ISO 1999:2013 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. The measure of exposure to noise for a population at risk is the noise exposure level normalized to a nominal 8 h working day, LEX,8h, for a given number of years of exposure. ISO 1999:2013 applies to noise at frequencies less than approximately 10 kHz which is steady, intermittent, fluctuating, irregular. Use of ISO 1999:2013 for sound pressures exceeding 200 Pa (140 dB relative to 20 µPa) is recognized as extrapolation.