oSIST prEN ISO 7933:2022
(Main)Ergonomics of the thermal environment - Analytical determination and interpretation of heat stress using calculation of the predicted heat strain (ISO/DIS 7933:2021)
Ergonomics of the thermal environment - Analytical determination and interpretation of heat stress using calculation of the predicted heat strain (ISO/DIS 7933:2021)
Ergonomie der thermischen Umgebung - Analytische Bestimmung und Interpretation der Wärmebelastung durch Berechnung der vorhergesagten Wärmebeanspruchung (ISO/DIS 7933:2021)
Ergonomie des ambiances thermiques - Détermination analytique et interprétation de la contrainte thermique fondées sur le calcul de l'astreinte thermique prévisible (ISO/DIS 7933:2021)
Ergonomija toplotnega okolja - Analitično ugotavljanje in razlaga toplotnega stresa z izračunom predvidene toplotne obremenitve (ISO/DIS 7933:2021)
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Standards Content (Sample)
SLOVENSKI STANDARD
oSIST prEN ISO 7933:2022
01-januar-2022
Ergonomija toplotnega okolja - Analitično ugotavljanje in razlaga toplotnega
stresa z izračunom predvidene toplotne obremenitve (ISO/DIS 7933:2021)
Ergonomics of the thermal environment - Analytical determination and interpretation of
heat stress using calculation of the predicted heat strain (ISO/DIS 7933:2021)
Ergonomie der thermischen Umgebung - Analytische Bestimmung und Interpretation der
Wärmebelastung durch Berechnung der vorhergesagten Wärmebeanspruchung
(ISO/DIS 7933:2021)
Ergonomie des ambiances thermiques - Détermination analytique et interprétation de la
contrainte thermique fondées sur le calcul de l'astreinte thermique prévisible (ISO/DIS
7933:2021)
Ta slovenski standard je istoveten z: prEN ISO 7933
ICS:
13.180 Ergonomija Ergonomics
oSIST prEN ISO 7933:2022 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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oSIST prEN ISO 7933:2022
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oSIST prEN ISO 7933:2022
DRAFT INTERNATIONAL STANDARD
ISO/DIS 7933.2
ISO/TC 159/SC 5 Secretariat: BSI
Voting begins on: Voting terminates on:
2021-11-12 2022-01-07
Ergonomics of the thermal environment — Analytical
determination and interpretation of heat stress using
calculation of the predicted heat strain
Ergonomie des ambiances thermiques — Détermination analytique et interprétation de la contrainte
thermique fondées sur le calcul de l'astreinte thermique prévisible
ICS: 13.180
This document is circulated as received from the committee secretariat.
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENT AND APPROVAL. IT IS
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THEREFORE SUBJECT TO CHANGE AND MAY
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STANDARD UNTIL PUBLISHED AS SUCH.
IN ADDITION TO THEIR EVALUATION AS
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WHICH REFERENCE MAY BE MADE IN
Reference number
NATIONAL REGULATIONS.
ISO/DIS 7933.2:2021(E)
RECIPIENTS OF THIS DRAFT ARE INVITED
TO SUBMIT, WITH THEIR COMMENTS,
NOTIFICATION OF ANY RELEVANT PATENT
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PROVIDE SUPPORTING DOCUMENTATION. © ISO 2021
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
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ii
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols . 2
5 Principles of the predicted heat strain (PHS) model . 4
6 Main steps of the calculation .5
6.1 Heat balance equation . 5
6.1.1 Metabolic rate, M . 5
6.1.2 Effective mechanical power, W . 5
6.1.3 Heat flow by respiratory convection, C . 5
res
6.1.4 Heat flow by respiratory evaporation, E . 5
res
6.1.5 Heat flow by conduction, K . 5
6.1.6 Heat flow by convection, C . 6
6.1.7 Heat flow by radiation, R . 6
6.1.8 Heat flow by evaporation, E . 6
6.1.9 Heat storage for increase of core temperature associated with the
metabolic rate, dS . 6
eq
6.1.10 Heat storage, S . 6
6.2 Calculation of the required evaporative heat flow, the required skin wettedness
and the required sweat rate . 7
7 Interpretation of required sweat rate . 7
7.1 Basis of the method of interpretation . 7
7.1.1 Stress criteria . 7
7.1.2 Strain criteria . 7
7.1.3 Reference values . 8
7.2 Analysis of the work situation . 8
7.3 Determination of allowable exposure time, D . 8
lim
Annex A (normative) Data necessary for the computation of thermal balance .9
Annex B (informative) Criteria for estimating acceptable exposure time in a hot work
environment .17
Annex C (informative) Metabolic rate .19
Annex D (informative) Clothing thermal characteristics .20
Annex E (informative) Computer programme for the computation of the predicted heat
strain model .22
Annex F (normative) Examples of the predicted heat strain model computations .27
Bibliography .28
iii
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 159, Ergonomics, Subcommittee SC 5,
Ergonomics of the physical environment.
