1. Where does heat stress occur in construction?

Heat stress can occur wherever construction operations involve heavy physical work in hot, humid environments. The locations may be indoors (mills, foundries, electrical vaults, interior renovation, etc.) or outdoors (road-building, excavation, roofing, home-building, etc.).

Asbestos removal and other operations that require workers to wear semi-permeable or impermeable protective clothing can contribute significantly to heat stress. Heat stress causes the body's core temperature to rise.


2. What happens when the body's core temperature rises?

The human body functions best within a narrow range of internal temperature. This core temperature varies from 36°C to 38°C. A construction worker performing heavy work in a hot environment builds up body heat. To get rid of excess heat, the body uses two cooling mechanisms:

  • The heart rate increases to move blood—and heat—from heart, lungs, and other vital organs to the skin.
  • Sweating increases to help cool blood and body. Sweating is the most important way the body gets rid of excess heat.

When too much sweat is lost through heavy labour or working under hot, humid conditions, the body doesn't have enough water left to cool itself. The result is dehydration. Core temperature rises above 38°C. A series of heat-related illnesses, or heat stress disorders, can then develop.


3. How can we recognize heat stress disorders?

Heat stress disorders range from minor discomfort to life-threatening conditions:

  • heat rash
  • heat cramps
  • heat exhaustion
  • heat stroke.

Heat rash

Heat rash-also known as prickly heat-is the most common problem in hot work environments. Symptoms include

  • red blotches and extreme itchiness in areas persistently damp with sweat
  • prickling sensation on the skin where sweating occurs.

Treatment – cool environment, cool shower, thorough drying. In most cases, heat rashes disappear a few days after heat exposure ceases. If the skin is not cleaned frequently enough the rash may become infected.


Heat cramps

Heat cramps – Under extreme conditions, such as removing asbestos from hot water pipes for several hours in heavy protective gear, the body may lose salt through excessive sweating. Heat cramps can result. These are spasms in larger muscles-usually back, leg, and arm. Cramping creates hard painful lumps within the muscles.

Treatment – stretch and massage muscles; replace salt by drinking commercially available carbohydrate/electrolyte replacement fluids.


Heat exhaustion

Heat exhaustion occurs when the body can no longer keep blood flowing to supply vital organs and send blood to the skin to reduce body temperature. Signs of heat exhaustion include

  • weakness
  • difficulty continuing work
  • headache
  • breathlessness
  • nausea or vomiting
  • feeling faint or actually fainting.

Workers fainting from heat exhaustion while operating equipment can injure themselves and others.

Treatment – heat exhaustion casualties respond quickly to prompt first aid. If not treated promptly, heat exhaustion can lead to heat stroke -- a medical emergency. It takes 30 minutes at least to cool the body down once a worker suffers heat exhaustion.

  • Call 911.
  • Help the casualty to cool off by
    • resting in a cool place
    • drinking cool water
    • removing unnecessary clothing
    • loosening clothing
    • showering or sponging with cool water.

Heat stroke

Heat stroke occurs when the body can no longer cool itself and body temperature rises to critical levels.

Heat stroke requires immediate medical attention.

The primary signs and symptoms of heat stroke are

  • confusion
  • irrational behaviour
  • loss of consciousness
  • convulsions
  • lack of sweating
  • hot, dry skin
  • abnormally high body temperature -- for example, 41°C.


  • Call 911.
  • Provide immediate, aggressive, general cooling.
  • immerse casualty in tub of cool water or
  • place in cool shower or
  • spray with cool water from a hose.
  • Wrap casualty in cool, wet sheets and fan rapidly.
  • Transport casualty to hospital.
  • Do not give anything by mouth to an unconscious casualty.

4. What factors are used to assess heat stress risk?

Factors that should be considered in assessing heat stress include

  • personal risk factors
  • environmental factors
  • job factors.

Personal risk factors

It is difficult to predict just who will be affected by heat stress and when, because individual susceptibility varies. There are, however, certain physical conditions that can reduce the body's natural ability to withstand high temperatures:



Workers who are overweight are less efficient at losing heat.


Poor physical condition

Being physically fit aids your ability to cope with the increased demands that heat puts on your body.


