, diabetes mellitus, renal disease, or rhabdomyolysis. 

(b) The use of prescribed medications that 

might interfere with heat tolerance or 

acclimatization (See Table 4-2). An 

alternate therapeutic regimen may be 

available that would be less likely to 

compromise the worker’s ability to work 

in a hot environment.

(c) The use of antihypertensive medications

that might affect heat tolerance. It may 

be prudent to monitor blood electrolyte values of workers who follow a 

salt-restricted diet or who take diuretic 

medications that affect serum electrolyte 

levels, especially during the initial phase 

of acclimatization to heat stress. The use 

of β-blockers (e.g., atenolol) for the treatment of hypertension may also limit performance on the job.

(d) A history of skin disease, an injury to a 

large area of the skin, or an impairment 

of the sweating mechanism that might 

impair heat elimination via sweat evaporation from the skin, specific evaluation 

may be advisable. Some people have 

defective sweating mechanisms (anhidrosis) and therefore are heat intolerant.

(e) Obesity may interfere with heat tolerance 

(see Chapter 4). An obese individual 

may require special supervision during 

the acclimatization period. Obesity can 

be defined as a body mass index (BMI) 

≥ 30. Measure height and weight to calculate body mass index according to the 

following formula:

BMI = weight (in pounds) × 703 / 

[height (in inches)]2

(Additional information on BMI is available at http://www.cdc.gov/healthyweight/

assessing/bmi/adult_bmi/index.html)

7.3.1.2 Periodic Medical Evaluations

All workers in the medical monitoring program should be provided with annual followup medical evaluations at regular intervals and 

at other times as deemed appropriate for the 

individual worker by the responsible healthcare 
90 Occupational Exposure to Heat and Hot Environments

7 . Medical Monitoring

provider. Evaluations should be based on data 

gathered in the preplacement medical evaluation, ongoing work history, new or changing 

symptoms, and when heat exposure change in 

the workplace. Any worker with signs or symptoms of heat-related illness should be examined 

immediately and may require more frequent 

screening and extensive testing. Evaluations 

should include the following:

(1) An occupational and medical history 

update and a physical examination focused 

on the cardiovascular, respiratory, nervous, 

and musculoskeletal systems and the skin, 

performed annually.

(2) Consideration of specific medical tests 

when deemed appropriate by the responsible healthcare provider.

7.3.1.3 Written Reports of

Medical Findings

Following each medical evaluation, the responsible healthcare provider should give each worker 

a written report containing the following:

■ The results of any medical tests performed on the worker.

■ A medical opinion in plain language 

about any medical condition that would 

increase the worker’s risk of heatrelated illness.

■ Recommendations for limiting the worker’s exposure to heat or hot environments.

■ Recommendations for further evaluation and treatment of medical conditions 

detected or for accommodation.

Following each medical evaluation, the 

responsible healthcare provider should give 

the employer a written report specifying 

the following:

■ Occupationally pertinent results of the 

medical evaluation.

■ A medical opinion as to whether the 

worker has any medical conditions that 

would increase the health risk of exposure to heat in the work environment.

■ An estimate of the individual’s tolerance 

to withstand hot working conditions (see 

6.2.3 Enhancing Tolerance to Heat and 

6.2.5 Screening for Heat Intolerance).

■ An opinion as to whether the worker can 

perform the work required by the job 

(i.e., physical fitness for the job).

■ Recommendations for reducing the 

worker’s risk for heat-related illness, 

which may include use of cooling measures, accommodations or limitations 

related to work/rest schedules and/or 

workload, medical recommendations for 

worksite medical or physiologic monitoring, or reassignment to another job, 

as warranted.

■ A statement that the worker has been 

informed by the responsible healthcare 

provider of the results of the medical 

evaluation and any medical conditions 

that require further explanation or treatment. The worker is cleared to work 

in the hot environment so long as no 

adverse health effects occur.

Specific findings, test results, or diagnoses 

that have no bearing on the worker’s ability 

to work in heat or a hot environment should 

not be included in the report to the employer. 

Safeguards to protect the confidentiality of the 

worker’s medical records should be enforced 

in accordance with all applicable regulations 

and guidelines. 
Occupational Exposure to Heat and Hot Environments 91

7 . Medical Monitoring

7.4 Medical Surveillance—

Periodic Evaluation 

of Data

Incidents of heat-related illness should be 

reported on first aid logs and the OSHA Form 

300-A and OSHA Form 301 Incident Report, 

if required to keep these forms, while following OSHA recordkeeping regulations. For 

sites under MSHA jurisdiction, MSHA Form 

7000-1 may be required. The responsible 

healthcare provider should evaluate data from 

incident reporting systems to look for trends 

in type of injury or illness (e.g., heat-related illness) or job title to target interventions. 

To ensure that control practices provide 

adequate protection to workers in hot areas, 

data from the medical monitoring program 

should be evaluated periodically in a systematic manner. Such evaluations may detect 

repeated incidents on the job, episodes of heatrelated disorders, or frequent absences that 

could be related to heat and could be used as 

the basis for appropriate worksite interventions. Job-specific clustering of heat-related 

illnesses or injuries should be followed up by 

the responsible healthcare provider in collaboration with others responsible for occupational 

safety and health at the worksite (e.g., industrial hygienists).

7.5 Employer Actions

The employer should ensure that the responsible healthcare provider’s recommended 

restriction of a worker’s exposure to heat or a 

hot environment or other workplace hazards 

is followed and that the RAL is not exceeded 

without taking additional protective measures. 

Efforts to encourage worker participation 

in the medical monitoring program and to 

promptly report any symptoms to the responsible healthcare provider are important to the 

program’s success. Medical evaluations performed as part of the medical monitoring program should be provided by the employer at no 

cost to the participating workers. Where medical removal or job reassignment is indicated, 

the affected worker should not suffer loss of 

wages, benefits, or seniority. 

7.6 Considerations 

Regarding Reproduction

7.6.1 Pregnancy

The medical literature provides limited data 

on potential risks for pregnant women and 

fertile women with heavy work and/or added 

heat stress within the permissible limits (e.g., 

where tre does not exceed 38°C or 100.4°F; see 

Chapter 5). However, because the human data 

are limited and because research data from 

animal experimentation indicate the possibility of heat-related infertility and teratogenicity, 

a woman who is pregnant or who may potentially become pregnant should be informed 

by the responsible healthcare provider that 

absolute assurances of safety during the entire 

period of pregnancy cannot be provided. 

The worker should be advised to discuss this 

matter with her own healthcare provider and 

discuss necessary accommodations with her 

supervisor if recommended.

7.6.2 Fertility

Heat exposure has been associated with temporary infertility in both females and males, 

with the effects being more pronounced in 

males [Rachootin and Olsen 1983; Levine 

1984]. In a study examining the time to pregnancy associated with heat exposure, the 

time was significantly prolonged in a subgroup of male welders and bakers [Thonneau 

et al. 1997]. Sperm density, motility, and the 

percentage of normally shaped sperm can 

decrease significantly when the temperature 
92 Occupational Exposure to Heat and Hot Environments

7 . Medical Monitoring

of the groin is increased above a normal temperature [Procope 1965; Henderson et al. 1986; 

Mieusset et al. 1987; Jung and Schuppe 2007]. 

Available data are insufficient to ensure that 

the REL protects against such effects. Thus, the 

responsible healthcare provider should question workers exposed to high heat loads about 

their reproductive histories.

7.6.3 Teratogenicity

The consequences of hyperthermia during 

pregnancy depend on the temperature elevation, the duration, and the stage of fetal development at the occurrence [Edwards 2006]. The 

body of experimental evidence reviewed by 

Lary [1984] indicates that, in the nine species 

of warm-blooded animals studied, prenatal 

exposure of the pregnant females to hyperthermia may result in a high incidence of embryo 

deaths and in gross structural defects, especially of the head and central nervous system 

(CNS). An elevation of the body temperature of the pregnant female to 39.5°C to 43°C 

(103.1°F–109.4°F) during the first week or two 

of gestation (depending on the animal species) 

resulted in structural and functional maturation defects, especially of the CNS, although 

other embryonic developmental defects were 

also found. It appears that some basic developmental processes may be involved, but selective cell death and inhibition of mitosis at 

critical developmental periods may be primary 

factors. Hyperthermia in these experimental 

studies did not adversely affect the pregnant 

females, but did adversely affect developing 

embryos. The length of hyperthermia in the 

studies varied from 10 minutes a day over a 

2- to 3-week period to 24 hours a day for 1 

or 2 days.

Retrospective epidemiologic studies have associated hyperthermia lasting from a day or less 

up to a week or more during the first trimester of pregnancy with birth defects, especially 

defects in CNS development (e.g., anencephaly) [Lary 1984]. In addition, a hyperthermic 

episode during pregnancy can result in embryonic death, spontaneous abortion, growth 

retardation, and other defects of development 

[Edwards 2006]. However, some of the information on hyperthermia’s effects on a pregnancy stems from women with fevers, so it 

is difficult to determine whether defects are 

caused by metabolic changes in the mother 

due to the infection [Clarren et al. 1979; Pleet 

et al. 1981; Edwards 2006]. 

It is important to monitor every hour or so 

the body temperature of a pregnant worker 

exposed to total heat loads above the RAL/

REL, to ensure that the core body temperature 

does not exceed 39°C to 39.5°C (102°F–103°F) 

during the first trimester of pregnancy. 

Regardless of sex, heat stress exposures with 

core body temperatures of 39°C (102°F) or 

greater should be avoided.
Occupational Exposure to Heat and Hot Environments 93

8 Basis for the Recommended Standard

The research data and information that served 

as the basis for the recommended standard are 

derived from an evaluation of: (a) the available 

scientific literature; (b) the many new technologies available for assessing heat stress and 

strain that are currently available; (c) suggested 

procedures for predicting risk of incurring 

heat-related disorders, of potentially unsafe 

acts, and of deterioration of performance; (d) 

accepted methods for preventing and controlling heat stress; and (e) domestic and international standards and recommendations for 

establishing permissible heat-exposure limits. 

This chapter includes a discussion of considerations that provide the basis for or impact the 

NIOSH recommended criteria for this standard for work in hot environments. 

8.1 The NIOSH RALs 

and RELs 

The NIOSH RELs were developed to protect most healthy workers (i.e., those who are 

physically and medically fit for the level of 

activity required and wearing the conventional 

one-layer work clothing ensemble) exposed to 

environmental and metabolic heat from developing adverse heat-related health effects (i.e., 

maintain thermal equilibrium). The total heat 

exposure to unacclimatized workers should be 

controlled so that unprotected healthy workers are not exposed to combinations of metabolic and environmental heat greater than the 

applicable RALs; see Figure 8-1. The total heat 

exposure to acclimatized workers should be 

controlled so that unprotected healthy workers 

are not exposed to combinations of metabolic 

and environmental heat greater than the applicable RELs; see Figure 8-2.

The RALs and RELs are represented by the 

following equations, where M represents the 

metabolic rate in Watts (W):

RAL [°C-WBGT] = 59.9 − 14.1 log10 M [W]

REL [°C-WBGT] = 56.7 − 11.5 log10 M [W]

When working for shorter intervals, workers 

may be able to work in higher temperatures 

without adverse heat-related health effects. 

Therefore, RALs and RELs take work/rest 

schedules into consideration so are shown 

for 60, 45, 30, and 15 minute time periods of 

work; see Figures 8-1 and 8-2. Working for 

shorter time periods and taking appropriate 

rest breaks slows down the body’s heat accumulation [Dukes-Dobos and Henschel 1973]. 

See Tables 6-2 and 6-3 for examples of work/

rest schedules.

The NIOSH RALs and RELs were derived 

using a physiologically based approach originating from a study identifying the work 

driven zones and environmentally driven 

zones, and determining the Upper Limit of 

the Prescriptive Zone (ULPZ) of mine rescue 

personnel during a series of experiments 

[Lind 1963; Dukes-Dobos and Henschel 1973; 

NIOSH 1973, 1986a]. Within the Prescriptive 

Zone, 95% of individuals are able to thermoregulate given the environmental conditions. 
94 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

Values shown are for a “standard man” of 70 kg (154 lb) body weight and 1.8 m2 (19.4 ft2) body surface. The “standard man” is used to normalize the data from the variability found in human beings. 

Both men and women adapt well to heat exposure, and given the similar physiological ability to 

tolerate heat, there are no significant differences between the sexes. 

Sources: [Leithead and Lind 1964; Wyndham 1974; Ramsey 1975; Strydom 1975; ISO 1982a; Spaul and Greenleaf 1984; 

ACGIH 1985].

Figure 8-1. Recommended heat stress alert limits (RALs) for unacclimatized workers

100

113 45

104 40

95 35

86 30

77 25

68 20

200 300 400 500

400 800 1200 1600 2000

116 233 349 465 580

60 min./h.

45 min./h.

30 min./h.

15 min./h.

oF oC

kcal/h.

RAL

Btu/h.

Watts

Metabolic heat

Environmental heat—WBGT

x

x

x
Occupational Exposure to Heat and Hot Environments 95

8 . Basis for the Recommended Standard

Values shown are for a “standard man” of 70 kg (154 lb) body weight and 1.8 m2 (19.4 ft2) body 

surface. The “standard man” is used to normalize the data from the variability found in human 

beings. Both men and women adapt well to heat exposure, and given the similar physiological ability to tolerate heat, there are no significant differences between the sexes.

Sources: [Leithead and Lind 1964; Wyndham 1974; Ramsey 1975; Strydom 1975; ISO 1982a; Spaul and Greenleaf 1984; 

ACGIH 1985].

Figure 8-2. Recommended heat stress exposure limits (RELs) for acclimatized workers 

100

113 45

104 40

95 35

86 30

77 25

68 20

200 300 400 500

400 800 1200 1600 2000

116 233 349 465 580

60 min./h.

45 min./h.

30 min./h.

15 min./h.

oF oC

kcal/h.

REL

Btu/h.

Watts

Metabolic heat

Environmental heat—WBGT
96 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

Above the Prescriptive Zone fewer workers 

are able to thermoregulate and maintain heat 

balance, leading to rises in the core body temperature. The Effective Temperatures (i.e., the 

environmental conditions that provide the 

horizontal axis when plotting the Prescriptive 

Zone) reported by Lind and others were converted to WBGTs, which then became the 

vertical axis for the RALs and RELs [DukesDobos and Henschel 1973; NIOSH 1973, 

1986a; ACGIH 2009]. The use of WBGT satisfied a set of criteria for developing a heat 

stress index discussed in Dukes-Dobos and 

Henschel [1973]: (a) applicability should be 

proven in practice; (b) all important factors 

must be included; (c) the measurements and 

calculations required should be simple and 

should not require physiological monitoring; 

(d) the exposure limits should reflect the magnitude of human response; and (e) it should be 

applicable for setting limits to a wide range of 

work conditions. The NIOSH WBGT–based 

limits for workers are similar to those of other 

national and international standards (see 8.5 

Recommendations of U.S. Organizations and 

Agencies and 8.6 International and Foreign 

Standards and Recommendations). 

The NIOSH RALs and RELs were re-evaluated 

for this update. No new data were identified to 

use as the basis for updated RALs and RELs. 

