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Heat Stress Prevention
Working in Hot Weather - Heat Stress Awareness &
Prevention Guidelines
Working, playing or exercising in the hot, muggy days of summer brings the risk of experiencing
heat-related disorders ranging from minor irritation of a heat rash to serious conditions such as
fainting, heat exhaustion or heat stroke.
This guideline was prepared to provide information and precautions for anyone working in hot
conditions and especially for those working outdoors. Review this information to learn about the
causes of heat stress; symptoms of heat-related disorders; first aid or emergency medical attention, and
most importantly - prevention measures. (Refer to Table #1 on page 6)
What is Heat Stress?
“Heat stress” refers to the overall heat burden to a person from all environmental sources (hot equipment
or processes, sunlight), physical activities performed and clothing worn. All these factors must be taken
into consideration when assessing the risk of heat stress.
Mild or moderate heat stress may cause discomfort, but is not harmful to health. However, the risk of
heat-related disorders increases as the heat stress increases and these can reach levels that are
dangerous to health.
WHO’S AT RISK?
Anyone! Even the young and fit can be affected by heat stress. Those generally at highest risk of
suffering a heat-related disorder are those who:






Work outdoors or in close proximity to significant heat sources such as furnaces, ovens or hot
asphalt.
Perform strenuous work as this contributes to the body’s overall heat burden.
Don’t self-pace or take regular breaks in cooler areas (e.g. shady or air-conditioned areas).
Don’t consume enough water.
Are susceptible because of a medical condition.
Don’t recognize or ignore symptoms of heat stress.
What Contributes to Heat Stress?
Temperature
As the air temperature increases, your body must work harder to keep cool.
Relative Humidity
While the ambient air temperature is an important factor, the relative humidity generally plays a more
significant role in the risk of heat stress. The reason for this is sweat does not evaporate as easily at
higher relative humidity levels and this reduces the body’s ability to cool itself.
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Heat Stress Prevention
Metabolic Heat Load (from physical effort)
Exercising, playing or performing physical activities generates body heat. So, the harder you work the
more body heat you generate. This metabolic heat adds to the overall heat stress burden and the body
must work harder to keep cool. Work categories can be grouped into light, moderate or heavy based on
the amount of energy used.
Radiant Heat Sources
Radiant heat refers to heat [energy] emitted from hot sources such as ovens, furnaces or other hot
processes, or from the sun if you are outside.
Clothing
Lightweight summer clothing typically consists of pants and a shirt. Additional clothing or body coverings,
hard hats, helmets or sports equipment for example, add to the overall heat load.
Individual or Medical Factors
Some personal medical conditions, low salt diets, medications and individual levels of fitness may affect
how you are able to cope with heat. Consult with your physician if you are concerned about working in
the heat.
How Does The Body Cope with Heat?
The body has mechanisms that help maintain a safe and relatively steady internal temperature of 37°C:
 Sweating (cooling effect of evaporation)
 Increasing blood flow to the extremities
 Increasing breathing rate.
 Acclimatization-getting used to hot conditions
Sweating and Dehydration
Sweating combined with inadequate fluid replacement is the biggest culprit when it comes to suffering
serious, adverse health effects. Sweating can lead to dehydration and subsequent complications. At
high humidity levels [muggy weather], sweat does not evaporate well and the body’s ability to cool itself
is reduced. This increases the risk of overheating and becoming dehydrated.
Acclimatization
Acclimatization refers to a gradual adaptation of an individual to tolerate heat stress. Acclimatization is a
slow, complicated process and only occurs after physical activity under heat stress conditions. The
longer you work hard in the heat, the better your body becomes at adjusting to the heat.
A person will not be fully acclimatized to a sudden higher level such as during a heat wave. In Ontario,
hot spells seldom last long enough to allow acclimatization and summers are not hot enough for workers
doing light work (sitting, standing, light arm work) to be considered acclimatized. This is important
because you must be careful to avoid intensely working or exercising if it suddenly becomes very hot
and/or humid.1
If you are not used to working in the heat then you should take a week or two to get “acclimatized” or
used to the heat.
1
Exposure to workplace heat sources may permit acclimatization.
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Heat Stress Prevention
There are a few ways to become acclimatized:

If you are experienced on the job, limit your time in hot working conditions to 50 per cent of the shift
on the first day, 60 per cent of the shift on the second day, and 80 per cent of the shift on the third
day. You should work a full shift the fourth day.

If you are not experienced on the job (ex. new employee) you should start off spending 20 per cent of
the time in hot working conditions on the first day and increase your time by 20 per cent each
subsequent day. You should be able to work a full shift on the fifth day.

