Effects of noise on health: Chapter 3 of a report on noise and health

The second edition of ISO 1999 "Acoustics -Determination of occupational noise exposure and estimation of noise-induced hearing impairment" (lS090) gives a calculation method for the determination of hearing threshold levels of populations exposed to all types of noise (steady-state, intermittent and impulse) during working hours. The noise exposure is characterized by the noise exposure level, LEX' In this report LEX is denoted by LEX,aw indicating the exposure concerns occupational noise. Relations are given between LEX,acc and noise-induced permanent threshold shift (NIPTS) for frequencies in the range from 500 to 6000 Hz, and for exposure times up to 40 years. These relations are expressed in statistical terms (median values of NIPTS as well as values from the 0.05 to the 0.95 fractile). The relations show that NIPTS is a phenomenon which occurs predominantly in the higher frequency range from 3000 to 6000 Hz; the effect is largest at a frequency of 4000 Hz. With increasing equivalent sound level and increasing exposure time, hearing loss also occurs at the lower frequencies, more specifically at 2000 Hz. For prolonged occupational noise exposure, ISO 1999 shows that permanent threshold shift is not induced by noise with LEX,acc values at and below Aweighted sound levels of 75 dB(A).