Studies on unacclimatized and acclimatized 

individuals demonstrate the validity of the 

ULPZ at various metabolic rates and WBGTs 

[Belding and Kamon 1973; Lind 1970; Lind et 

al. 1970]. It was determined that the current 

RALs for unacclimatized workers and RELs 

for acclimatized workers are still protective 

for most workers. However, as there are many 

factors that can affect how an individual will 

respond to hot temperatures, the RALs and 

RELs may not be protective of everyone (see 

3.3 Effects of Clothing on Heat Exchange and 

4.1 Physiologic Responses to Heat). For this 

reason, when assessing the level of heat stress 

and heat strain in workers, professional judgement is extremely important. ACGIH [2009] 

summarizes the recent data relevant to occupational exposure limits. Some studies suggest 

that current OELs may be unnecessarily restrictive [Khogali 1983; Parikh et al. 1976; Rastogi 

et al. 1992; Tranter 1998]. Ongoing research 

will provide additional data to continue to 

evaluate the effectiveness of the RALs and 

RELs. For summarized information pertaining 

to the RALs and RELs drawn from all chapters, 

see 1.1 Workplace Limits and Surveillance.

The NIOSH ceiling limit recommendations 

for acclimatized and unacclimatized workers were also re-evaluated. Data and scientific 

evidence were not identified to continue to 

support the NIOSH ceiling limit recommendations. Acclimatized workers live and work 

in temperatures above the ceiling limit without 

adverse health effects. The decision to remove 

the NIOSH ceiling limit recommendations was 

supported by scientific peer reviewers of the 

publicly available draft NIOSH document.

8.2 Estimation of Risks

The ultimate objective of a recommended 

heat stress standard is to limit the risk of heatrelated illness or injury for workers. Risk estimation has become more sophisticated in 

recent years but still lacks accuracy. Earlier 

estimation techniques were usually qualitative 

or, at best, only semiquantitative.

It is generally estimated that 2/1000 workers are 

at risk for heat stress [Parsons 2003] and that 

some occupations (firefighting, agriculture, 

construction, forestry, mining, manufacturing) confer an even greater risk for occupational exposure to heat stress because of the 

high physical (metabolic) workloads involved, 

as well as exposure to hot environments and 

the necessity of wearing PPE [Davies et al. 

1976; Slappendel et al. 1993; Kirk and Sullman 
Occupational Exposure to Heat and Hot Environments 97

8 . Basis for the Recommended Standard

2001; Parsons 2003; Maeda et al. 2006; Xiang 

et al. 2014]. One of the early semiquantitative 

procedures for estimating the risk of adverse 

health effects under conditions of heat exposure was designed by Lee and Henschel [1963]. 

The procedure was based on the known laws of 

thermodynamics and heat exchange. Although 

designed for the “standard man” under a standard set of environmental and metabolic conditions, it incorporated correction factors for 

environmental, metabolic, and worker conditions that differed from standard conditions. 

A series of graphs was presented that could be 

used to semiquantitatively predict the percentage of exposed individuals (of different levels 

of physical fitness and age) likely to experience 

health or performance consequences under 

each of 15 different levels of total stress. Part 

of the difficulty with early attempts to develop 

procedures for estimating risk was the lack of 

sufficient reliable industry-experience data to 

validate the estimates.

Much empirical data have been gathered on 

the relationship between heat stress and strain 

(including death from heat stroke) in South 

Africa’s deep, hot mines. From laboratory data, 

a series of curves has been prepared to predict 

the probability of a worker’s body temperature 

reaching dangerous levels during work under 

various levels of heat stress [Wyndham and 

Heyns 1973; Stewart 1979]. Based on these 

data and epidemiologic data on heat stroke 

among miners, estimates of probabilities of 

reaching dangerously high rectal temperatures 

were made. If a core body temperature of 40°C 

(104°F) is accepted as the threshold temperature 

at which a worker is in imminent danger of fatal 

or irreversible heat stroke, then the estimated 

probability of reaching this body temperature 

is 10-6 for a worker exposed to an effective temperature (ET) of 34.6°C (94.3°F), 10-4 at 35.3°C 

(95.5°F), 10-2 at 35.8°C (96.4°F), and 10-0.5 at 

36.6°C (97.9°F). If a body temperature of 38.5 

to 39.0°C (101.3–102.2°F) is accepted as the 

critical temperature, then the ET at which the 

body temperature reaches these values can also 

be derived for 10-1 to 10-6 probabilities. These 

ET correlates were established for conditions 

with RH near 100%; whether they are equally 

valid for these same ET values for low humidity has not been determined. Probabilities of 

body temperature reaching designated levels 

at various ET values have also been presented 

for unacclimatized men [Wyndham and Heyns 

1973; Strydom 1975; Stewart 1979]. Although 

these estimates have proven to be useful in preventing heat casualties under the conditions 

of work and heat found in the South African 

mines, their application to other workplaces 

may not be warranted.

A World Health Organization (WHO) scientific group on health factors involved in working under conditions of heat stress concluded 

that “it is inadvisable for deep body temperature to exceed 38°C (100.4°F) in prolonged 

daily exposure to heavy work”. The value 

of 38°C (100.4°F) tre incorporates a modest 

safety margin, which is required because of 

the degree of accuracy with which the actual 

environmental and metabolic heat loads are 

assessed. In closely controlled conditions, the 

deep body temperature may be allowed to rise 

to 39°C (102.2°F)” [WHO 1969]. This does 

not mean that when a worker’s rectal temperature (tre) reaches 38°C (100.4°F) or even 39°C 

(102.2°F), the worker will necessarily become 

a heat casualty. The physiological response to 

heat stress is quite variable in the human population. In fact, it is well documented that many 

motivated non-elite distance runners complete 

marathon-style runs with tre ≥41°C (105.8°F). 

In addition, tre of 41.9°C (107.4°F) have been 

measured in soccer players without any physical symptoms or lasting sequelae, whereas 

there are cases in which heat stroke and death 

have occurred in individuals with body core 

temperatures less than 40°C (104°F) who ran 

less than 10 km under mild environmental 
98 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

conditions [Armstrong et al. 2007b; Taylor 

et al. 2008]. Thus, heat injury is determined 

by both core temperature and symptoms, 

rather than core temperature alone. Although 

some individuals may not suffer heat illnesses 

under these conditions, if the tre exceeds 38°C 

(100.4°F), then the risk of heat casualties in 

a population increases. Nonthermal contributions to heat injury, such as poor acclimatization, dehydration, alcohol consumption, 

previous heat injury, age, and drug use, must 

also be determined [Armstrong et al. 2007b; 

Taylor et al. 2008].

Some safety margin is also justified by the recent 

finding that the number of unsafe acts committed by a worker increases with an increase 

in heat stress [Ramsey et al. 1983]. The data, 

derived by using safety sampling techniques to 

measure unsafe behavior during work, showed 

an increase in unsafe behavioral acts with an 

increase in environmental temperature. The 

incidence was lowest at WBGTs of 17°C to 

23°C (62.6°F – 73.4°F), whereas a WBGT that 

exceeds 28°C (82°F) confers the greatest risk 

of heat stress [Armstrong et al. 2007b]. Unsafe 

behavior also increased as the level of physical 

work of the job increased [Ramsey et al. 1983].

8.3 Correlation between 

Exposure and Effects

The large amount of data published from controlled laboratory studies and from workplace 

heat stress studies upholds the generality that 

heat strain increases as heat stress increases. 

All heat stress/heat strain indices are based 

on this relationship. This generally holds for 

heat-acclimatized and heat-unacclimatized 

individuals, for women and men, for all age 

groups, and for individuals with different levels 

of physical performance capacity and heat tolerance. In each case, differences between individuals or between population groups in the 

extent of heat strain resulting from a given 

heat stress relate to differences in heat acclimatization and physical work capacity. The individual variability may be large; however, with 

extreme heat stress, the variability decreases as 

the limits of the body’s systems for physiologic 

regulation are reached. 

Sophisticated models are available to predict 

heat strain as a function of heat load and physical activity and are capable of being modified by 

a variety of confounding factors. These models 

range from graphic presentations of relationships to programs for handheld and desk calculators and computers [Witten 1980; Kamon 

and Ryan 1981; Bernard and Pourmoghani 

1999]. The strain factors that can be predicted 

for the average worker are heart rate, body 

and skin temperature, sweat production and 

evaporation, skin wettedness, tolerance time, 

productivity, and required rest allowance. 

Confounding factors include amount, fit, insulation, and moisture vapor permeability of the 

clothing worn, physical work capacity, body 

hydration, and heat acclimatization. From 

some of these models, it is possible to predict 

when and under what conditions the physiologic strain factors will reach or exceed values 

that are considered acceptable from the standpoint of health.

These models are useful in industry to predict 

when any combination of stress factors is likely 

to result in unacceptable levels of strain, which 

then would require introduction of control and 

correction procedures to reduce the stress. The 

regression of heat strain on heat stress is applicable to population groups, and, with the use 

of a 95% confidence interval, it can be applied 

as a modified form of risk prediction. However, 

because of the variability in the human physiological response to heat stress (metabolic 

and/or environmental), the current models do 

not provide information on the level of heat 

stress at which one worker in 10, in 1,000, or in 

10,000 will incur heat exhaustion, heat cramps, 

or heat stroke.
Occupational Exposure to Heat and Hot Environments 99

8 . Basis for the Recommended Standard

8.4 Physiologic Monitoring of 

Heat Strain

When the NIOSH Criteria for a Recommended 

Standard: Occupational Exposure to Hot 

Environments was first published in 1972 (and 

revised in 1986), physiologic monitoring was 

not considered a viable adjunct to the WBGT 

index, engineering controls, and work practices for the assessment and control of workplace heat stress. However, by the time of the 

1986 revision, it was proposed that monitoring body temperature and/or the work and 

recovery heart rate of workers exposed to 

environmental conditions in excess of the 

ACGIH TLVs® could be a safe and relatively 

simple approach [Fuller and Smith 1980, 1981; 

Siconolfi et al. 1985]. All the heat stress indices 

assume that most workers will not incur heatrelated illnesses or injuries if they are exposed 

to hot work conditions that do not exceed the 

permissible value. Inherent in this is the recognition that a small proportion of the exposed 

workers may develop heat-related illness. The 

ACGIH TLV® is intended to protect nearly 

all healthy heat-acclimatized workers at heat 

stress levels that do not exceed the TLV®.

Physiologic monitoring, such as heart rate and/

or core temperature, of heat-exposure workers 

could help protect all workers, including the 

heat-intolerant worker exposed at hot worksites. In one field study, the recovery heart rate 

was taken with the worker seated at the end of a 

cycle of work from 30 seconds to 1 minute (P1), 

1.5 to 2 minutes (P2), and 2.5 to 3 minutes (P3). 

Oral temperature was measured with a clinical thermometer inserted under the tongue for 

4 minutes. The data indicate that, 95% of the 

time, the oral temperature was below 37.5°C 

(99.5°F) when the P1 recovery heart rate was 

124 bpm or less, and 50% of the time the oral 

temperature was below 37.5°C (99.5°F) when 

the P1 was less than 145 bpm. From these relationships, a table was developed for assessing 

heat strain and suggested remedial actions. If 

the P3 heart rate is lower than 90 bpm, then 

the heat stress conditions are satisfactory; if 

the P3 approximates 90 bpm and/or the P1–P3

recovery is approximately 10 bpm, it indicates 

that the work level is high but there is little 

increase in body temperature; if P3 is greater 

than 90 bpm and/or P1–P3 is less than 10 bpm, 

it indicates a no-recovery pattern—that is, the 

heat stress exceeds acceptable levels and corrective actions should be taken to prevent heat 

injury or illness [Fuller and Smith 1980, 1981]. 

The corrective actions may be of several types 

(engineering, work practices, etc.). 

In spite of the aforementioned field study findings, recent studies have indicated that body 

heat is still stored for up to 60 minutes of rest 

after cessation of work. Although Tre decreases, 

muscle temperature remains elevated, probably because of sequestration of warm blood in 

the muscle tissue. Therefore, even in recovery, 

subjects are still under heat stress [Kenny et al. 

2008]. This fact must be taken into consideration when any corrective actions (engineering 

controls, administrative controls, or the use of 

PPE) are adopted.

Historically, obtaining recovery heart rates at 

1- or 2-hour intervals or at the end of several 

work cycles during the hottest part of the workday of the summer season presented logistical 

problems, but available advanced technologies allow many of these problems to be overcome. Wearable sensors, capable of continuous 

monitoring and recording of physiological 

responses, have been introduced to the market. 

Probably the most common example is the 

heart-rate-recording wristwatch, which is 

used by many joggers and enables continuous 

automated heart-rate measurements in real 

time that are accurate and reliable. The data 

obtained from the heart rate–recording wristwatches can also be stored, downloaded onto a 

computer, and analyzed at a later time. 
100 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

The single-use disposable digital oral thermometer, which is capable of measuring oral 

temperatures of workers at regular intervals, 

makes monitoring of body temperature possible under most workplace situations without 

interfering with the normal work pattern. It 

would not be necessary to interrupt work to 

insert the thermometer under the tongue and to 

remove it after 4 to 5 minutes. However, ingestion of fluids and mouth breathing would have 

to be controlled for about 15 minutes before an 

oral temperature is taken. Moreover, oral temperatures are not the most accurate indicator 

of body core temperature, and such measurements may not be practical for a worker who is 

nauseated or has already vomited. 

A more accurate technology, involving ingestible capsules (CorTemp® Ingestible Core Body 

Temperature Sensors, Palmetto, FL) capable 

of recording and telemetering intestinal “core” 

temperature on a continuous basis, has been 

in use by the research community for ~20 

years and may eventually be used occupationally. The problem with ingestible temperature 

sensing capsules is that they must be ingested 

the evening before use and function only 

until passed from the body during defecation. 

Another drawback is the cost of the capsules 

and monitoring equipment. 

Other sophisticated wearable physiological 

sensor systems (e.g., LifeShirt®, VivoMetrics, 

Ventura, CA) have been or are under development, and one system has been evaluated for 

its accuracy against standard physiological 

monitoring systems found in modern laboratories [Coca et al. 2010]. A new system, called 

the Zephyr BioHarness® (Zephyr Bioharness, 

British Columbia, Canada), has moved from 

the research arena to commercial application. 

This device monitors heart rate, respiratory 

rate, skin temperature, ECG, body position, 

vector magnitude, and R-R interval. (The R-R 

interval is the time between 2 QRS waves in the 

ECG, in which the R-wave segment of the QRS 

complex is usually of the greatest magnitude. 

The time between two R waves corresponds 

to the heart rate.) These systems and others in 

development may revolutionize the real-time 

monitoring of workers in occupations that put 

them at risk for heat injury.

The advantages of such automated systems are 

that data could be immediately observed and 

trends established, from which actions could 

be initiated to prevent excessive heat strain. 

Disadvantages are that it takes time to attach 

and remove the transducers at the start and end 

of each workday; the transducers for rectal or 

ear temperature, as well as stick-on electrodes 

or thermistors, are not acceptable for routine 

use by some people; and electronic components require careful maintenance for proper 

operation. Also, the telemetric signals are often 

disturbed by the electromagnetic fields that 

may be generated by the manufacturing process. However, devices recently appearing on 

the market have addressed many of these problems, thus leading the way for common use of 

wearable physiological monitoring systems in 

occupations that expose workers to risk for 

heat injury.

8.5 Recommendations of 

U.S. Organizations 

and Agencies

8.5.1 The American Conference 

of Governmental Industrial 

Hygienists (ACGIH)

The ACGIH TLV® for heat stress refers to heat 

stress conditions under which it is believed that 

nearly all workers may be repeatedly exposed 

without adverse health effects [ACGIH 2014]. 

The TLV® goal is to maintain core body temperature within +1°C of normal (37°C), although 

exceptions can be made under certain circumstances [ACGIH 2014]. ACGIH suggests using 

a decision-making tree to evaluate the risk of 
Occupational Exposure to Heat and Hot Environments 101

8 . Basis for the Recommended Standard

heat stress and strain to workers. The guidance is based on the premises that (1) workers are acclimatized, adequately hydrated, and 

unmedicated and (2) that a healthy worker can 

be repeatedly exposed without adverse health 

effects, and (3) Action Limit guidance is protective of unacclimatized workers. 

