Respirator masks protect health but impact performance: A review

Abstract

Respiratory protective masks are used whenever it is too costly or impractical to remove airborne contamination from the atmosphere. Respirators are used in a wide range of occupations, form the military to medicine. Respirators have been found to interfere with many physiological and psychological aspects of task performance at levels from resting to maximum exertion. Many of these limitations have been investigated in order to determine quantitatively how much performance decrement can be expected from different levels of respirator properties. The entire system, including respirator and wearer interactions, must be considered when evaluating wearer performances. This information can help respirator designers to determine trade-offs or managers to plan to compensate for reduced productivity of wearers.

Full text

R E V I E W Open Access
Respirator masks protect health but impact
performance: a review
Arthur T. Johnson
Abstract
Respiratory protective masks are used whenever it is too costly or impractical to remove airborne contamination
from the atmosphere. Respirators are used in a wide range of occupations, form the military to medicine.
Respirators have been found to interfere with many physiological and psychological aspects of task performance at
levels from resting to maximum exertion. Many of these limitations have been investigated in order to determine
quantitatively how much performance decrement can be expected from different levels of respirator properties.
The entire system, including respirator and wearer interactions, must be considered when evaluating wearer
performances. This information can help respirator designers to determine trade-offs or managers to plan to
compensate for reduced productivity of wearers.
Keywords: Exercise, Respiration, Heat, Vision, Communications, Anxiety, Heart
Background
Respiratory protective masks (usually called respirators)
are used whenever airborne contaminants are present
and cannot be economically controlled by engineering
means or administrative controls. Respirators come in
many forms, including popular filtering facepiece respi-
rators (FFRs), one-quarter, one-half, and full facepiece
masks, and filtering air-purifying respirators (APR), air-
supplied respirators, blower powered air-purifying respi-
rators (PAPR), and self-contained breathing apparatus
(SCBA). They are used by personnel in homes, industry,
agriculture, mines, emergency first responses, medicine,
and the military wherever airborne contamination is a
possible threat [56]. The threats may be from gases, va-
pors, dusts, and particulates of various sizes, including
aerosols [19].
Although the protective mechanisms of respirators are
largely physical and sometimes chemical, wearing respi-
rators come with a host of physiological and psycho-
logical burdens [15]. These can interfere with task
performances and reduce work efficiency. These burdens
can even be severe enough to cause life-threatening con-
ditions if not ameliorated. Quantitative assessments of
these burdens have been made so that respirator design
trade-offs, wearer usages, and regulations can accommo-
date the needs of the wearer [7, 36, 55].
Understanding possible physiological and psycho-
logical effects of respirator wear requires a thorough un-
derstanding of the wearer and possible respirator effects
[23]. Respirators may appear to be rather simple, but
they can interfere with [36, 55]:
1. respiration
2. thermal equilibrium
3. vision
4. communication
5. feelings of well-being
6. personal procedures such as eating and sneezing
7. other equipment
There are two basic principles relevant to respirator
use:
1. Work cannot usually be performed as long or as
hard while wearing a respirator compared to when
respirators are not worn. Wearing protective
clothing plus respirators makes this situation even
worse. Either more time must be allowed for a
particular task or more workers must be assigned to
the same task.
Correspondence: artjohns@umd.edu
Fischell Department of Bioengineering, University of Maryland, College Park,
MD 20742, USA
© 2016 Johnson. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and
reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver
(http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
Johnson Journal of Biological Engineering (2016) 10:4
DOI 10.1186/s13036-016-0025-4
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2. There is a great deal of wearer variability. Some
wearers can tolerate respirator high inspiratory or
expiratory resistance or pressure levels, while others
cannot. Some wearers are much more anxious about
wearing respirators than others. Some wearers can
tolerate hot, humid conditions inside respirators,
whereas others cannot. Because of this variability,
each wearer must be treated as an individual.
Physiological reponses to work activity
A brief discussion of ergonomics and work physiology is
necessary to understand heavy exertion while wearing
respirators and protective clothing [12, 17, 27]:
1. Work/performance time tradeoff
Very hard work cannot be performed for as long a
time as work of lesser intensity (Fig. 1). This is
true even when unencumbered by protective
equipment. In the Figure, it can be seen that, for
different activity levels, there are corresponding
physiological limitations consisting of a
cardiovascular limitation for very intense work,
respiratory limitation for intense work, thermal
limitation for moderate work [21], and what is
generally called irritation limits for low-level
activity [13,17,27]. Protective masks and clothing
generally shorten the time that a particular activity
level can be sustained.
2. Physiological adjustments
The human body is attuned to performing
physical labor. What follows the start of muscular
activity is a coordinated series of adjustments [12]
involving all parts of the body, including the heart,
blood vessels, the lungs, digestive system, nervous
system, and the kidneys. The ones with most
direct bearing on exercise adjustments are
described below.
Metabolism
Muscular movement requires energy [17]. This energy
comes from an energy storage molecule called ATP (ad-
enosine triphosphate). When the supply of ATP is
exhausted, muscle activity ceases. It is important, there-
fore, to replenish the ATP supply as quickly as possible
in order to maintain muscular work. There is also an-
other energy-rich compound in the muscles called creat-
ine phosphate that can act to replenish the ATP supply
extremely quickly. When the muscle starts working
there is enough ATP in the muscles to sustain the work
for 0.5 sec. There is enough creatine phosphate present
to keep the muscle working for up to 2 min. After that,
other energy- transforming mechanisms are necessary to
replenish the ATP supply.
This other energy comes from stores of glucose in the
blood, glycogen (an animal form of starch) in the mus-
cles and liver, fats in the form of triglycerides in fat tis-
sue, and body proteins. In order to extract the energy
from these compounds, they must be respired at the cel-
lular level, and there are two kinds of cellular glucose
respiration: anaerobic and aerobic. The difference be-
tween the two is that aerobic respiration requires oxygen
and anaerobic respiration does not. Oxygen delivery to
the muscles begins in the lungs, continues in the blood,
and is finally delivered to the muscles (Fig. 2). If enough
oxygen can be delivered to the tissues, then aerobic res-
piration can keep up with the energy demands of the
muscles. However, there are limits to the rate that oxy-
gen can be supplied, called the maximum oxygen up-
take, and, once the maximum oxygen uptake is reached,
Fig. 1 Schematic representation of performance time while exercising wearing a protective mask
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additional muscular energy must come from anaerobic
respiration.
