ISO 1999:2013

INTERNATIONAL ISO
STANDARD 1999
Third edition
2013-10-01
Acoustics — Estimation of noise-
induced hearing loss
Acoustique — Estimation de la perte auditive induite par le bruit
Reference number
ISO 1999:2013(E)
©
ISO 2013

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ISO 1999:2013(E)

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© ISO 2013
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ii © ISO 2013 – All rights reserved

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ISO 1999:2013(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Principle . 3
5 Description and measurement of noise exposure . 3
6 Prediction of the effects of noise on hearing threshold . 4
6.1 Statistical distribution of hearing threshold levels of a noise-exposed population . 4
6.2 Databases for hearing threshold levels associated with age (HTLA) . 4
6.3 Calculation of noise-induced permanent threshold shift, N .5
7 Assessment of noise-induced hearing loss and disability . 8
7.1 Hearing loss . 8
7.2 Hearing disability . 8
7.3 Risk of hearing disability . 8
Annex A (informative) Calculation of database A, statistical distribution of hearing thresholds as a
function of age (HTLA) for an otologically normal population (highly screened) .10
Annex B (informative) Examples for database B .13
Annex C (informative) Example of assessment of risk of noise-induced hearing loss
and disability .17
Annex D (informative) Tables with examples for NIPTS data .20
Bibliography .22
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ISO 1999:2013(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. 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. 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.
The committee responsible for this document is ISO/TC 43, Acoustics.
This third edition cancels and replaces the second edition (ISO 1999:1990), of which it constitutes a
minor revision.
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ISO 1999:2013(E)

Introduction
This International Standard presents, in statistical terms, the relationship between noise exposures and
the “noise-induced permanent threshold shift” (NIPTS) in people of various ages. It provides procedures
for estimating the hearing loss due to noise exposure of populations free from auditory impairment other
than that due to noise (with allowance for the effects of age) or of unscreened populations whose hearing
capability has been measured or estimated. NIPTS is treated here as an additive term independent of
other components of hearing threshold levels. For any given noise exposure, it has a range of positive
values representing the variability of noise-damage susceptibility between individuals of a population.
Persons regularly exposed to noise can develop hearing loss of varying severity. Due to this hearing
loss, their understanding of speech, perception of everyday acoustic signals, or appreciation of music
may be impaired. With the exception of exposure to blast, high-impulse noise and extremely high levels
of steady noise, permanent impairment of the hearing organ takes time and is progressive over months,
years, or decades of exposure. NIPTS is usually preceded by a reversible temporary effect on hearing
called noise-induced “temporary threshold shift” (TTS). The severity of TTS and recovery from it depend
upon exposure level and duration. For a single individual, it is not possible to determine precisely
which changes in hearing threshold level are caused by noise and which changes are caused by other
factors, although, in doubtful individual cases, the data in this International Standard might provide an
additional means for estimating the most probable causes in audiological diagnosis. However, for a large
population exposed to a specific noise, changes in the statistical distributions of hearing threshold levels
can be determined. Parameters, such as the mean NIPTS and the median NIPTS, can be used to describe
differences in hearing threshold levels between two populations that are similar in all relevant respects
except that one population has had a well-defined (usually occupational) noise exposure. Throughout
this International Standard, the term NIPTS is applied to changes in the noise-induced permanent
threshold shift of statistical distributions of groups of people; it is not to be applied to individuals.
This International Standard can be applied to the calculation of the risk of sustaining hearing loss
due to regular occupational noise exposure or due to any daily repeated noise exposure. In some
countries, hearing loss caused by occupational noise exposure can have legal consequences with
respect to responsibility and compensation. The hearing threshold level at the various frequencies, at
which a hearing impairment is deemed to exist (fence), depends not only on the hearing loss per se,
but frequently on legal definitions and interpretations based on social and economic considerations. In
addition, the definition of a hearing impairment depends on the quality of speech recognition desired,
the average level of background noise, and with respect to the relative importance of the various
frequencies, perhaps even on the language. Consequently, this International Standard does not stipulate
(in contrast to the first edition of ISO 1999) a specific formula for assessment of the risk of impairment,
but specifies uniform methods for the prediction of hearing loss, which can be used for the assessment
of impairment according to the formula desired or stipulated in a specific country. The results obtained
by this International Standard may also be used for estimating the permanent effects of noise on the
perception of everyday acoustic signals, the appreciation of music, or the effect of one specific frequency
not necessarily stipulated by a hearing impairment formula.
Since noise-induced hearing loss is the result not only of occupational noise exposure but also of the
total noise exposure of the population, it may be important to take the non-occupational exposure of
individuals (during commuting to and from their jobs, at home, and during recreational activities) into
account. Only if this non-occupational exposure is negligible compared with the occupational exposure
does this International Standard allow prediction of the occurrence of hearing loss due to occupational
noise exposure. Otherwise, it should be used to calculate the hearing loss to be expected from the
combined (occupational plus non-occupational) total daily noise exposure. The contribution of the
occupational noise exposure to the total hearing loss can then be estimated, if desired.
The selection of maximum tolerable or maximum permissible noise exposures and protection
requirements, as well as the selection of specific formulae for impairment risk assessment or
compensation purposes, require consideration of ethical, social, economic, and political factors not
amenable to international standardization. Individual countries differ in their interpretation of these
factors and these factors are therefore considered outside the scope of this International Standard.
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ISO 1999:2013(E)