Those who are more tolerant of working in the 

heat and are under medical supervision may 

work under heat stress conditions that exceed 

the TLV®, but in no instance should the deep 

body temperature exceed the 38°C (100.4°F) 

limit for an extended period. However, acclimatized workers may be able to work safely 

under supervision with a core body temperature not to exceed 38.5°C (101.3°F). The TLV® 

permissible heat-exposure values incorporate both the environmental heat factors and 

metabolic heat production. The environmental 

factors are expressed as the WBGT. ACGIH 

provides instructions for adjusting the WBGT 

values on the basis of clothing type. The worker’s metabolic heat production is expressed as 

work-load category: light work = <180 kcal·h-1; 

moderate work = 180–300 kcal·h-1; heavy work 

= 300–415 kcal·h-1; and very heavy work = 

>520 kcal·h-1. 

Along with metabolic heat production, work 

demands need to be considered, with use of 

a table that includes WBGT values for 100% 

work, 75% work/25% rest, 50% work/50% rest, 

and 25% work/75% rest. If work demands vary 

or work and rest environments are different, a 

TWA should be calculated.

ACGIH provides additional guidance for limiting heat strain and for heat stress management. 

This guidance includes having general and 

job-specific controls in place and monitoring 

heart rate, core body temperature, heat stressrelated symptoms, profuse sweating rates, and 

weight loss.

8.5.2 Occupational Safety and Health 

Administration (OSHA)

Standards Advisory Committee on Heat 

Stress (SACHS)

In January 1973, the Assistant Secretary 

of Labor for OSHA appointed a Standards 

Advisory Committee on Heat Stress (SACHS) 

to conduct an in-depth review and evaluation of the NIOSH Criteria for a Recommended Standard: Occupational Exposure to Hot 

Environments and to develop a proposed standard that would establish work practices to 

minimize the effects of hot environmental conditions on working employees [Ramsey 1975]. 

The purpose of the standard was to minimize 

the risk of heat disorders and illnesses of workers exposed to hot environments so that the 

workers’ well-being and health would not be 

impaired. The 15 committee members represented worker, employer, state, federal, and 

professional groups.

The recommendations for a heat stress standard 

were derived by the SACHS by majority vote 

on each statement. Any statement that was disapproved by an overwhelming majority of the 

members was no longer considered for inclusion in the recommendations. The recommendations establish the threshold WBGT values 

for continuous exposure at three levels of physical work: light, <200 kcal·h-1 (<800 Btu·h-1), 

30°C (86°F); moderate, 200–300 kcal·h-1

(804–1200 Btu·h-1), 27.8°C (82°F); and heavy, 

>300 kcal·h-1 (>1200 Btu·h-1), 26.1°C (79°F), 

with low air velocities up to 1.52 m·s-1 (300 

fpm). These values are similar to the ACGIH 

TLVs®. When the air velocity exceeds 1.52 m·s-1

(300 fpm), the threshold WBGT values are 

increased 2.2°C (4°F) for light work and 2.8°C 

(5°F) for moderate and heavy work. The logic 

behind this recommendation was that the 

instruments used for measuring the WBGT 

index do not satisfactorily account for the 

increased heat removal when air velocity is 
102 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

increased beyond 1.52 m·s-1 (300 fpm). Kamon 

and Avellini [1979], presented data that did not 

support the assumption, because the clothing 

worn by the worker reduced the cooling effect 

of increased air velocity. However, under conditions in which heavy protective clothing or 

clothing with reduced air and/or vapor permeability is worn, higher air velocities may 

facilitate limited air penetration of the clothing 

and enhance convective and evaporative heat 

transfer. A modern WBGT with appropriate 

anemometry measurements could be used to 

take air velocity into account for this purpose.

The recommendations of the SACHS contained 

a list of work practices that were to be initiated 

whenever the environmental conditions and 

workload exceed the threshold WBGT values. 

The threshold WBGT values and work levels 

are based on a 120-minute TWA. Also included 

are directions for medical surveillance, training of workers, and workplace monitoring. The 

threshold WBGT values recommended by the 

OSHA SACHS were in substantial agreement 

with the ACGIH TLVs® and the ISO standard. 

However, the OSHA SACHS recommendations have not been promulgated in an OSHA 

heat stress standard. 

In 2011, OSHA and NIOSH cobranded an 

infosheet on protecting workers from heatrelated illness. This document included risk 

factors, health problems, first aid, and prevention. In May 2012, OSHA launched a 

nationwide education and outreach campaign (Water. Rest. Shade.) to educate workers and employers about the hazards of heat 

and preventing heat-related illnesses. OSHA 

worked with California OSHA (Cal/OSHA) 

and adapted many of that state’s campaign 

materials for national purposes. Many of these 

materials target at-risk populations and those 

with limited English proficiency. OSHA has 

also partnered with the National Oceanic and 

Atmospheric Administration (NOAA) on 

weather service alerts, requesting that worker 

safety precautions be included when extreme 

heat alerts are issued [OSHA-NIOSH 2011; 

OSHA 2012b]. A smart phone downloadable 

application was developed by OSHA to provide a way for employers or workers to calculate the heat index and view risk levels as well 

as protective measures [OSHA 2012a]. In 2014, 

the ongoing OSHA heat campaign also began 

to emphasize the importance of including 

acclimatization in heat illness prevention programs. OSHA has also been making efforts to 

reach out to state and local partners, national 

and local conferences, consultation programs, 

employers, trade associations, unions, community and faith-based organizations, consulates, universities, and health care and 

safety professionals.

8.5.2.1 Cal/OSHA

In 2005, the California Standards Board 

enacted emergency heat regulations based 

on a Cal/OSHA investigation. Cal/OSHA 

drafted the Heat Illness Prevention standard 

in collaboration with the Labor and Workforce 

Development Agency, worker and employer 

communities, the Standards Board, and other 

interested parties; and in 2006, the state of 

California adopted the heat stress standard 

[Wilson 2008]. The standard (Title 8, Chapter 

4, § 3395, Heat Illness Prevention) applies to all 

outdoor places of employment and addresses 

access to potable drinking water; access to 

shade; high heat procedures; and employee 

and supervisor training. Concerns over the 

Cal/OSHA standard have included a lack of 

heat stress threshold that accounts for humidity and lack of mandatory rest breaks.

In 2010, Cal/OSHA used a heat-related illness 

prevention campaign to target low-wage, non–

English speaking outdoor workers to reduce 

heat-related illnesses and fatalities. The campaign included media ads, radio spots, promotional items, posters, DVDs, postcards, training 

kits, and community and employer outreach 
Occupational Exposure to Heat and Hot Environments 103

8 . Basis for the Recommended Standard

and training. A subsequent evaluation of the 

campaign concluded that a sustained effort is 

needed in order to achieve long-term behavior change. Enforcement, as well as education, 

is important to producing an enduring impact 

and changing longstanding attitudes and cultural norms [Cal/OSHA 2010]. In addition, 

the report concluded that many immigrant 

workers are afraid of contacting government 

agencies about hazards at work, so Cal/OSHA 

is planning a hotline that will employ community sources to handle phone calls. Cal/OSHA 

launched another campaign in 2012 to prevent 

worker deaths and illnesses due to heat exposure in all outdoor workplaces in California.

8.5.3 American Industrial Hygiene 

Association (AIHA)

AIHA states that the best way to protect workers from the stresses of thermal environments 

is to help workers and supervisors understand 

the fundamentals of thermoregulation and 

exposure control [AIHA 2003]. AIHA’s The 

Occupational Environment: Its Evaluation, 

Control, and Management [AIHA 2003] contains a thorough overview of many of the heat 

exposure limits available, including WBGT 

recommendations, time-weighted averages, 

NIOSH recommendations, ACGIH TLVs®, 

and ISO recommendations. AIHA notes that 

the metabolic heat assumptions and WBGT 

threshold limit proposals are very similar: 

■ resting, 32°C–33°C; 

■ light, 30°C; 

■ moderate, 27°C–28°C;

■ heavy, 25°C–26°C; and

■ very heavy, 23°C–25°C. 

See Table 5-1. AIHA concludes that the WBGT 

threshold values are basically equivalent. 

8.5.4 The Armed Services

The 2003 publication entitled Heat Stress Control and Heat Casualty Management (TBMED 

507/AFPAM 48-52 (I)) addresses in detail 

the procedures for assessment, measurement, 

evaluation, and control of heat stress and the 

recognition, prevention, and treatment of heatrelated illnesses and injuries [DOD 2003]. The 

document may be applicable to many industrial- and outdoor worker–type settings. The 

WBGT index is used for measurement and 

assessment of the environmental heat load. 

The Navy Environmental Health Center [2007] 

developed a technical manual (NEHC-TMOEM 6260.6A) entitled Prevention and 

Treatment of Heat and Cold Stress Injuries. 

This document includes information on risk 

factors, hydration status, heat stress injuries, 

treatment, and follow-up. Like the aforementioned 2003 publication, this document uses 

the WBGT index.

In addition, both TBMED 507/AFPAM 48-52 

and NEHC-TM-OEM 6260.6A include examples of water intake tables based on WBGT, 

level of work, and number of minutes worked 

or the work/rest schedule (see Table 8-1).

8.5.5 American College of Sports 

Medicine (ACSM)

In 2007, the American College of Sports 

Medicine (ACSM) published a revised position statement, Exertional Heat Illness During 

Training and Competition [Armstrong et al. 

2007b]. To be competitive, the long distance 

runner must be in excellent physical condition, 

exceeding the physical fitness of most workers. 

For long distance races, such as marathons, 

the fastest competitors run at 12 to 15 miles 

per hour, which is classified as extremely hard 

physical work. When the thermal environment 

reaches even moderate levels, overheating can 

be a problem.
104 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

To reduce the risk of heat-related injuries and 

illnesses, the ACSM has prepared a list of recommendations to serve as advisory guidelines 

to be followed during distance running when 

the environmental heat load exceeds specific 

values. These recommendations include the 

following: (1) races of 10 km or longer should 

not be conducted when the WBGT exceeds 

28°C (82.4°F); (2) all summer events should 

be scheduled for early morning, ideally before 

8 a.m. or after 6 p.m.; (3) race sponsors must 

provide fluids; (4) runners should be encouraged to drink 300–360 mL of fluids 10 to 15 

minutes before the race; (5) fluid ingestion at 

frequent intervals during the race should be 

permitted, with water stations at 2- to 3-km 

intervals for races 10 km or longer, and runners should be encouraged to drink 100–200 

mL at each water station; (6) runners should 

be instructed on recognition of early signs and 

symptoms of heat-related illness; and (7) provisions should be made for the care of heatrelated illness cases.

In these recommendations, the WBGT is the 

heat stress index of choice. When the “red flag” 

high-risk WBGT index value reaches 23°C 

to 28°C (73.4°F–82.4°F) all runners must be 

made aware that heat injury is possible, and any 

person particularly sensitive to heat or humidity should be advised to not run. An “amber 

flag” indicates moderate risk with a WBGT of 

18°C to 23°C (64.4°F–73.4°F). It is assumed 

that the air temperature, humidity, and solar 

radiation are likely to increase during the day.

8.5.6 Washington State Department of 

Labor and Industries

In 2008, Washington State Department of 

Labor and Industries enacted the Outdoor 

Heat Exposure Rule, WAC 296-62-095. The 

rule applies to all employers with employees 

performing work in an outdoor environment 

from May 1 through September 30 annually. 

The rule also stipulates this is applicable only 

if employees are exposed to temperatures at 

or above 31.7°C (89°F), wearing double-layer 

woven clothes (e.g., coveralls, jackets, and 

sweatshirts) in temperatures at or above 25°C 

(77°F), or wearing nonbreathing clothes (e.g., 

vapor barrier clothing or PPE such as chemical resistant suits) in temperatures at or above 

11.1°C (52°F) [Washington State Legislature]. 

Table 8-1. Recommendations for fluid replacement during warm weather conditions

Easy work (250 W) Moderate work (425 W) Hard work (600 W)

WBGT

index (°F)

Work/Rest 

(min)

Water intake*

(qt·h-1)

Work/Rest 

(min)

Water intake 

(qt·h-1)

Work/Rest 

(min)

Water intake 

(qt·h-1)

78–81.9 Unlimited 0.5 Unlimited 0.75 40/20 0.75

82–84.9 Unlimited 0.5 50/10 0.75 30/30 1.0

85–87.9 Unlimited 0.75 40/20 0.75 30/30 1.0

88–89.9 Unlimited 0.75 30/30 0.75 20/40 1.0

90+ 50/10 1.0 20/40 1.0 10/50 1.0

*Fluid needs can vary on the basis of individual differences (± 0.25 qt·h-1) and exposure to full sun or full shade (± 0.25 qt·h-1). 

Fluid intake should not exceed 1.5 qt·h-1; daily fluid intake generally should not exceed 12 quarts. This is not to suggest limiting 

fluid intake by highly conditioned persons, who may require greater than 12 quarts daily.

Note: Rest = sitting or standing, in the shade if possible.

Adapted from DOD [2007].
Occupational Exposure to Heat and Hot Environments 105

8 . Basis for the Recommended Standard

These temperature limits were based on 

prevalent dew point temperatures across 

Washington, which are lower than many other 

U.S. locations, so these limits might not be generalizable to other areas of the United States.

The Outdoor Heat Exposure Rule states that an 

outdoor heat exposure safety program must be 

addressed in the employer’s written accident 

prevention program. Employers must also 

encourage their workers to drink water or other 

acceptable beverages and must provide at least 

1 quart of water per hour for each employee. 

Employers must also relieve from duty any 

workers showing signs or symptoms of heatrelated illness and provide a sufficient means 

to reduce their body temperature. The rule also 

states the necessity of providing appropriate 

training to workers before they begin work in 

excessive heat, as well as the need for appropriate training of supervisors.

8.5.7 Mine Safety and Health 

Administration (MSHA)

In 1976, MSHA published an Information 

Report, Heat Stress in Hot U.S. Mines and 

Criteria for Standards for Mining in Hot Environments [MSHA 1976]. MSHA also published a 

revised Safety Manual Number 6, Heat Stress 

in Mining, in 2001 [MSHA 2001]. MSHA notes 

a hot worksite can be defined as a WBGT 

exceeding 79°F (26°C). Heat problems can be 

reduced in mines by using administrative and 

engineering controls, advisable work practices, 

appropriate clothing, and PPE. The following 

work practices are recommended: 

■ Increasing workers’ heat tolerance by heat 

acclimatization, and by increasing their 

physical fitness.

■ A work/rest schedule with 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 

drinking water (50°F–60°F [10°C–15°C]), 

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 1 tablespoon salt to 15 quarts 

of water).

MSHA outlines two different acclimatization 

schedules based on whether the miner is new 

or has acclimatized and been on leave for 9 or 

more consecutive days (see Table 8-2).

Mine supervisors and workers should know 

the signs and symptoms of heat-related illness, 

how to administer first aid, and how to reduce 

heat stress. In addition, new miners should be 

Table 8-2. MSHA acclimatization schedules

Day on 

the job

Time spent working 

in hot site

1st 50%

2nd 60%

3rd 70%

4th 80%

5th 90%

6th 100%

If the acclimatized worker is returning from 

9 or more consecutive days of leave:

1st 50%

2nd 60%

3rd 90%

4th 100%

Adapted from MSHA [1976, 2001].
106 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

warned against trying to keep up with their 

acclimatized co-workers. In a medical examination for all workers assigned to a hot job for 

the first time, the physician should examine 

the heart, blood vessels, kidneys, liver, glands 

of internal secretion, respiratory system, and 

skin. Proper mine planning, ventilation, and 

air conditioning may reduce heat stress; and 

canopies can reduce solar heat and shielding 

can control radiant heat. Covered ditches and 

insulated piping can move hot ground water to 

the surface. Efficient blasting procedures will 

reduce additional heat into the air.

