Ergonomics of the thermal environment - Assessment of heat stress using the WBGT (wet bulb globe temperature) index (ISO 7243:2017)

ISO 7243:2017 presents a screening method for evaluating the heat stress to which a person is exposed and for establishing the presence or absence of heat stress.
It applies to the evaluation of the effect of heat on a person during his or her total exposure over the working day (up to 8 h).
It does not apply for very short exposures to heat.
It applies to the assessment of indoor and outdoor occupational environments as well as to other types of environment, and to male and female adults who are fit for work.

Ergonomie der thermischen Umgebung - Ermittlung der Wärmebelastung durch den WBGT-Index (wet bulb globe temperature) (ISO 7243:2017)

Ambiances chaudes - Estimation de la contrainte thermique de l'homme au travail, basée sur l'indice WBGT (température humide et de globe noir) (ISO 7243:2017)

ISO 7243:2017 décrit une méthode d'examen préalable permettant d'évaluer la contrainte thermique à laquelle est exposée une personne et d'établir la présence ou l'absence de contrainte thermique.
ISO 7243:2017 s'applique à l'évaluation de l'effet de la chaleur sur une personne pendant son exposition totale au cours d'une journée de travail (jusqu'à 8 heures).
ISO 7243:2017 ne s'applique pas à de très courtes expositions à la chaleur.
ISO 7243:2017 s'applique à l'évaluation des environnements de travail intérieur et extérieur ainsi qu'à d'autres types d'environnement, et aux hommes et femmes adultes et aptes au travail.

Ergonomija toplotnega okolja - Ocenitev toplotnega stresa na podlagi kazalnika WBGT (ISO 7243:2017)

Ta dokument predstavlja presejalno metodo za ocenitev toplotnega stresa, ki mu je izpostavljena oseba, ter za ugotavljanje prisotnosti ali odsotnosti toplotnega stresa.
Uporablja se za ovrednotenje vpliva toplote na osebo med njeno skupno izpostavljenostjo v delovnem dnevu (do 8 ur).
Ne uporablja se za zelo kratke izpostavljenosti toploti.
Uporablja se za ocenitev poklicnega okolja v zaprtih prostorih in na prostem ter druge vrste
okolij in za odrasle moške ter ženske, ki so sposobni za delo.

General Information

Status
Published
Publication Date
12-Sep-2017
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
13-Sep-2017
Completion Date
13-Sep-2017

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SLOVENSKI STANDARD
SIST EN ISO 7243:2018
01-februar-2018
1DGRPHãþD
SIST EN 27243:2001
Ergonomija toplotnega okolja - Ocenitev toplotnega stresa na podlagi kazalnika
WBGT (ISO 7243:2017)
Ergonomics of the thermal environment - Assessment of heat stress using the WBGT
(wet bulb globe temperature) index (ISO 7243:2017)
Ergonomie der thermischen Umgebung - Ermittlung der Wärmebelastung durch den
WBGT-Index (wet bulb globe temperature) (ISO 7243:2017)
Ambiances chaudes - Estimation de la contrainte thermique de l’homme au travail,
basée sur l’indice WBGT (température humide et de globe noir) (ISO 7243:2017)
Ta slovenski standard je istoveten z: EN ISO 7243:2017
ICS:
13.180 Ergonomija Ergonomics
SIST EN ISO 7243:2018 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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SIST EN ISO 7243:2018
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SIST EN ISO 7243:2018
EN ISO 7243
EUROPEAN STANDARD
NORME EUROPÉENNE
September 2017
EUROPÄISCHE NORM
ICS 13.180 Supersedes EN 27243:1993
English Version
Ergonomics of the thermal environment - Assessment of
heat stress using the WBGT (wet bulb globe temperature)
index (ISO 7243:2017)

Ambiances chaudes - Estimation de la contrainte Ergonomie der thermischen Umgebung - Ermittlung

thermique de l'homme au travail, basée sur l'indice der Wärmebelastung durch den WBGT-Index (wet bulb

WBGT (température humide et de globe noir) (ISO globe temperature) (ISO 7243:2017)

7243:2017)
This European Standard was approved by CEN on 1 July 2017.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this

European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references

concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN

member.