Previous heat illnesses

Workers are more sensitive to heat if they have experienced a previous heat-related illness.



As the body ages, its sweat glands become less efficient. Workers over the age of 40 may have trouble with hot environments. Acclimatization to heat and physical fitness can offset some age-related problems.


Heart disease or high blood pressure

In order to pump blood to the skin and cool the body, the heart rate increases. This can stress the heart.


Recent illness

Workers with recent illnesses involving diarrhea, vomiting, or fever have an increased risk of dehydration and heat stress because their bodies have lost salt and water.


Alcohol consumption

Alcohol consumption during the previous 24 hours leads to dehydration and increased risk of heat stress.



Certain drugs may cause heat intolerance by reducing sweating or increasing urination. People who work in a hot environment should consult their physician or pharmacist before taking medications.


Lack of acclimatization

When exposed to heat for a few days, the body will adapt and become more efficient in dealing with raised environmental temperatures. This process is called acclimatization. Acclimatization usually takes 6 to 7 days. Benefits include

  • lower pulse rate and more stable blood pressure
  • more efficient sweating (causing better evaporative cooling)
  • improved ability to maintain normal body temperatures.

Acclimatization may be lost in as little as three days away from work. People returning to work after a holiday or long weekend—and their supervisors—should understand this. Workers should be allowed to gradually re-acclimatize to work conditions.


Environmental factors

Environmental factors such as ambient air temperature, air movement, and relative humidity can all affect an individual's response to heat. The body exchanges heat with its surroundings mainly through radiation and sweat evaporation. The rate of evaporation is influenced by humidity and air movement.


Radiant Heat

Radiation is the transfer of heat from hot objects through air to the body. Working around heat sources such as kilns or furnaces will increase heat stress. Additionally, working in direct sunlight can substantially increase heat stress. A worker is far more comfortable working at 24°C under cloudy skies than working at 24°C under sunny skies.



Humidity is the amount of moisture in the air. Heat loss by evaporation is hindered by high humidity but helped by low humidity. As humidity rises, sweat tends to evaporate less. As a result, body cooling decreases and body temperature increases.


Air Movement

Air movement affects the exchange of heat between the body and the environment. As long as the air temperature is less than the worker's skin temperature, increasing air speed can help workers stay cooler by increasing both the rate of evaporation and the heat exchange between the skin surface and the surrounding air.


Job factors

Clothing and Personal Protective Equipment (PPE)
Heat stress can be caused or aggravated by wearing PPE such as fire- or chemical-retardant clothing. Coated and non-woven materials used in protective garments block the evaporation of sweat and can lead to substantial heat stress. The more clothing worn or the heavier the clothing, the longer it takes evaporation to cool the skin. Remember too that darker-coloured clothing absorbs more radiant heat than lighter-coloured clothing.



The body generates more heat during heavy physical work. For example, construction workers shoveling sand or laying brick in hot weather generate a tremendous amount of heat and are at risk of developing heat stress without proper precautions. Heavy physical work requires careful evaluation even at temperatures as low as 23°C to prevent heat disorders. This is especially true for workers who are not acclimatized to the heat.


5. Are there measures for evaluating heat stress risk?

To prevent heat stress, scientists from the World Health Organization (WHO) have determined that workers should not be exposed to environments that would cause their internal body temperature to exceed 38°C. The only true way of measuring internal body temperature is rectally (oral or inner ear measurements are not as accurate). As an alternative, the American Conference of Governmental Industrial Hygienists (ACGIH) has developed a method of assessing heat stress risk based on a wet bulb globe temperature (WBGT) threshold.

This method of assessment involves the three main components of the heat burden experienced by workers:

  • thermal environment
  • type of work
  • type of clothing.

Thermal environment

The first factor in assessing heat stress is the thermal environment as measured by WBGT index. WBGT is calculated in degrees Celsius using a formula which incorporates the following three environmental factors:

  • air temperature
  • radiant heat (heat transmitted to the body through the air from hot objects such as boilers or shingles heated by the sun)
  • cooling effects of evaporation caused by air movement (humidity).

