A Standard Threshold Shift is the single most consequential event in an OSHA 1910.95 hearing conservation program. When a worker’s annual audiogram shows an average 10 dB shift at 2,000, 3,000, and 4,000 Hz compared to their baseline — in either ear — a defined chain of employer obligations begins: notification within 21 days, review and upgrade of hearing protection, evaluation of engineering controls, and a recordability determination for the OSHA 300 Log under 29 CFR 1904.10. Getting STS right — the calculation, the age correction, the retest option, the notification, the revised baseline — is one of the highest-stakes technical requirements in occupational health compliance. This guide covers every element of OSHA’s STS framework in the order it applies.
STS Definition: What OSHA Means by “Standard Threshold Shift”
OSHA 1910.95(g)(10)(i) defines a Standard Threshold Shift as a change in hearing threshold, relative to the baseline audiogram for that employee, of an average of 10 decibels (dB) or more at 2,000, 3,000, and 4,000 Hz in either ear.
Four elements of this definition deserve emphasis:
- Relative to baseline: The comparison is always to the established baseline audiogram, not to the prior year’s audiogram. If the baseline was revised at a prior date, the comparison is to the revised baseline. STS is not a year-over-year progressive shift calculation; it measures cumulative change from the established reference point.
- Average of three frequencies: The STS calculation uses the average of thresholds at exactly 2,000, 3,000, and 4,000 Hz — no other frequencies. A shift at 4,000 Hz alone, no matter how large, does not constitute an STS if the average across the three frequencies does not reach 10 dB.
- Either ear independently: An STS in the right ear alone, or the left ear alone, triggers the full STS action protocol. The worker does not need to show shifts in both ears simultaneously.
- Relative to audiometric zero: Thresholds are expressed as hearing level in dB HL (hearing level), measured relative to audiometric zero as defined in ANSI S3.6. A threshold of 25 dB HL means the worker requires a tone 25 dB above the standard reference level to detect it — indicating mild hearing loss at that frequency.
How to Calculate an STS: Step by Step
The STS calculation is arithmetic but requires careful attention to which values are used. The process:
- Obtain the baseline thresholds at 2,000, 3,000, and 4,000 Hz for the ear being evaluated. These are the values from the established baseline audiogram for this worker.
- Obtain the current annual audiogram thresholds at the same three frequencies in the same ear.
- Calculate the shift at each frequency: Subtract the baseline threshold from the current threshold at each of the three frequencies. A positive result indicates threshold shift (worsening); a negative result indicates improvement.
- Average the three shifts: Sum the three individual frequency shifts and divide by 3. If the result is 10.0 dB or greater, an STS exists in that ear. If the result is less than 10.0 dB, no STS in that ear.
- Repeat for the other ear.
| Frequency | Baseline (dB HL) | Annual (dB HL) | Shift (dB) |
|---|---|---|---|
| 2,000 Hz | 10 | 15 | +5 |
| 3,000 Hz | 10 | 20 | +10 |
| 4,000 Hz | 15 | 30 | +15 |
| Average | — | — | +10.0 dB = STS |
In the example above, the average shift of exactly 10.0 dB meets the STS threshold. An average of 9.9 dB would not constitute an STS; 10.0 dB or greater does.
OSHA does not specify a rounding rule for the STS average. Best practice, and the approach used in most audiometric software, is to calculate to one decimal place without additional rounding. An average of 9.67 dB rounds to 9.7 dB and does not meet the STS threshold. An average of 10.0 dB meets it exactly. Document the calculation method used in your program records.
Age Correction Under OSHA Appendix F
OSHA 1910.95 Appendix F provides optional age-correction tables that employers may apply to STS calculations. Age correction adjusts the measured STS to account for the expected age-related hearing change between the baseline audiogram date and the current test date, separating the noise-induced component from the aging component.
When age correction applies
Age correction is optional, not required. Employers may apply it or not. The practical implication: if applying age correction would eliminate or reduce the STS to below the 10 dB threshold, using Appendix F may avoid the STS designation and the compliance obligations that follow. If age correction does not change the STS determination, there is no practical reason to apply it.
How to apply Appendix F age correction
- Identify the worker’s age at the time of the baseline audiogram and their age at the time of the current annual audiogram.
- Look up the expected threshold change for each frequency (2,000, 3,000, and 4,000 Hz) for a person of that sex moving from the baseline age to the current age, using the Appendix F tables.
- Subtract the age-correction value from the measured threshold shift at each frequency. If the age-correction value exceeds the measured shift, use zero (not a negative number) for that frequency.
- Average the three age-corrected shifts and compare to the 10 dB STS threshold.