Another option is to gradually increase the physical demands of the job over a week or two instead of
progressively increasing the exposure times.
CAUTION!
 It may take longer for you to acclimatize if you have health problems or are not in good physical
condition.
 Acclimatization can be reduced or lost quickly as your level of acclimatization begins to diminish
when the work under heat stress conditions stops. A noticeable loss occurs if you are away from
work for four days or more.
HEAT STRESS PREVENTION - COPING WITH HEAT AND HOT WEATHER
Here are some tips to help you cope with hot, humid weather and protect yourself from heat stress:





Take time to get used to working in the heat (see section on acclimatization)
Wear light coloured and lightweight clothing.
Keep hydrated by drinking plenty of cool fluids (preferably water). Drink at least one cup every 20
minutes - even if you aren’t thirsty!
Take rest breaks in cool or shady areas.
Schedule work for cooler periods of the day (early morning or evening). Avoid working in peak
temperatures if possible.
Adjust your work pace according to the environmental conditions and your abilities.
Reduce the amount of physical effort required to perform strenuous tasks.



Watch for signs of heat stress in co-workers.
Seek medical attention if you feel dizzy or light headed.
Contact your supervisor if you have concerns regarding working in the heat


Never ignore signs of heat stress!
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Heat Stress Prevention
Humidex Rating and Heat Stress Prevention Guidelines for Hot Weather
Humidex Rating
Environment Canada’s “Humidex” rating is a widely used subjective rating of how hot the temperature
“feels” based on combinations of temperature and relative humidity (refer to Table #2 on page 7 for
determining humidex levels). It is commonly reported in the media to provide heat stress awareness and
warnings for the general population2 based on various degrees of comfort/discomfort. Humidex ranges
and the corresponding degrees of comfort are summarized in the following table.
Table #3
Humidex and Comfort Levels
Humidex
Degree of Comfort
20 – 29
No discomfort
30 – 39
Some discomfort
40 – 45
Great discomfort; avoid exertion
46 and over
Dangerous; possible heat stroke
Humidex Based Response Plan
The Occupational Health Clinic for Ontario Workers (OHCOW) developed a simplified approach for
managing heat stress in the workplace by correlating recognized heat stress exposure guidelines3 with
humidex. This heat response plan involves simultaneously measuring temperature and relative humidity
(see note #3 below) and recommends appropriate rest periods and responses for various humidex
levels. The humidex based response plan is summarized in the table below.
Table #4
Summary Humidex Based Response Plan
Humidex 1
(Moderate Unacclimatized work
Action
& Heavy Acclimatized work)
45+
Only medically supervised work
42-44
Work with 45 min/hr relief
40-41
Work with 30 min/hr relief
38-39
Work with 15min/hr relief
34-37
Warn for symptoms & extra water
30-33
Alert for symptoms & extra water
25-29
Water as needed
*Humidex 45+: heat stress should be managed as per ACGIH TLVs
Humidex 2
(Moderate Acclimatized &
Light Unacclimatized work)
50+*
47-49*
45-46*
43-44
40-42
36-39
32-35
Refer to OHCOW’s full Heat Stress Response Plan document.
Notes:
(1) The ideal heat stress prevention plan allows workers to regulate their own pace of work, rest breaks and fluid
intake by “listening to their bodies” without the need for measurements.
(2) Use of the Humidex Response Plan may require adjustments for clothing or radiant heat sources and/or may
not be appropriate to use at all. Contact the Occupational Health and Safety Office if you have any questions or
require more information regarding heat stress in the workplace.
(3) Humidex is not measured directly, it is determined from charts using temperature and relative humidity. Also,
media reports of Humidex refer to the general outdoor conditions that may not be representative of your
particular location. So, it’s important to take site-specific temperature and relative humidity measurements to
determine the Humidex. Measurements must be taken in the shade.
2
3
People of all ages [e.g. young and elderly]
ACGIH WBGT TLVs
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Heat Stress Prevention
Recommended weather/environmental triggers for implementing hot weather plans include:
•
Humidex (local or site specific) reaching or exceeding 35
•
Environment Canada humidex advisory (air temperature exceeding 30ºC and humidex exceeding 40)
•
•
Environment Canada weather reports
Heat waves (three or more days of temperatures of 32ºC or more); and/or
•
Ontario Ministry of the Environment smog alert.
Click here to view the Ministry of Labour's Heat Stress Guideline
Click here to view CCOHS Information on Humidex and Heat Stress
Click here to view Environment Canada's Information on Heat and Humidity
Please contact the Occupational Health and Safety Department at extension 2225 if you have any
questions regarding working in the heat. Remember to always…
Think Safe! Work Safely!