8.6 International and 

Foreign Standards and 

Recommendations

International organizations and national organizations outside of the United States have 

developed and published standards, recommendations, and guidelines for limiting the 

exposure of workers to potentially harmful 

levels of occupational heat stress. These documents range from official international and 

national position standards to unofficial suggested practices and procedures and to unofficially sanctioned guidelines proposed by 

institutions, research groups, or individuals 

concerned with the health and safety of workers under conditions of high heat load. Most 

of these guidance documents have in common 

the use of (1) the WBGT as the index for 

expressing the environmental heat load and 

(2) some method for estimating and expressing the metabolic heat production. The permissible total heat load is then expressed as 

a WBGT value for all levels of physical work, 

ranging from resting to very heavy work.

8.6.1 The International Organization 

for Standardization (ISO)

As of 2012, the International Organization 

for Standardization (ISO) has members from 

164 countries that develop standards using a 

consensus-based approach. ISO standards are 

developed through a multi-stakeholder process with technical committees created from 

industry experts, consumer associations, academia, non-government organizations, and 

governments [ISO 2012].

8.6.1.1 ISO 7243

In 1989, the ISO revised ISO 7243: Hot Environments—Estimation of Heat Stress on Working 

Man, Based on the WBGT-Index (Wet Bulb 

Globe Temperature) [ISO 1989]. ISO 7243 

can be used to assess a hot environment with 

a simple method based on the WBGT. It can 

easily be used in a workplace environment 

for evaluating the stresses on an individual 

[ISO 1989].

The ISO standard index values are based on the 

assumption, as are most other recommended 

heat stress limit values, that the worker is a 

normal, healthy individual, physically fit for 

the level of activity being done, and wearing 

standard summer-weight work clothing with a 

thermal insulation value of about 0.6 clo (not 

including the still-air-layer insulation).

The environmental measurements specified in 

the ISO standard for calculation of the WBGT 

are (1) air temperature, (2) natural wet bulb 

temperature, and (3) black globe temperature. 

From these, WBGT index values can be calculated or can be obtained as a direct integrated 

reading with some types of environmental 

measuring instruments. The measurements 

must, of course, be made at the place and time 

of the worker’s exposure.

8.6.1.2 ISO 7933

ISO 7933, entitled Ergonomics of the Thermal 

Environment: Analytical Determination and 

Interpretation of Heat Stress Using Calculation 

of the Predicted Heat Strain, describes a 

method for predicting the sweat rate and the 
Occupational Exposure to Heat and Hot Environments 107

8 . Basis for the Recommended Standard

internal core temperature that the human body 

will develop in response to the working conditions [ISO 2004b]. The main objectives of 

ISO 7933:2004 include (1) the evaluation of 

the thermal stress in conditions likely to lead 

to excessive core temperature increase or water 

loss for the standard subject and (2) the determination of exposure times with which the 

physiological strain is acceptable (no physical 

damage is to be expected).

8.6.1.3 ISO 8996

ISO 8996, entitled Ergonomics of the Thermal 

Environment: Determination of Metabolic Heat,

last revised in 2004, specifies methods for 

determining the metabolic rate in a working 

environment, assessing working practices, 

and determining the energetic cost of a job or 

activity [ISO 2004c].

8.6.1.4 ISO 9886

ISO 9886, entitled Ergonomics: Evaluation of 

Thermal Strain by Physiological Measurements, 

describes methods for measuring the physiological strain on humans by considering four 

parameters [ISO 2004a]. ISO 9886 provides 

the principles and practical guidance for measuring body core temperature, skin temperatures, heart rate, and body mass loss.

8.6.1.5 ISO 9920

ISO 9920, entitled Ergonomics of the Thermal 

Environment: Estimation of Thermal Insulation 

and Water Vapour Resistance of a Clothing 

Ensemble, specifies methods for estimating the 

thermal characteristics (resistance to dry heat 

loss and evaporative heat loss) for a clothing 

ensemble on the basis of values for known garments, ensembles, and textiles [ISO 2007].

8.6.2 Canada

The Canadian Centre for Occupational Health 

and Safety uses two types of exposure limits: 

occupational exposure limits to protect industrial workers and thermal comfort limits to 

protect office workers. Some Canadian jurisdictions have adopted ACGIH TLVs® as occupational exposure limits and others use them 

as guidelines to control heat stress in the workplace. Thermal comfort limits are set by the 

CSA Standard CAN/CSA Z412-00 (R2005), 

Office Ergonomics, which gives acceptable 

ranges of temperature and RH for offices 

[Canadian Centre for Occupational Health 

and Safety 2011]. In addition to the standards, 

Health Canada, concerned with the changing 

climate and longer, more intense heat events, 

has been developing extreme heat event–

related materials to educate and raise awareness among workers and the general public. 

8.6.3 Japan

In Japan, the Society for Occupational Health 

decides the heat and cold stress threshold limit 

values, and the thermal standard for offices is 

decided by the Ministry of Health, Labor and 

Welfare [Tanaka 2007]. These standards are 

based on acclimatized, healthy male workers 

who wore normal working clothes for summer 

and drank adequate salt water (salt concentration of around 0.1%). The working period was 

either continuous for 1 hour or intermittent 

for 2 hours (See Table 8-3). 
108 Occupational Exposure to Heat and Hot Environments

8 . Basis for the Recommended Standard

Table 8-3. Occupational exposure limits for heat stress in Japan

OELs

Workload WBGT (°C) CET† (°C)

RMR* –1 (Very light, –130 kcal·h-1) 32.5 31.6

RMR –2 (Light, –190 kcal·h-1) 30.5 30.0

RMR –3 (Moderate, –250 kcal·h-1) 29.0 28.8

RMR –4 (Moderate, –310 kcal·h-1) 27.5 27.6

RMR –5 (Heavy, –370 kcal·h-1) 26.5 27.0

*

Relative Metabolic Rate (RMR) = (Metabolic energy expenditure during work—Metabolic energy expenditure at rest)/Basal 

metabolic rate corresponding to the work period.

†

Corrected effective temperature.

Adapted from Japan Society for Occupational Health [2005] and Tanaka [2007].
Occupational Exposure to Heat and Hot Environments 109

9 Indices for Assessing Heat Stress and Strain

During the past 75 years, several indices have 

been developed for assessing and/or predicting the level of heat stress and/or strain that 

a worker might experience when working at 

hot jobs. Some are based on the measurements 

of a single environmental factor (wet bulb), 

whereas others incorporate all of the important environmental factors (dry bulb, wet bulb, 

and mean radiant temperatures and air velocity). For all of the indices, either the level of 

metabolic heat production is directly incorporated into the index or the acceptable level of 

index values varies as a function of metabolic 

heat production.

To have workplace application, an index must 

meet the following minimum criteria:

■ Feasibility and accuracy must be proven 

with use.

■ All important factors (environmental, 

metabolic, clothing, physical condition, 

etc.) must be considered.

■ Required measurements and calculations 

must be simple.

■ The measuring instruments and techniques applied should result in data that 

accurately reflect the worker’s exposure 

but do not interfere with the worker’s performance.

■ Index exposure limits must be supported 

by corresponding physiologic and/or 

psychological responses that reflect an 

increased risk to safety and health.

■ The index must be applicable for setting 

limits under a wide range of environmental and metabolic conditions.

The measurements required, advantages, disadvantages, and applicability to routine workplace use of some of the more frequently used 

heat stress/heat strain indices will be discussed 

under the following categories: (1) Direct 

Indices, (2) Rational Indices, (3) Empirical 

Indices, and (4) Physiological Monitoring.

9.1 Direct Indices

9.1.1 Dry Bulb Temperature

The dry bulb temperature (ta) is commonly 

used for estimating comfort conditions for 

sedentary people wearing conventional indoor 

clothing (1.4 clo, including the surface air 

layer). With light air movement and RH of 

20% to 60%, air temperatures of 22°C to 25.5°C 

(71.6°F–77.9°F) are considered comfortable by 

most people. If work intensity is increased to 

moderate or heavy work, the comfort air temperature is decreased about 1.7°C (3°F) for 

each 25 kcal (100 Btu or 29 W) increase in the 

hourly metabolic heat production. Conversely, 

if the air temperature and/or the metabolic 

heat production is progressively increased 

above the comfort zone, then the level of heat 

stress and heat strain will increase.

Dry bulb temperature is easily measured, but 

its use when the temperature is above the comfort zone is not justified, except for work situations where the worker is wearing completely 

vapor- and air-impermeable encapsulating 
110 Occupational Exposure to Heat and Hot Environments

9 . Indices for Assessing Heat Stress and Strain

protective clothing. Even under these conditions, appropriate adjustments must be made 

when significant solar and long wave radiation 

are present [Goldman 1981].

9.1.2 Wet Bulb Temperature

The psychrometric wet bulb temperature (tpwb) 

may be an appropriate index for assessing 

heat stress and predicting heat strain under 

conditions where radiant temperature and air 

velocity are not large factors and where tpwb

approximates ta (high humidities). For normally clothed individuals at low air velocities, 

a wet bulb temperature of about 30°C (86°F) 

is the upper limit for unimpaired performance 

on sedentary tasks and 28°C (82.4°F is the 

upper limit) for moderate levels of physical 

work. As tpwb increases above these threshold 

values, performance deteriorates and accidents 

increase. The wet bulb temperatures under 

these hot, humid conditions have been used to 

predict risk of heat stroke occurring in South 

African and German mines [Stewart 1979].

Wet bulb temperature is easy to measure in 

industry with a sling or aspirated psychrometer, and it should be applicable in any hot, 

humid situation where tpwb approaches skin 

temperature, radiant heat load is minimal, and 

air velocity is light.

9.2 Rational Indices

9.2.1 Operative Temperature

The operative temperature (to) expresses the 

heat exchange between a worker and the environment by radiation and convection in a 

uniform environment as it would occur in an 

actual workplace environment. The to can be 

derived from the heat-balance equation where 

the combined convection and radiation coefficient is defined as the weighted sum of the radiation and convection heat-transfer coefficients, 

and it can be used directly to calculate heat 

exchange by radiation and convection. The to

considers the intrinsic thermal efficiency of the 

clothing. Skin temperature must be measured 

or assumed. The to presents several difficulties. For convective heat exchange, a measure 

of air velocity is necessary. Not included are 

the important factors of humidity and metabolic heat production. These omissions make 

its applicability to routine workplace use somewhat limited.

9.2.2 Belding-Hatch Heat Stress Index

The Belding and Hatch Heat Stress Index (HSI) 

[Belding and Hatch 1955] has had wide use in 

laboratory and field studies of heat stress. One 

of its most useful features is the table of physiological and psychological consequences of 

an 8-hour exposure at a range of HSI values. 

The HSI is essentially a derivation of the heatbalance equation that includes the environmental and metabolic factors. It is the ratio 

(times 100) of the amount of body heat that is 

required to be lost to the environment by evaporation for thermal equilibrium (Ereq), divided 

by the maximum amount of sweat evaporation allowed through the clothing system that 

can be accepted by the environment (Emax). It 

assumes that a sweat rate of about one liter per 

hour over an 8-hour day can be achieved by 

the average, healthy worker without harmful 

effects. This assumption, however, lacks epidemiologic proof. In fact, data indicate that a 

permissible eight liters per 8-hour day of sweat 

production is too high and, as the 8-hour sweat 

production exceeds five liters, workers will 

dehydrate more than 1.5% of the body weight, 

thereby increasing the risk of heat-related illness and injuries. The graphic solution of the 

HSI that has been developed assumes a 35°C 

(95°F) skin temperature and a conventional 

long-sleeved shirt and trouser ensemble. 

The worker is assumed to be in good health 

and acclimatized to the average level of daily 

heat exposure.
Occupational Exposure to Heat and Hot Environments 111

9 . Indices for Assessing Heat Stress and Strain

The HSI is not applicable for very high heat 

stress conditions. It also does not correctly 

differentiate between the heat stress resulting 

from hot dry or hot humid conditions. The 

heat stress resulting from metabolic vs. environmental heat is not differentiated. Because 

Ereq/Emax is a ratio, the absolute values of the 

two factors are not addressed, that is, the ratio 

for an Ereg and Emax of 300 or 500 or 1,000 each 

would be the same (100); yet the heat stress 

would be expected to be greater at the higher 

Ereq and Emax values. Therefore, if one uses a 

ratio of the evaporation required for thermoregulation to the maximal evaporation that can 

be achieved by the person, the ratio does not 

take into consideration the actual increase in 

heat stress imposed with a greater requirement 

for evaporation required for thermoregulation.

The environmental measurements require 

data on air velocity which provide, at best, an 

approximation under industrial work situations; in addition, ta, twb, and tr must be measured. Metabolic heat production must also be 

measured or estimated. The measurements are, 

therefore, difficult and/or time-consuming, 

which limits the application of the HSI as a 

field monitoring technique.

The heat transfer coefficients used in the original HSI have been revised by McKarns and 

Brief as a result of observations on clothed 

subjects [McKarns and Brief 1966]. Their 

modification of the HSI nomograph facilitates 

the practical use of the index, particularly for 

the analysis of factors contributing to the heat 

stress. The McKarns and Brief modification 

also permits the calculation of allowable exposure time and rest allowances at different combinations of environmental and metabolic heat 

loads; however, the accuracy of these calculations is affected by the limitations of the index 

mentioned above. HSI programs for programmable handheld calculators are available.

9.2.3 Skin Wettedness (%SWA)

Several of the rational heat stress indices are 

based on the concept that, in addition to the 

sweat production required for temperature 

equilibrium (Ereq) and the maximum amount 

of sweat that can be evaporated (Emax), the efficiency of sweat evaporation will also affect heat 

stress. The less efficient the evaporation, the 

greater will be the body surface area that has to 

be wetted with sweat to maintain the required 

evaporative heat transfer; the ratio of wetted 

to nonwetted skin area times 100% (SWA =

Ereq/Emax).

This concept of wettedness gives new meaning to the Ereq/Emax ratio as an indicator of 

heat strain under conditions of high humidity 

and low air movement where evaporation is 

restricted [Goldman 1973, 1978; Gonzalez et al. 

1978; Candas et al. 1979; Kamon and Avellini 

1979; ISO 1982b]. The skin wettedness indices 

consider the variables basic to heat balance (air 

temperature, humidity, air movement, radiative heat, metabolic heat, and clothing characteristics) and require that these variables 

be measured or calculated for each workplace 

situation where an index will be applied. These 

measurements require exacting and time-consuming procedures. In addition, wind speed 

at the worksite is difficult to measure with any 

degree of reliability; at best, it can generally be 

only an approximation. These indices are satisfactory as a basis for calculating the magnitude 

of heat stress and strain and for recommending 

engineering and work practice controls; however, as procedures for routine environmental 

monitoring, they are too complicated, require 

considerable recording equipment, and are 

time-consuming.

9.3 Empirical Indices

Some of the earlier and most widely used heat 

stress indices are those based upon objective 
112 Occupational Exposure to Heat and Hot Environments

9 . Indices for Assessing Heat Stress and Strain

and subjective strain response data obtained 

from individuals and groups of individuals exposed to various levels and combinations of environmental and metabolic heat 

stress factors.

