International Journal of lndustrial Ergonomics, 3 (1988) 89-102
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
89
HEAT STRESS INDICES: A REVIEW PAPER
Mohamed Youssef Beshir
Modern Co. for Textile Industries, 10 TH of Ramadan City (Egypt)
and Jerry D. Ramsey
Texas Tech University, Industrial Engineering Dept., Lubbock, TX 79409 (U.S.A.)
(Received January 18, 1987; accepted in revised form September 30, 1987)
ABSTRACT
This paper represents a review of the thermal
indices commonly used for assessing heat stress
conditions in an environment, e.g., Corrected Effective Temperature, New Effective Temperature, Heat
Stress Index, Wet Bulb Globe Temperature and Wet
Globe Temperature. The advantages and disadvantages of the indices are included, as well as an
example of calculations and procedures required to
determine the resulting values of each heat stress
index. Separate sections for Programmed Hand
Held Calculators, the Mean Equivalence Lines and
the International Organization for Standardization
(ISO) standards are also provided in the paper.
INTRODUCTION
also considered potential major factors contributing to the changes occurring in human physiological response due to heat exposure, i.e., heat
strain. There is a general agreement that a major
criterion for evaluating the effectiveness and usefulness of a heat stress index is its correlation with
heat strain (WHO, 1969; Peterson, 1970; Astrand
et al., 1975). The major physiological responses to
heat stress are increases in: (1) body temperature,
(2) heart rate, and (3) sweat production (Wyndham and Heyns, A973; Belding, 1976).
An ideal heat stress index which integrates all
the climatic, physical, and personal factors into a
single number and hence correlates them to one or
more physiological response has not yet been
found (Belding, 1970; D u k e s - D o b o s , 1976;
Kuhlemeier and Wood, 1979; Beshir, 1981). How-
Assessment of heat stress is most commonly
presented in terms of a heat stress index. A heat
stress index is a quantitative composite measure
which integrates into a single number one or more
of the thermal, a n d / o r physical, and personal
factors affecting heat transfer between the person
and the environment. Many heat stress indices
have been developed and these can be classified as
those based on: physical factors of the environment, thermal comfort assessment, "rational" heat
balance equations, and physiological strain
(Witherspoon and Goldman, 1974).
In addition to the climatic and physical factors,
personal factors such as: age, sex, physical fitness,
health status, clothing, and acclimatization are
0169-8141/88/$03.50
© 1988 Elsevier Science Publishers B.V.
90
ever, each of the available heat indices has some
advantages that make it more appropriate for use
in specific environmental conditions.
Some of the available heat stress indices are
useful for occupational and field measurements,
while others are more accurate in predicting heat
strain, and useful for research and laboratory
studies. Heat stress indices also provide a necessary composite measure of the thermal environment which is required when considering human
performance and worker safety (Hancock, 1987;
Ramsey et al., 1983). The principal criteria for a
recommended standard heat stress index for industrial use have been established by the National
Institute for Occupational Safety and Health
(NIOSH, 1972 and 1986).
The purpose of this paper is to review the
major heat stress indices reported in the literature.
The advantages and disadvantages of each index
have been summarized and a numerical example is
given to demonstrate the procedures and calculations required to compute each of these indices.
The Mean Equivalence Lines (MEL) (Wenzel,
1978), and the ISO standards for hot environments (ISO, 1983) are described in the paper.
CORRECTED
TURE (CET)
EFFECTIVE
globe temperature, so that the effective temperature is corrected for the radiant heat effects.
The CET, therefore, combines the climatic factors: air temperature, humidity, air velocity, and
radiation into a single reading. In measuring the
CET of an environment, globe temperature (Tg);
wet bulb temperature (Twb) and air velocity (v)
values are required. The CET of the environment
can be determined from a nomogram (Ellis et al.,
1972; Kerslake, 1972).
The advantages of the CET are its ability to
integrate the main four climatic factors in a single
reading (Goelzer, 1977); its simplicity and ease to
use (WHO, 1969) and it is thought by some to be
a useful index for engineers (Fuller and Smith,
1981). The disadvantages are its limited ability to
allow for the effects of clothing and metabolic
heat; different climates sharing the same CET
value do not necessarily impose the same heat
strain particularly below 40% rh (Goelzer, 1977);
it underestimates the adverse effect of humidity
and low air movement especially when the environmental conditions approach the tolerance
limit (Ramanathan and Belding, 1973) and the
updating findings cannot be assimilated into the
scale to extend its scope or improve its accuracy
(WHO, 1969).
TEMPERA-
Bedford (1946) modified the effective temperature scales (ET) developed by Houghten and
Yaglou (1923) and called them the Corrected Effective Temperature scales (CET). The ET scales
were based on equivalent subjective estimates of
thermal sensation of the thermal environment with
different combinations of air temperature, air
velocity, and humidity. All conditions having the
same thermal sensation were grouped together
under the same "Effective Temperature" (ET)
which is the air temperature of a still saturated
environment (i.e., zero air velocity and 100% relative humidity (rh)). Two ET scales were developed
by Houghten and Yaglou (1923): (1) the normal
scale, which is applied for men wearing ordinary
summer clothing, and (2) the basic scale, which is
applied for men stripped to the waist. The modifications of the ET scales as suggested by Bedford
(1946) were to replace the air temperature by the
SKIN WETTEDNESS (w)
Gagge (1937) defined the skin wettedness as the
fraction of the skin that is wet, and derived it from
the skin evaporative loss (Esk) according to the
following equation:
Esk = h e w ( P~k - ~ )
(1)
where E~k = total skin evaporative loss (W/m2),
h e = evaporative heat transfer coefficient ( W /
m 2°C), w = skin wettedness (no dimension), P~k
= saturated vapor pressure at T~k (mmHg), Pa =
saturated vapor pressure at Ta (mmHg), T~k = skin
temperature ( o C), and Ta = air temperature ( o C).