Very heavy exertion requires at least some anaerobic
respiration because oxygen demand exceeds the max-
imum oxygen uptake. This is called the anaerobic
threshold. Anaerobic respiration yields 18 times fewer
ATP molecules [18] than does aerobic respiration, and
so is not nearly as efficient. However, it does allow
movement to continue, at least for a while.
One of the end products of aerobic respiration is car-
bon dioxide, which can be removed during exhalation.
Carbon dioxide levels in the exhaled breath rarely reach
more than 4–5 % even at the extreme, but, if it did climb
much higher, carbon dioxide can cause disorientation,
confusion, and even death [1, 17].
The main end product of anaerobic respiration is lactic
acid that is released from the muscles into the blood
[17]. There are buffering mechanisms in the body that
tolerate lactic acid additions, but these mechanisms have
limited capacity. Once this capacity is reached, there is
no other source of energy for the muscles and all mus-
cular activity must cease. This capacity to tolerate lactate
is called the maximum oxygen debt because all the lactic
acid must be reformulated into pyruvate at the end of
exercise, and this requires oxygen.
Buffering the blood against lactic acid formation dur-
ing anaerobic respiration produces extra carbon dioxide
that can be exhaled. This extra carbon dioxide acts as a
respiratory stimulant that leads to hyperventilation, or
harder and deeper breathing.
All these processes proceed each time a person moves
actively. They are much more efficient for younger
people than for older people. Maximum oxygen uptake
for 20 year olds is about 2.5 l per minute, but declines
nearly linearly to about 1.7 l per minute at age 65 [17].
Well-trained individuals can have maximum oxygen up-
takes up to twice these values. In addition, the maximum
oxygen debt that can be incurred by an individual de-
clines with age and is also affected by training [12].
Metabolic responses during exercise, and especially
during emergencies, are modified by the release of the
adrenal hormones adrenalin (epinephrine) and cortisol.
These hormones increase metabolic rate, increase the
rate and force of heart contractions, enhance the avail-
ability of blood glucose, reroute blood from the gut to
the muscles, and mobilize the nervous system. The com-
bined actions of these hormones can affect physical,
emotional, and cognitive functions.
Muscular strength declines with age, making task
performance less efficient when more muscles must
be recruited to perform a task. Muscular power can
be restored relatively rapidly with strength training.
Drugs and medicines can also affect body metabolism,
as can illness. Products of cigarette smoking and caffeine
also affect metabolic rate [65].
Cardiovascular adjustments
The heart adjusts to the physical demands of exertion by
increasing its cardiac output, or the volume rate of
blood flow through the arteries, capillaries, and veins.
This is done to increase the rate of glucose and oxygen
supplied to, and removal of lactate and carbon dioxide
from, the muscles. The heart rate increases nearly
linearly with work rate, beginning to increase nearly as
soon as work rate increases. This is due to kinesthetic
neural sensors in the muscles and joints that signal the
fact that increased oxygen demand is on its way (feed-
forward control), despite the fact that there is as yet no
reduction in blood oxygen concentration or rise in car-
bon dioxide concentration. Once the concentrations of
these gases change, then control of heart response is de-
termined by chemical sensors in the aorta, in the carotid
arteries in the neck, and in the brain [17].
The stroke volume of the heart, or the volume of
blood pumped for each heart beat, increases initially at
the start of exercise, but soon reaches its maximum
level. Thereafter, increases in cardiac output are deter-
mined only by heart rate. Cardiac output at rest is about
5 or 6 l per minute; cardiac output can rise to 25 l per
minute during strenuous activity. Blood volume in a
somewhat smallish 150 lb (70 kg) person is about 5.6 l.
Hence, it takes about 1 min at rest and 12 s during exer-
cise for blood to make the loop of the whole circulatory
system.
Larger people generally have larger hearts and larger
stroke volumes [18]. Well-trained individuals have lower
resting heart rates and higher resting stroke volumes.
Older individuals can have somewhat lower cardiac effi-
ciencies than younger individuals [17].
If body temperature rises due to overheating, then
blood is shunted to body surface vessels and there is a
secondary rise in heart rate, which puts additional stress
on the heart. The water from sweat is derived from the
blood plasma, causing the blood to thicken somewhat
during prolonged exercise [17]. This also increases stress
Fig. 2 Oxygen delivery to the muscles is a multistep process,
beginning with gas exchange in the lungs (right), being transported
in the blood (middle), and finally being used in the muscles (left)
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on the heart, but is alleviated by drinking sufficient
amounts of liquid, some of which can be drunk before
or during work, if available.
Cardiovascular adjustments also include shunting the
blood from maintenance activities, such as digestion and
kidney function, to working muscles where it is needed.
Much of the blood in the circulatory system at rest is lo-
cated in the leg veins; during exercise, most of the blood
is shifted to the arteries. These changes occur very
quickly after activity begins. Release of the hormones
epinephrine and cortisol during psychological stress
speeds the heart and constricts some blood vessels to
shunt blood to the arms and legs.
Oxygen delivery to the working muscles can be limited
by the maximum cardiac output, given as the maximum
heart rate times the maximum stroke volume [17]. Once
this maximum has been reached, metabolism continues
anaerobically. Depending on the muscles being used and
the vascular structure serving those muscles, there may
be local regions of anaerobic metabolism occurring
while the muscles as a whole are still aerobic.
Cardiovascular limits
There is a maximum heart rate that can be achieved by
an individual. This is age dependent, generally being able
to be predicted as 220-(age of the individual). Younger
people therefore have higher maximum heart rates [17].
Once this maximum heart rate is reached, cardiac out-
put no longer increases, and oxygen delivery to the mus-
cles becomes static. Anaerobic metabolism is incurred,
terminating when the maximum oxygen debt in reached.
Cardiovascular-limited exercise normally terminates in 2
to 4 min.
Respirator effects
Data from multiple studies have shown that the use of
respirators by themselves have no effect on heart rates
of the wearers [46]. From this, it appears that respirators
do not impose additional stress on the heart. However,
for respirators and protective clothing with significant
weight, the additional weight can impose an ergonomic
burden that translates into cardiac stress. This additional
weight acts equivalently to body weight as long as it is
carried close to the body. Each kilogram (2.2 lb) of extra
weight can be expected to reduce the work performance
time by 2.5 min if walking at a high rate of speed [49].
If extra weight is carried awkwardly away from the
body, then the energetic penalty can be an additional
50–60 % of the energetic cost of carrying the load next
to the body [17]. Extra heavy loads add, as well, to the
nonproportional energy cost of carrying them. Loads
carried by the hands are less burdensome than loads car-
ried on the feet. Heavy protective clothing carries with it
a higher energy penalty than can be accounted for by its
weight alone. Apparently, bulk and friction of the clothes
is also an important factor.