9.3.1 The Effective Temperature 

(ET, CET, ET*, and P4SR)

The effective temperature (ET) index is the 

first and, until recently, the most widely used 

of the heat stress indices. The ET combines dry 

bulb and wet bulb temperatures and air velocity. In a later version of the ET, the Corrected 

Effective Temperature (CET), the black globe 

temperature (tg) is used instead of ta to take 

the effect of radiant heating into account. The 

index values for both the ET and the CET were 

derived from subjective impressions of equivalent heat loads between a reference chamber 

at 100% humidity and low air motion and an 

exposure chamber where the temperature 

and air motion were higher and the humidity 

lower. Recently a newer version of the effective 

temperature (ET*) was developed that uses 

50% reference RH rather than 100% reference 

RH used for the ET and CET. The ET* has all 

the limitations of the rational heat stress indices mentioned previously; however, it is useful 

for calculating ventilation or air-conditioning 

requirements for maintaining acceptable conditions in buildings.

The ET and CET have been used in studies 

of physical, psychomotor, and mental performance changes as a result of heat stress. In general, performance and productivity decrease as 

the ET or CET exceeds about 30°C (86°F). The 

WHO has recommended that for heat-unacclimatized individuals ET or CET values that 

exceed 30°C (86°F) for sedentary activities, 

28°C (82.4°F) for moderate work, and 25.5°C 

(79.7°F) for hard work are unacceptable. For 

fully heat-acclimatized individuals, the recommended limits are increased about 2°C (3.5°F).

The data on which the original ET was based 

came from studies on sedentary subjects 

exposed to several combinations of ta, tpwb, 

and Va, all of which approximated or slightly 

exceeded comfort conditions. The responses 

measured were subjective impressions of comfort or equal sensations of heat, which may or 

may not be directly related to values of physiologic or psychologic strain. In addition, the 

sensations were the responses to transient 

changes. The extrapolation of the data to various amounts of metabolic heat production has 

been based on workplace experience. The ET 

and CET have been criticized because they 

seem to overestimate the effects of high humidity and underestimate the effects of air motion 

and thus tend to overestimate the heat stress.

In the hot, humid mines of South Africa, heatacclimatized workers doing hard physical work 

showed a decrease in productivity beginning at 

ET of 27.7°C (81.9°F) (at 100% RH with minimal air motion), which is approximately the 

reported threshold for the onset of fatal heat 

stroke during hard work [Wyndham 1974; 

Strydom 1975]. These observations lend credence to the usefulness of the ET or CET as 

a heat stress index in mines and other places 

where the humidity is high and the radiant 

heat load is low.

The limitations of ET have led to the development of the concept of the four-hour sweat 

rate (P4SR). The P4SR was developed in environmental chambers at the National Hospital 

for Nervous Diseases in London and formally evaluated in Singapore for seven years 

[Macpherson 1960; Parsons 2003]. The P4SR 

is the approximate amount of sweat secreted 

by presumed healthy young men acclimatized 

to a particular environment for a duration of 

4 hours. This value is used as an index value 

of sweating but not as a predictor of the specific amount of sweat produced by a group of 

subjects. The P4SR is, therefore, an empirical 
Occupational Exposure to Heat and Hot Environments 113

9 . Indices for Assessing Heat Stress and Strain

index or measure that can be obtained by the 

following steps:

(1) Collect twb, tg, and ta.

(2) If tg ≠ ta, then increase the wet bulb temperature by 0.4 (tg – ta) °C.

(3) If the metabolic rate M is >63 Wm-2, then 

increase the wet bulb temperature by the 

amount indicated in a special nomogram.

(4) If the subjects are clothed, increase the wet 

bulb temperature by 1.5 Iclo (°C).

From these empirical measures, the basic 

four-hour sweat rate (B4SR) can be determined from a nomogram developed from this 

analysis. From this, the nomogram is used to 

calculate P4SR. (Nomogram can be found in 

Parsons [2003], Figure 10.4.)

Since it was determined that the sweat rate outside the prescriptive zone was not an adequate 

indicator of heat strain, the P4SR has been 

used to make adjustments to account for this 

inadequate level of prediction of heat strain. 

Although the P4SR is useful for defined conditions, its applicability is limited in many workplace settings since the effects of clothing on 

heat stress are oversimplified. Therefore, the 

P4SR is most useful as a heat storage index 

[Parsons 2003].

9.3.2 The Wet Bulb Globe 

Temperature (WBGT)

The WBGT index was developed in 1957 as a 

basis for environmental heat stress monitoring 

to control heat casualties at military training 

camps. Its advantages are that the measurements are few and easy to make; the instrumentation is simple, relatively inexpensive, 

and rugged; and the calculations of the index 

are straightforward. The data obtained from 

the WBGT can be collected as a continuous 

recording by a Squirrel data logging system 

(Grant Instruments, Ltd., Cambridgeshire, 

UK) [Åstrand et al. 2003]. For indoor use, only 

two measurements are needed: natural wet 

bulb and black globe temperatures (dry heat). 

For outdoors in sunshine, the air temperature 

must also be measured. 

The calculation of the WBGT for indoors is

WBGT = 0.7tnwb + 0.3tg

and the calculation of the WBGT for outdoors is

WBGT = 0.7tnwb + 0.2tg + 0.1ta.

The WBGT combines the effect of humidity 

and air movement (in tnwb), air temperature 

and radiation (in tg), and air temperature (ta) 

as a factor in outdoor situations in the presence 

of sunshine. If there is no radiant heat load (no 

sunshine), then the tg reflects the effects of air 

velocity and air temperature. Commercially 

available WBGT measuring instruments give 

ta, tnwb, and tg separately or as an integrated 

WBGT in a form for digital readouts. A printer 

can be attached to provide tape printouts at 

selected time intervals for WBGT, ta, tnwb, Va, 

and tg values.

The application of the WBGT index for determining training schedules for military recruits 

during the summer season resulted in a striking reduction in heat casualties [Minard 1961]. 

This dramatic control of heat casualty incidence stimulated its application to hot workplace situations.

In 1972, the first NIOSH Criteria for 

a Recommended Standard: Occupational Exposure to Hot Environments [NIOSH 1972] 

recommended the use of the WBGT index 

for monitoring workplace heat stress. The 

rationale for choosing the WBGT and the 

basis for the recommended guideline values 

was described in 1973 [Dukes-Dobos and 

Henschel 1973]. The WBGT was used as the 

index for expressing environmental heat load 

in the ACGIH TLVs® for heat stress, adopted 
114 Occupational Exposure to Heat and Hot Environments

9 . Indices for Assessing Heat Stress and Strain

in 1974 [ACGIH 1985]. Since then, the WBGT 

has become the index most frequently used 

and recommended for use throughout the 

world, including its use in the ISO document 

Hot Environments: Estimation of Heat Stress on 

Working Man Based on the WBGT Index (Wet 

Bulb Globe Temperature) 1982 [ISO 1982a] 

(see Chapter 9, Basis for the Recommended 

Standard, for further discussion of the adoption of the WBGT as the recommended heat 

stress index). However, when impermeable 

clothing is worn, the WBGT is not a relevant 

index because evaporative cooling (wet bulb 

temperature) will be limited. The air temperature or adjusted dry bulb temperature is the 

pertinent factor in the determination of heat 

transfer from the human body to the external environment.

The WBGT index meets the criteria of a heat 

stress index that are listed earlier in this chapter. In addition to the WBGT-based occupational exposure limit recommendations for 

continuous work in a hot environment, recommendations have also been made for limiting 

WBGT heat stress when 25%, 50%, and 75% of 

each working hour is at rest (ACGIH-TLVs®, 

OSHA, AIHA). Regulating work time in the 

heat (allowable exposure time) is a viable alternative technique for permitting necessary work 

to continue under heat stress conditions that 

would be intolerable for continuous exposure.

9.3.3 Wet Globe Temperature (WGT)

Next to the ta and twb, the wet globe thermometer, also known as the Botsball, is the simplest, most easily read, and most portable of 

the environmental measuring devices. The wet 

globe thermometer consists of a thermometer 

inside a hollow 3-inch copper sphere covered 

by two layers of black cloth, which is kept at 

100% wettedness from a water reservoir. The 

sensing element of a thermometer is located at 

the inside center of the copper sphere, and the 

temperature inside the sphere is read on a dial 

on the end of the stem.

Presumably, the wet sphere exchanges heat 

with the environment by the same mechanisms that a nude person with totally wetted 

skin would in the same environment; that is, 

heat exchange by convection, radiation, and 

evaporation are integrated into a single instrument reading [Botsford 1971]. The stabilization time of the instrument ranges from about 

5 to 15 minutes, depending on the magnitude 

of the heat-load differential (5 minutes for 

5°C [9°F] and 15 minutes for >15°C [59°F]). 

However, a study that investigated the relationship between the Botsball and WBGT in 

varied environmental conditions indicated 

that if good estimates of wind speed, humidity 

and radiant heat are available, WGT can predict WBGT with +/-0.4 degrees C with a 90% 

confidence level but this does not hold true 

when there is limited knowledge of the of environment conditions (e.g., humidity, heat, wind 

speed) [Ciricello and Snook 1977].

The WGT has been compared to the WBGT 

in many laboratory studies and field situations 

[Ciricello and Snook 1977; Johnson and Kirk 

1980; Beshir 1981; Beshir et al. 1982; Parker 

and Pierce 1984]. In general, the correlation 

between the two is high (r = 0.91 - 0.98); however, the relationship between the two is not 

constant for all combinations of environmental factors. Correction factors ranging between 

1°C (1.8°F) and 7°C (12.6°F) have been suggested. A simple approximation of the relationship is WBGT = WGT + 2°C for conditions of 

moderate radiant heat and humidity (22°C, 

50% relative humidity). These approximations 

are probably adequate for general monitoring 

in industry. If the WGT shows high values, 

it should be followed with WBGT or other 

detailed measurements. The WGT, although 

adequate for screening and monitoring, does 

not yield data for solving the equations for 

heat exchange between the worker and the 
Occupational Exposure to Heat and Hot Environments 115

9 . Indices for Assessing Heat Stress and Strain

workplace environment, but a color-coded 

WGT display dial provides a simple and rapid 

indicator of the level of heat stress.

9.3.4 Universal Thermal Climate 

Index (UTCI)

A more recent model has been developed to 

determine the thermal stress imposed by environmental conditions on the human body 

[Blażejczyk et al. 2013]. In this model, the 

human body is divided into two interacting 

thermoregulatory systems which respond to 

the environment (an active controlling and a 

passive controlled system). This model represents the human body as a complex series 

of segments containing information on anatomical and physiological properties which 

include heat balance. The model predicts the 

thermoregulatory responses (e.g., cutaneous 

blood flow, shivering thermogenesis, sweating responses) involved in human heat balance 

under various environmental conditions. The 

model defines UTCI as the air temperature 

of the reference condition causing the same 

model responses as the actual conditions. 

The deviation or offset from UTCI depends 

on actual values of air temperature and mean 

radiant temperature, wind velocity and relative 

humidity. Thus, the model takes into consideration not just the environmental conditions 

but the physiological responses to those conditions. Additional information and a UTCI 

calculator are available at http://www.utci.org.

9.3.5 Psychrometric Chart

In the event that the methods for obtaining estimates of environmental heat stress as described 

in this section are unavailable, good estimates 

of environmental heat stress can still be determined. A psychrometric chart can be used to 

determine the environmental heat stress (see 

5.1.5 Psychrometric Chart). The psychrometric chart is a graphical representation of the 

relationships among dry bulb temperature, 

wet bulb temperature, RH, vapor pressure, 

and dew point temperature. If any two of these 

variables are known, any of the others can be 

determined from the psychrometric chart.

9.4 Physiologic Monitoring

The objectives of a heat stress index are twofold: (1) to provide an indication of whether a 

specific total heat stress will result in an unacceptably high risk of heat-related illness or 

injuries and (2) to provide a basis for recommending control procedures. The physiologic 

responses to an increasing heat load include 

increases in heart rate, core body temperature, skin temperature, and sweat production. 

In a specific situation, any one or all of these 

responses to heat load may be elicited. The 

magnitude of the response(s) generally reflects 

the total heat load. An advantage of physiological monitoring is that it measures the individual’s responses to the environmental and 

metabolic heat load rather than extrapolating 

from the environmental conditions and the 

estimated work load. The individual integrates 

the stress of the heat load from all sources, 

and the physiologic responses (strain) to the 

heat load are the biological corrective actions 

designed to counteract the stress and thus 

permit the body to maintain an optimal internal temperature. Acceptable limits to increases 

in physiologic responses to heat stress have 

been recommended by several investigators 

[WHO 1969; Fuller and Smith 1980, 1981; 

Bernard and Kenney 1994; Logan and Bernard 

1999; Malchaire et al. 2000]. Measuring one or 

more of the physiologic responses (heart rate 

and/or core temperature) during work has 

been recommended and, in some industries, 

is used to ensure that the heat stress to which 

the worker is exposed does not result in unacceptable strain [Fuller and Smith 1980, 1981]. 

However, several of the physiologic monitoring procedures are invasive (i.e., ingestible 
116 Occupational Exposure to Heat and Hot Environments

9 . Indices for Assessing Heat Stress and Strain

plastic thermister used to determine intestinal 

temperature), socially unacceptable (i.e., rectal 

thermister), or interfere with communication 

(i.e., ear thermometer). Physiologic monitoring requires medical supervision and the consent of the worker. See Table 9-1, for examples 

of physiological monitoring used to prevent 

heat-related illnesses.

9.4.1 Work and Recovery Heart Rate

One of the earliest procedures for evaluating work and heat strain was introduced by 

Brouha [1960], in which the body temperature 

and pulse rate are measured during recovery 

following a work cycle or at specified times 

during the workday [Brouha 1960]. At the end 

of a work cycle, the worker sits on a stool, an 

oral thermometer is placed under the tongue, 

and the pulse rate is counted from 30 seconds 

to 1 minute (P1), from 1.5 to 2 minutes (P2), 

and from 2.5 to 3 minutes (P3) of seated recovery. If the oral temperature exceeds 37.5°C 

(99.5°F), the P1 exceeds 110 beats per minutes 

(bpm), and/or the P1–P3 is fewer than 10 bpm, 

the heat and work stress is assumed to be above 

acceptable values. These values are based on 

group averages and may or may not be protective for an individual worker or specific work 

situation. However, these values should alert 

the observer that further review of the job 

is desirable. 

A modified Brouha [1960] approach is being 

used for monitoring heat stress in some hot 

industries. An oral temperature and a recovery heart rate pattern have been suggested by 

Fuller and Smith [1980, 1981] as a basis for 

monitoring the strain of working at hot jobs. 

In this modification the primary criterion of 

high heat strain is an oral temperature exceeding 37.5°C (99.5°F). The heart rate recovery 

pattern is used to assist in the evaluation. If the 

P3 is 90 bpm or fewer, the job situation is satisfactory; if the P3 is about 90 bpm and the P1–P3

is about 10 bpm, the pattern indicates that the 

physical work intensity is high, but there is little 

if any increase in body temperature; if the P3 is 

greater than 90 bpm and the P1–P3 is fewer than 

10 bpm, the stress (heat + work) is too high for 

the individual, and corrective actions should 

be introduced. These individuals should be 

examined by a responsible healthcare provider, 

and the work schedule and work environment 

should be evaluated.

The field data reported by Jensen and DukesDobos [1976] corroborate the concept that the 

P1 recovery heart rate and/or oral temperature 

are more likely to exceed acceptable values 

when the environmental plus metabolic heat 

load exceeds the ACGIH TLVs® for continuous work. The recovery heart rate can be easily 

measured in workplace situations where being 

seated for about five minutes will not substantially interfere with the work sequence; in addition, the instrumentation required (a wearable 

electronic heart rate monitor) can be simple 

and inexpensive. The recovery and work heart 

rates can be used on some jobs as early indicators of the strain resulting from heat exposure 

in hot workplace jobs. The relatively inexpensive, noninvasive electronic devices now available (such as those used by joggers and others) 

make self-monitoring of work and recovery 

pulse rates practical for many workplaces. 

