Edgar A. G. Shaw
Inst. for Microstructural Sci., Nat. Res. Council, Ottawa, ON K1A 0R6, Canada
ISO 1999:1990 presents procedures for predicting noise-induced permanent threshold shift as a function of noise exposure which is defined in terms of the equivalent continuous A-weighted SPL, L[inf Aeq,T] over an average working day (assumed to be of 8-h duration) for a given number of years of exposure. Measurements are to be made with instruments capable of integrating high level impulses. These procedures are, of course, dependent on epidemiological data pertaining to the standard work week composed of approximately five 8-h work days. Nevertheless, daily exposure durations greater than 8 h and exposures that vary substantially from day to day are included in ISO 1999:1990, though subject to minor limitations. From another point of view, studies of temporary threshold shift, during and following exposure to steady noise for prolonged periods, seem to indicate that caution is justifiable. So, perhaps prolonged exposure durations should receive special treatment as suggested in the report prepared for the Special Advisory Committee on the Ontario Noise Regulation. [Work supported by APS707/NRCC No. 25051, Ottawa 1985.]