Translating the energy requirement of wearing pro-
tective clothing and carrying (or dragging) extra weight
into cardiac burden is not a straightforward procedure.
A lot depends on whether climbing up stairs, down
stairs, or walking on the level; the texture and composi-
tions of walking surfaces; the speed of movement; and
the body temperature of the wearer. Under relatively
easy walking conditions, the increase in heart rate while
carrying an extra 60 lb (27 kg) of weight is a heart rate
increase of 10 % of the maximum [17].
Respiration
Respiration also increases as exercise progresses, but re-
spiratory responses lag activity level changes by about
45 s [17]. There are many respiratory responses that
occur: the respiration rate increases [5, 41, 42], the tidal
volume (or the amount of air breathed during each
breath) increases up to a maximum amount, the respira-
tory waveform changes [40], there are adjustments to
the airways, and lung volumes change [14]. Many of
these changes appear to be stimulated by carbon dioxide
concentration of the blood, but initial respiratory adjust-
ments occur too quickly for that to be the only deter-
minant; kinesthetic sensors may also be important for
initial respiratory adjustments [64].
Respiration is a multistep process, whereby air is
breathed in, travels through the airways, reaches the al-
veoli (the sacs at the end of the lung where gas exchange
takes place), diffuses across the alveolar membrane, dis-
solves in the blood, and is absorbed by the hemoglobin
in the red blood cells. Carbon dioxide diffuses rapidly
into the blood, so the concentration of carbon dioxide in
the alveoli and the blood are always equal, even during
the most intense activity level. Oxygen, on the other
hand diffuses more slowly than carbon dioxide, so its
concentration in the blood is lower than in alveolar air
during inhalation [17]. Diffusion rates of both gases
change somewhat with activity level, with those for men
being somewhat higher than those for women.
Inhaled air is oxygen rich and carbon dioxide poor.
Exhaled air is oxygen poor and carbon dioxide rich. Be-
cause air flow in the airways is bidirectional, the first air
that reaches the alveoli is the same as the last air that
was exhaled during the previous exhalation. This is an
indication of the dead volume of the lung, or that vol-
ume that stores carbon dioxide from the previous
breath. Dead volume for average adults is about 180 ml,
but dead volume of respirators can add to the effective
dead volume of the respiratory system and affect per-
formance [52].
Carbon dioxide is a very powerful respiratory stimu-
lant. Increasing the concentration of inhaled carbon
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dioxide increases lung ventilation much more than does
oxygen deficiency. Metabolically-produced carbon diox-
ide is even more effective than inhaled carbon dioxide at
stimulating respiration. This is critical for additions of
external dead volume, which transforms exhaled meta-
bolic carbon dioxide into carbon dioxide inhaled during
the next breath. Once the anaerobic threshold is
reached, blood buffering makes it appear that metabolic
carbon dioxide increases, and respiration is stimulated
so much that lung ventilation increases dramatically as
work rate intensifies (Fig. 3).
Working muscles change their efficiencies over time
as they heat and tire. Additional oxygen demands of
muscles that have been worked for several minutes in-
creases the need for the respiratory system to respond.
This leads to a secondary rise in lung ventilation that
continues well into the exercise duration.
Moving the chest wall, lung tissue, and air in the air-
ways requires energy. This energy is equivalent to about
1–2 % of the total body oxygen consumption at rest, but
increases during intense activity to 8–10 %. For people
with obstructive pulmonary disease, the percentage at
rest can be 18–20 %. These people cannot perform
strenuous exercise. Adding external resistance or dead
volume from a respirator (APR), or external pressure
(SCBA or tight-fitting PAPR), increases the amount of
work that must be supplied to breathe. Oxygen to supply
the needs of the respiratory system cannot be used to
supply the working muscles, so respiratory demands can
definitely limit the rate of work that can be expected of
a wearer [13].
The work of respiration is supplied by the respiratory
muscles. These include the diaphragm, the intercostals,
and the abdominals. Inhalation is caused mainly due to
the straightening of the diaphragm in the chest. Exhal-
ation at rest for those without obstructive or restrictive
pulmonary diseases (normal, healthy individuals) is pas-
sive; that is, the force to propel the air to leave the lung
comes from the elasticity of the stretched lung. Exhal-
ation during exercise needs to happen a lot faster than
during rest, so becomes active when the abdominal mus-
cles push air out of the lung. Due to this difference, it is
much easier and comfortable to breathe against PAPR or
SCBA positive pressure during exertion than during rest.
The airways are reactive, and change during exercise.
They can constrict somewhat to reduce dead volume,
Fig. 3 Pulmonary ventilation increases linearly with oxygen uptake until the anaerobic threshold, and then dramatically greater once stimulated
by additional carbon dioxide
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and thus lower wasted breathing effort, but, as they con-
strict, they resist air flow and increase the work of
breathing [37], so there is a dynamic level of airway tone
that is achieved. These same airways may constrict to
protect against respiratory irritants reaching the lung,
and cause the same symptoms as a severe asthma attack.
Respiratory limits
Respiration does not usually limit work performances of
healthy individuals, but respiration can limit work time
when respirators are worn [44, 51]. The most important
function of the respiratory system is the removal of car-
bon dioxide from the body. Adjustments during exercise
increase depth and rate of breathing in order to expel
this gaseous end-product of aerobic metabolism. Exer-
cise exhalation becomes actively supported by the ab-
dominal muscles, spewing carbon dioxide at faster rates
as exercise intensifies. At some point, the rate at which
air can be exhaled becomes limited by the distensible
airways in the respiratory system. Any further increase
in abdominal pressure cannot increase expiratory flow
rate. Thus, for normal individuals, there is a limitation
when exhalation time decreases to one-half second or so
[22, 24]. Carbon dioxide cannot be expelled any faster
than this minimum exhalation time allows. Addition-
ally, some people suffer from respiratory impairments
that limit maximum pressures that can be generated by
the respiratory muscles when they breathe through
external resistances or against external pressures [59].
Respiratory-limited work usually lasts 5–20 min.