For reasons given above, this International Standard, by itself, does not comprise a complete guide
for risk assessment and protection requirements, and for practical use, it has to be complemented by
national standards or codes of practice delineating the factors which are here left open.
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INTERNATIONAL STANDARD ISO 1999:2013(E)
Acoustics — Estimation of noise-induced hearing loss
1 Scope
This International Standard specifies a method for calculating the expected noise-induced permanent
threshold shift in the hearing threshold levels of adult populations due to various levels and durations
of noise exposure; it provides the basis for calculating hearing disability according to various formulae
when the hearing threshold levels at commonly measured audiometric frequencies, or combinations of
such frequencies, exceed a certain value.
NOTE 1 This International Standard does not specify frequencies, frequency combinations, or weighted
combinations to be used for the evaluation of hearing disability; nor does it specify a hearing threshold level
(fence) which it is necessary to exceed for hearing disability to exist. Quantitative selection of these parameters is
left to the user. All sound pressure levels stated in this International Standard do not consider the effect of hearing
protectors which would reduce effective exposure levels and modify the spectrum at the ear.
The measure of exposure to noise for a population at risk is the noise exposure level normalized to a
nominal 8 h working day, L , for a given number of years of exposure. This International Standard
EX,8h
applies to noise at frequencies less than approximately 10 kHz which is steady, intermittent, fluctuating,
irregular. Use of this International Standard for sound pressures exceeding 200 Pa (140 dB relative to
20 µPa) is recognized as extrapolation.
Formulae are presented to calculate the hearing loss, including statistical distribution, at a range of
audiometric frequencies due to exposure to noise as a function of level of noise exposure and duration of
exposure (in years). The formulae do not distinguish between male and female populations.
NOTE 2 Although the models of hearing loss are based on data assumed to stem primarily from populations
exposed to occupational noise, they may be used, with some caution, for estimating the effects of comparable non-
occupational and combined exposures.
NOTE 3 The prediction method presented is based primarily on data collected with essentially broadband,
steady, non-tonal noise.
To calculate hearing threshold levels and the risk of acquiring hearing loss due to noise exposure, it is
necessary to make use of a comparable population. This International Standard contains a definition
of a highly screened otologically normal population (in accordance with ISO 7029) and three examples
of unscreened populations of three typical industrialized societies. The users of this International
Standard may choose a comparable population according to their particular requirements.
NOTE 4 All data and procedures presented in this International Standard are based on deliberate simplifications
of experimental data where the daily sound exposure duration did not exceed 12 h. The resulting approximations
restrict the validity to the stated ranges of the variables, percentages, sound exposure levels, and frequency ranges.
This International Standard is based on statistical data and therefore cannot be applied to the prediction
or assessment of the hearing loss of individual persons except in terms of statistical probabilities.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated
references, the latest edition of the referenced document (including any amendments) applies.
ISO 7029, Acoustics — Statistical distribution of hearing thresholds as a function of age
ISO 9612, Acoustics — Determination of occupational noise exposure — Engineering method
ISO/TR 25417, Acoustics — Definitions of basic quantities and terms
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ISO 1999:2013(E)

3 Terms and definitions
For the purposes of this document, the terms and definitions in ISO/TR 25417 and the following apply.
3.1
A-weighted noise exposure level normalized to a nominal 8 h working day
L
EX,8h
level, in decibels, given by the formula
LL=+ 10 lg ()TT/ dB
EX,,80hApTeq ee
where
L is the A-weighted equivalent continuous sound pressure level for T ;
pAeq,Te e
T is the effective duration of the working day in hours;
e
T is the reference duration (T = 8 h).
0 0
Note 1 to entry: The quantity “noise exposure level normalized to a nominal 8 h working day” may also be called
“daily noise exposure level”.
Note 2 to entry: If the exposure averaged over n days is desired, for example if noise exposure levels normalized to
a nominal 8 h working day for weekly exposures are considered, the average value of L , in decibels, over the
EX,8h
whole period may be determined from the values of (L ) for each day using the following formula:
EX,8h i
 n 
1
01,L()
 