The skin wettedness (w) is calculated from the
ratio between the total skin evaporative loss (E~k)
and the maximum evaporative loss from a fully
wetted skin surface (Emax) (i.e., w=Esk/Emax)
(Gagge, 1937). When Esk is defined as the
evaporative heat loss necessary for the regulation
of body temperature (i.e., heat storage ( S ) = 0),
91
then skin wettedness w x 100 has been defined by
Belding and Hatch (1955) as the Heat Stress Index
(HSI) (see Heat Stress Index section below). The
minimum value of w is 0.06 when there is skin
diffusion but no sweating, and the maximum value
of w is 1.0 when the skin is fully covered by sweat
(Gagge, 1981). It has been reported that the sweating efficiency is related to the skin wettedness
level, and that the decline in intensity of sweating
is linked to maximal inefficient sweat drippage
before the onset of hidromeiosis (Candas et al.,
1983).
NEW EFFECTIVE TEMPERATURE (ET*)
Gagge et al. (1971) developed the New Effective Temperature scale (ET*). The ET* is similar
to the ET scale, but it uses as a reference an
environment at 50% rh. The ET* is defined as the
dry bulb temperature at 50% rh in which the total
heat exchange from the skin surface would be the
same as in the actual environment (Gonzalez et
al., 1978). Quantitatively, ET* is the solution (by
iteration) of the following equation (Gagge and
Gonzalez, 1973):
ET* + w[2.2hc/(h~+h~)](O.5P~T.-Pw) = To
(2)
where E T * = new effective temperature ( ° C ) ; w
= skin wettedness (no dimension); h c = convective
heat transfer coefficient and is a function of air
velocity ( W / m 2°C). It may be evaluated by the
following equation (Nishi, 1977):
h~ = 8.6v °53
(3)
h~ = linear r a d i a t i o n exchange coefficient
( W / m 2° C). Its value is rather constant at normal
temperature range and is approximately 4.7
W / m 2 ° C (Nishi, 1977); P f f T * = saturation vapor
pressure at ET* (mmHg). It can be determined
from a psychrometric chart or by Antoine's formula (Nishi, 1977):
P~T* = exp[18.669
- 4030.18/(ET* + 235)] mmHg
of the net thermal effect of convection and radiation. To is determined from the air and mean
radiant heat temperatures according to the following equation (Gagge, 1981):
TO= ( h r T r + hcTa)/(h ~+ hc)
Tr = mean radiant temperature ( o C) (see Table 1).
Approximate values for w and h~ can be assumed, however, accurate determination of these
variables requires the measurements of skin temperature (Tsk), clothing temperature (T~I), oxygen
consumption (VO 2), and the subject's body weight
and height. It has been reported that ET* can be
applied in hyper and hypo environments (i.e., high
altitudes and lower than sea level as in underground mines, respectively) (Nishi and Gagge,
1977). The ET* scale is useful for comfort and
thermal sensation studies.
The advantages of ET* are: it is a good indicator of physiological strain and warmth discomfort
(Gonzalez et al., 1978); it provides a theoretically
accurate estimate of human heat transfer (Gagge
et al., 1971), and it is more comparable to our own
every day experience than the ET scale (Gagge
and Gonzalez, 1973). The disadvantages of the
ET*, however, are: it is difficult to apply and
requires complicated instruments, measurements
and computations; it is not suitable for occupational studies; it assumes a normally clothed
sedentary human subjects and its usefulness is
limited for exposure times shorter than an hour
(Gagge et al., 1971).
HEAT STRESS INDEX (HSl)
Belding and Hatch (1955) developed the Heat
Stress Index (HSI) (initially called the Belding and
Hatch Index (BHI)). The HSI is calculated from
the equation:
Heat Stress Index (HSI) =
Required evaporative heat ( Ereq )
Maximum evaporative heat (Ema x )
(4)
Pw = ambient vapor pressure at Ta (mmHg); and
To = operative temperature (°C). It is a single
temperature figure that is physically representive
(5)
X 100
(6)
where E r e q = evaporative heat loss required to
maintain the body in thermal equilibrium (i.e.,
S = 0), and E m a x = maximum evaporative capacity
of the climate.