Respirator effects
Air-purifying respirators (APRs) have inspiratory resis-
tances dominated by filter resistances, with a typical
value of 3.5 cm of water-seconds per liter (or 50 mm
H
2
O at 85 L/min flow rate). Exhalation resistances of
the exhalation valves may be somewhat less than 1.5 cm
H
2
O-sec/L. Powered air-purifying respirators (PAPRs)
may have much lower inhalation resistance but the same
exhalation resistance. Self-contained breathing appar-
atus (SCBA) may have zero or negative equivalent
resistance, but very high pressures to exhale against. Al-
though exhaling against high pressures is uncomfort-
able at rest, when respiration usually includes passive
exhalation, high-exhalation pressures can be tolerated
better during exercise when the respiratory muscles for
exhalation contract actively. Previous work seems to in-
dicate that inspiratory and expiratory resistance effects
are equivalent [3, 4, 50], although testing using very
high expiratory resistance resulted in severely degraded
performance [31]. No thresholds for respirator resist-
ance effects has ever been detected, so FFRs with very
low resistance values would still be expected to have
some effect.
The effects of APR inspiratory resistance on perform-
ance are felt most at very intense exercise (80–85 %
maximum oxygen consumption). Performance time de-
creases linearly with increased inspiratory resistance at
this exercise intensity [50]. A resistance level of 3.5 cm
H
2
O-sec/L is expected to result in a 30 % performance
decrement. Because of this, one might expect perform-
ance with PAPR to be better than with air-purifying res-
pirators, but this has yet to be definitively answered, and
the extra weight of the blower and tubing may counter-
act at least some of the advantage of lower resistance
[51, 63].
Extra inspiratory resistance [38] promotes
hypoventilation [2–4, 6, 16, 39, 50, 60] of the wearer
(lower volumes of air breathed and smaller amounts
of oxygen used). This can result in an earlier transi-
tion from aerobic (using oxygen) to anaerobic (no
oxygen needed) respiration [10, 32], and faster pro-
gress toward the maximum tolerance for exercise
(maximum oxygen debt).
Facepiece dead volume accumulates exhaled carbon
dioxide in the voids between the respirator and the face
and returns it to the respiratory system during the next
inspiration. This carbon dioxide then acts as a respira-
tory stimulant. Because carbon dioxide is a psychoactive
gas, dead volume may also produce discomfort and a
performance decrement at low-intensity work. A typical
value for full-facepiece APR respirator dead volume is
350 mL. Such a dead volume is expected to reduce per-
formance time by 19 % at a work rate of 80 to 85 % of
maximum oxygen uptake [52].
Intense exercise above the anaerobic threshold uses
more air than does moderate exercise, and because very
intense exercise metabolism has a higher anaerobic
component than does moderate exercise, the air that is
used is not consumed as efficiently as it is at lower in-
tensity[43].ThenetresultisthatSCBAtankairde-
pletes much more rapidly at high work rates than at
moderate work rates.
Thermal responses
The large skeletal muscles are only about 20 % efficient
[20]. Of the energy supplied to the muscles, approxi-
mately 80 % ends up as heat. Thus, heat loss mecha-
nisms are necessary to maintain thermal equilibrium of
the human body.
These mechanisms include vascular adjustments,
sweating, and voluntary responses. Voluntary responses
include moving to cooler locales, stretching out to lose
more heat, drinking cool liquids, or removing heavy
clothing. These responses may generally be unavailable
to workers or emergency responders who need to pro-
tect against threats of unknown type in uncontrolled
environments.
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There is a thermal mass to the body that requires
some time for heat to build up and cause dangerous
body temperatures. There is a normal 6–10 min of activ-
ity that can occur before deep body temperature rises
significantly [17]. Skin temperature probably increases
during this time. If sufficient heat cannot be lost to the
environment, then body temperature will continue to
rise until it reaches dangerous levels. A core body
temperature of 104 ° F (40 ° C) is expected to give a
50 % casualty rate [11]. This condition is characterized
by disorientation, convulsions, loss of body temperature
control, and death.
Heat can be lost from the body by convection (usually,
air movement), radiation (as to a cold clear sky), or
evaporation. Convection and radiation heat loss depends
on the difference in temperature between the surface
losing heat and the surrounding fluid (usually air, but, in
a pool, water). Thus, one adjustment the body makes
during thermal stress is to warm the skin surface. It does
this by shunting blood from deep veins into surface
veins. This is why veins on the surface of the hands seem
to stand out more in hot weather than in the cold. There
is also a small, but significant, amount of convective heat
loss from the respiratory system as air is breathed.
Evaporating water absorbs a large amount of heat, thus
making sweating effective as a heat loss mechanism.
Sweating heat loss on the surface of the skin is nearly
100 % effective for losing heat. Sweating through cloth-
ing cools the clothing surface where the evaporation ac-
tually takes place, and only partially cools the skin.
Sweat that drops from the skin is completely ineffective
for heat removal. The amount of sweating depends on
the cooling necessary, and different parts of the skin are
recruited at different times to produce sweat. When fully
recruited, the maximum cooling that can be obtained
from sweating is equivalent to nearly 12 times the body
heat production at rest (or 11.4 mets).
Women have higher percent body fat than do men.
They use this body fat as insulation between their body
cores and the outside environment. To lose heat, there-
fore, women depend more on vascular adjustments that
do men. Men sweat more than women and lose a larger
fraction of their heat in that way. Acclimation to hot envi-
ronments can improve sweating efficiency by increasing
both the rate of response and amount of sweat produced.
Some workers may not need to wear protective cloth-
ing with their respirators. However, covering up the en-
tire body, and possibly moving into a hot environment
eliminates nearly all possibility of heat loss natural to
the human body. Other means must be provided, such
as supply of cool air from an SCBA or PAPR, or body
temperatures must be closely monitored. An alternative
is to limit heat exposure time and to provide adequate
rest cycles.
Some work may be required in very cold temperatures
[66]. At the beginning, cold temperatures may limit
movement and dexterity. However, heat produced
during activity and the extra insulation afforded by
protective clothing and respirators soon overcome
cold temperature effects on the body. Surface blood
vessels in the head do not constrict in the cold, as do
similar blood vessels in other parts of the body.
Hence, nearly half of the body’sheatlossinthecold
can come from the uncovered head. Covering the
head and face with protective equipment helps to in-
sulate against this large amount of heat loss.
Thermal limits
The most important work limitation associated with
heat is deep body temperature. It must be prevented
from reaching 40 ° C. A conservative limit for adults
might be 39.2 ° C (102.5 ° F). Beyond this, thermal dis-
comfort becomes overwhelming and death could ensue.
Muscular efficiency is reduced at high temperatures and
judgment ability becomes impaired. Thus, the over-
heated individual cannot be expected to recognize his or
her own dangerous situation [62].