EX,8h i
L =10lg 10 dB
EX,8h  ∑ 
c
 
i=1
 
The value of c is chosen according to the purpose of the averaging process: it will be equal to n if an average value
is desired; it will be a conventional fixed number if the exposure is to be normalized to a nominal number of days
(for example, when n = 7, c = 5 will lead to a daily noise exposure level normalized to a nominal week of 5 eight-
hour working days). For consideration of irregular exposures over an extended time period, see ISO 9612.
3.2
hearing loss
deviation or a change for the worse of the threshold of hearing from normal
Note 1 to entry: The term hearing loss may sometimes only refer to a change.
3.3
hearing disability
effect of hearing loss on activities in daily living
Note 1 to entry: This is sometimes called “activity limitation” (WHO).
3.4
fence
hearing threshold level above which degrees of hearing disability are deemed to exist
3.5
risk of hearing disability
percentage of a population sustaining hearing disability
3.6
risk of hearing disability due to noise
risk of hearing disability in a noise-exposed population minus the risk of hearing disability in a population
not exposed to noise but otherwise equivalent to the noise-exposed population
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ISO 1999:2013(E)

3.7
hearing threshold level associated with age
HTLA
H
for a specified fraction of a population, the hearing threshold level observed as a function of age without
any exposure to occupational noise
Note 1 to entry: HTLA can be directly observed only in the absence of other causes of hearing impairment, for
example, pathological conditions or noise exposure.
3.8
noise-induced permanent threshold shift
NIPTS
N
for a specified fraction of a population, the permanent shift, actual or potential, in decibels, of the hearing
threshold level estimated to be caused solely by exposure to noise, in the absence of other causes
3.9
hearing threshold level associated with age and noise
HTLAN
H’
permanent hearing threshold level for a specified fraction of a population
Note 1 to entry: Hearing threshold levels (HTL), as defined in ISO 389, are expressed in decibels.
Note 2 to entry: The value HTLAN is a combination of the components associated with noise (NIPTS, see 3.8) and
with age (HTLA, see 3.7), as defined in 6.1.
4 Principle
Annex A gives the procedure for calculating the statistical distribution of hearing threshold levels
relative to the hearing threshold levels at the age of 18 years as a function of age for an otologically
normal population (highly screened) in accordance with ISO 7029.
Annex B gives three examples of the second database representing the statistical distribution of hearing
threshold levels as a function of age for unscreened populations of three typical industrialized societies.
These databases are derived from three recent studies in different countries and the data differ
significantly from those of database B in the previous edition of this International Standard. In two of
the examples, the test subjects have not been exposed to hazardous occupational noise but otherwise
represent all other factors that may affect hearing, e.g. age, genetic dispositions, non-occupational noise,
and ear diseases. The third database concerns a completely unscreened population, as explained in B.3.
Annex C describes an example of hearing risk assessment using this International Standard.
Annex D presents tables with examples of NIPTS as a function of exposure duration (10 years, 20 years,
3 2 4 2 4
30 years, and 40 years) and daily A-weighted sound exposure (3,64 × 10 Pa s, 1,15 × 10 Pa s, 3,64 × 10
2 5 2
Pa s and 1,15x 10 Pa s or equivalent continuous A-weighted sound pressure level for nominal 8 h
working day of 85 dB, 90 dB, 95 dB, and 100 dB) for six frequencies (0,5 kHz, 1 kHz, 2 kHz, 3 kHz, 4 kHz,
and 6 kHz) and three percentages (10 %, 50 %, and 90 %).
5 Description and measurement of noise exposure
Methods for the determination of occupational noise exposure are specified in ISO 9612.
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ISO 1999:2013(E)

6 Prediction of the effects of noise on hearing threshold
6.1 Statistical distribution of hearing threshold levels of a noise-exposed population
The hearing threshold level, in decibels, associated with age and noise (HTLAN), H’, of a noise-exposed
population is calculated, for the purposes of this International Standard, by using Formula (1):
HN×
HH'=+N− (1)
120
where
H is the hearing threshold level, expressed in decibels, associated with age (HTLA);
N is the actual or potential noise-induced permanent threshold shift (NIPTS), expressed in deci-
bels.
This formula is applicable only to corresponding percentage values of H’, H, and N.
NOTE The relationship expressed in Formula (1) is an approximation to the biological events and is considered
accurate enough for the purposes of this International Standard. The term (H × N)/120 starts to significantly
modify the result only when H + N is more than approximately 40 dB.
6.2 Databases for hearing threshold levels associated with age (HTLA)
6.2.1 General
The hearing of a non-noise-exposed population as a function of age depends on the degree to which other
factors besides aging are inadvertently included; diseases, history of ototoxic drugs, and unknown noise
exposure of occupational or non-occupational origin may modify the HTLA. Different approaches to
screening such data have been used and the selection of the most appropriate database depends on the
purpose of the application (see 6.2.4). This International Standard permits two databases (databases A
and B) to be used for HTLA in 6.1. Database A is fully specified, whereas database B is at the discretion
of the user. Three examples of database B are presented.
NOTE The databases presented in Annexes A and B are from populations of European and North American
countries. These populations may or may not be representative for the populations of other geographical areas.
Even if there are no differences in natural aging between different ethnic populations, differences in life style,
non-occupational noise exposure, incidence of disease, and ototoxic drugs are nevertheless liable to occur.
6.2.2 Database A
Database A derives from otologically normal persons, i.e. persons in a normal state of health who are
free from all signs or symptoms of ear disease and from obstructing wax in the ear canals and who
have no history of undue exposure to noise. The statistical distributions of the thresholds of such highly
screened populations have been standardized in ISO 7029 separately for male and female populations.
Formulae for calculating database A are specified in A.1. The selected values of the statistical distribution
of hearing threshold levels (in decibels) from database A are provided in Table A.3.
6.2.3 Database B
For database B, a set of data collected on a control population not occupationally exposed to noise of
the country under consideration, but exposed to other risk factors to a degree similar to occupationally
exposed populations, is recommended. When such an ideal control population is unavailable and in
countries where risk factors such as non-occupational noise are more prevalent in occupationally exposed
populations than in the general population, a totally unscreened control population may be preferable.
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ISO 1999:2013(E)