92
TABLE 1
HSI equations and coefficients (adapted from Ramsey and Chat, 1983)
Coefficients ( K )
R = K R (T r - 3 5 )
C = K c v0.6 ( T a - 3 5 )
E ...... = K F t,0.6 ( 4 2 - P~)
Shorts
Standard
clothing
Standard
plus coat
12.8(11.0)
13.6 (11.7)
27.1 (23.3)
7.7 (6.6)
8.1 (7.0)
16.3 (14.0)
6.2 (5.3)
6.5 (5.6)
13.0 (11.2)
rr = r~ +1.8,0s (Tg - r~l)
Ere q M + R + C
HSI = ( Ereq / E m a x ) X 100
=
Where T~, = air temperature ( o C), Tg = Vernon globe temperature ( o C), Tr = mean radiant temperature which can be estimated from
Tg ( o C), t, = air velocity ( m / s ) , Pw = water vapor pressure of ambient air (mmHg), M = metabolic rate of body heat production (W
(kcal/h)), R = radiant heat exchange (W (kcal/h)), C = convective heat exchange (W (kcal/h)), Ere q = an expression of stress in
terms of requirement for evaporation of sweat (W (kcal/h)), and Em~,x = m a x i m u m evaporative heat loss which can be achieved at a
given P,, and r, (W (kcal/h)).
Table 1 (Witherspoon and Goldman, 1974;
Ramsey and Chat, 1983) summarizes the equations and coefficients needed to calculate HSI.
The index (and coefficients in Table 1) assumes a
standard man (i.e., 70 kg weight, 1.7 m height, and
1.8 m 2 body surface area), dressed in shorts and
gym shoes, experiencing a skin temperature of
35 ° C (95 ° F), and uniformly wetted with sweat. If
the workers has a surface area (A) greatly different from that of the standard man, the coefficients
in Table 1 can be corrected if multiplied by A/1.8.
The coefficients for shorts and standard clothing
have been experimentally validated and the coefficients for standard clothing plus coat are an extrapolation of these values (Ramsey and Chat,
1983). Different authors, however, recommended
slightly different values for these coefficients.
Some investigators (Vogt et al., 1982) concluded that 3 6 ° C is a more accurate estimate of
mean skin temperature for use in heat transfer
equations, than the 35 ° C assumed by Belding and
Hatch (1955) for the HSI. K a m o n and Rayn (1981)
modified the HSI by assuming 36 ° C for the mean
skin temperature and called their index the Effective Heat Strain Index (EHSI).
The maximum sweat production that can be
maintained by the average man through an eight
hour period is assumed to be one liter per hour,
which is equivalent to an evaporite heat loss of
about 698 W (600 kcal/h). Therefore, the value of
E m a x c a n n o t exceed this value of 698 W. An HSI
of 100 was proposed by Belding and Hatch (1955)
as the m a x i m u m that a healthy young adult male
can tolerate for eight hour work shift, 70-90 indicates very severe heat strain, 40-60 represents
moderate to severe heat strain, and 10-30 represents a mild to moderate heat strain.
In addition to the analytical procedure (i.e.,
using the equations given in Table 1 to determine
the HSI values, nomographs are also available as
an aid to determine E r e q a n d E .... (McKarns and
Brief, 1966). Based on the values of E r e q and
E ..... the allowable exposure times (AET) in the
hot environments for a 1 ° C ( 2 ° F ) rise in body
temperature, and the minimum recovery times
(MRT) in a cooler rest location for restoration of
normal body temperature were suggested by McKarns and Brief (1966). It was assumed that the
average man can tolerate a I ° C body temperature
rise and that his temperature will rise I ° C for
each 73.25 W gained. The following equations
evaluate allowable exposure and minimum recovery times in minutes:
A E T = (73.25
× 60)/(Ere
M R T = (73.25 × 6 0 ) / ( E
q - Emax)
. . . .
--
Ereq )
(7)
(8)
The advantages of the .HSI are: it permits
estimation of tolerance time and required resting
time (Goelzer, 1977); it is useful in designing and
evaluating the efficiency of environmental control
systems (Goelzer, 1977); it can be continuously
93
improved in scope and accuracy as fresh information on heat exchange is acquired (WHO, 1969); it
differentiates between thermal conditions in correct order of physiological strain (Belding and
Hatch, 1955) and it has been used widely and
successfully as a tool for evaluating hot woi'k-environments (Ramsey and Beshir, 1987). The disadvantages are: it is difficult to apply to variable or
intermittent heat exposure (Goelzer, 1977); it is
validated only on young acclimatized when
(Goelzer, 1977); it involves difficult calculations
and requires more instruments than several other
indices (Goelzer, 1977); it underestimates the adverse effect of low wind speeds and hot humid
environments (Ramanathan and Belding, 1973)
and it does not correctly differentiate between
heat gained from work and that gained by convection or radiation (Ramsey and Beshir, 1985).
WET BULB
(WBGT)
GLOBE
TEMPERATURE
Yaglou and Minard (1957) developed the Wet
Bulb Globe Temperature (WBGT) index for use
in controlling heat casualties at military training
centers. It was not based on analysis of a new set
of prime data but was, in fact, derived from and
as a means for estimating the Corrected Effective
Temperature (CET).
The W B G T combines the effect of the four
main climatic factors contributing to heat stress:
air temperature, humidity, air velocity and radiation. Air temperature is measured directly by the
dry bulb temperature (Tab), while the combined
effect of humidity and air velocity is measured by
the natural wet bulb temperature (Tnwb) and radiation is measured by the globe temperature (Tg).
The W B G T at one time was calculated according
to the following equation (AIHA, 1975):
W B G T = 0.7Tnwb + 0 . 3 [ ( T g - TdblK + TdU]
(91
indoor, night or sunless day exposure, K equals 1
(Goelzer, 1977). W B G T can be measured either by
the method recommended by N I O S H (1972) or by
using an integrated electronic instrument.