To measure WBGT, a heat stress monitor consisting of three types of thermometers is required:

  • A normal thermometer called a dry bulb thermometer is used to measure air temperature.
  • Radiant heat is measured by a black bulb globe thermometer. This consists of a hollow, 6-inch diameter copper ball painted flat black and placed over the bulb of a normal thermometer.
  • A wet bulb thermometer measures the cooling effect of evaporation caused by air movement (wind or fan). It consists of a normal thermometer wrapped in a wick kept moist at all times. As air moves through the wet wick, water evaporates and cools the thermometer in much the same way that sweat evaporates and cools the body.

Heat stress monitor

Heat stress monitors currently available calculate WBGT automatically. The equipment required and the method of measuring WBGT can be found in the ACGIH booklet TLVs® and BEIs®: Threshold limit Values. The booklet also outlines permissible exposure limits for heat stress. Older instruments, however, require calculation by the operator.

Calculation depends on whether sunlight is direct (outdoors) or not (indoors).


Working outdoors in direct sunlight

For work in direct sunlight WBGT is calculated by taking 70% of the wet bulb temperature, adding 20% of the black bulb temperature, and 10% of the dry bulb temperature. WBGT (out) = 70% (0.7) x wet bulb temperature + 20% (0.2) x black bulb globe temperature + 10% (0.1) x dry bulb temperature


Working indoors (no sunlight)

For work indoors or without direct sunlight, WBGT is calculated by taking 70% of the wet bulb temperature and adding 30% of the black bulb temperature.

WBGT (in)= 70% (0.7) x wet bulb temperature + 30% (0.3) x black bulb globe temperature



Suppose it's a bright sunny day and a crew of roofers is working 20 feet above ground. Our assessment yields the following readings:

Wet bulb temperature(cooling effects of evaporation) = 20°C
Black bulb globe temperature (radiant heat) = 36°C
Dry bulb temperature (air temperature) = 33°C

Using the formula for work in direct sunlight, we calculate as follows:
WBGT = 0.7 x wet bulb temperature + 0.2 x black bulb globe temperature + 0.1 x dry bulb temperature
= 0.7 x 20 + 0.2 x 36 + 0.1 x 33
= 14 + 7.2 + 3.3
WBGT (outdoors) = 24.5 °C


Type of work

The second factor in assessing heat stress is the type of work being performed. Following are the four categories, with some examples of each:

Light work
  • Using a table saw
  • Some walking about
  • Operating a crane, truck, or other vehicle
  • Welding
Moderate work
  • Laying brick
  • Walking with moderate lifting or pushing
  • Hammering nails
  • Tying rebar
  • Raking asphalt
  • Sanding drywall
Heavy work
  • Carpenter sawing by hand
  • Shoveling dry sand
  • Laying block
  • Ripping out asbestos
  • Scraping asbestos fireproofing material
Very Heavy Work
  • Shoveling wet sand
  • lifting heavy objects

Type of clothing

Free movement of cool, dry air over the skin maximizes heat removal. Evaporation of sweat from the skin is usually the major method of heat removal. WBGT-based heat exposure assessments are based on a traditional summer work uniform of long-sleeved shirt and long pants. With regard to clothing, the measured WBGT value can be adjusted according to Table 1.


TABLE 1: Additions to measured WBGT values for some types of clothing

Clothing Type
Addition to WBGT
Summer work uniform
Cloth (woven material) overalls
Double-cloth overalls

These additions do not apply to encapsulating suits, thermal-insulated clothing, or clothing impermeable or highly resistant to water vapour or air movement. Special garments such as these, and multiple layers of clothing, severely restrict sweat evaporation and heat removal. As a result, body heat may produce life-threatening heat stress even when environmental conditions are considered cool.


6. How can heat stress be controlled?

Heat stress can be controlled through education, engineering, and work procedures. Controls will

  • Protect health
    Illness can be prevented or treated while symptoms are still mild.
  • Improve safety
    Workers are less liable to develop a heat-related illness and have an accident. Heat stress often creeps up without warning. Many heat-induced accidents are caused by sudden loss of consciousness.
  • Increase productivity
    Workers feel more comfortable and are likely to be more productive as a result.