A separate HTLA database for both men and women is required unless it can be shown that there are no
substantial gender differences. It is essential that the sample size be large enough to allow calculations
of a valid statistical distribution.
Therefore, the user should apply appropriate selection criteria to compile a database of hearing threshold
levels to compare to the levels given in Annex B of this International Standard. For instance, the average
HTLA of both ears or the ear showing the greatest hearing loss can be taken as the basis for database B.
Three examples of database B are presented in Annex B for an unscreened population (males and
females). These examples are compiled from representative data from three industrialized countries:
Sweden (B.2), Norway (B.3), and the United States (B.4).
It is emphasized that for practical situations the accuracy of the prediction of the hearing threshold level
of a noise-exposed population will largely be a function of the accuracy of the selected database for HTLA.
Since audiometric measurement techniques affect threshold measurements, the same measurement
technique should be used in the establishment of a specific HTLA database as might be used to obtain or
verify the threshold of hearing of the noise-exposed population.
6.2.4 Choice of database
Whether database A or B is the more appropriate (or whether the numerical examples for database
B in Annex B are suitable) depends on what question is to be answered. For example, if the amount
of compensation that could be due to a population of noise-exposed workers is to be estimated, and
otological irregularities and non-occupational noise exposure are not considered in compensation
cases, and in most cases when the occupationally exposed population is not highly screened, unscreened
populations will form the more appropriate databases.
6.3 Calculation of noise-induced permanent threshold shift, N
6.3.1 Calculation of N
50
The median potential noise-induced permanent threshold shift (NIPTS) values to be used in 6.1 are
functions of audiometric frequency, the exposure duration, the ratio t/t and the noise exposure level
0,
normalized to a nominal 8 h working day, L (see 3.1), and 5 days per week, averaged over the
EX,8h
exposure duration t.
For exposure durations between 10 years and 40 years, the median potential NIPTS values, N , in
50
decibels, are given for both genders by Formula (2):
2
 
Nu+−vtlg /tL L (2)
()
()
50 = 08EX, h 0
 
 
where
L is the noise exposure level normalized to a nominal 8 h working day (see 3.1), expressed
EX,8h
in decibels;
L is the sound pressure level, defined as a function of frequency in Table 1, expressed in
0
decibels, below which the effect on hearing is negligible;
t is the exposure duration, expressed in years;
t is 1 year;
0
u and v are given as a function of frequency in Table 1.
This formula applies to L greater than L . In cases where L is less than L , it shall be deemed
EX,8h 0 EX,8h 0
equal to L so that N is zero.
0 50
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ISO 1999:2013(E)

For exposure durations less than 10 years, N shall be extrapolated from the value of N for 10 years
50
using Formula (3):
lg()t+1
N = N (3)
50,,tt<=10 50 10
lg()11
Formula (3) is valid for exposure durations between 1 year and 10 years. For exposure durations of less
than 1 year, the results of Formula (3) represent an extrapolation.
Table 1 — Values of u, v, and L used to determine the NIPTS for the median value of the
0
population, N
50
Frequency u v L
0
Hz dB
500 –0,033 0,110 93
1 000 –0,020 0,070 89
2 000 –0,045 0,066 80
3 000 0,012 0,037 77
4 000 0,025 0,025 75
6 000 0,019 0,024 77
6.3.2 Statistical distribution of noise-induced permanent threshold shift, N
For the purposes of this International Standard, the statistical distribution of N is approximated by
two different halves of two normal (Gaussian) distributions. The upper half, for the percentage with
hearing worse than the median, is characterized by the parameter d ; the lower half has a smaller
u
dispersion characterized by the parameter percentage d . For a percentage of, Q, of the population such
l
that 5 ≤ Q < 50 %, the NIPTS is given by Formula (4):
NN=+kd (4)
Q 50 u
For a percentage of, Q, of the population such that 50 % < Q ≤ 95 %, the NIPTS is given by Formula (5):
NN=−kd (5)
Q 50 l
The values for the multiplier k corresponding to the normal (Gaussian) distribution are given in Table 2
of 6.3.2.1; d and d shall be calculated in accordance with 6.3.2.2.
u l
Tails of the statistical distributions for 0 % < Q < 5 % and for 95 % < Q < 100 % are unreliable and should
not be estimated since few experimental data exist to validate these ranges.
N
...

