What Is a Standard Threshold Shift Under OSHA?
Under 29 CFR 1910.95(g)(10)(i), a Standard Threshold Shift (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. The STS is not a diagnosis of hearing loss — it is a regulatory trigger. Once confirmed, it initiates a defined chain of actions that employers must take regardless of whether the employee reports any subjective hearing difficulty.
STS Quick Reference
- Threshold: ≥10 dB average shift at 2,000 + 3,000 + 4,000 Hz
- Applies to: Either ear, evaluated independently
- Baseline comparison: Revised baseline or original — whichever is lower
- Age correction: Optional under OSHA (Appendix F tables); required under some state plans
- Recordkeeping trigger: Work-relatedness presumption applies if noise-exposed
- Regulation: 29 CFR 1910.95(g)(8)–(g)(11)
How to Calculate an STS: Step-by-Step
Step 1: Identify the comparison baseline — use the most recent revised baseline if one exists; otherwise use the original baseline. Step 2: Calculate the average shift at 2,000, 3,000, and 4,000 Hz for each ear separately: STS = [(monitoring 2k − baseline 2k) + (monitoring 3k − baseline 3k) + (monitoring 4k − baseline 4k)] ÷ 3. If the result is ≥10 dB, a preliminary STS exists. Step 3: Apply age correction (optional) using Appendix F tables. Step 4: Retest within 30 days of a preliminary STS unless a physician or audiologist determines a retest is unnecessary.
STS Calculation Example: Right Ear
| Frequency | Baseline (dB HL) | Monitoring (dB HL) | Shift |
|---|---|---|---|
| 2,000 Hz | 10 | 20 | +10 |
| 3,000 Hz | 15 | 25 | +10 |
| 4,000 Hz | 20 | 35 | +15 |
Average shift = (10 + 10 + 15) ÷ 3 = 11.7 dB → STS confirmed. This employee must be notified within 21 days. Work-relatedness must be evaluated.
Employer Obligations After an STS Is Confirmed
Once a retest confirms the STS (or the 30-day window lapses without a retest), 29 CFR 1910.95(g)(8) requires: (1) Notify the employee in writing within 21 days. (2) Evaluate work-relatedness for OSHA 300 Log — OSHA presumes work-relatedness for employees with noise exposure at or above the action level; rebuttal requires written medical documentation. (3) Refer for further audiological evaluation if the STS is not work-related or if recommended. (4) Review hearing protection adequacy and refit or upgrade to higher-NRR devices if needed. (5) Consider baseline revision if the STS is persistent. (6) Evaluate program effectiveness if multiple STS events occur in the same exposure group.
OSHA Recordkeeping: When Is an STS Recordable?
| Condition | Recordable on OSHA 300? |
|---|---|
| STS confirmed + employee noise-exposed at or above action level | Yes (presumptively work-related) |
| STS confirmed + no occupational noise exposure | No |
| STS confirmed + physician/audiologist documents non-occupational cause | No (rebuttal documented in file) |
| STS confirmed + total threshold at 2k/3k/4k averages ≤25 dB HL after age correction | No (OSHA exclusion under 1904.10(b)(1)) |
Common STS Compliance Failures
| Failure | Regulatory Consequence |
|---|---|
| Failing to retest within 30 days of preliminary STS | Violation of 29 CFR 1910.95(g)(7)(ii) |
| Notifying employee more than 21 days after STS confirmation | Violation of 29 CFR 1910.95(g)(8)(ii) |
| Not recording a work-related STS on the OSHA 300 Log | Violation of 29 CFR 1904.10 |
| Failing to revise baseline after persistent STS | Ongoing recordkeeping inaccuracy |
| Not providing upgraded HPDs after STS in high-noise area | Violation of 29 CFR 1910.95(g)(8)(iii) |
OSHA defines an STS as an average change of 10 dB or more at 2,000, 3,000, and 4,000 Hz in either ear, compared to the employee’s baseline audiogram. The calculation is done separately for each ear.
No. An STS is only recordable if it is work-related. OSHA presumes work-relatedness for noise-exposed employees, but this can be rebutted with medical documentation. Additionally, if the total hearing threshold after the STS averages 25 dB HL or less (age-corrected), it is not recordable under 29 CFR 1904.10(b)(1).
Employers must notify employees of a confirmed STS within 21 days of determination. Notification must be in writing. Verbal notification alone does not satisfy the regulatory requirement.
Yes. Applying age correction using the Appendix F tables can reduce the calculated shift below the 10 dB STS threshold for recordkeeping purposes. However, age correction is optional under federal OSHA and not available under all state plans.
Failure to retest within 30 days of a preliminary STS — without a physician or audiologist determining a retest is unnecessary — is a violation of 29 CFR 1910.95(g)(7)(ii). The preliminary STS result becomes the basis for all subsequent obligations.
Automate STS Detection and 21-Day Follow-Up Tracking
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