Heat Stress:
Defining, Evaluating and
Preventing
Glenn E. Lamson CIH
OSHA Health Response Team
2012 OSHA Oil & Gas Safety Conference
Defining
• Heat Stress
– The net heat load that a worker is exposed to
from the combination of metabolic heat,
environmental factors and clothing
• Heat Strain
– The body’s overall physiological response
from heat stress.
Defining
• Core Temperature
– Temperature of blood circulating around the
hypothalamus, aka the “body’s thermostat”
– Best way to measure heat stress
– 370 C or 98.60 F
• Equilibrium
– Heat gain = heat loss
Defining
• Metabolic heat loss pathways
– Sweating (evaporative cooling)
– Blood flow (radiant cooling)
– Hormonal (regulation of salt & H2O balance)
• Anti-diuretic hormones reduce water and salt
losses through the kidneys
Defining: Heat balance equation
Δ S = (M-W) ± C ± R – E
• Δ S is the change in the body’s heat content
• (M-W) is metabolism minus the work being
performed and is always positive
• C is convective heat being exchanged
• R is radiant heat being exchanged
• E is evaporative heat loss and is always negative
Defining: (M-W) Metabolism
– Energy necessary to keep the body functioning and
accomplishing work
– Estimated by oxygen consumption ~ 5 kcal/liter of O2
• Metabolism is best derived from the table in the
ACGIH’s TLV® booklet.
Defining
• ± C Convective heat loss or gain
– Heat transfer between skin and air
– Function of air velocity
– Skin temperature is 350 C so ∆C will be positive when
air temperature is over 350 C
• – E Evaporative heat loss
• Perspiration evaporation causes body heat reduction
• Function of air motion humidity
• If humidity is 100% E=zero
Defining: ± R Radiant heat
– Heat form electromagnetic radiation when it
strikes an object.
– From skin and surrounding surfaces
– DOES NOT heat the air it passes though
– Important in hot environments
• and easily controlled
Defining: Some rules of thumb
± C: When ambient temp rises above skin
temp, heat loss by convection is eliminated.
– E: When humidity levels increase heat loss
by evaporation is markedly reduced
– As EH&S practitioners we have the most
control over metabolism and radiant heat.
Defining: Heat disorders
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Heat stroke
Heat exhaustion
Heat syncope
Heat cramps
Heat rash
Defining: Heat disorders – Heat Stroke
• Heat stroke is the most serious heat-related disorder. It
occurs when the body becomes unable to control its
temperature
• The body's temperature rises rapidly, the sweating
mechanism fails, and the body is unable to cool down.
• When heat stroke occurs, the body temperature can rise
to 106 degrees Fahrenheit or higher within 10 to 15
minutes.
• Heat stroke can cause death or permanent disability if
emergency treatment is not given.
Defining: Heat disorders – Heat Stroke
• A medical emergency - call 911!
• Symptoms
– Hot, dry skin, hyperthermia, brain disorders
• Treatment
– Move the victim to a cool area
– Cool body rapidly
– Treat for shock
– Transport to hospital
Defining: Heat disorders – Heat exhaustion
• Heat exhaustion is the body's response to
an excessive loss of the water and salt,
usually through excessive sweating.
• Workers most prone to heat exhaustion
are those that are elderly, have high blood
pressure, and those working in a hot
environment.
Defining: Heat disorders – Heat exhaustion
• Symptoms
– Clammy moist skin – usually with a raised
body temperature
– Fatigue, weakness, nausea, headache,
fainting
• Treatment
– Remove from heat
– Give fluids
– Fainting implies hospital visit
Defining: Heat disorders – Heat syncope
• Fainting (syncope) episode or dizziness
that usually occurs with prolonged
standing or sudden rising from a sitting or
lying position.
• Factors that may contribute to heat
syncope include dehydration and lack of
acclimatization.
