12.6 Hearing Conservation Programs and Standard Threshold Shifts

Hearing Conservation Programs and Standard Threshold Shifts

Engineering control reduces the hazard; the hearing conservation programme manages the residual risk and produces the audiometric evidence that the programme is or is not working. OSHA prescribes the elements of that programme and the arithmetic of a standard threshold shift.

1. OSHA Hearing Conservation Program (29 CFR 1910.95) Mandatory Elements

Whenever employee noise exposure equals or exceeds an 8-hour TWA of 85 dBA (50% dose or 85 dBA Action Level), the employer must implement a comprehensive Hearing Conservation Program.

   +-------------------------------------------------------------------------+
   |               OSHA 29 CFR 1910.95 HEARING CONSERVATION PROGRAM           |
   +-------------------------------------------------------------------------+
   |                                                                         |
   |  1. EXPOSURE MONITORING:                                                |
   |     - Conduct representative 8-hr dosimetry (80 dBA threshold).         |
   |     - Re-monitor upon process, equipment, or control modifications.     |
   |                                                                         |
   |  2. AUDIOMETRIC TESTING (Free to workers):                              |
   |     - Baseline Audiogram: Within 6 months of exposure (14 hr quiet).    |
   |     - Annual Audiograms: Compared against baseline for STS.             |
   |                                                                         |
   |  3. HEARING PROTECTORS:                                                 |
   |     - Provided free of charge; minimum 2 types of plugs & 1 muff.       |
   |     - Mandatory wear for: TWA >= 90 dBA, or TWA >= 85 dBA if no baseline|
   |       yet established, or if employee has experienced an STS.           |
   |                                                                         |
   |  4. EMPLOYEE TRAINING:                                                  |
   |     - Annual training on noise hazards, HPD selection, fit, and testing.|
   |                                                                         |
   |  5. RECORDKEEPING:                                                      |
   |     - Noise measurements retained for AT LEAST 2 YEARS.                 |
   |     - Audiometric test records retained for DURATION OF EMPLOYMENT.     |
   +-------------------------------------------------------------------------+

Audiometric Testing Program Requirements

1. Baseline Audiogram:
   • Must be established within 6 months of an employee's first exposure at or above the Action Level (85 dBA TWA), or within 1 year if using mobile test vans (workers must wear HPDs after 6 months until tested).
   • Mandatory 14-Hour Quiet Period: The baseline test must be preceded by at least 14 hours of quiet free from workplace noise to ensure temporary threshold shift (TTS) has resolved. Wearing hearing protectors with adequate attenuation is legally permitted to satisfy this 14-hour quiet period.
2. Annual Audiogram:
   • Tested at pure-tone frequencies: 500, 1000, 2000, 3000, 4000, 6000 Hz in both left and right ears.
   • Compared directly against the baseline audiogram to detect hearing deterioration.
3. Audiometer Calibration Schedule:
   • Daily Biological Check: Test a person with stable hearing; shift ≥ 10 dB requires acoustic calibration.
   • Annual Acoustic Calibration: Performed annually using an artificial ear/sound level meter.
   • Biannual Exhaustive Calibration: Full factory electronic recalibration every 2 years.

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2. Standard Threshold Shift (STS) Determination and Follow-Up

   +-------------------------------------------------------------------------+
   |                  STANDARD THRESHOLD SHIFT (STS) AUDIOGRAM               |
   +-------------------------------------------------------------------------+
   |                                                                         |
   |  Hearing Level (dB HL)                                                  |
   |      0 |                                                                |
   |     10 |---[ Baseline: 10 dB, 10 dB, 15 dB -> Avg = 11.7 dB ]           |
   |     20 |                                                                |
   |     30 |---\- - - - - - - - - - - - - - - - - - - - - - - - - - - -     |
   |     40 |    \ [ Annual: 25 dB, 30 dB, 35 dB -> Avg = 30.0 dB ]          |
   |     50 |     \                                                          |
   |     60 |      v  Shift = 30.0 - 11.7 = 18.3 dB (>= 10 dB -> STS!)       |
   |        +--------------------------------------------------------------  |
   |           500 Hz   1 kHz   2 kHz   3 kHz   4 kHz   6 kHz   8 kHz        |
   +-------------------------------------------------------------------------+

Standard Threshold Shift (STS) Definition

A Standard Threshold Shift (STS) is defined by OSHA 29 CFR 1910.95(g)(10) as:

$\mathbf{\text{Average Shift } \ge 10.0\text{ dB at } 2,000\text{ Hz}, 3,000\text{ Hz},\text{ and } 4,000\text{ Hz} \text{ in either ear relative to baseline.}}$

$\text{Shift} = \frac{(\Delta \text{HL}_{2000} + \Delta \text{HL}_{3000} + \Delta \text{HL}_{4000})}{3} \ge 10.0\text{ dB}$

Presbycusis Age Correction (OSHA Appendix F)

• Normal age-related hearing loss (presbycusis) can be mathematically deducted from the annual audiogram using standardized values in OSHA 1910.95 Appendix F (stratified by age and sex).
• If the age-corrected shift is < 10.0 dB, an STS has not occurred.