This European Standard exists in three official versions (English, French, German). A version in any other language made by

translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management

Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2017 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 7243:2017 E

worldwide for CEN national Members.
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SIST EN ISO 7243:2018
EN ISO 7243:2017 (E)
Contents Page

European foreword ....................................................................................................................................................... 3

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SIST EN ISO 7243:2018
EN ISO 7243:2017 (E)
European foreword

This document (EN ISO 7243:2017) has been prepared by Technical Committee ISO/TC 159

“Ergonomics” in collaboration with Technical Committee CEN/TC 122 “Ergonomics” the secretariat of

which is held by DIN.

This European Standard shall be given the status of a national standard, either by publication of an

identical text or by endorsement, at the latest by March 2018, and conflicting national standards shall

be withdrawn at the latest by March 2018.

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. CEN shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN 27243:1993.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the

following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,

Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,

France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,

Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,

Turkey and the United Kingdom.
Endorsement notice

The text of ISO 7243:2017 has been approved by CEN as EN ISO 7243:2017 without any modification.

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SIST EN ISO 7243:2018
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SIST EN ISO 7243:2018
INTERNATIONAL ISO
STANDARD 7243
Third edition
2017-08
Ergonomics of the thermal
environment — Assessment of heat
stress using the WBGT (wet bulb globe
temperature) index
Ambiances chaudes — Estimation de la contrainte thermique de
l'homme au travail, basée sur l'indice WBGT (température humide et
de globe noir)
Reference number
ISO 7243:2017(E)
ISO 2017
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2017, Published in Switzerland

All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized otherwise in any form

or by any means, electronic or mechanical, including photocopying, or posting on the internet or an intranet, without prior

written permission. Permission can be requested from either ISO at the address below or ISO’s member body in the country of

the requester.
ISO copyright office
Ch. de Blandonnet 8 • CP 401
CH-1214 Vernier, Geneva, Switzerland
Tel. +41 22 749 01 11
Fax +41 22 749 09 47
copyright@iso.org
www.iso.org
ii © ISO 2017 – All rights reserved
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Method ............................................................................................................................................................................................................................ 2

5 Determination of WBGT ................................................................................................................................................................................ 3

6 Determination of metabolic rate ......................................................................................................................................................... 3

7 Determination of effects of clothing ................................................................................................................................................. 3

8 Timing and duration of measurements ........................................................................................................................................ 4

8.1 Timing of measurements ............................................................................................................................................................... 4

8.2 Duration of measurements ........................................................................................................................................................... 4

9 Spatial and temporal variations ........................................................................................................................................................... 5

9.1 Measurement specifications relating to heterogeneity of environment

(spatial variations) .............................................................................................................................................................................. 5

9.2 Measurement specifications relating to time variations of WBGT index ............................................. 5

9.3 Measurement specifications relating to time variations of metabolic rate ........................................ 5

9.4 Measurement specifications relating to time variations of clothing ........................................................ 5

10 Interpretation .......................................................................................................................................................................................................... 6

Annex A (informative) Reference values of the WBGT heat stress index ....................................................................... 7

Annex B (normative) Measurement of parameters used in the WBGT index and

specification of instruments..................................................................................................................................................................... 9

Annex C (informative) Alternative globe thermometers ..............................................................................................................11

Annex D (informative) Prediction of natural wet bulb temperature ...............................................................................13

Annex E (informative) Estimation of metabolic rate ........................................................................................................................15

Annex F (informative) Clothing adjustment values (CAVs) ........................................................................................................16

Bibliography .............................................................................................................................................................................................................................17

© ISO 2017 – All rights reserved iii
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SIST EN ISO 7243:2018
ISO 7243:2017(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 cancels and replaces the second edition (ISO 7243:1989), which has been technically

revised and contains the following changes:

— in Annex A, for information, additional exposure limits are represented in Figure A.1, together with

reference equations;
— the assessment of heat stress now includes the effects of clothing;

— the potential errors and adjustments for non-standard globe temperature sensors are described;

— a method for predicting the natural wet bulb temperature is provided.
iv © ISO 2017 – All rights reserved
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
Introduction

This International Standard provides a method for the assessment of heat stress. It is one of a series of

standards intended for use in the assessment of thermal environments. These include standards for the

assessment of hot, moderate and cold environments involving both the principles of assessment and

their practical application.