Training and education

According to the National Institute of Occupational Safety and Health (NIOSH), heat stress training should cover the following components:

  • knowledge of heat stress hazards
  • recognition of risk factors, danger signs, and symptoms
  • awareness of first-aid procedures for, and potential health effects of, heat stroke
  • employee responsibilities in avoiding heat stress
  • dangers of using alcohol and/or drugs (including prescription drugs) in hot work environments.

Engineering controls

Engineering controls are the most effective means of preventing heat stress disorders and should be the first method of control. Engineering controls seek to provide a more comfortable workplace by using

  • reflective shields to reduce radiant heat
  • fans and other means to increase airflow in work areas
  • mechanical devices to reduce the amount of physical work.

Given the constantly changing nature of construction sites, engineering controls are not usually feasible. Proper work procedures are therefore required to prevent heat stress disorders.


Work procedures

The risks of working in hot construction environments can be diminished if labour and management cooperate to help control heat stress.



  • Give workers frequent breaks in a cool area away from heat. The area should not be so cool that it causes cold shock-around 25°C is ideal.
  • Increase air movement by using fans where possible. This encourages body cooling through the evaporation of sweat.
  • Provide unlimited amounts of cool (not cold) drinking water conveniently located.
  • Allow sufficient time for workers to become acclimatized. A properly designed and applied acclimatization program decreases the risk of heat-related illnesses. Such a program exposes employees to work in a hot environment for progressively longer periods. NIOSH recommends that for workers who have had previous experience in hot jobs, the regimen should be
    • 50% exposure on day one
    • 60% on day two
    • 80% on day three
    • 100% on day four.
  • For new workers in a hot environment, the regimen should be 20% on day one, with a 20% increase in exposure each additional day.
  • Make allowances for workers who must wear personal protective clothing and equipment that retains heat and restricts the evaporation of sweat.
  • Schedule hot jobs for the cooler part of the day; schedule routine maintenance and repair work in hot areas for the cooler seasons of the year.
  • Consider the use of cooling vests containing ice packs or ice water to help rid bodies of excess heat.


  • Wear light, loose clothing that permits the evaporation of sweat.
  • Drink small amounts of water—8 ounces (250 ml) —every half hour or so. Don't wait until you're thirsty.
  • Avoid beverages such as tea, coffee, or beer that make you pass urine more frequently.
  • Where personal PPE must be worn,
    • use the lightest weight clothing and respirators available
    • wear light-colored garments that absorb less heat from the sun
    • use PPE that allows sweat to evaporate.
  • Avoid eating hot, heavy meals. They tend to increase internal body temperature by redirecting blood flow away from the skin to the digestive system.
  • Don't take salt tablets unless a physician prescribes them. Natural body salts lost through sweating are easily replaced by a normal diet.

7. What are the responsibilities of workplace parties regarding heat stress?



The Occupational Health and Safety Act and its regulations do not specifically cover worker exposure to heat. However, under the Occupational Health and Safety Act employers have a general obligation to protect workers exposed to hot environments. Employers should develop a written health and safety policy outlining how workers in hot environments will be protected from heat stress. As a minimum, the following points should be addressed.

  • Adjust work practices as necessary when workers complain of heat stress.
  • Make controlling exposures through engineering controls the primary means of control wherever possible.
  • Oversee heat stress training and acclimatization for new workers, workers who have been off the job for a while, and workers with medical conditions.
  • Provide worker education and training, including periodic safety talks on heat stress during hot weather or during work in hot environments.
  • Monitor the workplace to determine when hot conditions arise.
  • Determine whether workers are drinking enough water.
  • Determine a proper work/rest regime for workers.
  • Arrange first-aid training for workers.

When working in a manufacturing plant, for instance, a contractor may wish to adopt the plant's heat stress program if one exists.



  • Follow instructions and training for controlling heat stress.
  • Be alert to symptoms in yourself and others.
  • Avoid consumption of alcohol, illegal drugs, and excessive caffeine.
  • Find out whether any prescription medications you're required to take can increase heat stress.
  • Get adequate rest and sleep.
  • Drink small amounts of water regularly to maintain fluid levels and avoid dehydration.