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Working in the Heat

What is Heat Stress?

Heat stress is defined as the net (overall) heat burden on the body from the combination of the body heat generated while working, environmental sources (air temperature, humidity, air movement, radiation from the sun or hot surfaces/sources) and clothing requirements1 . It is obvious to see why heat stress is an issue for employees who work around radiant heat sources or have direct physical contact with hot objects like in iron and steel foundries or outdoor construction work, but is also important for the service sector. For example, workers in bakeries, commercial kitchens, outdoor operations, and laundries may be exposed to heat stress.

Effects of Heat Exposure

It is important to know how the body reacts to heat and the symptoms which may put your health at risk. Age, weight, degree of physical fitness, degree of acclimatization, metabolism, use of alcohol or drugs, and a variety of medical conditions such as hypertension all affect a person's sensitivity to heat, and even the type of clothing worn must be considered3.

Heat Exhaustion

  • Dehydration
  • Thirst, nausea, fatigue, headache, dizziness, muscle cramps, profuse sweating
  • Increased pulse rate and breathing rate
  • Core body temperature < 38oC
  • Treatment: place in cool place, fluids, rest at least 24 hours
  • May progress to heat stroke

Heat Stroke

  • Medical emergency
  • Irritability, confusion, bizarre behaviour, weakness, nausea, vomiting, collapse, seizures, coma, death
  • Lack of sweating; hot, dry skin
  • Core body temperature > 40oC
  • Dangers: damage to muscles, kidney, liver, brain, heart; increased bleeding; brain damage; and death
  • Treatment: call 911; need hospitalization

Heat Cramps

  • Painful muscle cramps involving muscles used for strenuous work
  • Involved muscles feel like hard, stony lumps
  • Moist, cool skin
  • Treatment: Move to cool place, drinking commercial carbohydrate-electrolyte liquids relieves cramps

Heat Fainting or Heat Collapse

  • Sudden unconsciousness
  • May occur following at least 2 hours of strenuous work
  • Rest in cool place lying down, fluids

Heat Rash

  • The most common problem in hot work environments
  • Red bumpy rash with itching
  • Appears in areas where clothing is restrictive
  • Treatment: wash skin, change into dry clothes
  • Also: sunburn with outdoor work

Control Measures

Engineering Controls

  • General Ventilation
  • Isolation
  • Localized cooling of workstation
  • Shielding

Administrative Controls and Work Practices

  • Training including knowledge of the hazards of heat stress, and awareness of first-aid procedures and the potential health effects of heat stroke.
  • Re-scheduling tasks
  • Work-rest scheduling; cool rest areas (25oC)
  • Follow-up of heat-related incidents


The process by which the body can adapt to heat exposure is called acclimatization. After a period of acclimatization, the same activity will produce fewer cardiovascular demands. The worker will sweat more efficiently (causing better evaporative cooling), and thus will more easily be able to maintain normal body temperatures.

Fluid Replacement:

Cool (10-15C) water or any cool liquid (except alcoholic beverages) should be made available to workers to encourage them to drink small amounts frequently (e.g.8 ounces every hour)

Personal Protective Clothing

  • Light porous clothing, or
  • In high radiant environment, reflective clothing.

Employer Prevention Program

Employers need to evaluate their workplaces to ensure workers are protected in hot environments. Elements of a hot environment prevention program include4:

  • Worker training
  • Environmental Evaluation
  • Engineering & process controls; cooling fans
  • Re-scheduling tasks
  • Drinking water: 8 ounces every ½ hour
  • Work-rest scheduling; cool rest areas (25oC)
  • Appropriate clothing and ppe
  • Acclimatization time
  • Follow-up of heat-related incidents

For more information on prevention, check out the OSSA's "Working in Heat" hazard kit: Working in Heat.

Exposure Limits and Legislation

  • In Ontario, the Ministry of Labour (MOL) requires employers to protect workers in hot environments.
  • Orders are issued under Section 25(2)(h) of the Occupational Health and Safety Act.
  • The MOL uses the exposure guidelines recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) using the Wet Bulb Globe Temperature (WBGT). See Table 1 5.
  • WBGT is the only index to evaluate air temperature, humidity, air flow and radiant heat load, and there are modifications for work load, clothing and acclimatization.

Table 1
Screening Criteria for Heat Stress Exposure
(WBGT values in C)
for 8 hour work day five days per week with conventional breaks

  Acclimatized Unacclimatized
Work Demands Light Moderate Heavy Very Heavy Light Moderate Heavy Very Heavy
100% work 29.5 27.5 26.0 - 27.5 25.0 22.5 -
75% work; 25% rest 30.5 28.5 27.5 - 29.0 26.5 24.5 -
50% work; 50% rest 31.5 29.5 28.5 27.5 30.0 28.0 26.5 25
25% work; 75% rest 32.5 31.0 30.0 29.5 31.0 29.0 28.0 26.5

* For unacclimatized workers, the permissible heat exposure TLV should be reduced by 2.5C
Examples of work loads:
Light work -- sitting or standing to control machines, performing light hand or arm work Moderate work -- walking about with moderate lifting and pushing Heavy work -- pick and shovel work, digging

Adopted from: 2001 TLVs and BEIs: Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. Cincinnati, Ohio: American Conference of Governmental Industrial Hygienists, 2001. p. 171-172.

Guides & Tool Downloads

  • Heat Stress Education Poster by the Occupational Health and Safety Council of Ontario (OHSCO): Click to download
  • Heat Stress Guide 22 pages by the Occupational Health and Safety Council of Ontario (OHSCO): Click to download
  • Heat Stress Awareness Tool by the Occupational Health and Safety Council of Ontario (OHSCO): Click to download (Please use high quality colour printer when printing)
  • Ministry of Labour guidelines on heat stress: Click to download

1 Reference: American Conference of Governmental Industrial Hygienists.
2 Reference: Presentation "Working in Hot Environments" by Leon Genesove, Provincial Physician Ontario Ministry of Labour, OSSA - June 19, 2002
3 U.S. Dept. of Labour, OSHA Technical Manual, Web site: OSHA Technical Manual
4 Reference: Presentation "Working in Hot Environments" by Leon Genesove, Provincial Physician Ontario Ministry of Labour, OSSA - June 19, 2002
5 Source: Canadian Centre for Occupational Health and Safety Web site: CCOHS Physical Agents

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