Appendix F age correction is most useful for workers in their 40s and 50s, where age-related threshold changes at 3,000 and 4,000 Hz are significant enough to account for a meaningful portion of the measured shift. For workers under 35, the age correction values are small and rarely affect the STS determination. For workers over 60, the correction can be substantial — particularly at 4,000 Hz where age-related changes are most pronounced.
Interactive STS Calculator
Enter baseline and annual audiogram thresholds at 2,000, 3,000, and 4,000 Hz for each ear. The calculator determines whether an STS has occurred and applies optional age correction.
This calculator implements the OSHA 1910.95(g)(10)(i) STS definition: average shift of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear, relative to baseline. Age correction per Appendix F is not applied in this tool. Results are for educational purposes; all STS determinations should be reviewed by a qualified professional supervisor.
The 30-Day Retest Option
OSHA 1910.95(g)(7)(ii) gives employers a one-time option to schedule a retest within 30 days of the STS determination. If the retest does not confirm the STS — if the retest audiogram shows an average shift of less than 10 dB at the same three frequencies — the employer may use the retest result in place of the original annual audiogram and is not required to treat the case as an STS.
The retest option is valuable because temporary threshold shift (TTS) from recent noise exposure can cause a spurious STS result on the annual audiogram. A worker who was exposed to loud noise the morning of their annual test — or who did not observe the required 14-hour quiet period before testing — may show a temporary shift that would resolve within days. The retest conducted after adequate quiet time can confirm whether the shift is permanent (STS confirmed) or temporary (no STS).
| Retest scenario | Outcome | Required action |
|---|---|---|
| Retest average < 10 dB shift from baseline | STS not confirmed | No notification required; retain original and retest results; note in file |
| Retest average ≥ 10 dB shift from baseline | STS confirmed | Proceed with all STS obligations: 21-day notification, HCP review, recordability determination, baseline revision consideration |
| No retest conducted within 30 days | Original STS stands | Proceed with all STS obligations based on original annual audiogram |
Ensure the worker observes at least 14 hours of no noise exposure above 80 dBA before the retest. Document the quiet period in the retest record. A retest conducted without a verified quiet period has the same vulnerability to TTS as the original annual audiogram that prompted the retest.
The 21-Day Notification Requirement
OSHA 1910.95(g)(8) requires the employer to notify each employee who experiences an STS in writing within 21 days of the determination. The 21-day clock begins when the STS is determined — which OSHA interprets as when the audiologist, physician, or other professional reviewer confirms the STS, not when the raw audiometric test was conducted.
The notification must:
- Be in writing
- Inform the worker that they have experienced a Standard Threshold Shift
- Be understandable to the worker (translated if necessary for non-English-speaking workers)
OSHA does not specify the exact content of the notification beyond informing the worker of the STS. Best practice is to also include the specific thresholds showing the shift, a brief explanation of what an STS means for the worker’s hearing health, and the follow-up steps the employer is taking (HPD review, referral if indicated).
OSHA inspectors consistently cite failure to notify workers within 21 days as one of the most frequently found violations during HCP audits. The failure is almost always process-related, not intentional: the audiogram results come back from the testing provider, sit in a file, and never get formally reviewed and acted upon. Soundtrace’s platform flags STS-positive audiograms in real time and generates the notification documentation, keeping the 21-day clock visible and manageable.
Required HCP Actions After an STS Determination
An STS triggers a specific set of mandatory program actions under 1910.95. These are not optional and must be documented:
1. Hearing protector review and upgrade
OSHA 1910.95(g)(8)(ii) requires that employees who experience an STS be fitted or refitted with hearing protectors, shown how to use them, and required to use them if they are not already enrolled in the HCP or if they are already enrolled but not yet required to use HPDs (i.e., workers between the action level and PEL). For workers already required to use HPDs, the employer must evaluate whether the current HPD is adequate and, if not, upgrade to a protector that provides sufficient attenuation.
2. Engineering and administrative controls evaluation
OSHA 1910.95(f) requires employers to use feasible administrative or engineering controls to reduce noise exposure when employee noise exposures exceed the PEL. While this obligation is not newly triggered by every STS — it is an ongoing obligation — an STS in a worker at or above the PEL is a practical signal to re-evaluate whether current controls are adequate.
3. Audiologist or physician referral
OSHA 1910.95(g)(8)(i) requires the employer to provide the employee with ear, nose, and throat (ENT) or audiological referral if the professional reviewer determines that additional evaluation is necessary. Not every STS requires clinical referral — the reviewing audiologist or physician makes that determination based on the audiogram pattern — but the employer must make the referral available when indicated.