This third edition supersedes the second edition (ISO 7933:2004), which has been technically revised.
The main changes compared to the previous edition are as follows:
— The maximum sweat rate SW described in section B.4 of Annex B is fixed; that is, it is no longer
max
adjusted for metabolic rate.
— As the model has not been extensively validated for conditions with unsteady environmental
parameters, metabolic rate and/or clothing, a caution was added for cases where these parameters
vary substantially with time.
iv
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Introduction
[1]
ISO 15265 describes the assessment strategy for the prevention of discomfort or health effects in
[2]
any thermal working condition, while ISO 16595/WP recommends specific practices concerning
hot working environments. For these hot environments, these standards propose to rely on the wet
[3]
bulb globe temperature (WBGT) heat stress index described in ISO 7243 as a screening method for
establishing the presence or absence of heat stress, and on the more elaborate method presented in this
document, to make a more accurate estimation of stress, to determine the allowable durations of work
in these conditions, and to optimize the methods of protection. This method, based on an analysis of the
heat exchange between a person and the environment, is intended to be used directly when it is desired
to carry out a detailed analysis of working conditions in heat.
This document makes it possible to predict the evolution of a few physiological parameters (skin and
rectal temperatures, as well as sweat rate) over time for a person working in a hot environment. This
prediction is made according to the climatic parameters, the energy expenditure of the person and his/
her clothing. This prediction is made for an average person and should be used to assess the risk of heat
stress for a group of people; and it cannot predict a particular person’s responses.
This document is based on the latest scientific information. Future improvements concerning the
calculation of the different terms of the heat balance equation, or its interpretation will be taken into
account in the future when they become available.
Occupational health specialists are responsible for evaluating the risk encountered by a given individual,
taking into consideration their specific characteristics that can differ from those of a standard person.
[4]
ISO 9886 describes how physiological parameters are used to monitor the physiological behaviour of
[5]
a particular person and ISO 12894 describes how medical supervision is organized.
v
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oSIST prEN ISO 7933:2022
DRAFT INTERNATIONAL STANDARD ISO/DIS 7933.2:2021(E)
Ergonomics of the thermal environment — Analytical
determination and interpretation of heat stress using
calculation of the predicted heat strain
1 Scope
This document describes a model (the predicted heat strain (PHS)model) for the analytical
determination and interpretation of the thermal stress (in terms of water loss and rectal temperature)
experienced by an average person in a hot environment and determines the “maximum allowable
exposure times”, with which the physiological strain is acceptable for 95 % of the exposed population
(the maximum tolerable rectal temperature and the maximum tolerable water loss are not exceeded by
95 % of the exposed people).
The various terms used in this prediction model, and in particular in the heat balance, show the
influence of the different physical parameters of the environment on the thermal stress experienced
by the average person. In this way, this document makes it possible to determine which parameter or
group of parameters can be changed, and to what extent, in order to reduce the risk of physiological
strains.
In its present form, this method of assessment is not applicable to cases where special protective
clothing (such as fully reflective clothing, active cooling and ventilation, impermeable coveralls…) is
worn.