Because of the thermal capacity of the body to store
heat, it takes a while before body temperature rises to
the point where it can become limiting. Heat-limited
work usually occurs in the 10 min to 2 h time range.
Respirator effects
Use of respirators in nontemperate conditions can lead
to special problems. [35] Cold conditions can cause fog-
ging of full facepiece respirators, which leads to severe
dissatisfaction with respirator use. Nose cups inside the
facepiece are designed to eliminate fogging, but are not
always effective. Fog-proof lenses are available on some
models. Fog-proofing solutions that can be applied to
the face shield are also available. Cold can also cause
valve sticking and stiffen the rubber facepiece material
to the point that it prevents a good facial seal. Cold rub-
ber has a higher thermal conductivity than does still air,
so in still, cold air the face may be cooled by the respir-
ator. In a cold wind, however, the facepiece may add a
small amount of insulation to the face.
Use of respirators in hot conditions leads to several
difficulties. Discomfort has been related to facial temper-
atures inside the facepiece. Facial skin temperatures are
more important for comfort than skin temperatures in
other parts of the body. PAPR blowers send filtered air
over the face that evaporates sweat and cools the face
[51]. SCBA air expands and cools when released from
the cylinder; this cool air can also help to alleviate facial
discomfort. Some SCBA have coolant packs used to fur-
ther cool supplied air before it reaches the facepiece.
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APRs, however, have been found to be uncomfortable in
the heat because they do not supply cool air.
At moderate work rates (50 to 70 % of maximum oxy-
gen uptake, or maximum exercise capacity), respirators
impede the loss of heat from the face and can result in
hyperthermia occurring sooner than it otherwise would.
This is not usually a problem except when the rest of
the body is sealed in protective clothing. With no easy
means to lose heat, the body can overheat, especially in
hot and active conditions.
Sweat produced inside the facepiece can accumulate
and cause discomfort, interfere with breathing, and
cause exhalation valve sticking [48]. Accumulated sweat
can cause a respirator face piece to slip on the face and
promote leakage.
There is also an effect of heat on the ability to
recognize dangers, make coordinated movements, and
perform manual tasks [35]. As deep body temperature
increases, dexterity, cognition, and motor skills degrade
significantly (Fig. 4). One of the most dangerous effects
of overheating is disorientation, and not being able to
recognize the direction to safety in the event of extreme
danger. This inability to recognize safe passage has con-
tributed to past deaths. Respirators can contribute to
this disorientation by adding other sensory burdens (es-
pecially vision and hearing) to the wearer in this
environment.
Prolonged activity
Some wearers will be assigned tasks that are physically
not very intense. These people will have no trouble with
maximum oxygen debt, maximum oxygen uptake, or
(most likely, unless the ambient temperature is ex-
tremely warm) excessive body temperature. Different
challenges confront these wearers. First of all, discomfort
is felt more strongly when attention is not directed else-
where. There can be a considerable amount of discom-
fort associated with wearing respirators, gloves, boots,
and protective suits. Those individuals prone to anxious
feelings may have their anxieties made worse during pe-
riods of inactivity. Anxieties are the most important
threat to protective equipment wear, and extremely anx-
ious people should not be asked to wear respirators, if
possible. Studies have shown that anxiety level is a very
reliable indicator of difficulty encountered while wearing
a respirator. Extremely anxious individuals do not per-
form for as long or at the same work rate as low-anxiety
wearers [28, 61].
For those who can tolerate the discomfort and claus-
trophobic feelings when wearing respirators, there will
nonetheless be physical effects of prolonged wear
[54, 57]. Many respirators require a tight face seal in
ordertoassureadequateprotection.Thesiteofthe
face seal may produce rashes and edema in sur-
rounding skin areas. These will disappear with time
once the equipment is removed.
Vision can be important at low work rates. There may
be tasks to be performed that require a broad visual field
or fine discrimination among various lights, switches, or
objects. Respirators interfere with vision in various ways,
but visual acuity at low work rates can be compromised
by lens fogging, dust or films on the lenses, or wearing
of improper corrective lenses. Sweating people wearing
respirators in cold drafts can easily incur moisture con-
densation inside the facepiece. Dusts and precipitates
that are of no respiratory consequence to the wearer can
obscure vision if not able to be wiped from the lenses.
Prolonged activity limits
Physiological limits to long term exercise deal with limi-
tations on blood glucose levels and muscle glycogen
stores. Dehydration or electrolyte depletion may occur
[17]. These are difficult to quantify for any individual,
but frequent eating and drinking can deter them from
happening [30].
Fig. 4 Effect of body temperature on dexterity, cognition, and motor skills
Johnson Journal of Biological Engineering (2016) 10:4 Page 8 of 12
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

Psychological effects are also important. Feelings of
fatigue are common, as are feelings of anxiety and dis-
content [54, 57].
Respirator effects: vision
Sharp vision is important for some of the tasks required
during an emergency. There is a natural tunneling of
vision that occurs during intense exertion: attention is
focused on objects straight ahead. Consequently, deg-
radation of vision due to respirator use during high ex-
ertion has little effect on the ability to complete the
required task. Under normal conditions, this might be
advantageous to task performance. In a situation where
dangers can come flying from all directions, there may
be difficulty recognizing peripheral threats.
Vision is extremely important for performing some
tasks, such as computer work, console monitoring, and
reading [9, 29, 33, 34]. There are many aspects of vision,
including visual acuity, peripheral vision, and color de-
tection, and some or all of these may be needed. Respi-
rators should be selected to accommodate requirements
for peripheral vision, acuity, and color recognition.
Workers requiring corrective lenses while wearing full
facepiece respirators must not wear spectacles with tem-
ple bars or straps that come between the sealing surface
of the respirator and the face. Instead, special corrective
lens mounting kits may be used with full facepiece respi-
rators. These may not be entirely satisfactory for some
wearers. Those who can wear contact lenses can usually
do so while wearing a respirator mask. As long as the in-
sides of full facepiece respirators are kept clean, dust
particles will not be present to cause difficulties with
contact lenses.
Dust, mist, smoke, condensation, or water flowing
down over the facepiece lenses can degrade visual acuity
during an emergency. Under such conditions, task per-
formance can be expected to be seriously degraded (Fig. 5),
and extra training under these conditions might be war-
ranted [29]. Disorientation in a low-visibility environment
is common, and may make it difficult to know how to
move or which direction is the safest to go.