1. NIOSH Method
N I O S H (1972) recommended the W B G T index
as the standard heat stress index for industrial use.
This recommendation was based on the principal
criteria established by N I O S H (1972) for a recommended standard heat stress index. N I O S H also
suggested the instruments and procedures for
measuring W B G T so that it could be used as the
parameter in determining the environmental conditions for implementation of work practices. The
standard tree for measuring WBGT, as suggested
by NIOSH, consists of a tripod, a mercury-in-glass
thermometer to measure Tdh, a 15 cm (6 in.) globe
with mercury-in-glass thermometer placed in the
center of the globe to measure Tg and a mercuryin-glass thermometer with its bulb covered by a
clean wetted wick to measure Tnwb. The wick is
immersed in a 125 ml flask filled with distilled
water.
NIOSH selected values for K in eqn. (9) equal
to 1 and 2 / 3 for indoor and outdoor heat exposures, respectively. Consequently, N I O S H recommended the following equations to calculate
the WBGT:
For indoor exposure, or outdoor exposure with
no solar load:
W B G T = 0.7T, wb + 0.3Tg
For outdoor sunlit exposure:
W B G T = 0.7Tnwb + 0.2Tg + 0.1Tdb
(11)
In continuous heat exposure, the time-weighted
average W B G T values are calculated on an hourly
basis, and in a two-hour basis in intermittent heat
exposure. The time-weighted average W B G T is
determined by the equation (NIOSH, 1972):
Av. WBGT
For outdoor exposures with solar load, the value
of K is related to the clothing or the type of skin,
e.g., 0.75 for green or gray outer clothing, 0.65 for
military khaki uniform, and 0.45 for clean white
clothes. For seminude, K equals 0.82 for negroid,
0.78 for Hindu, and 0.60 for white skin. For
(10)
WBGTlXtl+WBGT2×t2+'"+WBGT"×tn
tl + t2 + ... + t n
(12)
where: W B G T D W B G T 2. . . . . WBGT. are calculated values of W B G T for various work or rest
areas occupied during the total time period; and
94
t 1, t 2. . . . . t, are the elapsed times in minutes spent
in the corresponding areas which are determined
by a time study.
The WBGT index, its measuring techniques
and calculation procedures as recommended by
NIOSH (1972), were later supported by the Occupational Safety and Health Administration
(OSHA) advisory committee (Ramsey, 1975), and
the American Conference of Governmental Industrial Hygienists (ACGIH, 1987). The original
NIOSH criteria document (1972) was revised to
reflect acclimatized versus unacclimatized workers
and exposure limits versus alert limits (NIOSH,
1986). Values for ceiling limits were given in the
revised criteria. Following these recommendations,
many investigators have used the WBGT index for
evaluating heat stress (Ramsey and Beshir, 1987).
The WBGT has been applied for evaluating heat
stress in many industrial plants, e.g., aluminum
reduction plants (Horvath, 1976), steel plants
(Parker and Pierce, 1984; Minard, 1976), glass
container plants (Polhemus, 1976), and chemical
plants (Rodgers, 1976) as well as in underground
mines (Ramsey et al., 1986). Recently, the WBGT
index has been adopted by the International
Organization for Standardization (ISO, 1982) as
the international standard heat stress index. The
WBGT has been used for assessing the effects of
workplace thermal conditions on safe work behavior (Ramsey et al., 1983).
2. Integrated electronic instruments
Integrated electronic instruments for measuring
W B G T are also commercially available (e.g.,
Reuter Stokes models RSS-211, 212, 213, 214 and
217, Yellow Springs Heat Stress Instrument, and
Vista Scientific Corporation Heat Stress Monitor).
Such instruments provide a direct or digital readout of WBGT, and for some models individual
thermal measurements as Tdb, Twb, Tg and v can
be obtained as well as an access to a self contained
data logger for loading directly to a printer or a
computer. The stabilization time required for the
integrated electronic instruments is usually around
5 rain since all the sensors are resistance thermometers and the globe has a small diameter
(about 4.2 cm) (Kuehn and MacHattie, 1975).
The advantages of WBGT are: it is simple to
measure and calculate heat stress (Astrand et al.,
1975; ACGIH, 1987); the need for measuring each
climatic the need for measuring each climatic factor separately (i.e., air temperature, humidity and
radiation) in order to determine the W B G T provides information which are useful for evaluating
efficiency of environmental control systems if
coversion factors between wet-bulb and natural
wet-bulb temperatures are used (Ramsey and Chai,
1983); air velocity does not have to be measured
separately (Dukes-Dobos, 1976; Goelzer, 1977); it
is a reliable indicator and has a reasonable degree
of precision (Ramsey, 1976); it is practical for
industrial purposes (Astrand et al., 1975); it apparently correlates well with the resulting physiological reactions due to heat exposure (Ramsey,
1976; Onkaram et al., 1980); it has proved to be of
value in eliminating adverse effects of heat at
military training centers (Onkaram et al., 1980);
its applicability in industrial use has been proven
(Horvath, 1976; Minard, 1976; Polhemus, 1976;
Rodgers, 1976) and the integrated electronic instruments have small size, require short stabilization time, and are simple to use (Kuehn and
MacHattie, 1975).
