Criteria for a Recommended Standard: 

Occupational Exposure to Heat and 

Hot Environments

Revised Criteria

Brenda L. Jacklitsch, MS

Paul A. Schulte, PhD

Education and Information Division

Public Meeting, February 13, 2014

Cincinnati, OH

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This information is distributed solely for the purpose of pre dissemination peer review under applicable 

information quality guidelines. It has not been formally disseminated by the National Institute for 

Occupational Safety and Health. It does not represent and should not be construed to represent any 

agency determination or policy.
Why Update the Criteria Document?

 Injuries, illnesses, and deaths in a large worker population.

 Document was last updated in 1986.

 In recent years, including during the Deepwater Horizon oil spill 

response of 2010, questions raised regarding the need for revision.

 Health Hazard Evaluation (HHE) requests and CDC Info requests.

 Additional info relating to the physiological changes that result from 

heat stress.

 Updated info from relevant studies.

 Need to redefine heat stroke and associated symptoms.

 Updated info on physical monitoring and Personal Protective 

Equipment (PPE).

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History of NIOSH Heat Stress Activities

 1972 – Original Criteria Document published

 1986 – Revised Criteria Document

 2005 – Research activity on PPE and heat stress begins

 2008 – Heat Stress Web Topic page revised; MMWR on heat-related 

deaths among crop workers

 2010 – NIOSH Fast Facts card; participation in the Deepwater Oil Spill 

Response; Start reviewing content of Criteria Document

 2011 – OSHA-NIOSH Infosheet; Medscape Article; NIOSH Blog; E-News 

Director’s Desk; AIHce Table Talk; Begin work on Criteria Document

 2013 – Workplace Solution document

 2014 – Criteria Document Public Meeting and Tentative Publication

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Definitions

 Heat Stress

 The net heat load to which a person may be exposed from the 

combined contributions of metabolic heat, environmental factors, 

and clothing requirements which may result in an increase in heat 

storage in the body.

 Heat Strain

 The physiological response to the heat load experienced by a 

person, which attempts to increase heat loss from the body in 

order to maintain a stable body temperature.

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Occupational Relevance of

Heat Stress

 Potential for worker exposure

 Outdoor workers

• Ex. Construction; Roofers; Farmers/Agricultural; Lawn 

care/Landscapers; Foresters; Emergency Responders

 Indoor workers in hot environments

• Ex. Firefighters; Factory/Foundry; Bakery; Anyone in hot, 

confined spaces

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Occupational Relevance of

Heat Stress

 Heat-related illness, such as:

 Heat stroke

 Heat exhaustion

 Heat cramps

 Heat rashes

 Increased risk of injury, resulting from:

 Sweaty palms

 Fogged-up safety glasses

 Dizziness

 Contact with hot surfaces or steam

 Increased risk of death

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Factors Affecting Risk of Heat-related Illness

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Purpose of a Criteria Document

NIOSH is mandated by the Occupational Safety and Health 

(OSH) Act of 1970:

“...to develop criteria dealing with toxic materials and harmful physical 

agents and substances which will describe exposure levels that are safe for 

various periods of employment, including but not limited to the exposure 

levels at which no employee will suffer impaired health or functional 

capacities or diminished life expectancy as a result of his work experiences.”

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[OSH Act, 20 USC 22 (a)(3)]
Updating this Criteria Document

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Literature 

Search

Chapter 

Updates

Internal 

Review

Public 

Comment

External 

Peer 

Review

Publication
Criteria Document Topics

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Fundamental Thermal Physiology 

Concepts and Established NIOSH Limits

 Heat Balance Equation

 Modes of Heat Exchange

 Thermoregulatory Pathways

 Basic Effects of Clothing on Heat Exchange

 NIOSH Recommended Alert Limit (RAL) and 

Recommended Exposure Limit (REL)

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Heat Balance Equation

 Normal body function requires that the deep body core 

temperature be maintained at about 37°C (98.6°F) ± 1°C 

(1.8°F).

 Body temperature equilibrium requires constant exchange 

of heat between the body & environment.

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Change in 

body heat 

content

=

Total 

Metabolism -

External 

Work ( Performed )

Convection

Heat

Exchange 

(C)

Radiative

Heat

Exchange 

(R)

Evaporative 

Heat Loss (E) ± ± -

C = Difference between the ambient air & the mean weighted skin temp. & the rate of air movement over 

the skin.

R = Temp. gradient between the mean radiant temp. of the surroundings & the mean weighted skin temp.

E = Air motion & water vapor pressure difference between ambient air & wetted skin at skin temp.
Major Modes of Heat Exchange

 Convection

 Transfer of heat between skin and ambient air immediately surrounding the skin.

 Rate of air movement is a factor.

 Radiation

 Transfer of heat between the radiant temperature of the surroundings and the 

skin.