Occupational Health & Safety Office
May 2015
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Heat Stress Prevention
Table #1
HEAT STRESS SYMPTOMS, TREATMENT AND PREVENTION4
Cause
Symptoms
Heat Rash
Hot humid environment;
plugged sweat glands.
Heat
Cramps
Heavy sweating from
strenuous physical
activity drains a person's
body of fluids and salt,
which cannot be replaced
just by drinking water.
Cramps occur from salt
imbalance resulting from
failure to replace salt loss
from heavy sweating.
Fainting
Fluid loss and inadequate
water intake and standing
still, resulting in
decreased blood flow to
brain. Usually occurs in
unacclimatized persons.
Heat
Exhaustion
Fluid loss and inadequate
salt and water intake
causes a person's body's
cooling system to start to
break down.
Heat
Stroke
If a person’s body has
used up all its water and
salt reserves, it will stop
sweating. This can cause
body temperature to rise.
Heat stroke may develop
suddenly or may follow
from heat exhaustion.
Red bumpy rash with
severe itching.
Painful cramps commonly
in the most worked muscles
(arms, legs or stomach)
which occur suddenly at
work or later at home.
Heat cramps are serious
because they can be a
warning of other more
dangerous heat-induced
illnesses.
Sudden fainting after at
least two hours of work;
cool moist skin; weak pulse.
Heavy sweating; cool moist
skin; body temperature over
38°C; weak pulse; normal
or low blood pressure;
person is tired and weak,
and has nausea and
vomiting; is very thirsty; or
is panting or breathing
rapidly; vision may be
blurred.
High body temperature
(over 41 C) and any one of
the following: the person is
weak, confused, upset or
acting strangely; has hot,
dry, red skin; a fast pulse;
headache or dizziness. In
later stages, a person may
pass out and have
convulsions.
Treatment
Change into dry clothes often and
avoid hot environments. Rinse skin
with cool water.
Move to a cool area; loosen
clothing, gently massage and
stretch affected muscles and drink
cool salted water (1/4-1/2 tsp. salt
in 1 litre of water) or balanced
commercial fluid electrolyte
replacement beverage. If the
cramps are severe or don't go away
after salt and fluid replacement,
seek medical aid. Salt tablets are
not recommended.
Wash regularly to keep
skin clean and dry.
Reduce activity levels
and/or heat exposure.
Drink fluids regularly.
Workers should check on
each other to help spot the
symptoms that often
precede heat stroke.
GET MEDICAL ATTENTION.
Assess need for CPR. Move to a
cool area; loosen clothing; make
person lie down; and if the person
is conscious, offer sips of cool
water. Fainting may also be due to
other illnesses.
Reduce activity levels
and/or heat exposure.
Drink fluids regularly.
Move around and avoid
standing in one place for
too long. Workers should
check on each other to help
spot the symptoms that
often precede heat stroke.
GET MEDICAL ATTENTION.
This condition can lead to heat
stroke, which can kill. Move the
person to a cool shaded area;
loosen or remove excess clothing;
provide cool water to drink; fan
and spray with cool water. Do not
leave affected person alone.
Reduce activity levels
and/or heat exposure.
Drink fluids regularly.
Workers should check on
each other to help spot the
symptoms that often
precede heat stroke.
CALL AMBULANCE. This
condition can kill a person quickly.
Remove excess clothing; fan and
spray the person with cool water;
offer sips of cool water if the
person is conscious.
Reduce activity levels
and/or heat exposure.
Drink fluids regularly.
Workers should check on
each other to help spot the
symptoms that often
precede heat stroke.
Never ignore signs of heat stress!
4
Prevention
Ministry of Labour Heat Stress Health and Safety Guidelines, May 2013
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Heat Stress Prevention
Table #2
Determining Humidex Rating and Comfort Ranges
21 °C
100%
29
95%
29
90%
28
85%
27
80%
27
75%
26
70%
26
65%
24
60%
24
55%
23
50%
23
45%
22
22 °C
31
29
29
28
28
27
26
26
24
24
23
23
23 °C
33
32
32
31
30
29
28
27
27
26
25
24
23
24 °C
35
34
33
33
32
31
30
29
28
28
27
26
26
25
25 °C
37
36
35
34
33
33
32
31
30
29
28
27
27
26
26 °C
39
38
37
36
35
34
33
32
31
31
29
28
28
27
27 °C
41
40
39
38
37
36
35
34
33
32
31
30
29
28
28
28 °C
43
42
41
41
39
38
37
36
35
34
33
32
31
29
28
29 °C
46
45
44
43
42
41
39
38
37
36
34
33
32
31
30
30 °C
48
47
46
44
43
42
41
40
38
37
36
35
34
33
31
31
31 °C
50
49
48
46
45
44
43
41
40
39
38
36
35
34
33
31
32 °C
52
51
50
49
47
46
45
43
42
41
39
38
37
36
34
33
33 °C
55
54
52
51
50
48
47
46
44
43
42
40
38
37
36
34
34 °C
58
57
55
53
52
51
49
48
47
45
43
42
41
39
37
36
58
57
56
54
52
51
49
48
47
45
43
42
41
38
37
58
57
56
54
53
51
50
48
47
45
43
42
40
38
58
57
55
53
51
50
49
47
45
43
42
40
57
56
54
52
51
49
47
46
43
42
40
56
54
53
51
49
47
45
43
41
57
54
52
51
49
47
44
43
56
54
52
50
48
46
44
56
54
52
50
48
46
56
54
51
49
47
35 °C
36 °C
37 °C
38 °C
39 °C
40 °C
41 °C
42 °C
43 °C
Humidex
Degree of Comfort
20 – 29
No discomfort
30 – 39
Some discomfort
40 – 45
Great discomfort; avoid exertion
46 and over
Dangerous; possible heat stroke
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40%
35%
30%
25%
20%