The disadvantages of the WBGT are: the
WBGT estimate gets progressively poorer under
low humidity conditions (Ramsey, 1976; Goelzer,
1977); the same W B G T value does not have consistent physiological meaning independent of the
climatic factors (Ramanathan and Belding, 1973;
AIHA, 1975; Goelzer, 1977); higher air temperatures and work rates would exaggerate these inconsistencies (Ramanathan and Belding, 1973); it
does not consider the metabolic workload (Ramsey
and Beshir, 1985); the standard tree is bulky,
awkward, and requires 20 min estabilization period
(Astrand et al., 1975; Onkaram et al., 1980); the
integrated electronic instruments have high initial
cost and sensitive to electronic failure (Ciriello
and Snook, 1977; Beshir et al., 1982); the electronic circuit a n d / o r the plastic case of the integrated instrument may be damaged by the high
temperatures that would be experienced if the
instrument were left exposed for a long period
(Parker and Pierce, 1984) and when NIOSH determined that the studies of Lind (1963) provided
the best basis for establishing a heat standard, the
environmental conditions were measured in Lind's
study by the ET scale and NIOSH declared ET to
be impractical and substituted W B G T for it,
95
Study
therefore, W B G T is a secondary measure and ET
was the primary unit (Botsford, 1976).
WET GLOBE TE M PER A T U R E
Botsford (1971) developed the Wet Globe Temperature (WGT) which is measured with a Botsball (BB) thermometer. The W G T combines the
effect of the four basic climate factors: air temperature, humidity, air velocity, and radiation into a
single reading. The main concept considered in
developing this index was to design a simple device that exchanges heat with the thermal environment in a manner qualitatively similar to the
human.
The Botsball (BB) consists of a dial thermometer with its metal probe inserted in a plastic tube
which is surrounded by an aluminum tube. The
heat sensor at the end of the metal probe is placed
in the center of a 6 cm (2~ in.) diameter hollow
copper sphere connected to the plastic and
aluminum tubes. The copper sphere is painted
black and covered with a double layer of black
cloth which is saturated with water. The BB has a
water reservoir of about 7 cc capacity. If the BB
will be in continuous use for long periods, a
siphon into an auxiliary water source (e.g., the
plastic bottle furnished with each Botsball) should
be used in order to prevent the dryness of the
cloth which causes inaccurate measurements.
The stabilization time for the BB is approximately 5 min when the temperature differential
between subsequent readings is small ( 5 - 1 0 ° C ) ,
but may require 15 rain when this differential is
large ( > 15°C) (Beshir et ai., 1982). A summary
for the advantages and disadvantages of the W G T
and the W B G T observed by numerous investigators and a comprehensive comparison between the
two indices has been reported (Beshir, 1981).
Since N I O S H (1972) recommended the W B G T
as a standard index for industrial use, and because
of the potential advantages of the Botsball, several
researchers have studied the relationship between
the two indices indoor (Brief and Confer, 1971;
Sundin et al., 1972; Mutchler and Vecchio, 1977;
Ciriello and Snook, 1977; Beshir et al., 1982;
Parker and Pierce, 1984) and outdoor (Onkaram
et al., 1980). Predictive equations with high coefficient of correlation (ranging from 0.913 to 0.980)
No.
RegressionEquation
No. of
Obs.
WBGT
Range
°C
r
(°C)
O
Beshir, e t . . l .
r'l
Clde~lo & Snook
~,
Mutchler & Vsc©hio
O
13489
W B G T = 1.01 W G T + 2.60
0 - 37
0.956
210
W B G T = 1.07 W G T + 0.80
16 - 4 8
0.948
1023
W B G T = 1.05 W G T + 1.14
11 - 4 2
0.913
W G T = 0.958 W B G T - 2.52
21 - 41
0.957
W B G T = 0.0212 ( W G T ) 2
+ 0.192 ( W G T ) + 9.5
18 - 56
0.976
Bdel & Conler
34
Sundin, et.al.
379
--
Rsmsey & Beshir
W B G T = W G T + 3.00
---
Rsmsey
W B G T = 1.1 W G T
O
5O
//
/
W B G T = 1.1 W G T - ~ I ~ . / / & / ~
40
W B G T = W G T + 3.0
O
ov
I-
30
m
2O
/
A
10
o
I
,o
,'o
1o
I
,o
5'0
W G T (° C)
Fig. 1. Relationships between W B G T and WGT.
for the relationship between the W B G T and W G T
have been reported. All the reported predictive
equations are linear except in one study (Sundin et
al., 1972). Ramsey et al. (1982) suggested that as a
simple approximation for indoor measurements,
the W B G T value can be obtained by adding 3 ° C
to the Botsball reading. G o l d m a n (1981) suggested
adding 1 ° C to the Botsball to approximate the
W B G T value for outdoor measurements. Based on
the relationships between the two indices which
have been reported by the previous investigators, a
simple relationship was suggested for indoor use
(Ramsey, 1987):
W B G T = 1.1 W G T ° C
(13)
Figure 1 summarizes the relationships between
W B G T and W G T reported in several individual
studies. Any of these equations are basically valid,
but the last two ( W B G T = W G T + 3 . 0 ° C or
W B G T = 1.1 W G T ) are much simpler to remember and use and could be argued to be as valid as
any of the individual studies. A word of caution is
in order, however, since this correlation implies a
moderate level of both humidity and radiant heat;
96
at both high and low levels for either of these two
variables the well behaved relationship shown in
Fig. 1 may not be valid. For example, environments with moderate heat, high humidity and no
radiant heat, have WBGT and W G T values which
are almost equal (Ramsey, 1987).