SLOVENSKI STANDARD
oSIST ISO/DIS 1999:2012
01-januar-2012
Akustika - Ugotavljanje izgube sluha zaradi hrupa
Acoustics -- Estimation of noise-induced hearing loss
Acoustique -- Estimation de la perte auditive induite par le bruit
Ta slovenski standard je istoveten z: ISO/DIS 1999
ICS:
13.140 Vpliv hrupa na ljudi Noise with respect to human
beings
17.140.20 Emisija hrupa naprav in Noise emitted by machines
opreme and equipment
oSIST ISO/DIS 1999:2012 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST ISO/DIS 1999:2012

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oSIST ISO/DIS 1999:2012

DRAFT INTERNATIONAL STANDARD ISO/DIS 1999
ISO/TC 43 Secretariat: DS
Voting begins on Voting terminates on

2011-09-01 2012-02-01
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION    МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ    ORGANISATION INTERNATIONALE DE NORMALISATION


Acoustics — Estimation of noise-induced hearing loss
Acoustique — Estimation de la perte auditive induite par le bruit
[Revision of second edition (ISO1999:1990)]
ICS 13.140









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Secrétariat central de l'ISO au stade de publication.



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REFERRED TO AS AN INTERNATIONAL STANDARD UNTIL PUBLISHED AS SUCH.
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©  International Organization for Standardization, 2011

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oSIST ISO/DIS 1999:2012
ISO/DIS 1999

Copyright notice
This ISO document is a Draft International Standard and is copyright-protected by ISO. Except as permitted
under the applicable laws of the user’s country, neither this ISO draft nor any extract from it may be
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oSIST ISO/DIS 1999:2012
ISO/DIS 1999
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.2
3 Definitions .2
4 Description and measurement of noise exposure.4
5 Prediction of the effects of noise on hearing threshold.4
5.1 Statistical distribution of hearing threshold levels of a noise-exposed population .4
5.2 Data bases for hearing threshold levels associated with age (HTLA).4
5.2.1 General .4
5.2.2 Data base A .4
5.2.3 Data base B .5
5.2.4 Choice of data base.5
5.3 Calculation of noise-induced permanent threshold shift, N.5
5.3.1 Calcuation of N .5
0,50
5.3.2 Statistical distribution of noise induced permanent threshold shift, N.6
6 Assessment of noise-induced hearing loss and disability.8
6.1 Hearing impairment.8
6.2 Hearing disability.8
6.3 Risk of hearing disability.9
Annex A (informative) Calculation of data base A, statistical distribution of hearing thresholds as
a function of age (HTLA) for an otologically normal population ("highly screened") .10
A.1 Specification of data base A .10
A.2 Selected values from data base A .12
Annex B (informative) Examples for data base B.14
B.1 Selected values from data base B1 .14
B.2 Selected values from data base B2 .16
B.3 Selected values from data base B3 .17
Annex C (informative) Example of assessment of risk of noise-induced hearing impairment .19
Annex D (informative) Tables with examples for NIPTS data.21
Bibliography.24