Defining: Heat disorders – Heat syncope
• Symptoms
– Light-headedness
– Dizziness
– Fainting
• Treatment
– Sit or lie down in a cool place at onset of
symptoms
– Slowly drink water or a sports beverage
Defining: Heat disorders – Heat cramps
Muscle spasms caused by electrolyte loss
• Symptoms:
– Muscle pains or spasms usually in the
abdomen, arms or legs
• Treatment:
– Stop activity and sit in a cool place
– Drink clear juice or a sports beverage
– Do NOT return to strenuous work for a few
hours after cramps subside
Defining: Heat disorders – Heat rash
AKA “Prickly heat” a skin irritation from
excessive sweating
• Symptoms:
– red cluster of pimples or small blisters
– Most likely on the neck and upper chest,
groin, elbow crease and below breasts
• Treatment:
– Keep the affected area dry
– Dusting powders can increase comfort
Evaluating: Measures of heat stress
– Wet bulb globe temperature
• Traditional measure of heat stress, easy to
measure and basis for ACGIH TLV®
• Incorporates air temperature, radiant heat, air
movement and humidity
– Indoors
– WBGT = 0.7tnwb+0.3tg
– Outdoors (and indoors when there is a radiant heat source)
– WBGT = 0.7tnwb+0.2tg+0.1tdb
Evaluating: Wet bulb globe temperature
• Forget that formula this is all you need
Evaluating: Measures of metabolic rate
Work category
Rest
Metabolic rate
115
Work examples
Sitting
Light
180
Sitting, standing, light
arm/hand work
occasional walking
Moderate
300
Normal walking,
moderate lifting
Heavy
415
Heavy material handling
walking at a fast pace
520
Pick and shovel work
Very Heavy
Evaluating: Clothing effects
• Clothing alters convective and evaporative
heat exchange
• Insulating effects estimated using clothing
adjustment factors (CAFs)
Evaluating: Clothing Adjustment Factors
• CAFs are a function of clothing thickness
– Work Clothes (long sleeves and pants) =
– Coveralls (w/ only underwear under) =
– Double-layer woven clothing =
– Polyolefin coveralls =
– Limited-use vapor-barrier coveralls =
0
0
3
1
11
• Can not be added and do not apply to
completely encapsulating suits (level A)
Evaluating: Applying CAF’s to WBGTs
• Once WBGT is calculated or measured
you need to adjust the value to the
worker’s dress level
• Add the clothing adjustment factor to the
WBGT value (all values in °C) provides the
Effective WBGT (WBGTeff)
Evaluating: Acclimatization
• Gradual physiological adaptation that improves
ability to tolerate heat stress
• Acquired by time lost even faster
– Gained when working in heat 2 hours a day for 5 of
the last 7 days to 10 of last 14 days
– Loss begins when work in heat is discontinued.
– Noticeable loss in 4 days, complete loss in 3-4 weeks.
• Situations to monitor
– New hires
– Returning vacationers
Evaluating: Tying all of that together
• How to us all those calculations..
• ACGIH TLV®
– WBGTeff
– Metabolic rate
• Action Limit
– Replaces unacclimatized TLV
– Work below action limit is presumed to be low
stress for any healthy worker
Evaluating: Tying all of that together
• Lets take an example...
– Joe is a 38 year welder who has been
working on a turnaround for 2 months
– Joe wears jeans, a long sleeve shirt under a
pair of flame retardant coveralls
– WBGT is 27 and it has been hot for a month
– Calculation: WBGTeff = 27+3 or 30
– Assuming that his work is “moderate”, using
the TLV® he should be further evaluated for
susceptibility for heat strain.
Evaluating: Tying all of that together
• Lets take a further look at that example...
– Variables:
• Acclimatization
– @ W=300 30 ∆ between TLV® & AL
• Metabolic work rate
– TLV® has a ~ 30 ∆ between W=300 and W=415
• CAF
– CAF for double-layer woven clothing is 3
Evaluating:
• One more thing…
Evaluating: Measures of heat stress
– NOAA’s Heat index
• A measure of how hot it
feels when the
amplifying effects of
increased humidity is
factored into the actual
air temperature.
• Reported daily
Evaluating: Measures of heat stress
• Heat Index: Limitations
– Developed for shady, light wind
conditions the heat index does
not take into account site
specific factors such as
metabolic work rate, radiant
heat, acclimatization or
clothing. Exposure to full
sunshine can increase the heat
index values by up to 15° F.
Preventing: Safety & Health Plan
• Heat injury safety and health plans
– Essential after exceeding the TLV®
– Should be written and spell out controls
• Anticipate – Evaluate – Control
• Engineering – Administrative – PPE
Preventing: Safety and Health Plan
• Goal: Protect all workers from heat strain
– Targets: workers over the AL and in PPE
• Symptoms triggering action
– Sustained HR >180 bbm – age
– Core temp >38.50 C or 38.0 C (unacclimatized)
– Recovery HR >120 bbm
– Sudden and severe fatigue, nausea, dizziness
or lightheadedness
Preventing: Engineering Controls
• Ventilation: removing the heat or adding
cooling
• Shielding: protecting workers from radiant
heat waves
– Important in processing plants
• 15 times the radiant temperature minus skin temp
– SHADE!!!
Preventing: Administrative Controls
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Work-rest cycles
Shade
Proper hydration
Acclimatization
Worker monitoring
Preventing: Personal protective equipment
• Cool vests
• Vortex cooling suits
Glenn E Lamson CIH
OSHA HRT
801-233-4914
lamson.glenn@dol.gov