Recently, a NIOSH Workshop on Recommended Heat Stress Standards (NIOSH, 1980)
concluded that "the primary heat stress index to
be used for monitoring the industrial environment
should be the WGT, with the alternative of using
the WBGT, the ET or equivalent". The W G T was
suggested as a basic index because of its simplicity
and ease of use in industry. The primary purpose
for using a single and simple index would be to
establish a point at which some other more detailed observations must be made.
The advantages of W G T are: the Botsball is a
practical, rugged, simple and reliable device (Beshir et al., 1982); the instrument's small size provides for non-invasive placement near the worker
and enhances accurate assessment of the environment (Beshir et al., 1982); the required stabilization time is relatively short (Ciriello and Snook,
1977; Beshir et al., 1982); it combines the effects
of the four main climatic factors into a single
reading (Gershoni, 1979); it requires no calculations (Ramsey and Beshir, 1987); it is less expensive than the WBGT standard tree or the integrated electronic instruments (Onkaram et al.,
1980) and high correlations between W G T and
other indices, especially WBGT, have been reported (Ramanathan and Belding, 1973; Botsford,
1976; Beshir, 1981; Lee and Ramsey, 1987).
The disadvantages of W G T are: correlation
between W G T and physiological responses to heat
exposures has not been fully established
(Ramanathan and Belding, 1973); although predictive relationship between W G T and WBGT
have been developed (Beshir et al., 1982; Lee and
Ramsey, 1987), some combinations of environment can result in large errors (Ciriello and Snook,
1977); when the wick is dry, erroneous measurements will be obtained (Beshir et al., 1982); lack
of positive adjustment for flow rate and wick
wettedness (Beshir et al., 1982); the small size of
the globe resulted in a much higher sensitivity to
air velocity than man exhibits (Hatch, 1973) and
only an integrated measure of the environment is
obtainable, rather than individual climatic factors
which may be useful in further in-depth analysis
(Ramsey and Beshir, 1987).
PROGRAMMED HAND HELD CALCULATORS
Programmed hand held calculators are increasingly being used for evaluating occupational environments. The heat stress indices utilizing such
calculators, are those which are well correlated to
human physiological responses but have complicated computations such as the New Effective
Temperature (Nishi, 1977) and the Heat Stress
Index (Kamon and Ryan, 1981). For example,
instead of applying the tedious computations suggested by Gagge and Gonzalez (1973) to determine the value of ET*, the programmed hand
held calculator developed by Nishi (1977) to determine this value can be used. This calculator
performs those boring computations and needs as
input data: Za, Twb (or rh), To, h~, h r, metabolic
rate, human mechanical efficiency, effective clothing insulation and mean skin temperature.
Programmed calculators provide the user not
only with the heat stress level, but may also provide work practice information such as the time
limits of exposure and the rest period required.
Computer programs have been also developed for
determination of some heat stress indices (Gagge,
1973; Chai, 1981; Cvejanovich, 1983). The use of
programmed hand held calculators and computers
for assessment of occupational heat and thermal
indices is expected to be more widely practiced in
the future.
MEAN EQUIVALENCE LINES (MEL)
Influencing factors of heat ~tress can be classified as climatic variables and non-climatic variables. Climatic variables consist of: air temperature, air velocity, humidity and radiant heat while
non-climatic variables include: activity level (i.e.,
heat production in the body) and thermal resistance of the clothing. In order to assess the heat
stress effects, it is necessary to know how far the
various influencing factors compensate each other,
or in other words, what combinations of the variables produce equal effect.
97
Wenzel (1978) investigated the reactions of a
number of men to systematically varied combinations of all the climatic and non-climatic variables
mentioned above, to drive those combinations
which produce equal effects on various physiological reactions and to check the statements of all
the indices in use. The author developed the Mean
Equivalence Lines (MEL) which are represented
in psychrometric charts and describe combinations of climatic and non-climatic conditions in
which equal heart rates as well as equal rectal and
skin temperatures would be reached.
Wenzel (1978) reported that the combinations
of ambient temperatures and humidities that were
found to be equivalent under the given conditions
of physical activity corresponded particularly well
with the Index of Physiological Effect (Robinson
et al., 1945). There was also good agreement with
the Predictive 4 hour Sweat Rate index (McArdle
et al., 1947). The combinations corresponded with
other indices (i.e., HSI, ET and WBGT) only
within limited ranges of climate depending upon
work level.
INTERNATIONAL ORGANIZATION FOR
STANDARDIZATION (ISO) STANDARDS
The ISO has recently issued a standard for hot
environments. ISO (1982) specifies the required
measurements and procedures for the estimation
of the heat stress on working man, based on the
W B G T index.
The ISO has also proposed a method of analytical evaluation and interpretation of the thermal stress experienced by a subject in a hot environment (ISO, 1983). The method is based on
the comparison between the required skin wettedness and the required sweat rate as a result of the
working conditions, and the skin wettedness and
sweat rate which it is physiologically possible to
achieve.