Although visual acuity has little to no effect on per-
formance of intense physical activity [33], wearing a full
facepiece respirator while walking, running, or driving
can erode visual acuity somewhat, probably due to the
pull of the shaking facepiece on the face. Recognition of
objects or signs while wearing a respirator and walking
or driving cannot be expected to happen as quickly as
without a respirator.
Respirator effects: communications
Full facepiece respirators can interfere with visual cues
during speaking and listening. It thus becomes more dif-
ficult not only to recognize what is said, but also who is
saying it. Distance and intelligibility are interrelated; lon-
ger distances between communicating individuals result
in less intelligibility. Speakers and listeners should talk
in sentences where the message can be conveyed by con-
text as well as by word recognition. Sentence context al-
lows speakers and listeners to be separated by 10 times
the distance compared to communicating by single words.
Simple words and phrases are unable to be understood
27 % of the time at distances as close as 2 ft [8].
When telephones or radios are used for long-distance
communication, expect a 10 % error rate in recognition
of words and a 50 % increase in the time required to
recognize the words [45, 47, 53]. Because standard tele-
phone and radio equipment dimensions are not entirely
compatible with respirator facepieces, protocols should
be established to let the user know when to move the
earpiece from the ear and to move the mouthpiece in
front of the speech diaphragm. Training in the use of
these protocols is essential [58].
Special communication equipment is available from
some manufacturers and some respirators have speech
diaphragms or are made of materials that enhance
speech transmission.
If workers are close enough to be able to see each
other, a lot of communication can take place with hand
signals. There are some generally-accepted hand signals
that denote easily-understood simple messages (examples
of these are thumbs up for agreement, a finger across the
throat for danger, an upright palm to indicate “stop”,and
pointing to indicate direction). These will be harder to see
in a dusty or smoky environment and with gloves on, so
there is a distance penalty even with hand signals.
One of the most difficult impediments to clear com-
munication is accented speech. If speech cannot be
Fig. 5 Performance for several tasks as visual acuity varies while
wearing respirators. The Snellen eye chart denotes better vision for
higher line numbers. Control panel recognition and performance
ability is particularly sensitive to visual acuity
Johnson Journal of Biological Engineering (2016) 10:4 Page 9 of 12
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

clearly understood without a respirator, it will be nearly
impossible with a respirator. Hand signals may serve to
overcome speech understanding, but different cultures
may also have different interpretations of hand signals.
Respirator effects: bulk
Respirators can interfere with worker activities because
of their bulk or weight. Use of respirators in tight places
is difficult and can temporarily disrupt facial seals when
bumping against other objects. Respirators may interfere
with sighting equipment or with other measuring de-
vices. Contrarily, the impact resistance of the lenses of
many full face piece respirators can be a positive attri-
bute in situations where objects or debris may possibly
fall to the face.
Protective clothing can be bulky and heavy, and can
impede worker progress. Small spaces must be larger for
a protected worker to fit into. Gloves make fine hand or
finger movements nearly impossible.
Respirator effects: personal procedures
The facial area inside a respirator is usually not access-
ible from the outside unless the face seal is broken.
Thus, eating, drinking, scratching one’s face, blowing
one’s nose, or rubbing an eye are not possible while
wearing full facepiece respirators. One exception to this
is certain respirators that have a drinking tube incorpo-
rated into their designs.
As long as periodic breaks are allowed, respirators
should not add to the fatigue that accompanies long
term work [30]. Food or drink can be ingested during
those breaks, and energy levels maintained. While it is
unlikely that responders would be needed to work for
hours at a time without breaks, if such were the case,
then blood glucose could fall to dangerously low levels
(hypoglycemia), and work could not continue efficiently.
The inaccessibility of the face may generate consider-
able tension in the mind of the wearer, especially if the
reason to access the face is due to some particularly sen-
sitive need. Dust or dryness in the eyes of contact lens
wearers, runny noses, or unbearable pressure to parts of
the face can be particularly distressing. If the situation
does not allow the wearer to leave the hazardous envir-
onment to take care of the problem, then considerable
anxiety may develop.
Work/rest cycles
As given in Fig. 1, more intense work cannot be sus-
tained as long a time as can less intense work. If workers
are expected to work very hard for a while, they must
also be in a position to rest or, at least, slow down for a
while. This can be a problem if the worker cannot con-
trol the rate of work, because anaerobic work continued
for too long can result in the maximum oxygen debt
being reached. Then the worker would not be able to
work any more until he or she recovers sufficiently.
The amount of time that a person can be expected to
work is related to the fraction of the maximum oxygen
uptake represented by the task being performed [17, 26].
Thus, performance time involves the size of the individ-
ual as well as age, sex, and physical conditioning. In gen-
eral, men have higher maximum oxygen uptakes than
women, but they have larger sized bodies that use more
oxygen to move around. Older people have lower max-
imum oxygen uptakes than younger people. Wearers in
better physical condition have higher maximum oxygen
uptakes, and, additionally, are able to perform tasks with
lower oxygen use than are less physically-able wearers.
Work performance times can range from forever at
rest, to 4 h walking at 3 miles per hour, to 23 min for
cross-country running, to 10 min climbing stairs. These
are typical times for an unencumbered 40 year old man
[17]. The addition of extra protective equipment can re-
duce these times to one-half or less of the values given,
depending on the types of equipment worn.
Rest times are also dependent on the intensity of the
task and the maximum oxygen uptake of the individual
[17]. In general, the more intense the work, the longer
will be the recovery time, but the relationship is nonlin-
ear. A task that can be performed for an hour requires
at least a 10 min rest period. More intense tasks (with
shorter performance times) require longer rest times.
Conclusion
Physical exertion involves the entire body in a coordi-
nated fashion. Adjustments made during work or exer-
cise can be profound, but the limitations of exercise can
be modified or overcome by training and proper selec-
tion of equipment. Familiarity with the physiological ad-
justments that occur can lead to enhanced effectiveness
and larger return on investment for both manpower and
equipment [25]. As long as humans are involved in per-
forming physical or mental work, accommodation must
be made for the adjustments that characterize their
physical abilities. Training is important to improve the
wearer’s ability to respond to work conditions, but does
not eliminate the basic physiological and psychological
limits to performance.
Competing interests
The author declares that he has no competing interests.
Author’s contributions
The sole author did all the work for this manuscript. He has read and approved
the final manuscript.
Acknowledgement
There are no acknowledgements to be made.
Received: 16 June 2015 Accepted: 1 February 2016
Johnson Journal of Biological Engineering (2016) 10:4 Page 10 of 12
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

References
1. Billings CO. Atmosphere. In: Parker JF, West VR, editors. Bioastronautics Data
Book. Washington, DC: NASA; 1973. p. 35–63.