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oSIST ISO/DIS 1999:2012
ISO/DIS 1999
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
This second edition cancels and replaces the first edition ISO 1999:1990.
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Introduction
This International Standard presents, in statistical terms, the relationship between noise exposures and the
"noise-induced permanent threshold shift" (NIPTS) in people of various ages. It provides procedures for
estimating the hearing impairment due to noise exposure of populations free from auditory impairment other
than that due to noise (with allowance for the effects of age) or of unscreened populations whose hearing
capability has been measured or estimated. NIPTS is treated here as an additive term independent of other
components of hearing threshold levels. For any given noise exposure, it has a range of positive values
representing the variability of noise-damage susceptibility between individuals of a population.
Persons regularly exposed to noise can develop hearing loss of varying severity. Due to this hearing loss their
understanding of speech, perception of everyday acoustic signals or appreciation of music may be impaired.
With the exception of exposure to blast, high-impulse noise and extremely high levels of steady noise,
permanent impairment of the hearing organ takes time and is progressive over months, years or decades of
exposure. NIPTS is usually preceded by a reversible temporary effect on hearing, called noise-induced
"temporary threshold shift" (TTS). The severity of TTS and recovery from it depend upon exposure level and
time. For a single individual, it is not possible to determine precisely which changes in hearing threshold level
are caused by noise and which changes are caused by other factors, although, in doubtful individual cases,
the data in this International Standard might provide an additional means for estimating the most probable
causes in audiological diagnosis. However, for a large population exposed to a specific noise, changes in the
statistical distributions of hearing threshold levels can be determined. Parameters such as the mean NIPTS,
the median NIPTS, etc., can be used to describe differences in hearing threshold levels between two
populations that are similar in all relevant respects except that one population has had a well defined (usually
occupational) noise exposure. Throughout this International Standard, the term "NIPTS" is applied to changes
in the noise-induced permanent threshold shift of statistical distributions of groups of people; it is not to be
applied to individuals.
This International Standard can be applied to calculation of the risk of sustaining hearing impairment due to
regular occupational noise exposure or due to any daily repeated noise exposure. In some countries hearing
impairment caused by occupational noise exposure can have legal consequences with respect to
responsibility and compensation. The hearing threshold level at the various frequencies, at which a hearing
impairment is deemed to exist ("fence"), depends not only on the impairment per se, but frequently on legal
definitions and interpretations based on social and economic considerations. In addition, the definition of a
hearing impairment depends on the quality of speech recognition desired, the average level of background
noise and, with respect to the relative importance of the various frequencies, perhaps even on the language.
Consequently, this International Standard does not stipulate (in contrast to the first edition of ISO 1999) a
specific formula for assessment of the risk of impairment, but specifies uniform methods for the prediction of
hearing loss, which can be used for the assessment of impairment according to the formula desired or
stipulated in a specific country. The results obtained by this International Standard may also be used for
estimating the permanent effects of noise on the perception of everyday acoustic signals, the appreciation of
music or the effect of one specific frequency not necessarily stipulated by a hearing impairment formula.
Since noise-induced hearing impairment is the result not only of occupational noise exposure but of the total
noise exposure of the population, it may be important to take the non-occupational exposure of individuals
(during commuting to and from their jobs, at home and during recreational activities) into account. Only if this
non-occupational exposure is negligible compared with the occupational exposure does this International
Standard allow prediction of the occurrence of hearing loss due to occupational noise exposure. Otherwise, it
should be used to calculate the hearing loss to be expected from the combined (occupational plus non-
occupational) total daily noise exposure. The contribution of the occupational noise exposure to the total
hearing loss can then be estimated, if desired.
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The selection of maximum tolerable or maximum permissible noise exposures, and protection requirements
as well as the selection of specific formulae for impairment risk assessment or compensation purposes,
require consideration of ethical, social, economic and political factors not amenable to international
standardization. Individual countries differ in their interpretation of these factors and these factors are
therefore considered outside the scope of this International Standard.
For reasons given above this International Standard by itself does not comprise a complete guide for risk
assessment and protection requirements, and for practical use it has to be complemented by national
standards or codes of practice delineating the factors which are here left open.
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DRAFT INTERNATIONAL STANDARD ISO/DIS 1999