The wet bulb globe temperature (WBGT) is a heat stress index and its value represents the thermal

environment to which an individual is exposed. This index is easy to determine in most environments.

It should be regarded as a screening method to establish the presence or absence of heat stress.

A method of estimating the thermal stress, based on an analysis of the heat exchange between a person

and the environment, allows a more accurate estimation of stress and an analysis of the methods of

protection (see ISO 7933). Such a method should be used either directly when it is desired to carry

out an intensive analysis of working conditions in heat, or in addition to the method presented in this

standard, which is based upon the WBGT index, when the WBGT values obtained exceed the reference

values shown.
© ISO 2017 – All rights reserved v
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SIST EN ISO 7243:2018
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SIST EN ISO 7243:2018
INTERNATIONAL STANDARD ISO 7243:2017(E)
Ergonomics of the thermal environment — Assessment
of heat stress using the WBGT (wet bulb globe
temperature) index
1 Scope

This document presents a screening method for evaluating the heat stress to which a person is exposed

and for establishing the presence or absence of heat stress.

It applies to the evaluation of the effect of heat on a person during his or her total exposure over the

working day (up to 8 h).
It does not apply for very short exposures to heat.

It applies to the assessment of indoor and outdoor occupational environments as well as to other types

of environment, and to male and female adults who are fit for work.
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 7933, Ergonomics of the thermal environment — Analytical determination and interpretation of heat

stress using calculation of the predicted heat strain
ISO 13731, Ergonomics of the thermal environment — Vocabulary and symbols
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 13731 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at http://www.iso.org/obp
— IEC Electropedia: available at http://www.electropedia.org/
3.1
wet bulb globe temperature
WBGT

simple index of the environment that is considered along with metabolic rate to assess the potential for

heat stress among those exposed to hot conditions

Note 1 to entry: The WBGT combines the measurement of two derived parameters: natural wet-bulb temperature

(t ) and black globe temperature (t ). Where the sensors are influenced by direct incident radiation from the

nw g

sun (solar load), either outdoors or indoors, the weighting of the globe temperature is reduced by including air

temperature (t ).
© ISO 2017 – All rights reserved 1
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
3.2
effective wet bulb globe temperature
effective WBGT
WBGT
eff
WBGT value adjusted for the effects of clothing

Note 1 to entry: It gives the WBGT environment when the actual clothing worn is equivalent to that when

standard work clothing is worn (thermal insulation index I = 0,6 clo, i = 0,38). See ISO 9920.

cl m
3.3
clothing adjustment value
CAV

adjustment to the WBGT value to account for the effects of clothing that has different thermal properties

from that of standard work clothing
4 Method
The degree of heat stress to which a person is exposed depends on

a) the characteristics of the environment governing heat transfer between the ambient environment

and the body,
b) the production of heat inside the body as a result of physical activity, and
c) the clothing worn, which modifies the exchange of heat with the environment.

A detailed analysis of the influence of the environment on heat stress requires knowledge of the

following four basic parameters: air temperature, mean radiant temperature, air velocity, and absolute

humidity (ISO 7726). However, an estimation of this influence can be made by measuring parameters

derived from these basic parameters and which are a function of the physical parameters of the

environment investigated. The WBGT index is used to give a first approximation of the heat stress on a

person (see Clause 5).

The internal thermal load is the result of metabolic energy caused by activity. The rate of metabolic

heat production is usually estimated (see Clause 6).

The heat stress threshold assumes a long sleeve cotton shirt and cotton trousers/pants. An adjustment

shall be made for other clothing (see Clause 7).

This method for estimating heat stress is based on the assessment of these different parameters and

the calculation of mean values taking into account changes in location, duration and activity, as well as

variations in time (see Clause 8).

The WBGT reference values (exposure limits) presented correspond to levels of sustained exposure for

up to 8 h.