 Ex. Heat from hot ovens or the sun.

 Evaporation

 Water evaporating from the skin resulting in heat loss.

 Ex. Evaporation of sweat.

 Conduction

 Transfer of heat from a warmer object to a cooler object by direct contact.

 Ex. Body contact with hot tools, working in water, or in supine positions.

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Physiological Responses to Heat

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Skin

Temperature

Core

Temperature

Peripheral

Thermoreceptors

(in skin)

Central

Thermoreceptors

(in hypothalamus, 

CNS, abdominal 

organs)

Skin Blood 

Vessels Sympathetic

Nervous System

Sweat

Glands

Vasodilation Sweating

Heat Loss Heat Loss

Hypothalamus

(acts like thermostat)
Effects of Clothing on Heat Exchange

 Clothing serves as a barrier between skin and the environment 

to protect against hazards (i.e., chemical, physical, and biologic 

agents).

 Clothing systems alter the rate and amount of heat exchange

between the skin and the ambient air by convection (C), 

radiation (R), and evaporation (E).

 The thicker, and greater the air and vapor impermeability of 

the clothing layers, the greater its interference with C, R, & E 

heat exchange.

 During exertion there is an increased metabolic heat 

production that can be trapped inside clothing, creating a hot 

microenvironment.

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NIOSH RAL and REL

 Recommended Alert Limit (RAL) 

for unacclimatized workers.

 Recommended Exposure Limit 

(REL) for acclimatized workers.

 Still protective.

 No new data identified to use as 

the basis for an update.

 Wet Bulb Globe Temperature 

(WBGT)-based threshold values 

for acclimatized workers are 

similar to OSHA, ACGIH, AIHA, 

and ISO.

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Updated Items

 Hydration and Fluid Requirements

 Urine Color Chart

 Psychrometric Chart

 Body Fat and Obesity

 Acclimatization

 Medications

 Caffeine Usage

 Exertional Heat Stroke

 HHE Reports and Case Studies

 Physiological Monitoring

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Hydration and Fluid Requirements

• Drink 8 oz. of water or other fluids every 15-20 

minutes (moderate activity in moderate 

conditions).

• During prolonged sweating lasting several hours, 

drink a sports drink that contains balanced 

electrolytes (electrolytes/carbohydrates should 

not exceed 8% by volume).

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Urine Color Chart

Adapted from [Mayo Clinic 2011; Medline Plus 2011; Watson 2011].

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Addition of the Psychrometric Chart

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Adapted from ISO [1993] and Coolerado [2012].
Body Fat and Obesity

 Body fat is an insulator and alters heat exchange with the 

environment.

 Obesity predisposes individuals to heat disorders -- heat 

disorders occur 3.5 times more frequently in the obese 

than in the lean individual (Leithead and Lind 1964; 

Henschel 1967; Chung and Pin 1996; Kenney et al. 2010).

 More fat → More weight → Greater energy expenditure 

for a given task

 Body surface to body weight ratio.

 Store thermal energy at a greater rate.

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Acclimatization

Topics Additional Information

Acclimatization Plan

 Gradually increase exposure time in hot environmental conditions over a 7-14 day period.

 For new workers, the schedule should be no more than 20% 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 a 50% exposure on day 1, 60% on day 2, 80% on day 3, and 100% on 

day 4. 

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 increase likelihood of acute dehydration, illness, or fatigue.

 Can be regained in 2-3 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 which also apply in hot, desert 

environments, and vice versa.

 Air conditioning will not affect acclimatization.

Adapted from [Moseley 1994; Armstrong and Stoppani 2002; TBMed 2003; Casa et al. 2009; ACGIH 2011; OSHA-NIOSH 2011].

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Medications

Drug or drug class Proposed mechanism of action

Anticholinergics Impaired sweating

Antihistamines Impaired sweating

Phenothiazines (Thorazine®, Stelazine®, Trilafon®) Impaired sweating, (possibly) disturbed 

hypothalamic temperature regulation

Tricyclic antidepressants (imipramine, 

amitriptyline)

Impaired sweating, increased motor activity and 

heat production

Amphetamines, cocaine, “Ecstasy” Increased psychomotor activity, activated 

vascular endothelium

Ergogenic stimulants (ephedrine/ephedra) Increased heat production

Lithium Nephrogenic diabetes insipidus and water loss

Diuretics Salt depletion and dehydration

Antihypertensives (atenolol, carvedilol) 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

Adapted from Heat Stress Control and Heat Casualty Management (DOD 2003).

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Caffeine Usage

 Past: Caffeine is a mild diuretic and may contribute to 

heat stress by reducing fluid volume and resulting in 

cardiovascular strain during exposure to the heat (Serafin 

1996).