9.4.2 Body Temperature

The WHO scientific group on Health Factors 

involved in Working Under Conditions of 

Heat Stress recommended that the deep body 

temperature should not, under conditions of 

prolonged daily work and heat, be permitted 

to exceed 38°C (100.4°F) or the oral temperature to exceed 37.5°C (99.5°F), although the 

tolerance to elevated body temperature is quite 

variable [Taylor et al. 2008]. The limit has generally been accepted by the experts working 

in the area of workplace heat stress and strain. 
Occupational Exposure to Heat and Hot Environments 117

9 . Indices for Assessing Heat Stress and Strain

Given worker permission, body temperature 

monitoring can be used in many hot jobs.

9.4.2.1 Core Temperature

Monitoring the core body temperature is 

a direct, objective, and reliable approach. 

Measuring internal body temperature in the 

rectum or esophagus may be unacceptable to 

workers. However, newer technologies, involving an ingestible plastic thermister capable 

of telemetering “core” (intestinal) temperatures, are in wide use (e.g., CorTemp, HQInc., 

Palmetto, FL). The disadvantage of the ingestible thermisters that it takes several hours to 

migrate from the mouth to the small intestine 

before accurate temperatures can be recorded 

[Lee et al. 2000; Williams et al. 2011].

It must be noted that tolerance to increased 

Tre varies widely in individuals. Non-elite runners have been documented to have completed 

a marathon run with a Tre of >41°C (105.8°F), 

and a Tre of 41.9°C (107.4°F) has been recorded 

in soccer players with no adverse physiological 

consequences [Armstrong et al. 2007b; Taylor 

et al. 2008]. Therefore, recovery Tre will be different in heat tolerant individuals than in those 

who are less heat tolerant (see Chapters 5 and 9 

for more detailed discussions).

9.4.2.2 Oral Temperature

Oral temperatures are easy to obtain with the 

inexpensive disposable oral thermometers 

available. However, to obtain reliable oral temperatures requires a strictly controlled procedure. The thermometer must be correctly 

placed under the tongue for 3 to 5 minutes 

before the reading is made, mouth breathing 

is not permitted during this period, no hot or 

cold liquids should be consumed for at least 

15 minutes beforehand, and the thermometer 

must not be exposed to an air temperature 

higher than the oral temperature either before 

the thermometer has been placed under the 

tongue or until after the thermometer reading 

has been taken. In hot environments, this may 

require that the thermometers be kept in a 

cool insulated container or immersed in alcohol, except when in the worker’s mouth. Oral 

temperature is usually lower than deep body 

temperature by about 0.55°C (0.8°F). With 

the advent of digital oral thermometers, oral 

temperatures may be obtained within <30 seconds, thus avoiding some of the issues found 

with standard alcohol oral thermometers. 

Evaluation of the significance of any oral temperature must follow established medical and 

occupational hygiene guidelines.

9.4.2.3 Skin Temperature

The use of skin temperature (Tsk) as a basis for 

assessing the severity of heat strain and estimating tolerance can be supported by thermodynamically and field-derived data. To move 

body heat from the deep tissues (core) to the 

skin (shell), where it is dissipated to the ambient environment, requires an adequate heat 

gradient. As the skin temperature rises and 

approaches the core temperature, this temperature gradient is decreased and the rate of heat 

moved from the core to the shell is decreased 

and the rate of core heat loss is reduced. To 

restore the rate of heat loss or core skin heat 

gradient, the core body temperature would 

have to increase. Circulation of warm blood 

from central (core) body compartments to the 

skin results in an increase in skin temperature. The increase in skin temperature results 

in heat transfer to the environment through 

conduction, convection, and radiation as discussed in previous sections of this document. 

As heat is transferred to the environment, 

cutaneous blood cools and returns to the body 

core resulting in a decrease in body core temperature. Under certain environmental conditions, insufficient heat is transferred from 

the skin to the environment which can result 

in an increase in core body temperature. As 

the core temperature is increased above 38°C 
118 Occupational Exposure to Heat and Hot Environments

9 . Indices for Assessing Heat Stress and Strain

(100.4°F), the risk of an ensuing heat-related 

illness is increased.

From these observations, it has been suggested 

that a reasonable estimate of tolerance time 

for hot work could be made from the equilibrium lateral thigh or chest skin temperature 

[Iampietro 1971; Shvartz and Benor 1972; 

Goldman 1978, 1981, 1985b, 1985a]. Under 

environmental conditions where evaporative 

heat exchange is not restricted, skin temperature would not be expected to increase much, 

if at all. Also, in such situations, the maintenance of an acceptable deep body temperature 

should not be seriously jeopardized, except 

under very high metabolic loads or restricted 

heat transfer. However, when convective and 

evaporative heat loss is restricted such as when 

wearing impermeable protective clothing, an 

estimate of the time required for skin temperature to converge with deep body temperature 

should provide an acceptable approach for 

assessing heat strain and for predicting tolerance time. , Moreover, although Tsk is generally 

2°C to 4°C below body core temperature (Tcore), 

Tsk can be used to estimate Tcore when other 

methodologies are not available [Lenhardt and 

Sessier 2006]. In fact, skin and core temperatures are used as concepts for convenience, 

because the temperature varies over parts of 

the body (i.e., both over skin and internally) 

[Parsons 2003]. One only gives averages of 

skin or internal temperature. Several authors 

have attempted to use HR and skin temperature to estimate core temperature, with varying degrees of success. In practice, skin or 

internal temperatures should be measured 

directly by techniques discussed elsewhere in 

this document. 

9.4.2.4 Aural Temperature

Aural temperature is measured with an infrared (IR) thermometer in the ear canal. Because 

the IR thermometer should not come into 

direct contact with the tympanic membrane, it 

does not provide a true tympanic temperature. 

Measurements taken at the peak temperature 

can be compared with the after-work temperature to compare with the temperature obtained 

during their work shift to potentially establish 

work-related hyperthermia. The efficacy of 

aural temperature measurement to monitor for 

heat stress is uncertain because it consistently 

underestimates core temperature, but it is 

simple and noninvasive [Huggins et al. 2012]. 

See Table 9-1 for more information on physiological monitoring.

9.4.3 Dehydration

Under heat stress conditions where sweat production may reach 6 to 8 liters in a workday, 

voluntary replacement of the water lost in the 

sweat is usually incomplete. The normal thirst 

mechanism is not sensitive enough to urge us 

to drink enough water or other fluids to prevent dehydration. If dehydration exceeds 1.5% 

to 2% of body weight, then tolerance to heat 

stress begins to deteriorate, heart rate and 

body temperature increase, and work capacity decreases [Greenleaf and Harrison 1986]. 

When dehydration exceeds 5%, it may lead to 

collapse and to dehydration heat-related illness. Since thirst is not an adequate guide for 

water replacement, workers in hot jobs should 

be encouraged to drink water or other fluids 

every 15 to 20 minutes. The fluids should be 

less than 15°C (59°F). For work that requires 

an increased level of activity in a hot environment for a prolonged period of time (≥2 

hours), sports drinks that contain carbohydrates and electrolytes (e.g., Gatorade) should 

be used in place of water in order to replace 

the electrolytes lost from sweating and to 

avoid hyponatremia (serum sodium concentration <136 mEq·L-1) from excessive consumption of plain water [DOD 2003; Montain 

and Cheuvront 2008]. Drinking from disposable drinking cups may be preferable to using 

drinking fountains, as the cups can be taken 
Occupational Exposure to Heat and Hot Environments 119

9 . Indices for Assessing Heat Stress and Strain

with the worker and can provide a visual estimate for how much fluid was consumed. The 

amount of dehydration can be estimated by 

measuring body weight at intervals during 

the day or at least at the beginning and end 

of the work shift. The worker should drink 

enough water to prevent a loss in body weight. 

However, because this may not be feasible 

in all situations, following a recommended 

water-drinking schedule is usually satisfactory. See 4.1.4.1 Water and Electrolyte Balance 

and the Influence of Endocrines, for a more 

complete discussion of recommended hydration strategies.
120 Occupational Exposure to Heat and Hot Environments

See footnotes at end of table. (Continued)

9 . Indices for Assessing Heat Stress and Strain

Monitoring 

metho

d

How

assessed

When 

assessed

Additional 

information Limitations

Heat exposure history

■ Worker interview 

or questionnaire

■ Report from 

the responsible healthcare provider

■ Workers should be 

cleared by the responsible healthcare provider before starting and/or returning to work in a hot environment

■ A history of heat-related 

illness increases the risk of a repeat occurrence

■ Acclimatization status must 

be assessed

■ Include pre-existing 

medical conditions, prescription and over-thecounter medications, and supplements that could affect heat tolerance

■ Self-reporting by workers carries risk of 

either intentional or accidental omission of medical history or conditions that would increase risk of injury during work in a hot environment

Heart rate(pulse rate)

■ Radial pulse check: 

count the number of beats per minute (bpm) by feeling the pulse on the inside of the wrist

■ Continuous 

monitoring with chest strap containing a heart rate sensor

■ Before work begins and 

while at rest to determine baseline; then after heat exposure (e.g., minutes 1 and 3 after the work period ends)

■ Chest strap data can be 

taken over the entire work shift

■ Heat strain is indicated by 

a sustained HR > 180 bpm minus the worker’s age

■ Heart rate should fall 

rapidly, approaching the baseline once the worker is resting in a cool environment

■ Heart rate will remain 

elevated in a worker experiencing a heatrelated illness

■ Many factors independent of heat stress 

can cause an increased heart rate (e.g., physical exertion, physical condition, fever from an infection, pain from an injury or heart attack, dehydration)

■ Medications (e.g., beta blockers) or 

pre-existing medical conditions (e.g., sinus bradycardia, hypothyroidism, pacemaker) can suppress heart rate

■ Chest straps must be properly fitted; 

data loss can occur when contact with skin is lost during exertion or excessive sweating

Temperature ■ Increased temperature 

indicates the body is not cooling as rapidly as necessary or failure of cooling mechanisms

■ Body temperatures usually reduce more 

slowly than heart rate in response to heat stress

Table 9-1. Examples of physiological monitoring used to prevent heat-related illness
Occupational Exposure to Heat and Hot Environments 121

See footnotes at end of table. (Continued)

9 . Indices for Assessing Heat Stress and Strain

Monitoring 

metho

d

How

assessed

When 

assessed

Additional 

information Limitations

Temperature (Continued)

■ Oral temperature:

measured with oral thermometer

■ Initial baseline at rest in a 

cool environment

■ At expected peak ambient 

workplace temperature or during the highest anticipated heat load and again after the work period

■ Inaccurate if the 

measurement is taken soon after ingesting cool beverages

■ Oral temperatures are generally 0.5°F 

lower than true core body temperature

■ Aural temperature: 

measured with infrared (IR) thermometer placed in the ear canal but does NOT come in direct contact with the ear drum (i.e., the tympanic membrane)

■ At expected peak ambient 

workplace temperature and again after the work period

■ More accurate measure of 

core temperature than oral

■ Measurement from an IR thermometer 

should not be interpreted to be the same as data obtained by a thermometer that places a sensor directly on the tympanum

■ False, low readings can occur if there is 

excessive wax in the ear

■ Should not be used if there is an ear 

infection or recent ear surgery

■ Core body 

temperature (CBT): measured with inert electronic sensor that transmits real-time CBT data as it travels through the gut

■ Continuous sensing devices 

measure temperature during both work and rest periods

■ Sensor is swallowed 

before starting work and excreted with normal bowel movements

■ Gives most accurate measure 

of body temperature

■ Some CBT systems have 

automated alarm capability to alert in real time if specific CBTs are exceeded, allowing for immediate removal of workers to a cool environment

■ Some systems may only 

allow CBT data to be seen after download at end of work shift

■ Workers must be screened for conditions 

that may not allow them to safely ingest the sensor

■ Sensor should be taken with food or 

water to ensure it leaves the stomach and enters the small intestine before data are collected

■ Artificial drops in transmitted 

temperature data occur when cool liquids are ingested; data need to be properly cleaned before analysis

■ May have only partial shift data if sensor 

is excreted with bowel movement during the work shift

Table 9-1 (Continued). Examples of physiological monitoring used to prevent heat-related illness
122 Occupational Exposure to Heat and Hot Environments

See footnotes at end of table. (Continued)

9 . Indices for Assessing Heat Stress and Strain

Monitoring 

metho

d

How

assessed

When 

assessed

Additional 

information Limitations

Body weight ■ Can use bathroom 

scale with good precision

■ Ideally, use a 

calibrated digital scale for accuracy

■ Initial baseline and 

immediately after heat exposure

■ Daily weight loss 

can indicate fluid losses are exceeding replacement efforts

■ Body weight should be measured on 

calibrated scales while person is dressed in only underwear to minimize error due to weight of sweat absorbed by clothing

Blood pressure

■ Automated blood 

pressure cuff Count breaths per minute with stop watch

■ Initial baseline and again 

after the work period

■ Blood pressure does not 

recover as quickly when a worker is suffering heatrelated illness

■ Posture can affect blood 

pressure in workers with heat-related illness and must be taken into account when using various physiological monitoring methods

■ Must be sure arm is extended, proper 

size cuff used, and legs uncrossed during measurement

■ Dehydration can impact blood pressure, 

so needs to be included in the assessment

Respiratory rate

■ Count breaths 

per minute with stop watch

■ Initial baseline and again 

after the work period

■ Breathing rate does not 

return to baseline as quickly when a worker is suffering heat-related illness

■ Impacted by other physiological factors 

(e.g., metabolic acidosis from lactate production in anaerobic exertion)

■ Pre-existing medical conditions (e.g., 

asthma or tobacco smoking) can impact lung function, resulting in abnormally high respiratory rate and/or prolonged recovery time

Table 9-1 (Continued). Examples of physiological monitoring used to prevent heat-related illness
Occupational Exposure to Heat and Hot Environments 123

9 

. Indices for Assessing Heat Stress and Strain

Monitoring 

method

How

assessed

When 

assessed

Additional 

information Limitations

Alertness ■ Changes in 

mental status (e.g., confusion, odd behavior, seizure, loss of consciousness) in a hot environment indicate heat stroke.

■ At regular intervals during 

work cycles and after the work period

■ If work/rest cycles are 

implemented, then workers should be checked as they enter and before leaving the rest period

■ If there is any change in 

mental status, the worker should be immediately moved to a cool area and be sent for immediate medical treatment

■ Heat stroke victims are often unaware 

of their decreased functional capacity; the buddy system is vital so coworkers can continuously monitor each other for symptoms

Adapted from [OSHA; ACGIH 2014].

Table 9-1 (Continued). Examples of physiological monitoring used to prevent heat-related illness
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Occupational Exposure to Heat and Hot Environments 125

10 Research Needs

The past decade has brought an enormous 

increase in our knowledge of heat stress and 

strain, their relation to health and productivity, techniques and procedures for their assessment, and their health risks. In spite of this, 

there are several areas where further research 

can contribute to reducing occupational heatrelated health and safety problems.

10.1 Exposure Times 

and Patterns

In some hot industries, the workers are 

exposed to heat most of the day; other workers may be exposed only part of the time. 

Although there is general agreement that 

there is a relationship between heat stress 

and the health and safety risks for continuous 

exposure during an 8-hour workday, there is 

still uncertainty over acceptable levels of heat 

stress for intermittent exposure, where the 

worker may spend only part of the working 

day in the heat. Issues that need more information include:

■ Is a 1-hour, a 2-hour, or an 8-hour 

TWA required for calculating risk of 

health effects? 

■ How long are acceptable exposure times 

for various total heat loads? 

