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), EA,T, and the related equivalent continuous A-weighted sound pressure level, LAeq,T , over an average working day (assumed to be of 8 h duration, LEX,8h), for a given number of years of exposure. 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 364 Pa2s. to 1,15 x 105 Pa2s. (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 nontonal 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 nontonal 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. 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 and 40 years) and daily A-weighted sound exposure (3,64 x 103 Pa2s, 1,15 x 104 Pa2s, 3,64 x 104 Pa2s and 1,15x 105 Pa2s, 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).