The WBGT values obtained using the method are compared with WBGT reference values (exposure

limits). If the values are greater than the reference values, then the risk of heat-related disorders

increases and it will be necessary to either

— reduce directly the heat stress or strain at the workplace by appropriate methods, or

— carry out a detailed analysis of the heat stress using ISO 7933.

It should be noted that the exposure thresholds described in this document are designed to reduce the

risk of heat-related illness and that this does not preclude the possibility of other outcomes associated

with heat stress exposures (e.g. risk of burns and accidents, loss of productivity, or lack of comfort).

2 © ISO 2017 – All rights reserved
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
5 Determination of WBGT

Formulae (1) and (2) provide equations for the calculation of WBGT and show the relationship between

the different parameters:
— without solar load
WBGT = 0,7t + 0,3t (1)
nw g
— with solar load
WBGT = 0,7t + 0,2t + 0,1t (2)
nw g a

Globe temperature assesses the total radiant heat load from the sun and other sources. Formula 2

accounts for an overestimation of direct radiant heat from the sun (solar load). That is, the provisions

of this document are applicable where there is radiant heat load with or without direct solar radiation

[Formulae (1) and (2)].

The reference values were selected so that the level of heat stress could be sustained during the total

exposure over the working day (up to 8 h). The time interval for analysis is about 1 h, representative

of the exposure. If there are spatial and/or temporal variations in the environment, it is necessary to

adjust for those variations, as described in 9.1 (spatial variation) and 9.2 (temporal variation).

Annex B presents the requirements for sensors associated with the measurement of the WBGT.

NOTE 1 There are variations in the design of actual sensors in instrumentation that are used to assess WBGT.

The common variations in design are described in Annex C, along with a discussion of the design implications

when compared with the design adopted in this document and specified in Annex B.

NOTE 2 The preferred method for determining WBGT values is direct measurement using the sensors

specified in Annex B. However, it is sometimes of interest to predict WBGT values from the four parameters, air

temperature, mean radiant temperature, relative humidity and air velocity. (See Annex C and D)

6 Determination of metabolic rate

The quantity of heat produced inside the body is an important contributor to heat stress and a valid

estimate of this is essential for the assessment. Metabolic rate, which represents the total quantity of

energy consumed inside the body over time, is a good estimation for most situations (i.e. the energy

consumed can be assumed to be the heat produced, as the energy used for other functions such as

external work is usually negligible by comparison).

Metabolic rate may be classified as resting, low metabolic rate, moderate metabolic rate, high metabolic

rate or very high metabolic rate according to Annex E. The values provided in Table E.1 are based on

continuous work at the described levels of effort. In the case of intermittent work, a time-weighted

averaging shall be performed in accordance with 9.3.

If a more detailed estimation is required, then the methods presented in ISO 8996 should be used.

7 Determination of effects of clothing

The reference values (exposure limits) provided in Annex A were developed with cotton work

clothes (0,6 clo and i = 0,38) as the reference clothing. Different clothing, especially with a different

evaporative resistance, is likely to have a different effect on the heat stress level. For clothing materials

and configurations different from standard work clothing, clothing adjustment values (CAVs) in WBGT

temperature units are provided. The CAV is added to the measured WBGT to produce an effective

© ISO 2017 – All rights reserved 3
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SIST EN ISO 7243:2018
ISO 7243:2017(E)

WBGT (WBGT ) that represents an estimate of the heat stress provided by the actual clothing worn as

eff
an equivalent environment, i.e.
WBGT = WBGT + CAV (3)
eff

Annex F provides a list of CAVs. It should be remembered that the effects of clothing can be complex and

that the CAV is a simple adjustment and a first approximation to taking account of the heat stress on a

person as determined from laboratory results.

There may be a clothing ensemble for which a CAV is not directly known. In this case, the CAV may be

estimated from clothing with similar thermal properties. The thermal properties of a wide range of

clothing are provided in ISO 9920.

For clothing ensembles for which the CAV cannot be determined, this document shall not be used and a

detailed analysis of the heat stress, using ISO 7933, shall be carried out.

