Temporary Threshold Shift and Damage-Risk Criteria for Intermittent Noise Exposures

Twelve normal-hearing students were exposed for up to 8 h to steady and intermittent noises that, according to the CHABA damage-risk criteria (DRC) proposed in 1966, should produce an average TTS2 (temporary threshold shift measured 2 min after exposure) no greater than 10 dB at 1 kHz or below, 15 dB at 2 kHz, or 20 dB at 3 kHz or above. This objective appears to have been attained for single uninterrupted exposures and for intermittent exposures involving short (below 3–5 min) noise bursts and recovery periods. However, the cited limits of TTS2 tend to be exceeded when the noise bursts are quite long (10 min or more), owing to an erroneous previous assumption about the course of recovery during the quieter intervals between successive bursts. Modification of the DRC is therefore indicated. However, a more vexing finding is that intermittent exposures to high-frequency high-energy noise—either long bursts or short—often produce a delayed recovery; that is, more than 16 h of rest is required to restore the initial sensitivity. Thus, in an industrial situation, some workers in such a noise may in essence have a chronic TTS. In case such a chronic auditory fatigue may lead to permanent damage, extreme caution should be exercised in exposing men to levels above 100 dB in the 1500-Hz range or higher, even when the ratio of noise duration to pause duration is quite small. It is proposed that limiting values of TTS30 or TTS1000 rather than TTS2 might be more valid indicators of relative noxiousness of various exposure patterns. Alternate DRC presently under consideration are also discussed.