Acoustics — Estimation of noise-induced hearing loss
1 Scope
This International Standard specifies a method for calculating the expected noise-induced permanent
threshold shift in the hearing threshold levels of adult populations due to various levels and durations of noise
exposure; it provides the basis for calculating hearing impairment according to various formulae when the
hearing threshold levels at commonly measured audiometric frequencies, or combinations of such frequencies,
exceed a certain value.
NOTE 1 This International Standard does not specify frequencies, frequency combinations or weighted combinations to
be used for the evaluation of hearing impairment; nor does it specify a hearing threshold level ("fence") which must be
exceeded for hearing impairment to exist. Quantitative selection of these parameters is left to the user. All sound pressure
levels stated in this International Standard do not consider the effect of hearing protectors which would reduce effective
exposure levels and modify the spectrum at the ear.
The measure of exposure to noise for a population at risk is the A-weighted sound exposure (time-integrated
squared sound pressure), E , and the related equivalent continuous A-weighted sound pressure level, L ,
A,T Aeq,T
over an average working day (assumed to be of 8 h duration, L ), for a given number of years of exposure.
EX,8h
This International Standard applies to noise at frequencies less than approximately 10 kHz which is steady,
intermittent, fluctuating, irregular or impulsive in character. Use of this International Standard for sound
pressures exceeding 200 Pa (140 dB relative to 20 µPa) should be recognized as extrapolation.
For the assessment of hearing impairment due to exposure to noise, formulae are presented to calculate the
NIPTS for audiometric frequencies from 0,5 kHz to 6 kHz for 8 h per day daily A-weighted sound exposure of
2 . 2 .
5
364 Pa s to 1,15 x 10 Pa s (equivalent continuous A-weighted sound pressure level for a normal 8 h working
day from 75 dB to 100 dB), and periods of exposure lasting from 0 years to 40 years. Extrapolations to higher
levels are not supported by quantitative data. The median values of NIPTS as well as the statistical
distribution above and below the median value from the 0,05 to the 0,95 fractile are specified. The NIPTS data
are the same for male and female populations.
NOTE 2 Although the NIPTS data are based on data assumed to stem from primarily occupationally noise-exposed
populations, they may be used, with some caution, for estimating the effects of comparable non-occupational and
combined exposures.
NOTE 3 The prediction method presented is based primarily on data collected with essentially broad-band steady non-
tonal noise. The application of the data base to tonal or impulsive/impact noise represents the best available extrapolation.
Some users may, however, wish to consider tonal noise and/or impulsive/impact noise about as harmful as a steady non-
tonal noise that is approximately 5 dB higher in level.
To calculate hearing threshold levels and the risk of acquiring hearing impairment due to noise exposure, the
threshold of hearing of a non-noise-exposed population of comparable age has to be known. Since different
criteria can be applied to the selection of this population, this International Standard allows for two possibilities
presented by two different data bases:
a) an otologically normal population, that is, "highly screened" (see ISO 7029),
b) any other population selected by the user of the International Standard as being appropriate.
NOTE 4 All data and procedures presented in this International Standard are based on deliberate simplifications of
experimental data where the daily sound exposure duration did not exceed 12 h. The resulting approximations restrict the
validity to the stated ranges of the variables, fractiles, sound exposure levels and frequency ranges.
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This International Standard is based on statistical data and therefore shall not be used to predict or assess the
hearing impairment of individual persons except in terms of statistical probabilities.
Annex A gives the procedure for calculating the statistical distribution of hearing threshold levels as a function
of age for an otologically normal population ("highly screened") in accordance with ISO 7029.
Annex B gives three examples of the second data base representing the statistical distribution of hearing
threshold levels as a function of age for unscreened populations of three typical industrialized societies. These
data bases are derived from three recent studies in different countries, and the data differ significantly from
those of Data base B in the previous edition of this International Standard. In two of the examples, the test
subjects have not been exposed to hazardous occupational noise but otherwise represent all other factors that
may affect hearing, e.g. age, genetic dispositions, non-occupational noise, ear diseases. The third data base
concerns a completely unscreened population, as explained in Annex B.3.
Annex C describes an example of hearing risk assessment using this International Standard.
Annex D presents tables with examples of NIPTS as a function of exposure time (10 years, 20 years, 30 years
3 2 4 2 4 2
and 40 years) and daily A-weighted sound exposure (3,64 x 10 Pa s, 1,15 x 10 Pa s, 3,64 x 10 Pa s and
5 2
1,15x 10 Pa s, or equivalent continuous A-weighted sound pressure level for nominal 8 h working day of 85
dB, 90 dB, 95 dB and 100 dB) for six frequencies (0,5 kHz, 1 kHz, 2 kHz, 3 kHz, 4 kHz and 6 kHz) and three
fractiles (0,1, 0,5 and 0,9).
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are
indispensable for its application. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 7029, Acoustics – Statistical distribution of hearing thresholds as a function of age
ISO 9612, Acoustics – Determination of occupational noise exposure – Engineering method
ISO/TR 25417, Acoustics – Definitions of basic quantities and terms
3 Definitions
In addition to the definitions in ISO/TR 25417, for the purposes of this document, the following definitions
apply.
3.1
noise exposure level normalized to a nominal 8 h working day, L
EX,8h
level, in decibels, given by the formula
L = L + 10 lg (T /T ) dB
,
EX 8h pAeq,Te e 0
where
L is the equivalent continuous A-weighted sound pressure level
pAeq,Te
T is the effective duration of the working day;
e
T is the reference duration (T = 8 h).
0 o
NOTE 1 If the effective duration of the working day, T , is equal to 8 h, then L equals L . The quantity “noise
e X,8h Aeq,8h
E p
exposure level normalized to a nominal 8 h working day” may also be called “daily noise exposure level”
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NOTE 2 If the exposure averaged over n days is desired, for example if noise exposure levels normalized to a nominal
8 h working day for weekly exposures are considered, the average value of LEX,8h, in decibels, over the whole period may
be determined from the values of (L ) for each day, using the following equation:
X,8h i
E
n
⎡⎤1
01()L
EiX,8h
L = 10 lg 10 dB
E,X8h ∑
⎢⎥
c
⎣⎦i=1
The value of c is chosen according to the purpose of the averaging process: it will be equal to n if an average value is
desired; it will be a conventional fixed number if the exposure is to be normalized to a nominal number of days (for
example, c = 5 will lead to a daily noise exposure level normalized to a nominal week of 5 eight-hour working days).
3.2
hearing loss
hearing impairment
deviation or a change for the worse of the threshold of hearing from normal.
NOTE The term hearing loss may sometimes only refer to a change.
3.3
hearing disability
hearing impairment that affects activities in daily living
NOTE This is sometimes called ‘activity limitation’ (WHO)
3.4
fence
hearing threshold level above which degrees of hearing disability are deemed to exist.
3.5
risk of hearing disability
fractile of a population sustaining hearing disability (see 6.3)
3.6
risk of hearing disability due to noise
risk of hearing disability in a noise-exposed population minus the risk of hearing disability in a population not exposed to
noise, but otherwise equivalent to the noise-exposed population
3.7
hearing threshold level associated with age and noise (HTLAN), H'
permanent hearing threshold level, for a specified fraction of a population

