Temporary vs Permanent Threshold Shift: Safety Manager Guide

Understanding Temporary Threshold Shift (TTS) and Permanent Threshold Shift (PTS) is essential for interpreting audiometric results under OSHA 1910.95. According to CDC/NIOSH, approximately 22 million U.S. workers are exposed to hazardous occupational noise annually.

TTS vs PTS: The Fundamental Difference

The 14-Hour Quiet Period Requirement

OSHA 1910.95(g)(5) requires workers to have 14 hours free from workplace noise before baseline audiograms. This ensures TTS has fully resolved, so the baseline reflects true permanent threshold status. For workers who cannot achieve 14 hours of work-quiet, hearing protectors must be worn during that period. Annual audiograms should also avoid being conducted immediately after noisy shifts — a worker coming to testing directly from a 95 dBA press room may show apparent threshold elevation from TTS that would generate a false-positive STS.

When TTS Converts to PTS

TTS is the cochlea's recoverable response to noise. But repeated TTS events — shift after shift — can eventually exceed the hair cells' repair capacity. This is why occupational hearing loss accumulates gradually: each shift may produce only TTS, but years of repeated TTS cause cumulative PTS. The audiometric program catches PTS when it first manifests as an STS; the hearing conservation program is designed to prevent the repeated TTS events that precede it.

Re-Test After Suspected STS

When an annual audiogram shows a potential STS, the Professional Supervisor may recommend re-testing after a full quiet period to confirm whether the shift is PTS or TTS. A shift that resolves on re-test was TTS; one that persists is PTS and triggers STS follow-up obligations. This re-test provision prevents unnecessary STS notifications and HPD refitting for recoverable threshold changes. See: standard threshold shift: OSHA definition and action guide.