A 15 dB shift in hearing threshold at any frequency constitutes a Significant Threshold Shift requiring follow-up

A Significant Threshold Shift (STS) is defined by NIOSH as a 15 dB change in hearing threshold at any single audiometric frequency compared to the baseline audiogram. If an STS is detected at annual testing, the worker must be retested within 30 days after a 12-hour quiet period to distinguish a temporary threshold shift (reversible — caused by recent loud exposure) from a permanent one. Workers with confirmed STS must be notified and further exposure prevented through hearing protection refitting, training, or reassignment. Annual testing is ideally done at shift’s end to catch temporary shifts before they become permanent. Baseline testing establishes the comparison reference and should be preceded by a 12-hour quiet period.

Examples

A sound technician’s annual audiogram shows hearing at 4 kHz dropped from 10 dB HL (baseline) to 26 dB HL — a 16 dB shift. This exceeds the 15 dB STS criterion and triggers retesting and program action.

Assessment

A worker’s 4 kHz baseline threshold was 15 dB HL. This year’s test shows 30 dB HL. Determine whether this constitutes an STS. State the next required action under NIOSH guidelines.