ISO (1983) recommends that ambient conditions acceptable for an 8 hour should satisfy the
following conditions:
Ep = Ere q
(14)
Wreq < Wlim
(15)
SWp <
(16)
Omax/8
where
E p = predicted evaporation rate ( W m - 2 ) ,
required evaporation rate ( W m - 2), w req
required skin wettedness (dimensionless), w lim =
skin wettedness compatible with a steady state of
b o d y t e m p e r a t u r e s (dimensionless), SWp =
predicted sweat rate ( W m - 2 ) , and Dmax =
m a x i m u m acceptable dehydration (Win-2).
Equations for calculating the above mentioned
variables are included. N o time limits have been
suggested for the work shift, if the three conditions are satisfied. In this case SWp can be used as
a comparison index for the equivalence of heat
stress conditions.
ISO (1983) emphasizes that it is necessary to
calculate an allowable exposure time (DLE), if
one or other of these three conditions is not
satisfied. The D L E can be determined from the
m a x i m u m heat storage (Qmax) compatible on the
one side with the normal execution of a task and
on the other with the absence of pathological
effects. The raising of the deep body temperature
corresponding to this m a x i m u m heat storage is
taken to be between 0.8 and 1°C.
If the required evaporation rate is not achievable
Ere q =
=
D L E 1 = 60Q ..... / ( Ereq - -
Ep)
(17)
If the required skin wettedness is excessive and
incompatible with a stable regime of body temperatures
DLE 2 =
60Qrnax//(Wreq
-
Wlim)
(18)
If the required sweat rate involves an exaggerated dehydration
D E E 3 = 60Oma,,/SWp
(19)
The allowable exposure time (DLE) which is to
be taken into consideration in the limiting duration of work, is the shortest DLE, i.e., the minim u m value of D L E 1, D L E 2 and D L E 3. When
D L E 1 or D L E 2 are the determining factors, it is
advised to allow the worker a rest period sufficient
to bring about a return to normal body temperature before being exposed again to a hot environment. When D L E 3 is the determining factor, no
further exposure should be allowed during the
day.
This proposed standard provides a programme
in BASIC which allows the calculation of the
98
required sweat rate and of the D L E for all situations where the following parameters are known:
the metabolic heat production, the thermal insulation of the clothing and climatic parameters.
The method of analytical estimation and interpretation of thermal stress (ISO, 1983) allows a
more precise approach than the method based on
the W B G T (ISO, 1982) as well as a more rational
choice of means of protection. However, at the
present state of techniques of measurement and
calculation, its application takes longer and is
more difficult. The ISO (1983) procedures can be
used either when an in-depth analysis of hot working conditions is needed, or as a complement of
the ISO (1982) standard when its reference values
are exceeded.
A NUMERICAL EXAMPLE
The following example reviews the procedures
and calculations required to determine the values
of the heat stress indices discussed in this paper.
The climatic factors of an indoor industrial
workplace were measured with the appropriate
instruments as follows:
Air temperature (Ta) = 36 ° C
Natural wet bulb temperature (Tnwb) = 30 ° C
Psychrometric wet bulb temperature (Twb)=
29°C
Globe temperature (T~) = 40 ° C
Air velocity (v) = 0.5 m / s
Botsball reading ( W G T ) = 29.5 o C
The worker was wearing a normal summer
clothing (i.e., clo = 0.6), and performing a m o d erate workload task and his metabolic energy was
250 W.
1. Corrected Effective Temperature (CET)
The corrected effective temperature value can
be determined from a n o m o g r a m (Ellis et al.,
1972; Kerslake, 1972) by applying Tg = 40, Twb =
29 and v = 0.5:
C E T = 31.6 ° C
2. Skin wettedness (w)
The skin wettedness can be determined from
the relation:
w = HSI/100
= 199.5/100
= 1.995
(see section 4 below for the calculation of HSI)
Since w cannot exceed 1, therefore the value of
the skin wettedness in this example is 1. This
means that the worker's b o d y was fully covered by
sweat.
3. New effective temperature ( E T * )
In this example, the value of E T * is determined
by the iteration procedure (Gagge and Gozalez,
1973) to demonstrate the complexity of the computations. However, it is agreed that the computations would not be so hard, if they were performed on a p r o g r a m m e d hand held calculator
(Nishi, 1977).
The new effective temperature can be obtained
by solving eqn. (2) (by iteration).
E T * + w [ 2 . 2 h c / ( h c + hr) ] (0.5P~T. -- Pw) = To
hc = 8.6v °53
= 8.6(0.5) 0.53 --- 5 . 9 5 6 W / m 2 o C
Assuming h r = 4 . 7 W / m 2 ° C , then the operative temperature (To) can be determined from
eqn. (5):
To=(hrTr+hcTa)/(hr+hc)
= (4.7 × 45.09 + 5.956 X 3 6 ) / ( 4 . 7 + 5.956)
= 40°C
(see section 4 below for the calculation of T~)
The term w
[ 2 . 2 h c / ( h c + h r ) ] = 112.2 X 5 . 9 5 6 / ( 4 . 7 + 5.956)]
= 1.23
Knowing T~=36°C
and T w b = 2 9 ° C , then
from a psychrometric chart, the value of Pw is
equal to 26.5 m m H g . F r o m the above c o m p u t e d
values, eqns. (2) and (4) become:
If the effective temperature value is required,
use Ta instead of Tg:
E T * + 1.23(0.5P~T. -- 26.5) = 40
ET = 30.5° C
P~v* = exp(18.669 - 4 0 3 0 . 1 8 / [ E T * ]) m m H g
99
Therefore,
C = KeY° 6(Ta - 3 5 )
E T * + 1.23{0.5 exp[18.669 - 4 0 3 0 . 1 8 / ( E T * + 235)]
-
26.5
} =
40
The value of E T * can be determined from this
equation by trial and error.