■ Are the health effects (heat-related 

illnesses) and risks the same for intermittent as for continuous heat exposure? 

■ Do workers exposed intermittently each 

day to various lengths and amount of 

heat stress develop heat acclimatization 

similar to that achieved by continuously 

exposed workers? 

■ Are the electrolyte and water balance problems the same for intermittently versus continuously heatexposed workers?

10.2 Deep Body 

Temperature

The WHO Scientific Group recommended 

that “it is considered inadvisable for a deep 

body temperature to exceed 38°C (100.4°F) 

for prolonged daily exposures (to heat) in 

heavy work” [WHO 1969] and that a deep 

body temperature of 39°C (102.2°F) should 

be considered reason to terminate exposure, 

even when deep body temperature is being 

monitored. Issues that need more information include:

■ Are these values equally realistic for 

short-term acute heat exposures as for 

long-term chronic heat exposures? 

■ Are these values strongly correlated 

with increased risk of incurring heatrelated illnesses? 

■ Are these values considered maximal, 

which are not to be exceeded, mean population levels, or 95th percentile levels? 

■ Is the rate at which deep body temperature rises to 38°C or 39°C (100.4°F 

or 102.2°F) important in the healthrelated significance of the increased 

body temperature? 
126 Occupational Exposure to Heat and Hot Environments

10 . Research Needs

■ Does a 38°C or 39°C (100.4°F or 102.2°F) 

deep body temperature have the same 

health significance if reached after only 

one hour of exposure as when reached 

after more than one hour of exposure?

10.3 Electrolyte and 

Water Balance

The health effects of severe acute negative 

electrolyte and water balance during heat 

exposure are well documented. However, the 

health effects of the imbalances, when derived 

slowly over periods of months or years, are 

not known; nor are the effects known for 

long-term electrolyte loading with and without hyperhydration or hypohydration. An 

appropriate electrolyte and water regimen 

for long-term work in the heat requires more 

data derived from further laboratory and epidemiologic studies.

10.4 Effects of Chronic 

Heat Exposure

All of the experimental and most of the epidemiologic studies of the health effects of heat 

stress have been directed toward short exposures of days or weeks and toward acute heatrelated illnesses. Little is known about the 

health consequences of living and working in 

a hot environment for a working lifetime. Do 

such long exposures to heat have any morbidity or mortality implications? Does experiencing an acute heat-related illness have any 

effects on future health and longevity? It is 

known that individuals with certain health 

disorders (e.g., diabetes or cardiovascular 

disease) are less heat tolerant. There is some 

evidence that the reverse may also be true; for 

example, chronic heat exposure may render 

an individual more susceptible to both acute 

and chronic diseases and disorders [Redmond 

et al. 1979; Dukes-Dobos 1981]. The chronic 

effect of heat exposure on blood pressure is a 

particularly sensitive problem because hypertensive workers may be under treatment with 

diuretics and on restricted salt diets. Such 

treatment may be in conflict with the usual 

emphasis on increased water and salt intake 

during heat exposure. 

10.5 Circadian Rhythm 

of Heat Tolerance

The normal daily variation in core body 

temperature from the high point in the 

mid-afternoon to the low point in the early 

morning is about 0.5°C [Cheung et al. 2000]. 

Superimposed on this normal variation in 

body temperature are increases due to heat 

exposure. In addition, the WHO report recommends that the 8-hour TWA body temperature of workers in hot industries should not 

exceed 38°C (100.4°F) [WHO 1969]. Issues 

that need more information include: 

■ Is this normal daily increase in body 

temperature additive to the increase 

resulting from heat stress? 

■ Does tolerance to increased body temperature and the connected health risk 

follow a similar diurnal pattern? 

■ Would it be necessary to establish different permissible heat exposure limits 

for day and night shift workers in 

hot industries? 

Also, in addition to the circadian rhythm 

variations in heat tolerance, there are also 

rhythmic changes in body temperature and 

heat tolerance associated with the menstrual 

cycle [McArdle et al. 2010b]. The effect of the 

menstrual cycle on core temperature variation (as much a 0.5°C diurnally), particularly 

as it potentially impacts heat stress and heat 

tolerance, needs to be better understood. 
Occupational Exposure to Heat and Hot Environments 127

10 . Research Needs

10.6 Heat Tolerance 

and Shift Work

It has been estimated that about 30% of workers are on some type of work schedule other 

than the conventional day work (9 a.m. to 

5 p.m.). Shift work, long days with a short week, 

and double shifts alter the usual living patterns 

of the worker and result in some degree of 

sleep deprivation. The effects of these changes 

in living patterns on heat tolerance are mostly 

undocumented. Before these changes in work 

patterns are used, it would be prudent to understand their health and safety implications.

10.7 Detection, Treatment, 

and Prevention of 

Heat-related Illnesses

Discovering means of quickly and accurately 

identifying individuals experiencing or about 

to experience adverse effects of heat stress 

exposure—specifically, mechanisms and methods that are inexpensive and that can screen 

groups of workers quickly—should be a priority. In addition, more could be known about 

heat cramp etiology and prevention.

Treatment of heat stroke could be improved. A 

greater understanding of the mechanisms and 

why, after a certain point, it becomes irreversible could lead to more effective treatments. In 

addition, are there any medications that might 

help prevent the body’s inflammatory reactions that often result in death?

10.8 The Effects of Global 

Climate Change

Global climate change could have a significant 

effect on indoor and outdoor workers, such 

as those in agriculture, fishing, construction, 

and many service areas. In particular, workers 

located in densely populated areas closer to the 

equator are at increased risk for heat-related 

injuries and illnesses, as temperatures are 

expected to rise in relation to the changing climate and other factors (e.g., low- and middleincome, limited air-conditioning, economic 

drivers to maintain productivity) [Lucas et 

al. 2014]. Climate change will not necessarily result in new hazards; however, it may add 

to the severity, prevalence, and distribution of 

the already known hazards [Schulte and Chun 

2009; Schulte et al. 2015]. Schulte and Chun 

identify seven categories of climate-related 

hazards: (1) increased ambient temperature, 

(2) air pollution, (3) ultraviolet exposure, (4) 

extreme weather, (5) vector-borne diseases and 

expanded habitats, (6) industrial transitions 

and emerging industries, and (7) changes in the 

built environment. The relationship between 

these categories and the possible occupational 

health effect outcomes can be seen in Figure 

10-1. In addition, another result of climate 

change is a reduced work capacity and productivity in heat-exposed jobs, with resulting loss 

of income, which is also likely to cause mental 

health and economic effects [Kjellstrom 2009; 

Kjellstrom et al. 2009b; Berry et al. 2010; 

Kjellstrom et al. 2010; McMichael 2013].

How climate change effects on the workforce 

can be addressed is a relatively new area of 

research that should be pursued. Some climate 

change-related risks may be reduced by general 

improvements in public health, whereas other 

risks can be managed by ‘adaptation policies 

and actions’ [Kjellstrom et al. 2009b; Kjellstrom 

and Weaver 2009; Nilsson and Kjellstrom 

2010]. In addition, as relative humidity is 

expected to remain constant in a changing 

climate, WBGT-based occupational exposure 

limits may remain unchanged based on this 

factor [Ingram 2002]. The idea to develop a 

program to capture the growing evidence on 

climate change and health emerged at a 1998 

Intergovernmental Panel on Climate Change 
128 Occupational Exposure to Heat and Hot Environments

10 . Research Needs

meeting, and it was eventually presented as 

the ‘high occupational temperature health and 

productivity suppression’ (Hothaps) program 

[Kjellstrom et al. 2009a]. The Hothaps program is a multi-center health research and prevention program used to quantify the extent 

to which workers are affected by, or adapt to, 

heat exposure while working, and how global 

heating during climate change may increase 

such effects. Programs like Hothaps and others 

will help to capture current heat-related events, 

likely leading to new heat-related occupational 

safety and health recommendations and regulations in the future.

10.9 Heat Stress 

and Toxicology

Exposure to heat can affect how chemicals are 

absorbed into the body. Since the 1890s, animal 

studies have shown that exposure to heat exacerbates chemical absorption and toxicity [Leon 

2008]. Leon states that changes to the body’s 

core temperature can alter absorption, distribution, metabolism, and excretion of the 

toxicants. Increases in respiration can lead to 

further toxicant exposure through inhalation, 

whereas increases in sweat and skin blood 

flow can lead to more efficient transcutaneous 

Figure 10-1. Relationship between climate change and occupational safety and health 

Adapted from Schulte and Chun [2009].
Occupational Exposure to Heat and Hot Environments 129

10 . Research Needs

absorption of some toxicants [Gordon 2003; 

Leon 2008]. The relationships between how 

heat and other factors can affect the physiological response to toxicants can be seen in 

Figure 10-2.

Animal studies examining thermal stress and 

the effects on chemical toxicity show that heat 

plays a role in toxicant absorption, but are difficult to interpret when trying to extrapolate 

to humans. Test animals tend to be sedentary 

with no option for exercise, are acclimatized to 

ideal environmental conditions, and use hypothermia as their predominant thermoregulatory response to chemical toxicants [Gordon 

Adapted from Gordon [2003].

Figure 10-2. How heat, humidity, work, and thermoregulation 

affect the physiological response to toxicants
130 Occupational Exposure to Heat and Hot Environments

10 . Research Needs

2003; Leon 2008]. In vitro and in vivo studies 

have suggested that heat stress, with or without exercise, will activate thermoeffectors (e.g., 

skin blood flow, sweating, respiration) that, in 

turn, accelerate pesticide absorption in humans 

[Gordon and Leon 2005]. Gordon and Leon 

[2005] also mention an in vitro model used to 

show blood flow, temperature, and RH and the 

effect on absorption of the pesticide parathion, 

as well as human studies showing the accelerating effects of perspiration on the absorption 

of organophosphorus compounds. Pesticides 

are also a hazard to workers in the heat, as high 

temperatures accelerate dispersion, increase 

the density of airborne particles, and some 

workers remove their PPE due to discomfort in 

the heat [Gordon 2003]. The following may be 

at risk of heat stress in the presence of chemicals: farmers and other agricultural workers, 

food-processing laborers, construction workers, fire fighters, and foundry and manufacturing workers [Bourbonnais et al. 2013].

Most of what is known about toxicants is 

derived from animal studies in which the animals were kept in comfortable temperatures; 

therefore, a better understanding of the mechanisms involved between heat exposure and 

toxicants in humans is needed [Gordon 2003; 

Gordon and Leon 2005]. With changes in the 

climate and hotter temperatures, the need for 

more information on toxicants and their relationship to heat stress will become increasingly 

important [Leon 2008]. 

10.10 Other Research Needs

Other suggested research needs include validity/sensitivity and specificity of personal 

monitoring methods, effective management 

methods (e.g., value and content of training), and epidemiological studies to evaluate the RAL and REL where outcomes can be 

heat-related disorders, productivity, and injuries [Bernard 2014]. In addition, research is 

needed on the development of more effective 

and cheaper PPE to protect workers against 

heat illness and injury. These activities are 

ongoing, and the future will no doubt bring 

new technologies to mitigate the effects of heat 

on workers and reduce the incidence of heatinduced injury, illness, and death.
Occupational Exposure to Heat and Hot Environments 131

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Appendix A

Heat Exchange Equation Utilizing the SI Units
This appendix is referred to in Chapter 3 of the current document. It is reprinted in full from the 

1986 NIOSH criteria document: 

NIOSH [1986a]. Criteria for a recommended standard: occupational exposure to hot environments: revised criteria 1986. Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control, National Institute for Occupational Safety and Health.

Convection (C) SI Units

The rate of heat exchange between a person and the ambient air can be stated algebraically:

C = hc(ta−tsk)

Where

hc is the mean heat transfer coefficient

ta = air temperature (°C)

t sk = air temperature (°C)

The value of hc differs for the different parts of the body [Nishi 1981], depending mainly on the 

diameter of the body part; for example, at the torso the value of hc is about half of what it is at the 

thighs. The value used for hc is generally the average of the hc values for the head, chest, back, upper 

arms, hands, thighs, and legs. The value of hc varies between 2 and 12, depending on body position 

and activity.

Other factors that influence the value of hc are air speed, air direction, and clothing. The value of 

t sk can also vary according to the method used for the measurements, the number and location of 

the measuring points over the body, and the values used for weighting the temperatures measured 

at the different locations.

Numerous investigations have tried to simplify the calculation of convection heat exchange. The 

ISO Working Group on the Thermal Environment (ISO-WGTE) developed a draft standard for the 

Analytical Determination of Heat Stress [ISO 1982b]*. One of the simplifications they adopted is to 

use only the following three values for hc,which are expressed in units of Wm-2°C-1, corresponding 

to the SI system.

(a) When air speed is very low and is due only to natural convection, 

hc = 2.38(t sk−ta)0.25

(b) In forced convection when relative air speed (Var) is less than 1m·,

hc = 3.5 + 5.2Var

(c) In forced convection, 

when Var is greater than 1m·,

hc = 8.7Var0.6

*This standard has been revised. See Ergonomics of the Thermal Environment: Analytical Determination and Interpretation 

of Heat Stress Using Calculation of the Predicted Heat Strain (Standard No. ISO 7933) [ISO 2004b].

Appendix A

146 Occupational Exposure to Heat and Hot Environments
The expression Var is defined as the ratio of the air velocity relative to the ground and the speed of 

the body or parts of the body relative to the ground. If the body movement is due to muscular work, 

Var can be calculated by the following equation:

Var= Va + 0.0052(M−58)

where

Va = air velocity in ms-1

M = metabolic heat production (Wm-2)

For simplicity, however, it is recommended to add 0.7 ms-1 to Va as a correction for the effect of 

physical work.

The ISO-WGTE recommends also to include in the equation for calculating the convection heat 

exchange a separate coefficient for clothing, called a reduction factor for loss of sensible heat 

exchange due to the wearing of clothes (Fcl), which can be calculated by the following equation:

Fcl = 1/1 + (hc + hr)Icl(dimensionless)

where

hr= the heat transfer coefficient for radiant heat exchange

Icl = the thermal insulation of clothing

Both hr and Icl will be explained later in this appendix in more detail. The ISO-WGTE recommended the use of 36°C (96.8°F) for tsk on the assumption that most workers engaged in hot jobs 

would have a tsk very close to this temperature; thus, any error resulting from this simplification will 

be small. They also assumed that most corrected for different body positions when calculating the 

convective heat exchange of workers.

The final equation for C to be used according to the ISO-WGTE is

C = hcFcl (ta−36) (Wm-2)

Radiation (R) SI Units

The rate of radiant heat exchange between a person and the surrounding solid objects can be 

stated algebraically:

R = hr (Tr−Tsk)4

where

hr = the coefficient for radiant heat exchange

Tr = the mean radiant temperature in °K

Tsk = the mean weighted skin temperature in °K

Appendix A

Occupational Exposure to Heat and Hot Environments 147
The value of hr depends on the body position of the exposed worker and on the emissivity of the 

skin and clothing, as well as on the insulation of clothing. The body position will determine how 

much of the total body surface will be actually exposed to radiation, and the emissivity of the skin 

and clothing will determine how much of the radiant heat energy will be absorbed on those surfaces. The insulation of clothing determines how much of the radiant heat absorbed at the surface 

of the garments will actually be transferred to the skin.

The ISO-WGTE recommended a linearized equation for calculating the value of R, using SI units:

R = hr Fcl(tr−tsk) (Wm-2 / °C-1)

The effect of insulation and emissivity of the clothing material on radiant heat exchange is covered by the addition of the clothing coefficient Fcl, which is also used in the equation for C as 

described above.

