How to Calculate Standard Threshold Shift (STS) — Step-by-Step Guide

Standard Threshold Shift (STS) is the metric OSHA uses to identify meaningful changes in an employee's hearing over time. Defined as an average shift of 10 dB or more at 2000, 3000, and 4000 Hz in either ear compared to the baseline audiogram, STS detection is the trigger for employer notification requirements, hearing protector re-evaluation, and potential OSHA 300 log recording.

Getting STS calculations right is critical. False positives create unnecessary alarm and administrative burden; false negatives mean genuine hearing loss goes undetected. Here's exactly how the calculation works.

Step 1: Gather the Audiogram Data

You need two audiograms for each ear:

• Baseline audiogram: The reference audiogram established when the employee first entered the hearing conservation program
• Current annual audiogram: The most recent audiometric test

For each audiogram, you'll use the hearing thresholds (in dB) at three specific frequencies: 2000 Hz, 3000 Hz, and 4000 Hz. These are the frequencies most affected by noise-induced hearing loss.

Step 2: Calculate the Shift at Each Frequency

For each ear, subtract the baseline threshold from the current threshold at each frequency:

• Shift at 2000 Hz = Current 2K threshold − Baseline 2K threshold
• Shift at 3000 Hz = Current 3K threshold − Baseline 3K threshold
• Shift at 4000 Hz = Current 4K threshold − Baseline 4K threshold

Example (Right Ear):

• Baseline: 2K = 10 dB, 3K = 15 dB, 4K = 10 dB
• Current: 2K = 20 dB, 3K = 30 dB, 4K = 25 dB
• Shifts: 2K = +10, 3K = +15, 4K = +15

Step 3: Average the Three Shifts

Average the shifts at 2K, 3K, and 4K Hz:

(10 + 15 + 15) / 3 = 13.3 dB

If this average is 10 dB or greater, an STS has occurred in that ear.

In our example, 13.3 dB exceeds the 10 dB threshold, so this employee has an STS in the right ear.

Step 4: Apply Age Correction (Optional)

OSHA allows — but does not require — employers to apply age correction factors from Table F-1 (Appendix F of 29 CFR 1910.95). Age correction accounts for presbycusis, the natural hearing loss that occurs with aging.

How age correction works:

1. Look up the Table F-1 values for the employee's age at baseline and current age, at each frequency (2K, 3K, 4K), using the correct gender table
2. Calculate the age correction at each frequency: Current age value − Baseline age value
3. Subtract the age correction from the raw threshold shift at each frequency
4. Average the age-corrected shifts

Continuing our example (male employee, age 35 at baseline, age 42 now):

• Age correction at 2K: 5 − 3 = 2 dB
• Age correction at 3K: 7 − 4 = 3 dB
• Age correction at 4K: 10 − 6 = 4 dB
• Corrected shifts: 2K = 10−2 = 8, 3K = 15−3 = 12, 4K = 15−4 = 11
• Average corrected shift: (8 + 12 + 11) / 3 = 10.3 dB

Even with age correction, this employee still has an STS (10.3 ≥ 10). In some cases, age correction can reduce the average below 10 dB, eliminating the STS determination.

Step 5: Determine OSHA Recordability

An STS becomes OSHA recordable (must be logged on the OSHA 300 log) when both conditions are met:

1. An STS exists (average shift ≥ 10 dB at 2K/3K/4K)
2. The current hearing level at 2K, 3K, and 4K averages 25 dB or more in the affected ear

Checking our example:

• Current thresholds: 2K = 20, 3K = 30, 4K = 25
• Average current level: (20 + 30 + 25) / 3 = 25.0 dB

Since the average is exactly 25 dB (meeting the threshold), this STS is OSHA recordable.

Common STS Calculation Mistakes

• Using wrong frequencies: STS uses only 2K, 3K, and 4K — not 500, 1000, 6000, or 8000 Hz
• Comparing to wrong baseline: If the baseline has been revised, use the revised baseline. If not, use the original
• Applying age correction inconsistently: If you use age correction, apply it the same way across all employees. Don't cherry-pick
• Forgetting to check both ears: STS is evaluated independently for each ear
• Ignoring retest option: OSHA allows a retest within 30 days of the annual audiogram. If the retest shows no STS, the retest result stands
• Missing the recordability check: Not all STS cases are recordable — you must check the 25 dB criterion

When to Revise the Baseline

A persistent STS may warrant revising the baseline audiogram. The reviewing audiologist or physician should consider revising when the STS appears to be permanent (confirmed on retest) and represents the employee's new stable hearing level. A revised baseline becomes the new reference for future STS calculations.