The Standard Threshold Shift is the central compliance event in OSHA 29 CFR 1910.95. When an STS is identified, it triggers a specific chain of required employer actions — and when those actions are not taken, it generates citation exposure. When an STS is not identified when it should be, it generates WC liability exposure. According to CDC/NIOSH, approximately 22 million U.S. workers face hazardous noise annually, and a significant fraction develop audiometric threshold shifts that require STS determinations and follow-up actions each year.
What Constitutes an STS Under OSHA 1910.95
An STS is defined in 29 CFR 1910.95(g)(10) as a change in hearing threshold relative to the baseline audiogram of an average of 10 decibels (dB) or more at 2000, 3000, and 4000 Hz in either ear. The calculation:
- Average the thresholds at 2000, 3000, and 4000 Hz from the current annual audiogram in one ear
- Average the thresholds at 2000, 3000, and 4000 Hz from the baseline audiogram in the same ear
- Subtract the baseline average from the current average
- If the result is 10 dB or more, an STS has occurred in that ear
Once an STS is confirmed — either from the initial test or after a retest that also shows the STS — the employer must notify the worker in writing within 21 days. Failure to provide timely written notification is a separate citation basis from the underlying STS determination failure. Maintain a log of notification dates for each STS determination; this is part of the auditable HCP record set.
The Retest Option
OSHA 1910.95(g)(7)(ii) provides an employer the option to retest a worker within 30 days of an audiogram that shows an STS. If the retest does not confirm the STS, the employer is not required to take the STS follow-up actions. Retest conditions: the retest must be conducted under standardized conditions that meet the same technical requirements as the original test.
Retesting is most useful when the original test shows an apparent STS that may reflect temporary threshold shift from noise exposure before the test, a non-standardized test environment, or other transient factors. Retesting is not appropriate as a routine strategy to avoid STS determinations when genuine progressive shifts are occurring.
STS Follow-Up Actions Required by OSHA
| Action Required | Timing | Notes |
|---|---|---|
| Fit worker with HPDs if not already provided | Promptly after STS confirmation | If worker was already in HCP with HPDs, ensure HPDs are adequate for actual TWA |
| Refit or retrain if HPDs were inadequate | Promptly after STS confirmation | Reassess HPD selection; consider REAT-based fit testing to verify attenuation |
| Notify worker in writing | Within 21 days of STS determination | Written notification required; oral notification alone insufficient |
| Refer to physician or audiologist | As warranted per professional supervisor | Professional supervisor makes the referral determination; not automatic for every STS |
STS vs. OSHA 300 Log Recordability
Not every 1910.95 STS is recordable on the OSHA 300 log. A work-related hearing loss case is recordable under 29 CFR 1904 when it meets both criteria:
- The threshold shift meets the STS definition (10 dB average at 2000, 3000, 4000 Hz), AND
- The combined average hearing level at those frequencies is 25 dB HL or more
A worker whose baseline was 20 dB HL average at 2000/3000/4000 Hz who shifts to 30 dB HL average has an STS under 1910.95 (10 dB shift) but is not recordable under 1904 (30 dB HL, but below 25 dB HL threshold... actually this example exceeds 25 dB HL so it would be recordable). Age correction may also be applied under both standards.
For every STS determination, document: the baseline and annual audiogram values used for calculation, the STS calculation itself, whether a retest was conducted and results, the professional supervisor’s determination, the date of written notification to the worker, any HPD refitting or retraining actions taken, any medical referral made, and whether the case is recordable under 1904. This documentation trail is the employer’s evidence of compliance if the STS is later cited or litigated.
Frequently Asked Questions
Professional Supervisor STS Determinations — Every Audiogram
Soundtrace licensed audiologist Professional Supervisors calculate STS for every annual audiogram, make all required determinations, and generate the documentation trail for OSHA compliance and WC defense.
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