 Update: Recent studies suggest that caffeine may have 

less effect on heat tolerance than previously suspected 

(Roti et al. 2006; Armstrong, Casa, Maresh et al. 2007; Ely et 

al. 2011).

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Heat Stroke: Classical vs. Exertional

Adapted from Heat Stress Control and Heat Casualty Management [DOD 2003].

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Characteristic Classical Heat Stroke Exertional Heat Stroke

Age Young children or elderly 15-45 years

Health Chronic illness common Usually healthy

Activity Sedentary Strenuous exercise

Sweating Usually absent Often present

Prevailing weather Frequent in prolonged 

heat waves

Variable

Rhabdomyolysis Seldom severe Common, may be severe

Acute renal failure Fairly rare Common
HHE Reports and Case Studies

 HHE Reports

 Aluminum Smelter Potrooms (NIOSH 2006)

 Automobile Parts Manufacturing Facility (NIOSH 2003)

 Glass Bottle Manufacturer (NIOSH 2003)

 National Park (NIOSH 1999)

 Case Studies

 Landscaping (NIOSH 2002)

 Migrant Farm Workers (NIOSH 2006, CDC 2008)

 Construction (NIOSH 2004)

 Fire Fighter (NIOSH 1997)

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Physiological Monitoring

Monitoring 

Method

When 

Assessed

How 

Assessed

Additional Information

Heat Exposure 

History

 Before work begins  Interview or questionnaire  A history of heat-related illness increases 

the risk of a repeat occurrence, so worker 

should be monitored more closely.

 Some workers might choose to alert their 

employers of medical conditions which 

increase the risk of heat-related illnesses.

Heart Rate

(Pulse Rate)

 Before work begins to 

determine baseline and then 

after heat exposure (e.g., 1st

minute and 3rd minute after 

the work period ends)

 Count the number of beats per minute 

(use a watch), or monitor with heart 

rate sensor

 The heart rate should fall rapidly, 

approaching the baseline.

 Heart rate will remain elevated in a worker 

experiencing a heat-related illness.

Temperature  Increased temperature indicates that the 

body is not cooling itself as rapidly as 

necessary.

 Initial baseline and again 

after the work period

 Oral temperature – measure with an 

oral thermometer

 Oral temperature is inaccurate if the 

workers drinks cool beverages frequently 

(as is recommended).

 Initial baseline and again 

after the work period.

 Tympanic temperature – measure with 

an infrared thermometer

 Tympanic temperature is a more reliable 

indicator of core temperature than oral.

 Continuous sensing devices 

measure temperature during 

both work and rest periods

 Core temperature – measure with 

electronic or color-changing sensing 

devices (e.g., ingestible, in-ear, or part 

of skin patches)

 Core temperature is the most reliable 

measure of body temperature.

 Modern advances in sensing technology 

are making core temperature 

measurements increasingly practical.

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Future Research Needs

 Effects of climate change on outdoor workers.

 Increased ambient temperatures.

 Extreme weather.

 Reduced work capacity and productivity in heatexposed jobs.

 How heat stress affects toxic response to 

chemicals.

 Heat can affect absorption of chemicals into the body.

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Timeline

 Federal Register Notice requesting public comments 

posted on December 27, 2013.

 Public meeting on February 13, 2014.

 Receive all public comments by February 25, 2014.

 Receive all peer review comments by 

March 28, 2014.

 Anticipate completion of FINAL Criteria Document in 

2014.

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Charge to Peer Reviewers

 The following questions should be considered:

 Is worker acclimatization clearly explained and presented?

 Are there any additional recommendations that should be made? 

 Is there any additional information on hydration that should be 

considered?

 Are there any additional risk factors for heat-related illnesses that 

should be discussed?

 Are there any additional examples of auxiliary body cooling and 

protective clothing that should be included?

 Are there any additional research needs that should be 

mentioned? 

 Are there any additional references that should be included?

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Criteria Document Update Team

 Authors

 W. Jon Williams (NPPTL)

 Nina Turner (NPPTL)

 Aitor Coca (NPPTL)

 Jung-Hyun Kim (NPPTL)

 Kristin Musolin (DSHEFS)

 Support

 Thomas J. Lentz (EID)

 Kathleen MacMahon (EID)

 Ralph Zumwalde (EID)

 Lauralynn Taylor McKernan (EID)

 HeeKyoung Chun

 Barbara Dames (EID)

 Sherry Fendinger (EID)

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For updates visit the NIOSH Docket Heat Stress web page:

http://www.cdc.gov/niosh/docket/review/docket266/default.html

Written comments may be submitted by :

Federal eRulemaking Portal: http://www.regulations.gov

Enter: CDC-2013-0025

Mail: 

NIOSH Docket Office

Robert A. Taft Laboratories

4676 Columbia Parkway, MS C–34

Cincinnati, OH 45226

Thank you!

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