Evaluation of guidelines for safe exposure to continuous noise of moderate and high intensity

In a 1974 report the Environmental Protection Agency made predictions, based upon extrapolations from the literature, as to what constitutes a 5 dB equal 90th percentile TTS2 curve, i.e., a "safe exposure" curve. However, because of a dearth of data for brief exposures to noise, they were uncertain as to the accuracy of some of their predictions. Therefore, in the present investigation, the TTS2 consequences of brief exposures to noise were systematically evaluated. Specifically, 40 subjects were tested at each of 10 appropriately spaced durations of exposure to noise. The 90th percentile TTS2 at 4000 Hz was determined for each duration of exposure and then a multiple regression equation was fitted to these values. From this equation, a 5 dB equal 90th percentile TTS2 curve was calculated, which represents our predictions as to the trade-offs between noise intensity and duration of exposure necessary to produce 90th percentile TTS2S of 5 dB. A comparison of our predictions to those of the EPA (in the area of uncertainty) suggests that the conservative EPA predictions may be too conservative while the EPA's modified and extended CHABA-criterion predictions (from Nat. Acad. Sci., Nat. Res. Council Committee on Hear., Bioacoustics, Biomechanics) may not be sufficiently conservative. Although our predictions reflect the trends which were actually observed in the present investigation, it would be unreasonable to assume that our predictions are more than "ball park" estimates of reality. They do suggest, though, that in reality a 5 dB equal 90th percentile TTS2 curve would probably be intermediate between the conservative EPA curve and EPA's modified and extended CHABA criterion curves.