Industrial Hygiene Week 7 Article Review

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E H S To d ay I J U N E 2 0 1 0 I W W W. E H S TO DAY. C O M 45

How Heat Stress Affects Performance Even a 2 percent dehydration level caused by heat stress can dramatically reduce a worker’s reac-

tion times and ability to focus.

N IOSH notes in its publication,

Occupational Exposure to Hot

Environments (1986), that

although workers can acclimatize

themselves to different levels of heat,

each worker has an upper limit for

heat stress beyond which that worker

can become a heat casualty.

It’s accepted that businesses like

foundries, heavy machine manufac-

turing, ship building and a variety

of others have areas that subject

employees to heat stress. In tempera-

tures as low as 80 F, the human body

compensates for heat levels in the

inner core by pumping blood to the

skin for cooling. When combined

with the fact that most people (an es-

timated 80 percent of the U.S. popula-

tion) start the day in a dehydrated

state, heat stress is a major contribut-

ing factor in preventable accidents

and work-related injury.

Many types of businesses encoun-

ter daily activity that can cause heat

stress in the people that work for

them, and they do not even know

it. “We have always done things this

way” is a quote that is all too famil-

iar when asked why preventive mea-

sures were not taken to prevent heat

stress in the workplace.

HOW THE BODY

RELEASES HEAT

Sixty-five percent of the body’s heat

is released through radiation. This oc-

curs when ambient air temperature

is lower than the body’s skin temper-

ature. Radiation is the movement of

heat energy from a warmer object to

a cooler object, such as when heat

B Y B R U C E B A K E R A N D J O H N L A D U E

Active cooling products, such as the shirt worn by the worker above, help prevent heat stress by using conduction to enhance the body’s capacity to cool.

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4 6 W W W. E H S TO DAY. C O M I J U N E 2 0 1 0 I E H S To d ay

>> HOW HEAT STRESS AFFECTS PERFORMANCE radiates from the sun to the earth.

Convection accounts for approxi-

mately 10 percent of heat loss. Convec-

tion is the transfer of heat energy from

a warmer object or space to a cooler

object or space through differences in

density and the action of gravity.

Approximately 23 percent of heat

loss is due to evaporation of perspira-

tion from the skin. Evaporation is the

cooling of a surface through the pro-

cess of a liquid changing to a vapor

and leaving that surface. Conduction

will add another 2 percent to the heat

loss total. Conduction is the transfer of

heat energy from a warmer object to a

cooler object through direct contact.

When the ambient temperature of

the surrounding air is 95 F or higher,

radiation, convection and conduction

stop working. Evaporation is all that is

left to cool the body. Protective cloth-

ing used by welders, firefighters, racers

and hazmat workers will make the heat

situation even worse.

A performance study by NASA using

telegraph key operators showed that in

temperatures of 80 F, the operator will

make five errors an hour and 19 mis-

takes after 3 hours. At 90 F, the opera-

tors made nine mistakes per hour and

27 after 3 hours. At 95 F, the mistakes

went to 60 in 1 hour and 138 in 3 hours.

Although errors made by telegraph key

operators may not be critical, this same

hot environment will produce a pro-

portional amount of errors regardless

of the task.

When a person is in a hot environ-

ment, up to 48 percent of the blood

is pumped by the heart to the skin

for cooling. The first effect is to re-

lease heat, but water also is released

through perspiration. If an individual

loses 2 percent of body weight due to

perspiring, that person is considered

to be in a heat-exhausted state. A study

by Wasterlund and Chaseling 1 placed

forest workers in a controlled environ-

ment, where one group was properly

hydrated and the other group was de-

hydrated to an extent of 1 percent of

body weight loss. The test included

the time taken to debark and stack 2.4

cubic meters of plywood. They found

a 12 percent decrease in productivity

from the dehydrated group.

Another study by Gopinthan et al 2

focused on mental performance and

the effects of dehydration on the de-

cision-making process and could be

related to an increase in work-related

accidents. The study concluded that

with 2 percent of body weight loss, vi-

sual motor tracking, short term mem-

ory, attention and arithmetic efficiency

all were impaired. In the extreme, the

study notes that a 23 percent reduction

in reaction time occurred with a 4 per-

cent body fluid loss.

WHEN THE BODY

CAN’T KEEP UP

At the ambient temperature of 95 F, the

body can no longer keep up with its in-

ternal heat generation levels and the in-

ner core temperature begins to rise. The

only mechanism to release body heat

from the inner core is for up to 48 per-

cent of the body’s blood to be pumped

to the skin to create perspiration.

This creates two problems. The first

is blood loss to the organs, muscles

and brain. The second problem is de-

hydration. When the brain, muscles and

major organs are receiving half of the

blood they normally receive, the heart

must work much harder to try to deliver

the same volume of blood to those or-

gans to keep them nourished by beat-

ing up to 150 times a minute. When you

factor in a thickening of the blood due

to fluid loss (dehydration), you begin

to understand why heart attacks are a

major byproduct of heat stress.

When an employee performs heavy

physical work, fluid intake may not

overcome the effects of sweat output.

Employees who perform duties in fully

encapsulated protective clothing may

have increased sweat rates of 2.25 liters

per hour.