They also recommend a simplified equation for calculating an approximate value for hr:

hr = 4Esk Ar /ADu [(tr+ tsk)/2 + 273]3

= the universal radiation constant

= (5.67 × 10-8) Wm-2 °K-4

The effect of the emissivity of the skin on radiant heat exchange is covered by the expression 

E sk, which has the value of 0.97 in the infrared range. The effect of body position is covered by the 

expression Ar/ADu, which is the ratio of the skin surface area exposed to radiation and the total skin 

surface area, as estimated by DeBois’ formula:

ADu = 0.00718 × Weight0.425 / Height0.725

In this equation, body weight must be expressed in kg and height in cm, and the value of ADu is then 

obtained in m2. Values given for the ratio Ar/ADu by the ISO-WGTE include these:

Standing: 0.77

Seated: 0.70

Crouched: 0.67

The value of tr (mean radiant temperature) can be calculated by the following equation:

tr= tg + 1.8Va

0.5(tg – ta)

For further simplification, the value of tsk can be assumed to be 36°C, just as it was in the equation 

for convection.

Evaporation (E) SI Units

Ereq is the amount of heat that must be eliminated from the body by evaporation of sweat from 

the skin in order to maintain thermal equilibrium. However, there are major limitations to the 

Appendix A

148 Occupational Exposure to Heat and Hot Environments
maximum amount of sweat that can be evaporated from the skin (Emax):

(a) The human sweating capacity

(b) The maximum vapor uptake capacity of the ambient air

(c) The resistance of the clothing to evaporation

As described in Chapter 5†, the sweating capacity of healthy individuals is influenced by age, sex, 

state of hydration, and acclimatization.

The draft ISO-WGTE [ISO 1982b]‡ standard recommends that an hourly sweat rate of 650 grams 

for an unacclimatized person and 1,040 grams for an acclimatized person are the maximum permissible for the average worker while performing physical work in heat. However, these sweating 

rate limits are not the maximum sweating capacities; they are related to levels of heat stress at which 

the risk of heat-related illnesses is minimal.

For a full work shift, the total sweat output should not exceed 3,250 grams for an unacclimatized 

person and 5,200 grams for an acclimatized person to prevent deterioration in performance due to 

dehydration. It follows from the foregoing that if heat exposure is evenly distributed over an 8-hour 

shift, then the maximum acceptable hourly sweat rate is about 400 grams for an unacclimatized 

person and 650 grams for an acclimatized person.

Thus, if the worker’s heat exposure remains within the limits of the recommended standard, then 

the maximum sweating capacity will not be exceeded, and the limitation of evaporation will be due 

only to the maximum vapor uptake capacity of the ambient air. The Emax can be described with the 

equation recommended by the ISO-WGTE:

Emax = (psk,s-pa)/Re

where

Emax = maximum water vapor uptake capacity (Wm-2)

psk,s = saturated water vapor pressure at 36°C

 skin temperature (5.9 kPa)

pa = partial water vapor pressure at ambient air temperature (kPa)

Re = total evaporative resistance of the limiting layer of air and clothing (m2kPa W-1), which 

can be calculated by the following equation:

Re = 1 / 16.7 / hc / Fpcl

where

hc = convective heat exchange coefficient (Wm-2 / C-1)

Fpcl = reduction factor for loss in latent heat exchange due to clothing (dimensionless) which 

†

Chapter 4 of the current document.

‡

This standard has been revised. See Ergonomics of the Thermal Environment: Analytical Determination and 

Interpretation of Heat Stress Using Calculation of the Predicted Heat Strain (Standard No. ISO 7933) [ISO 2004b].

Appendix A

Occupational Exposure to Heat and Hot Environments 149
can be calculated by the following equation:

Fpcl = 1 / 1 + 0.92hc / Icl

where

Icl = Thermal insulation of clothing (m2 °C W-1)

What this means is that the maximum vapor uptake capacity of the air depends on the temperature, 

humidity, and velocity of the ambient air and clothing worn. However, the relationship of these 

variables with respect to human heat tolerance is quite complex. Further complications are caused 

by the fact that in order to be able to evaporate a certain amount of sweat from the skin, it is necessary to sweat more than that amount, because some of the sweat will drip off the skin or will be 

picked up by the clothing. To calculate the additional amount of sweat required due to dripping, the 

ISO-WGTE recommended the following equations:

Sreq = Ereq

where

Sreq = required sweat (Wm-2). This quantity can also be expressed as (g h-1 m-2) × 0.68

Ereq = required evaporation (Wm-2), which can be calculated by the equation Ereq = M + C + R

η = Evaporative efficiency of sweating of a nude person, which can be calculated by the following equation:

η = 1 – 0.5 / e-6.6(1-w)

where

e = the base of natural logarithm

w = Ereq/Emax, also called the “wettedness index”

The experimental data are insufficient to calculate the loss of evaporative efficiency of sweat due 

to the wicking effect of clothing. However, if the workers wear thin knitted cotton underwear, this 

can actually enhance the cooling efficiency of sweat, because after wicking the sweat off the skin, 

it spreads it more evenly over a larger area, thus enhancing evaporation and preventing dripping. 

Since the thin knitted material clings to the skin, the evaporative cooling will affect the skin without much loss to the environment. If a loosely fitting garment wicks up the sweat, there may be a 

substantial loss in evaporative cooling efficiency. However, if the heat exposure (M+C+R) remains 

below the human sweating capacity, then the exposed worker’s sweat excretion can increase to 

compensate for the loss of its cooling efficiency. A compensatory increase of sweating does not add 

much to the physiologic strain if water and electrolytes are replaced satisfactorily and if the water 

vapor uptake capacity of the ambient air is not exhausted.

In order to make sure that in the Sreq index the wettedness modifies the value of Sreq only to the 

extent to which it increases physiologic strain, the Ereq/Emax ratio affects the value of Sreq in an exponential manner.

Appendix A

150 Occupational Exposure to Heat and Hot Environments
The closer the value of Ereq comes to Emax, the greater the impact of w on Sreq. This is in accord with 

the physiologic strain as well as the subjective feeling of discomfort.

In this manner, the Sreq index is an improvement over other rational heat stress indices, but at the 

same time the calculations involved are more complex. With the availability of pocket-sized programmable calculators, the problem of calculations required is greatly reduced. However, it is questionable whether it is worthwhile to perform a complex calculation with variables that cannot be 

measured accurately. These variables include the mean weighted skin temperature, the velocity and 

direction of the air, the body position and exposed surface area, the insulation and vapor permeability of the clothing, and the metabolic heat generated by the work.

For practical purposes, simplicity of the calculations may be preferable to all-inclusiveness. Also, 

the utilization of familiar units such as kcal, Btu, and W, (the British units or metric units instead 

of the SI suggested), to express energy in heat production may increase the use of the calculations. 

They can be useful in analysis of a hot job for determining the optimal method of stress reduction 

and for prediction of the magnitude of heat stress so that proper preventive work practices and 

engineering controls can be planned in advance.

Appendix A

Occupational Exposure to Heat and Hot Environments 151
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Appendix B

Urine Chart
Urine charts can be implemented as a training tool to demonstrate the concept of color change 

between the urine of a well-hydrated worker and that of a dehydrated worker. When conducting an 

investigation to evaluate the validity and sensitivity of urine color, Armstrong et al. [1998] found 

that urine color was as good an index as urine osmolality, urine specific gravity, urine volume, 

plasma osmolality, plasma sodium, and plasma total protein for tracking changes in body water 

and hydration status. In an earlier study, the author suggested that urine color could be used in 

workplace settings where close estimates of urine specific gravity or urine osmolality are acceptable 

[Armstrong et al. 1994, 2010].

Although the urine chart is a good indicator of hydration status for most workers with normal pale 

yellow to deep amber urine, urine color can also be affected by diet, medications, and illnesses or 

disorders. See Table B-1.

Urine Color Chart

Are you hydrated?

Drink more water now

HYDRATED DEHYDRATED

v4.1

Urine Color Chart

Are you hydrated?

HYDRATED DEHYDRATED

v6

URGENT: Drink more water

Appendix B

154 Occupational Exposure to Heat and Hot Environments
Table B-1. Causes of abnormal colors in urine

Causes

Color Diet Medications Medical conditions

Clear ■ Diuretics

Cloudy or 

milky

■ Urinary tract infection

■ Bacteria

■ Crystals

■ Fat

■ White or red blood cells

■ Mucus

Yellow ■ Vitamins

Orange ■ B complex vitamins

■ Carotene

■ Carrots

■ Rifampin

■ Sulfasalazine (Azulfidine)

■ Phenazopyridine (Pyridium)

■ Laxatives

■ Chemotherapy drugs

■ Medical conditions 

(liver or bile duct)

Red or pink ■ Beets

■ Blackberries

■ Rhubarb

■ Rifampin 

(Rifadin, Rimactane)

■ Phenazopyridine

■ Laxatives containing senna

■ Blood (infection 

or cancer)

■ Toxins (chronic lead or 

mercury poisoning)

Port wine 

or purple

■ Porphyria 

(inherited disease)

Green or 

blue

■ Food dyes ■ Amitriptyline

■ Indonethacin (Indocin)

■ Propofol (Diprivan)

■ Medications containing 

methylene blue

■ Familial hypercalcemia 

(inherited disorder)

■ Urinary tract infection 

with Pseudomonas sp.

Brown ■ Fava beans

■ Rhubarb

■ Aloe

■ Kidney or liver 

disorders (cirrhosis)

■ Antimalarial drugs 

(chloroquine, primaquine)

■ Antibiotics (metronidazole, 

nitrofurantoin)

■ Laxatives containing cascara 

or senna

■ Methocarbamol 

(muscle relaxant)

■ Urinary tract infections

Adapted from Mayo Clinic 2011; Medline Plus 2011; Watson 2011.

Appendix B

Occupational Exposure to Heat and Hot Environments 155
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Appendix C

Heat Index
The National Oceanic and Atmospheric Administration (NOAA) issues heat alerts based on the 

heat index values, as seen in the chart above. The Heat Index is a measure of how hot it feels when 

RH is taken into account with the actual air temperature. Since heat index values were devised for 

shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F.

NOAA may also issue an extreme heat advisory:

■ Excessive Heat Outlook

Extended excessive heat (heat index of 105°F−110°F [41°C−43°C]) over the next 3 to 7 days.

■ Excessive Heat Watch

Excessive heat may occur within the next 24 to 72 hours.

■ Excessive Heat Warning

The heat index will be life threatening in the next 24 hours. Excessive heat is imminent or 

has a high probability of occurring.

■ Excessive Heat Advisory

The heat index may be uncomfortable but not life threatening if precautions are taken.

NOAA uses four bands of colors associated with four risk levels. The risk level–related measures in 

Table C-1 have been modified by OSHA for use on worksites.

Adapted from NOAA [2012].

40

45

50

55

60

65

70

75

80

85

90

95

100

80(27)

80(27)

80(27)

80(27)

82(28)

82(28)

82(28)

84(29)

84(29)

84(29)

86(30)

86(30)

87(31)

81(27)

82(28)

83(28)

84(29)

84(29)

85(29)

86(30)

88(31)

89(32)

90(32)

91(33)

93(34)

95(35)

83(28)

84(29)

85(29)

86(30)

88(31)

89(32)

90(32)

92(33)

94(34)

96(36)

98(37)

100(38)

103(39)

85(29)

87(31)

88(31)

89(32)

91(33)

93(34)

95(35)

97(36)

100(38)

102(39)

105(41)

108(42)

112(44)

88(31)

89(32)

91(33)

93(34)

95(35)

98(37)

100(38)

103(39)

106(41)

110(43)

113(45)

117(47)

121(49)

91(33)

93(34)

95(35)

97(36)

100(38)

103(39)

105(41)

109(43)

113(45)

117(47)

122(50)

127(53)

132(56)

94(34)

96(36)

99(37)

101(38)

105(41)

108(43)

112(46)

116(47)

121(49)

126(52)

131(55)

97(36)

100(38)

103(39)

106(41)

110(43)

114(46)

119(48)

124(51)

129(54)

135(57)

101(38)

104(40)

108(42)

112(44)

116(47)

121(49)

126(52)

132(56)

105(41)

109(43)

113(45)

117(47)

123(51)

128(53)

134(57)

109(43)

114(46)

118(48)

124(51)

129(54)

136(58)

114(46)

119(48)

124(51)

130(54)

137(58)

119(48)

124(51)

131(55)

137(58)

124(51)

130(50)

137(58)

130(54)

137(58)

136(58)

80(27) 82(28) 84(29) 86(30) 88(31) 90(32) 92(34) 94(34) 96(36) 98(37) 100(38) 102(39) 104(40) 106(41) 108(43) 110(47)

Likelihood of Heat Disorders with Prolonged Exposure or Strenuous Activity

Relative Humidity (%)

Caution Extreme Caution Danger Extreme Danger

NOAA’s National Weather Service 

Heat Index

40

45

50

55

60

65

70

75

80

85

90

95

100

80(27)

80(27)

80(27)

80(27)

82(28)

82(28)

82(28)

84(29)

84(29)

84(29)

86(30)

86(30)

87(31)

81(27)

82(28)

83(28)

84(29)

84(29)

85(29)

86(30)

88(31)

89(32)

90(32)

91(33)

93(34)

95(35)

83(28)

84(29)

85(29)

86(30)

88(31)

89(32)

90(32)

92(33)

94(34)

96(36)

98(37)

100(38)

103(39)

85(29)

87(31)

88(31)

89(32)

91(33)

93(34)

95(35)

97(36)

100(38)

102(39)

105(41)

108(42)

112(44)

88(31)

89(32)

91(33)

93(34)

95(35)

98(37)

100(38)

103(39)

106(41)

110(43)

113(45)

117(47)

121(49)

91(33)

93(34)

95(35)

97(36)

100(38)

103(39)

105(41)

109(43)

113(45)

117(47)

122(50)

127(53)

132(56)

94(34)

96(36)

99(37)

101(38)

105(41)

108(43)

112(46)

116(47)

121(49)

126(52)

131(55)

97(36)

100(38)

103(39)

106(41)

110(43)

114(46)

119(48)

124(51)

129(54)

135(57)

101(38)

104(40)

108(42)

112(44)

116(47)

121(49)

126(52)

132(56)

105(41)

109(43)

113(45)

117(47)

123(51)

128(53)

134(57)

109(43)

114(46)

118(48)

124(51)

129(54)

136(58)

114(46)

119(48)

124(51)

130(54)

137(58)

119(48)

124(51)

131(55)

137(58)

124(51)

130(50)

137(58)

130(54)

137(58)

136(58)

80(27) 82(28) 84(29) 86(30) 88(31) 90(32) 92(34) 94(34) 96(36) 98(37) 100(38) 102(39) 104(40) 106(41) 108(43) 110(47)

Likelihood of Heat Disorders with Prolonged Exposure or Strenuous Activity

Relative Humidity (%)

Caution Extreme Caution Danger Extreme Danger

NOAA’s National Weather Service 

Heat Index

Appendix C

158 Occupational Exposure to Heat and Hot Environments
Table C-1. Heat index–associated protective measures for worksites

Heat index Risk level Protective measure

Less than 91°F 

(33°C)

Lower (caution) Basic health and safety planning

91°F to 103°F

(33°C to 39°C)

Moderate Implement precautions and heighten awareness

103°F to 115°F

(39°C to 46°C)

High Additional precautions to protect workers

Greater than 115°F 

(46°C)

Very high to extreme Even more aggressive protective measures

Adapted from OSHA [2012c].

Additional information about protective measures mentioned in the above table can be found on OSHA’s website. 

Note: The presence of a radiant heat source may decrease the accuracy and usefulness of the above heat index.

Appendix C

Occupational Exposure to Heat and Hot Environments 159
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DHHS (NIOSH) Publication No. 2016-106