The model has not been extensively validated for conditions with unsteady environmental parameters,
metabolic rate and/or clothing and therefore must be used cautiously in cases where these parameters
vary substantially with time. It does not permit to determine validly the duration of time needed for an
average person whose rectal temperature has risen to 38 °C or more, to recover a rectal temperature of
36,8 °C.
This document does not predict the physiological response of an individual person, but only considers
average persons in good health and fit for the work they perform. It is therefore intended to be used
by ergonomists, industrial hygienists, etc. as the outcomes may require expert interpretations.
Recommendations about how and when to use this model are given in ISO 16595/WP.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 7726, Ergonomics of the thermal environment — Instruments for measuring physical quantities
ISO 8996, Ergonomics of the thermal environment — Determination of metabolic rate
ISO 9886, Ergonomics — Evaluation of thermal strain by physiological measurements
ISO 9920, Ergonomics of the thermal environment — Estimation of thermal insulation and water vapour
resistance of a clothing ensemble
ISO 13731, Ergonomics of the thermal environment — Vocabulary and symbols
ISO 13732-1, Ergonomics of the thermal environment — Methods for the assessment of human responses to
contact with surfaces — Part 1: Hot surfaces
1
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ISO/DIS 7933.2:2021(E)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 13731 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
4 Symbols
The symbols and abbreviated terms are listed in Table 1
Table 1 — Symbols and units conforming to ISO 13731
Symbol Term Unit
α fraction of the body mass at the skin temperature —
α skin-core weighting at time t —
i i
α
skin-core weighting at time t —
i–1 i–1
ε emissivity of outer clothing surface assuming this is non-reflective —
cl
ε emissivity of outer clothing surface —
cl,r
θ angle between walking direction and wind direction —
A age years
2
A DuBois body area surface m
Du
A
fraction of the body surface covered by the reflective clothing —
p
A 2
effective radiating area of a body m
r
−2
C convective heat flow W⋅m
−1
c water latent heat of vaporization J⋅kg
e
Corr,i correction factor for the static moisture permeability index —
m
Corr,I correction factor for the static boundary layer thermal insulation —
a
Corr,I correction factor for the static clothing thermal insulation —
cl
Corr,I correction factor for the static total clothing thermal insulation —
T
−1 −1
c specific heat of dry air at constant pressure J⋅kg ⋅K
p
−1 −1
c specific heat of the body J⋅kg ⋅K
p,b
−2
C respiratory convective heat flow W⋅m
res
D allowable exposure time min
lim
D allowable exposure time for heat storage min
lim,tcr
D allowable exposure time for water loss, 95 % of the working population min
lim,loss
D maximum water loss g
max
D maximum water loss to protect 95 % of the working population g
max,95
−2
dS body heat storage at the time i W⋅m
i
body heat storage rate due to increase of core temperature associated with the
−2
dS W⋅m
eq
metabolic rate
−2
E evaporative heat flow at the skin surface W⋅m
−2
E maximum evaporative heat flow at the skin surface W⋅m
max
−2
E predicted evaporative heat flow at the skin surface W⋅m
p
−2
E required evaporative heat flow at the skin surface W⋅m
req
−2
E respiratory evaporative heat flow W⋅m
res
f
clothing area factor —
cl
2
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
Table 1 (continued)
Symbol Term Unit
F reflection coefficients for different special materials —
r
H body height m
b
−2 −1
h dynamic convective heat transfer coefficient W⋅m ⋅K
c,dyn
−2 −1
h radiative heat transfer coefficient W⋅m ⋅K
r
2 −1
I resultant boundary layer thermal insulation m ⋅K⋅W
a,r
2 −1
I static (or basic) boundary layer thermal insulation m ⋅K⋅W
a
2 −1
I resultant clothing thermal insulation m ⋅K⋅W
cl,r
2 −1
I static (or basic) clothing thermal insulation m ⋅K⋅W
cl