NOTE 1 Hearing threshold levels (HTL) as defined in ISO 389 are expressed in decibels
NOTE 2 The value HTLAN is a combination of the components associated with noise (NIPTS, see 3.7) and with age
(HTLA, see 3.8) as defined in 5.1.
3.8
noise-induced permanent threshold shift (NIPTS), N
for a specified fraction of a population, the permanent shift, actual or potential, in decibels, of the hearing
threshold level estimated to be caused solely by exposure to noise, in the absence of other causes.
3.9
hearing threshold level associated with age (HTLA), H
for a specified fraction of a population, the hearing threshold level observed as a function of age. without any
exposure to occupational noise.
NOTE HTLA can be directly observed only in the absence of other causes of hearing impairment; for example,
pathological conditions or noise exposure.
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4 Description and measurement of noise exposure
Detailed requirements for the correct determination of occupational noise exposure are specified in ISO 9612.
5 Prediction of the effects of noise on hearing threshold
5.1 Statistical distribution of hearing threshold levels of a noise-exposed population
The hearing threshold level, in decibels, associated with age and noise (HTLAN), H’, of a noise-exposed
population is calculated, for the purposes of this International Standard, by using the following empirical
formula:
H× N
H'=+H N−          (1)
120
where
H is the numerical value of the hearing threshold level, expressed in decibels, associated with age (HTLA);
N is the numerical value of the actual or potential noise-induced permanent threshold shift (NIPTS),
expressed in decibels;
The formula is applicable only to corresponding fractile values of H', H and N.
NOTE The additive relationship is an approximation to the biological events and is considered accurate enough for
H×N
the purposes of this International Standard. Frequently, the term N− is called the actual NIPTS.
( )
120
H× N
The term  starts to modify the result significantly only when H + N is more than approximately 40 dB.
120
5.2 Data bases for hearing threshold levels associated with age (HTLA)
5.2.1 General
The hearing of a non-noise-exposed population as a function of age depends on the degree to which other
factors besides aging are inadvertently included; diseases, history of ototoxic drugs and unknown noise
exposure of occupational or non-occupational origin may modify the HTLA. Different approaches to screening
such data have been used, and the selection of the most appropriate data base depends on the purpose of
the application (see 5.2.4). This International Standard permits two data bases (data bases A and B) to be
used for HTLA in 5.1. Data base A is fully specified, whereas data base B is at the discretion of the user.
Three examples of data base B are presented.
NOTE The data bases presented in annexes A and B are from populations of European and North American
countries. These populations may or may not be appropriate for the populations of other geographical areas. Even if there
are no differences in natural aging between different ethnic populations, differences in life style, non-occupational noise
exposure, incidence of disease, and ototoxic drugs are nevertheless liable to occur.
5.2.2 Data base A
Data base A derives from otologically normal persons, i.e. persons in a normal state of health who are free
from all signs or symptoms of ear disease and from obstructing wax in the ear canals and who have no history
of undue exposure to noise. The statistical distributions of the thresholds of such "highly screened"
populations have been standardized in ISO 7029 separately for male and female populations. Formuale for
calculating data base A are specified in A.1. A table with selected values is given in A.2.
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5.2.3 Data base B
For data base B, a set of data collected on a control population not occupationally exposed to noise of the
country under consideration, but exposed to other risk factors to a degree similar to occupationally-exposed
populations, is recommended. When such an ideal control population is unavailable, and in countries where
risk factors such as non-occupational noise are more prevalent in occupationally-exposed populations than in
the general population, a totally unscreened control population may be preferable.
A separate HTLA data base for both men and women is required unless it can be shown that there are no
substantial sex differences. It is essential that the sample size be large enough to allow calculations of a valid
statistical distribution.
Therefore the user should apply appropriate selection criteria to compile a database of hearing threshold
levels to compare to the levels given in Annex B of this International Standard. For instance, the average
HTLA of both ears or the more sensitive ear alone can be taken as the basis for data base B.
NOTE It is emphasized that for practical situations the accuracy of the prediction of the hearing threshold level of a
noise-exposed population will largely be a function of the accuracy of the selected data base for HTLA. Since audiometric
measurement techniques affect threshold measurements, the same measurement technique should be used in the
est
...