ISO DIS 1999

Description / Abstract: 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 handicap 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 handicap; nor does it specify a hearing threshold level ("fence") which must be exceeded for hearing handicap 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 the levels effective at the ear.

The measure of exposure to noise for a population at risk is the averaged A-weighted sound exposure (time-integrated squared sound pressure), E_A,T, and the related equivalent continuous A-weighted sound pressure level, L_Aeq,T, over an average working day (assumed to be of 8 h duration), for a given number of years of exposure. This International Standard applies to audio frequency (less than approximately 10 kHz) noise which is steady, intermittent, fluctuating, irregular or impulsive in character. Use of this International Standard for instantaneous sound pressures exceeding 200 Pa (140 dB relative to 20 μPa) and for higher sound pressures 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 364 Pa^2.s to 1,15 x 10⁵ Pa^2.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 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.

NOTES

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 nonoccupational and combined exposures. (The length of the working day should be stated.)

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 or handicap 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.

This International Standard is based on statistical data and therefore shall not be used to predict or assess the hearing impairment or hearing handicap of individual persons.

Annex A gives the procedure for calculating the age-related hearing threshold levels for an otologically normal population ("highly screened") in accordance with ISO 7029.

Annex B gives as an example of the second data base the procedure for calculating the age-related threshold levels for an unscreened population of a typical industrialized society.

Annex C gives an example of selected values of the hearing threshold levels of a specific unscreened population, which, when used with the procedures of this International Standard, results in approximately the same risk of hearing handicap as the one predicted by the first edition of ISO 1999.

Annex D describes an example of hearing risk assessment using this International Standard.

Annex E presents tables with examples of NIPTS as a function of exposure time (10, 20, 30 and 40 years) and daily A-weighted sound exposure (3,64 x 10³, 1,15 x 10⁴ 3,64 x 10⁴ and 1,15x 10⁵ Pa^2.s, or equivalent continuous A-weighted sound pressure level for nominal 8 h working day of 85, 90, 95 and 100 dB) for six frequencies (0,5, 1, 2, 3, 4 and 6 kHz) and three fractiles (0,1, 0,5 and 0,9).

A bibliography is given in annex F.