i resultant moisture permeability index —
m,r
i static (or basic) moisture permeability index —
m
incr time increment from time t to time t min
i–1 i
2 −1
I resultant total clothing thermal insulation m ⋅K⋅W
T,r
2 −1
I static (or basic) total clothing thermal insulation m ⋅K⋅W
T
−2
K conductive heat flow W⋅m
k time constant of the increase of the sweat rate min
sw
time constant of the variation of the core temperature as function of the met-
k min
tcreq
abolic rate
k time constant of the variation of the skin temperature min
tsk
−2
M metabolic rate W⋅m
p water vapour partial pressure at air temperature kPa
a
p
saturated water vapour pressure at skin temperature kPa
sk,s
−2
R radiative heat flow W⋅m
2 −1
R resultant clothing total water vapour resistance m ⋅Pa⋅W
e,T,r
r required evaporative efficiency of sweating —
req
−2
S body heat storage rate W⋅m
body heat storage for increase of core temperature associated with the meta-
−2
S W⋅m
eq
bolic rate
−2
SW maximum sweat rate capacity W⋅m
max
−2
SW predicted sweat rate W⋅m
p
−2
SW predicted sweat rate at time t W⋅m
p,i i
−2
SW predicted sweat rate at time t W⋅m
p,i–1 i–1
−2
SW required sweat rate W⋅m
req
t time min
t air temperature °C
a
t clothing surface temperature °C
cl
t core temperature °C
cr
t steady-state core temperature as a function of the metabolic rate °C
cr,eq
t
t core temperature as a function of the metabolic rate at time °C
cr,eq i i
t
t core temperature as a function of the metabolic rate at time °C
cr,eq i–1 i–1
t steady-state value of core temperature as a function of the metabolic rate °C
cr,eq,m
t core temperature at time t °C
cr,i i
t core temperature at time t °C
cr,i-1 i–1
t expired air temperature °C
ex
t mean radiant temperature °C
r
3
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
Table 1 (continued)
Symbol Term Unit
t rectal temperature °C
re
t maximum rectal temperature °C
re,max
t rectal temperature at time t °C
re,i i
t rectal temperature at time t °C
re,i–1 i–1
t skin temperature °C
sk
t steady-state mean skin temperature °C
sk,eq
t steady-state mean skin temperature for clothed person °C
sk,eq,cl
t steady-state mean skin temperature for nude person °C
sk,eq,nu
t mean skin temperature at time t °C
sk,i i
t mean skin temperature at time t °C
sk,i–1 i–1
−1
V expired volume flow rate L⋅min
ex
−1
v air velocity m⋅s
a
−1
v relative air velocity m⋅s
ar
−1
v walking speed m⋅s
w
w skin wettedness —
−2
W effective mechanical power W⋅m
W humidity ratio of inhaled air kg /kg
a water air
W body mass kg
b
W humidity ratio of expired air kg /kg
ex water air
w maximum skin wettedness —
max
w predicted skin wettedness —
p
w required skin wettedness —
req
5 Principles of the predicted heat strain (PHS) model
The PHS model is based on the thermal energy balance of the body which requires the values of the
following parameters:
a) the parameters of the thermal environment as measured or estimated according to ISO 7726:
— air temperature, t ;
a
— mean radiant temperature, t ;
r
— water vapour partial pressure, p ; and
a
— air velocity, v .
a
b) the metabolic rate, M, as measured or estimated using ISO 8996 or other methods of equal or
greater accuracy;
c) the static clothing thermal characteristics, as measured or estimated using ISO 9920 or other
methods of equal or greater accuracy.
Clause 6 describes the principles of the calculation of the different heat exchanges occurring in the
heat balance equation, as well as those of the water loss necessary for the maintenance of the thermal
equilibrium of the body. The mathematical expressions given in Annex A shall be used for these
calculations.
Clause 7 describes the method for interpreting the results from Clause 6, which leads to the
determination of the predicted sweat rate, the predicted rectal temperature and the allowable exposure
4
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oSIST prEN ISO 7933:2022
ISO/DIS 7933.2:2021(E)
times. The determination of the allowable exposure times is based on two strain criteria: maximum
allowable rectal temperature and maximum allowable body water loss, given in Annex B.