First trial
Assume E T * = 4 0 ° C ,
equation will be:
then the L.H.S. of the
40 + 1.23{0.5 exp[18.669 - (4030.18/275)]
- 26.5 } = 41.449
Therefore, by assuming E T * = 40 ° C, the
L.H.S. of the equation (i.e., 41.449) becomes
greater than the R.H.S. (i.e., 40).
= 8.1(0.5)°6(36 - 35)
=5.3W
Ere q = M + R + C
= 250 + 77.7 + 5.3
= 333 W
Emax =
KE°°'6( 42 -- Pw)
= 16.3(0.5)°"6(42 - 26.5)
= 166.9 W
(the value of Pw (i.e., 26.5) is determined from a
psychrometric chart, see section 3 above)
Heat Stress Index ( H S I ) =
(Ereq/Emax) X 100
= (333/166.9) X 100
Second trial
Assume E T * = 39 ° C, the L.H.S. of the equation will be:
= 199.5
5. Wet Bulb Globe Temperature (WBGT)
39 + 1.23{0.5 e x p [ 1 8 . 6 6 9 - (4030.18/274)]
- 26.5 } = 38.676
By assuming E T * = 3 9 ° C , the L.H.S. of the
equation (i.e., 38.676) becomes smaller than the
R.H.S. (i.e., 40).
Third trial
Assume E T * = 39.5°C, the L.H.S. of the equation will be:
39.5 + 1.23{0.5 exp[18.669 - (4030.18/274.5)]
-
26.5
} =
40.05
In this iteration, the L.H.S. (i.e., 40.05) is approximately equal to the R.H.S. (i.e., 40), hence:
The wet bulb globe temperature is obtained
from eqn. (10):
W B G T = 0.7Tnw b + 0.3Tg
= 0.7 X 30 + 0.3 X 40
= 33°C
6. Wet Globe Temperature (WGT)
The wet globe temperature is obtained directly
from the Botsball reading:
W G T = 29.5 ° C
ET* = 39.5°C
Conclusions
4. Heat Stress Index (HSI)
The heat stress index can be obtained by using
the equations and coefficient given in Table 1:
Tr = Tg + 1.8v°5(Tg - Ta)
= 40 + 1.8(0.5)°'3(40 - 36)
= 45.09 ° C
R = KR(g
- 35)
= 7.7(45.09 - 35)
= 77.7 W
As described above, heat stress is the aggregate
of climatic (i.e., air temperature, humidity or vapor
pressure, air velocity and radiation) and physical
factors. Assessment of heat stress is performed by
measuring one or m o r e of these factors and then
utilizing the appropriate heat stress index. The
objective of assessing the occupational heat stress
by one of the available heat indices is to establish
safe heat exposure limits, and regulatory rules.
Several heat indices have been developed over
the years. Some of these indices were discussed in
this paper (i.e., CET, w, E T * , HSI, W B G T , W G T
100
and programmed hand held calculators). The measurements required, advantages and disadvantages
for each index were discussed. A numerical example was given to illustrate the procedures and
calculations required to compute each index. Mean
Equivalence Lines (MEL) developed by Wenzel
(1978) and the International Organization for
Standardization (ISO) standards for hot environments were also provided in the paper.
Other heat stress indices reported in the literature and not discussed in this paper include: the
cooling power of air measured by the wet-kata
thermometer, the Index of Physiological Effect
(IPE), the predicted 4 hour sweat rate (P4SR), the
wet bulb-dry index (W.D.), the temperature
humidity index (THI), the index of physiological
effect ( E p ) , the index of thermal stress (ITS), the
relative strain index (RS), the reference index (RI)
and others (Witherspoon and Goldman, 1974;
Ramsey and Beshir, 1987).
A heat stress index as defined above is a
numerical evaluation of the hot environment.
Therefore, by increasing the level of one or more
of the climatic factors, the numerical value of a
heat index will increase. Consequently, it is logical
to expect that strong correlations exist among
most of the heat indices, and this has been reported in several laboratory and field studies (Brief
and Confer, 1971; Jensen and Heins, 1976;
Mutchler and Vecchio, 1977; Pulket et al., 1980).
NIOSH (1972) recommended the WBGT among
the available heat stress indices as the standard
index for industrial use. This recommendation was
later widely accepted and supported by different
institutions and researchers nationally and internationally (Beshir and Hafez, 1984). It has been
proposed that the WBGT index, corrected for
energy expenditure and air velocity, can be used
as an international standard heat stress index
(Brown and Dunn, 1977). Recently, the WGT is
being highly recommended to be the primary heat
stress index to be used for monitoring the industrial environment (NIOSH, 1980; Beshir, 1981).
This is basically because of the simplicity, small
size and reliability of the Botsball. However, the
primary purpose for using a simple index such as
the WGT, or the WBGT would be to establish a
point at which some other more detailed observations and in-depth analysis of the working environment must be made.
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