Temporary Threshold Shift
Occurs when a person has been exposed for a few hours to noise levels of about 80 dB and above. These often leave a ringing the ears for some time afterwards. The greater part of the hearing loss occurs soon after exposure and, simply, recovery occurs largely in the 30 minutes following removal from the noise. Persons exposed to continuous noise at a level of 100 dB(A) for an 8 hour working day could show a temporary threshold shift up to 40 dB in that part of the spectrum most affected. Such a shift may be caused by other means such as use of aspirin or other drugs.
Significant Threshold Shift
A shift in hearing threshold, outside the range of audiometric testing variability (5 dB), that warrants follow-up action to prevent further hearing loss. The National Institute for Occupational Safety and Health defines significant threshold shift as an increase in the hearing threshold level of 15 dB or more at any frequency (500, 1000, 2000, 3000, 4000, or 6000 Hz) in either ear that is confirmed for the same ear and frequency by a second test within 30 days of the first test.
Permanent Threshold Shift
A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level. Occurs when the ear is subjected to high intensity noise day after day, causing more lasting damage. It is possible that a person may not recover full hearing between exposures and what started as a temporary threshold shift eventually becomes permanent.
Subjects: Noise & Vibration