CAV is an approximation of the effect of wearing clothing that differs from “ordinary work clothes” for

which the reference values given in Annex A apply without any adjustment for clothing (CAV = 0). In

general, the CAV increases with increasing evaporative resistance (or decreasing permeability index).

Other effects are radiant heat, air velocity, body movements, clothing configurations and humidity. Of

these, the CAV is greatly affected by a combination of high evaporative resistance and humidity. In this

case, and because of the simplistic nature of the adjustment, the CAV should be a high estimate to allow

for a margin of safety. The effects of radiant heat on the CAV are not known.
8 Timing and duration of measurements
8.1 Timing of measurements

The determination of the WBGT index allows only the estimation of the heat stress to which a worker

is subjected at the time the measurements were carried out. Consequently, it is recommended that

measurements are carried out at the time of the year when heat stress is most likely to occur: during

the hot summer period. For the same reason, the representative period of the exposure is best selected

during the middle of the day, or the period of the exposure which is most likely to induce heat stress.

If the work over a day is divided into distinctly different types or categories then it may be necessary to

make separate measurements and separate assessments of the different types of work.

EXAMPLE When there is mainly light work in the morning and heavy work in the afternoon, or when the

WBGT values are significantly different for periods of over an hour.
8.2 Duration of measurements

A measurement of the WBGT is required over a representative period of about 1 h. The duration of

each measurement depends on the response time of the sensor, which on certain occasions can be

considerable (globe temperature especially). A steady-state value for all sensor readings should be

established prior to recording the values assigned for that reading. The total duration of measurement

may therefore be greater than the single hour used as the time base in the analysis (see 9.2).

It is possible to record environmental measurements with high resolution (e.g. every second or minute)

and store large amounts of data in digital form.

Time constants, accuracy and sensitivity of instrumentation need to be taken into consideration when

measuring the value of any parameter.
4 © ISO 2017 – All rights reserved
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SIST EN ISO 7243:2018
ISO 7243:2017(E)
9 Spatial and temporal variations

9.1 Measurement specifications relating to heterogeneity of environment (spatial

variations)

The WBGT values should normally be measured at the position of the abdomen (ISO 7726) of those

exposed to the heat. When parameters in the space surrounding these people are not homogeneous,

measurement should be made at the position where heat stress is highest.

In the case where it is impossible to situate the sensors at the normal place of work, they should be

situated where they will be exposed to the same influence from the environment.
9.2 Measurement specifications relating to time variations of WBGT index

If the analyses of the environment and of the activity have shown that a parameter does not exhibit a

constant value in time, a representative mean value has to be determined.

The most accurate procedure consists in measuring the continuous development of this parameter as

a function of time and deducing from it the mean value by integration. As this method can only be used

with difficulty in many cases, the variations of each parameter are classified into almost constant levels.

The mean value of the parameter considered is then obtained by weighting the levels of the different

categories by the total time during which each of these levels was obtained.

The time base T for the calculation of the mean values is a period of about 1 h, which is representative

of the possible heat stress exposure. The mean value of a parameter, p — for example, air temperature,

natural wet bulb temperature, globe temperature or WBGT in the case of simultaneous measurement of

the three parameters of the environment — for which the development as a function of time has been

broken down into n levels is therefore expressed by Formula (4):
pt× +×pt ++... pt×
() () ()
11 22 nn
p = (4)
tt++...t
12 n

where p , p , ... p is the level of the parameter obtained during time t , t , ... t ;

1 2 n 1 2 n
t + t + ... = T = 1 h (5)
1 2

The number of measurements to be carried out depends on the variation speed of the parameters, the

response characteristics of the sensors used and the desired accuracy of measurement.

9.3 Measurement specifications relating to time variations of metabolic rate

Formula (4) applies to the determination of the time-weighted mean value of the metabolic rate based

on values measured or estimated from reference tables. The metabolic rate is classified under one of the

five main classes presented in Annex E. The mean metabolic rate level is determined from Formula (4),

where the parameter is metabolic rate, by taking, for each elementary activity, the mean value of the

metabolic rate given in Table E.1.

When there is doubt with regard to the metabolic rate value to be adopted, the refere

...

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