2. Brokaw E, Johnson A, McCuen R. Improved prediction of minute volume
flow rates. J Internat Soc Respir Prot. 2011;28:40–7.
3. Caretti DM, Coyne K, Johnson A, Scott W, Koh F. Performance when
breathing through different respiratory inhalation and exhalation resistances
during hard work. J Occupat Environ Hyg. 2006;3:214–24.
4. Caretti DM, Scott WH, Johnson AT, Coyne KM, Koh F. Work performance
when breathing through different respirator exhalation resistances. Am
Indus Hyg Assoc J. 2001;62:411–5.
5. Christie RV. Dyspnoea in relation to the visco-elastic properties of the lung.
Proc Roy Soc Med. 1953;46:381–6.
6. Coyne K, Caretti D, Scott WH, Johnson A, Koh F. Inspiratory flow rates
during hard work when breathing through different respirator inhalation
and exhalation resistances. J Occupat Environ Hyg. 2006;3:490–500.
7. Coyne KM, Johnson AT. Review of recent research on improved design of
respiratory protective masks based upon human performance data. Res Adv
Biomed Eng. 2000;1:91–8.
8. Coyne KM, Johnson AT, Yeni-Komshian GH, Dooly CR. Respirator performance
ratings for speech intelligibility. Am Indus Hyg Assoc J. 1998;59:257–60.
9. Dooly CR, Johnson AT, Brown EY. Performance decrement due to altered
vision while wearing a respiratory face mask. Mil Med. 1994;159:408–11.
10. Dooly CR, Johnson AT, Dotson CO, Vaccaro P, Soong P. Peak oxygen
consumption and lactate threshold in full mask versus mouth mask
conditions during incremental exercise. Eur J Appl Physiol. 1996;73:311–6.
11. Goldman RF. Predicting the Effects of Environment, Clothing, and Personal
Equipment on Military Operations. India: Paper presented to the Eleventh
Commonwealth Defense Conference on Operational Clothing and Combat
Equipment; 1975.
12. Hurley BF, Johnson AT. Factors affecting mechanical work in humans. In:
Brozino JD, editor. Biomedical Engineering fundamentals. Boca Raton: Taylor
and Francis; 2006. p. 66-1–66-10.
13. Johnson AT. The energetics of mask wear. Am Indus Hyg Assoc J. 1976;37:
479–88.
14. Johnson AT. The change in initial lung volume during exercise. IEEE Trans
Biomed Eng. 1995;42:278–81.
15. Johnson AT. Human factors and ergonomic aspects of respirator wear. In:
Colton CE, Brosseau LM, editors. Respiratory Protection: A Manual and
Guideline. Fairfax: Amer. Indus. Hyg. Assoc; 2001. p. 87–92.
16. Johnson AT. Why peak flow matters. J Internat Soc of Respir Prot. 2006;23:
100–10.
17. Johnson AT. Biomechanics and Exercise Physiology: Quantitative Modeling.
Boca Raton: Taylor and Francis; 2007.
18. Johnson AT. Biology for engineers. Boca Raton: Taylor and Francis; 2011.
19. Johnson AT. CBRN equipment and the wearer, Chapter 7. In: NIOSH CBRN
Handbook. Atlanta: National Institute for Occupational Safety and Health,
Centers for Disease Control and Prevention; 2012. p. 95–118.
20. Johnson AT, Benjamin MB, Silverman N. Oxygen consumption, heat
production, and muscular efficiency during uphill and downhill walking.
Appl Ergon. 2002;33:485–91.
21. Johnson AT, Berlin HM (1973) Interactive Effects of Heat Load and Respiratory
Stress on Work Performance of Men Wearing CB Protective Equipment,
Edgewood Arsenal Technical Report ED-TR-83059 APG, MD 2l0l0
22. Johnson AT, Berlin HM. Exhalation time characterizing exhaustion while
wearing respiratory protective masks. Am Indus Hyg Assoc J. 1974;5:
463–7.
23. Johnson AT, Cummings EG. Mask design considerations. Am Indus Hyg
Assoc J. 1975;36(3):220–8.
24. Johnson AT, Curtis AV. Minimum exhalation time with age, sex, and physical
condition. Am Indus Hyg Assoc J. 1978;39:820–4.
25. Johnson AT, Dooly CR. Design of respirator masks to incorporate
physiological effects: a review. J Internat Soc Respir Prot. 1995;13:8–22.
26. Johnson AT, Dooly CR. Design of respiratory protective masks to improve
human performance. In: Bronzino JD, editor. Handbook of Biomedical
Engineering. Boca Raton: CRC Press; 1995. p. 2321–34.
27. Johnson AT, Dooley CR. Exercise Physiology. In: Bronzino JD, editor. Biomedical
Engineering Fundamentals. Boca Raton: Taylor and Francis; 2006. p. 65-1–65–9.
28. Johnson AT, Dooly CR, Blanchard CA, Brown EY. Influence of anxiety level
on work performance with and without a respirator mask. Am Indus Hyg
Assoc J. 1995;56:858–65.
29. Johnson AT, Dooly CR, Brown EY. Task performance with visual acuity while
wearing a respirator mask. Am Indus Hyg Assoc J. 1994;55:818–22.
30. Johnson AT, Dooly CR, Caretti DM, Green M, Scott WH, Coyne KM, et al.
Individual work performance during a 10-hour period of respirator wear.
Am Indus Hyg Assoc J. 1997;58:345–53.
31. Johnson AT, Dooly CR, Coyne KM, Sahota MS, Benjamin MB. Work
performance when breathing through very high exhalation resistance. J
Internat Soc Respir Prot. 1997;15:25–9.
32. Johnson AT, Dooly CR, Dotson CO. Respirator mask effects on exercise
metabolic measures. Am Indus Hyg Assoc J. 1995;56:467–73.
33. Johnson AT, Dooly CR, Sahota MS, Coyne KM, Benjamin MB. Effect of
altered vision on constant load exercise performance while wearing a
respirator. Am Indus Hyg Assoc J. 1997;58:578–86.
34. Johnson AT, Dooly CR, Simpson CR. Generating the Snellen chart by
computer. Comput Meth Prog Biomed. 1998;57:161–6.
35. Johnson AT, Grove CM, Weiss RA. Respirator performance rating tables for
nontemperate environments. Am Indus Hyg Assoc J. 1992;53:548–55.