The accuracy with which the predicted sweat rate and the exposure times are estimated is a function
of the model (i.e. of the expressions in Annex A) and the maximum values which are adopted. It is also
a function of the accuracy of estimation and measurement of physical parameters, metabolic rate and
thermal insulation of the clothing.
6 Main steps of the calculation
6.1 Heat balance equation
The thermal energy balance of the human body can be written as Formula (1):
M − W = C + E + K + C + R + E + S (1)
res res
This equation expresses that the internal heat production of the body, which corresponds to the
metabolic rate, M, minus the effective mechanical power, W, are balanced by the heat exchanges in the
respiratory tract by convection, C , and evaporation, E , as well as by the heat exchanges on the skin
res res
by conduction, K, convection, C, radiation, R, and evaporation, E.
If the balance is not satisfied, some energy is stored in the body, S.
The different terms of Formula (1) are successively reviewed in 6.1.1 to 6.1.10 in terms of the principles
of calculation (normative expressions for the computations are provided in Annex A).
6.1.1 Metabolic rate, M
The estimation or measurement of the metabolic rate is described in ISO 8996. Indications for the
evaluation of the metabolic rate are given in Annex C.
6.1.2 Effective mechanical power, W
In most industrial situations, the effective mechanical power is small and can be neglected.
6.1.3 Heat flow by respiratory convection, C
res
The heat flow by respiratory convection may be expressed, in principle, by Formula (2):
tt−
ex a
Cc=×0,000 02 V × (2)
resp ex
A
Du
6.1.4 Heat flow by respiratory evaporation, E
res
The heat flow by respiratory evaporation can be expressed, in principle, by Formula (3):
WW−
ex a
Ec=×0,000 02 V × (3)
resee x
A
Du
6.1.5 Heat flow by conduction, K
Heat flow by thermal conduction occurs on the body surfaces in contact with solid objects. It is usually
quite small and ignored.
[6]
Note ISO 13732-1 deals specifically with the risks of pain and burns when parts of the body contact hot
surfaces.
5
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ISO/DIS 7933.2:2021(E)
6.1.6 Heat flow by convection, C
The heat flow by convection on the bare skin may be expressed by Formula (4):
C = h × (t – t) (4)
c sk a
For clothed person, the heat flow by convection occurs at the surface of the clothing and is expressed by
Formula (5):
C = h × f × (t – t) (5)
c cl cl a
Annex D provides some indications for the evaluation of the clothing thermal characteristics.
6.1.7 Heat flow by radiation, R
The heat flow by radiation may be expressed by Formula (6):
R = h × f × (t – t) (6)
r cl cl a
where h is the radiative heat transfer coefficient and takes into account the clothing characteristics,
r
(e.g. emissivity and the presence of reflective clothing) and the effective radiating area of the person
related to the posture (e.g. standing, seated, crouching person).
6.1.8 Heat flow by evaporation, E
The maximum evaporative heat flow, E , is that which can be achieved in the hypothetical case of the
max
skin being completely wetted. In these conditions, Formula (7) applies:
pp−
sk,s a
E = (7)
max
R
e,T,r
where the dynamic clothing total water vapour resistance, R , takes into account the clothing
e,T,r
characteristics as well as the movements of the person and the air.
The actual evaporation heat flow, E, depends upon the fraction, w, of the skin surface wetted by sweat
and is given by Formula (8):
E = w × E (8)
max
6.1.9 Heat storage for increase of core temperature associated with the metabolic rate, dS
eq
Even in a neutral environment, the core temperature rises towards a steady-state value, t , as a
cr,eq
function of the metabolic rate.
The core temperature reaches this steady-state temperature exponentially with time. The heat storage
t t
associated with the increase from time to time , dS does not contribute to the onset of sweating
i–1 i eq,
and should therefore be deducted from Formula (1).
6.1.10 Heat storage, S
The heat storage of the body is given by the algebraic sum of the heat flows defined previously.
6
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ISO/DIS 7933.2:2021(E)
6.2 Calculation of the required evaporative heat flow, the required skin wettedness and
the required sweat rate
Because conduction (K) is ign
...
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