4. Medical pathology review
OSHA 1910.95(g)(8)(i) also requires informing the worker if a medical pathology of the ear is believed to be present when that pathology may be unrelated to occupational noise exposure. If the audiogram pattern suggests a medical condition (asymmetric loss, low-frequency loss, sudden hearing change) rather than noise exposure, the reviewing professional should note this and recommend medical evaluation.
OSHA 300 Log Recordability: When an STS Becomes a Recordable Case
An STS does not automatically become a recordable case on the OSHA 300 Log. Recordability requires two conditions to be met simultaneously under 29 CFR 1904.10:
- Work-relatedness: The hearing loss is work-related (i.e., an event or exposure in the work environment either caused or contributed to the hearing loss or significantly aggravated a pre-existing condition).
- Total hearing level threshold: The employee’s total hearing level, averaged at 2,000, 3,000, and 4,000 Hz in the affected ear, is 25 dB or more above audiometric zero — even after any STS has been accounted for.
| Frequency | Annual Threshold (dB HL) |
|---|---|
| 2,000 Hz | 20 |
| 3,000 Hz | 25 |
| 4,000 Hz | 35 |
| Average | 26.7 dB HL — Recordable if work-related |
The work-relatedness determination for hearing loss requires a case-by-case assessment. OSHA’s rule is that a case is presumptively work-related if the work environment caused or contributed to the loss. The employer has the ability to rebut this presumption by demonstrating that a non-occupational cause accounts for the hearing loss — which is where pre-employment audiograms and occupational history documentation become critical evidence.
Revised Baseline Rules
OSHA 1910.95(g)(9) allows or requires revisions to the established baseline audiogram under specific circumstances. Understanding when the baseline should be revised is essential to maintaining an accurate STS detection system:
Mandatory revised baseline
A revised baseline is mandatory when an audiologist, otolaryngologist, or physician determines that the employee’s observed threshold shift is persistent and that the standard threshold shift is not caused by the use of hearing protectors. In practice, this means: once an STS is confirmed and determined to be a permanent hearing change (not a temporary shift from TTS), the annual audiogram showing the STS becomes the new baseline for future STS comparisons.
Optional revised baseline (improved thresholds)
The employer may revise the baseline upward — to a better-hearing baseline — if an employee’s current audiogram shows significantly improved thresholds compared to the established baseline. This can occur when the original baseline was elevated due to TTS that was not properly controlled for. Revision to a better-hearing baseline benefits both the worker (who has a more accurate representation of their hearing health) and the employer (who has a more accurate STS detection threshold).
Revising the baseline to incorporate a confirmed STS is required — but it also resets the comparison point. A worker whose baseline is revised to reflect an STS will now require an additional 10 dB average shift from the new baseline before their next STS is triggered. This is correct OSHA compliance, but it means that a worker with progressive hearing loss will show fewer STS events as the baseline chases the deteriorating thresholds. Annual audiograms tell the full story; the baseline revision is a compliance mechanism, not a health monitoring substitute.
Frequently Asked Questions
Under OSHA 1910.95, an STS is a change in hearing threshold relative to the baseline audiogram of an average of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear. An STS triggers worker notification within 21 days, hearing protection review, potential OSHA 300 Log recordability, and consideration of baseline revision.
No. An STS is recordable only if it is work-related and the employee’s total hearing level averaged at 2,000, 3,000, and 4,000 Hz in the affected ear is 25 dB or more above audiometric zero. An STS in a worker whose average thresholds remain below 25 dB HL is not a recordable case under 29 CFR 1904.10.
Yes. OSHA 1910.95 Appendix F age correction is optional but may be applied before the STS determination. If age correction reduces the average threshold shift to below 10 dB, no STS has occurred. If the age-corrected average still meets or exceeds 10 dB, the STS stands. Document whether age correction was applied in the worker’s audiometric record.
OSHA requires the employer to provide the written notification; it does not require the worker to sign an acknowledgment. Best practice is to document that notification was provided (date, method, content) regardless of whether the worker signs. If the worker refuses to accept the notification, document the attempt and the refusal in the record.
OSHA uses the term Standard Threshold Shift (STS). Some audiometric testing providers and audiologists use “significant threshold shift” (SigTS) as a different designation — typically defined as a 15 dB or greater shift at a single frequency in either ear. SigTS is not an OSHA-defined term and does not trigger the specific OSHA action sequence. The OSHA STS definition — 10 dB average at 2k/3k/4k Hz — is the operative standard for compliance purposes.
STS Detection Built Into Every Test
Soundtrace calculates STS automatically against each worker’s established baseline, flags confirmed shifts in real time, generates the 21-day notification documentation, and routes flagged audiograms to the professional supervisor for review — so nothing falls through the compliance cracks.
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