36. Johnson AT, Grove CM, Weiss RA. Mask performance rating table for specific
military tasks. Mil Med. 1993;158:665–70.
37. Johnson AT, Jones SC, Pan JJ, Vossoughi J. Variation of respiratory resistance
suggests optimization of airway caliber. IEEE Trans Biomed Eng. 2012;59:
2355–61.
38. JohnsonAT,KohFC,ScottJrWH,MackeyKM,ChiouKYS,RehakJ.
Inhalation flow rates during strenuous exercise. J Inter nat Soc Respir Prot.
2005;22:79–96.
39. Johnson AT, Mackey KR, Scott WH, Koh FC, Chiou KYH, Phelps SJ. Exercise
performance while wearing a tight-fitting powered air purifying respirator
with limited flow. J Occupat Environ Hyg. 2005;2:368–73.
40. Johnson AT, Masaitis C. Prediction of inhalation time/exhalation time ratio
during exercise. IEEE Trans Biomed Eng. 1976;23:376–80.
41. Johnson AT, McCuen RH. A comparative model study of respiratory period
prediction on men exercising while wearing masks. IEEE Trans Biomed Eng.
1980;27:430–9.
42. Johnson AT, McCuen RH. Prediction of respiratory period on men exercising
while wearing masks. Am Indus Hyg Assoc J. 1981;42:707–10.
43. Johnson AT, Phelps SJ, Scott Jr WH, Koh FC. Using self-contained self-
rescuers at high work rates. J Internat Soc Respir Prot. 2005;22:106–11.
44. Johnson AT, Scott WH, Caretti DM. Review of recent research on respiration
while wearing a respirator. J Internat Soc Respir Prot. 2000;18:22–30.
45. Johnson AT, Scott WH, Caretti DM. Review of recent research on
communications while wearing a respirator. J Internat Soc Respir Prot.
2000;8:31–6.
46. Johnson AT, Scott WH, Caretti DM, Danisch SG. Do respirators stress the
cardiovascular system? J Internat Soc Respir Prot. 2003;20:26–36.
47. Johnson AT, Scott WH, Coyne KM, Koh FCS, Rebar JE. Telephone
communications with several commercial respirators. Am Indus Hyg Assoc
J. 2001;62:685–8.
48. Johnson AT, Scott WH, Coyne KM, Sahota MS, Benjamin M, Rhea PL, et al.
Sweat rate inside a full-facepiece respirator. Am Indus Hyg Assoc J. 1997;58:
881–4.
49. Johnson AT, Scott WH, Koh FC, Francis EB, Lopresti ER, Phelps SJ. Effects of
PAPR helmet weight on voluntary performance time at 80-85 % of maximal
aerobic capacity. J Internat Soc Respir Prot. 2006;23:111–
8.
50. Johnson AT, Scott WH, Lausted CG, Benjamin MB, Coyne KM, Sahota MS, et
al. Effect of respirator inspiratory resistance level on constant load treadmill
work performance. Am Indus Hyg Assoc J. 1999;60:474–9.
51. Johnson AT, Scott WH, Lausted CG, Coyne KM. Comparison of treadmill
exercise performance times for several types of respirators. J Internat Soc
Respir Prot. 1999;17:19–23.
52. Johnson A, Scott WH, Lausted CG, Coyne KM, Sahota MS, Johnson MM.
Effect of external dead volume on performance while wearing a respirator.
Am Indus Hyg Assoc J. 2000;61:678–84.
53. Johnson AT, Scott WH, Lausted CG, Coyne KM, Sahota MS, Johnson MM, et
al. Communication using a telephone while wearing a respirator. Am Indus
Hyg Assoc J. 2000;61:264–7.
54. Johnson AT, Scott WH, Phelps SJ, Caretti DM, Koh FC. How is respiratory
comfort affected by respiratory resistance? J Internat Soc Respir Prot. 2005;
22:38–46.
55. Johnson AT, Weiss RA, Grove C. Respirator performance rating table for
mask design. Am Indus Hyg Assoc J. 1992;53:193–202.
Johnson Journal of Biological Engineering (2016) 10:4 Page 11 of 12
Content courtesy of Springer Nature, terms of use apply. Rights reserved.

56. Johnston AR. Introduction to selection and use. In: Colton CE, Brosseau LM,
editors. Respiratory Protection: A Manual and Guideline. Fairfax: Amer. Indus.
Hyg. Assoc; 2001. p. 13–24.
57. Koh FC, Johnson AT, Scott WH, Phelps SJ, Francis EB, Cattungal S. The
correlation between personality type while wearing a respirator. J Occupat
Environ Hyg. 2006;3:317–22.
58. LaTourrette T, Patterson DJ, Bartis JT, Jackson BA, Houser A. Protecting
Emergency Responders, vol. 2: Community Views of Safety and Health Risks
and Personal Protection Needs. Santa Monica: RAND Science and Policy
Institute; 2003.
59. Lausted CG, Johnson AT, Scott WH, Johnson MM, Coyne KM, Coursey DC.
Maximum static inspiratory and expiratory pressures with different lung
volumes. Biomed Eng Online. 2006;5:29 (5 May).
60. Mackey KRM, Johnson AT, Scott WH, Koh FC. Over breathing a loose-fitting
PAPR. J Internat Soc Respir Prot. 2005;22:1–10.
61. Morgan WP, Raven PB. Prediction of distress for individuals wearing
industrial respirators. Am Indus Hyg Assoc J. 1985;46:363–8.
62. MSHA. Metal and Nonmetal Safety and Health Fatal Investigation Report;
Storm Exploration Decline. Washington, DC: Barrick Goldstrike Mines, Inc.,
US Mine Safety and Health Administration; 2003.
63. Rebar JE, Johnson AT, Russek-Cohen E, Caretti DM, Scott WH. Effect of
differing facial characteristics on breathing resistance inside a respirator
mask. J Occupat Environ Hyg. 2004;1:343–8.
64. Saunders KB, Bali HN, Carson ER. A breathing model of the respiratory
system: the controlled system. J Theor Biol. 1980;84:135–61.
65. Scott Jr WH, Coyne KM, Johnson MM, Lausted CG, Sahota M, Johnson AT.
Effects of caffeine on performance of low intensity tasks. Percep Motor
Skills. 2002;94:521–32.
66. Scott Jr WH, Koh FC, Chiou KYS, Mackey KRM, Johnson AT. Quantifying
energy expenditure during water-immersion in non-trained cyclists. Biol
Eng. 2008;1:281–9.
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