Mandatory Follow-Up Actions upon STS Detection

1. Retest Option: Employer may conduct an immediate or scheduled retest within 21 calendar days of the annual test. If the retest does not confirm the shift, the STS is negated.
2. Written Employee Notification: Employer must notify the affected employee in writing within 21 calendar days of the STS determination.
3. HPD Enforcement & Refitting:
   • Employees not currently wearing HPDs must be fitted with HPDs, trained in their use, and mandated to wear them.
   • Employees already wearing HPDs must be refitted, retrained, and provided with higher-attenuation HPDs if current protection is inadequate.
4. Clinical Referral: Refer employee for audiological or otological evaluation if ear pathology or medical conditions are suspected.
5. OSHA 300 Log Recordability (29 CFR 1904.10):
   • An STS must be recorded on the employer's OSHA Form 300 Log within 7 calendar days if all three of the following criteria are met:
     1. Work-Related: The hearing loss is caused or aggravated by workplace noise exposure.
     2. STS Confirmed: A confirmed STS (≥ 10 dB average shift at 2k, 3k, 4k Hz, with optional age correction) is identified in one or both ears.
     3. Total Audiometric Hearing Level Threshold: The employee's current overall hearing level in the affected ear averages 25 dB or greater above audiometric zero across 2000, 3000, and 4000 Hz on the current audiogram.

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3. Worked Step-by-Step Calculation Examples

Worked Example 11.3.1: Room Reverberation and Noise Reduction via Sabine Formula

Problem: A plant stamping bay measures L = 60 ft, W = 40 ft, and H = 20 ft (Volume V = 48,000 ft³). The total internal surface area is S = 2(60× 40 + 60× 20 + 40× 20) = 8,800 ft². The unconditioned hard concrete walls, floor, and metal deck ceiling have an average acoustic absorption coefficient of α1 = 0.05.

1. Calculate the initial room absorption (A1) and reverberation time (T(60,1)).
2. If acoustic fiberglass baffles are suspended from the ceiling to add 2,200 Sabins of absorption (A2 = A1 + 2200), calculate the new reverberation time (T(60,2)).
3. Calculate the noise reduction (Δ Lrev) achieved in the reverberant sound field.

Solution Steps:

1. Calculate initial room absorption and reverberation time: $A_1 = S \times \alpha_1 = 8,800\text{ ft}^2 \times 0.05 = 440\text{ Sabins}$ $T_{60,1} = \frac{0.049 V}{A_1} = \frac{0.049 \times 48,000}{440} = \frac{2,352}{440} = \mathbf{5.35\text{ seconds}}$

2. Calculate new absorption and reverberation time with ceiling baffles: $A_2 = A_1 + 2,200 = 440 + 2,200 = 2,640\text{ Sabins}$ $T_{60,2} = \frac{0.049 \times 48,000}{2,640} = \frac{2,352}{2,640} = \mathbf{0.89\text{ seconds}}$

3. Calculate reverberant sound level reduction (Δ L = 10 log10[A2 / A1]): $\Delta L = 10 \log_{10}\left( \frac{2,640}{440} \right) = 10 \log_{10}(6.0) = 10 \times 0.77815 = \mathbf{7.78\text{ dB} \approx 7.8\text{ dBA}}$

Result: Adding acoustic baffles reduces reverberation time from 5.35 s down to 0.89 s and attenuates reverberant field noise by 7.8 dBA.

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Worked Example 11.3.2: Dual Hearing Protection Calculation and Protected Exposure

Problem: A boilermaker is exposed to continuous metal gouging noise at an ambient level of Lp = 104.0 dBA. The worker wears formable foam earplugs with a manufacturer labeled NRR = 31.0 dB combined with circumaural earmuffs with an NRR = 25.0 dB.

1. Calculate the OSHA Field Derated NRR for the dual protection combination.
2. Calculate the worker's estimated protected noise exposure level (Lprot).
3. Determine if the dual protection adequately protects the worker to below the OSHA PEL (90.0 dBA) and Action Level (85.0 dBA).

Solution Steps:

1. Calculate Dual Protection Field NRR:

   • Identify the higher NRR device: Earplugs (extNRR = 31.0 dB).
   • Apply OSHA derating to the higher device: Field NRRprimary = (31.0 - 7)/2 = 24.0/2 = 12.0 dB.
   • Add the standard 5.0 dB dual protection credit for the secondary device: $\text{Field NRR}_{\text{dual}} = 12.0\text{ dB} + 5.0\text{ dB} = \mathbf{17.0\text{ dB}}$

2. Calculate Protected Exposure Level (Lprot = Ambient dBA - Field NRRdual): $L_{\text{prot}} = 104.0\text{ dBA} - 17.0\text{ dB} = \mathbf{87.0\text{ dBA}}$

3. Evaluate Protection Adequacy:

   • Protected exposure is 87.0 dBA, which is below the OSHA PEL of 90.0 dBA (compliant for 8-hour shift).
   • Note: While compliant with the 90.0 dBA PEL, it remains above the 85.0 dBA Action Level, meaning the worker must remain enrolled in the Hearing Conservation Program.

Result: The derated dual protection provides 17.0 dB of effective attenuation, reducing worker exposure from 104.0 dBA to 87.0 dBA.

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Worked Example 11.3.3: Evaluating Audiometric Test Data for STS with Age Correction

Problem: A 48-year-old male machinist receives an annual audiogram. His baseline audiogram (established at age 40) and current annual audiogram show the following pure-tone hearing threshold levels (HTL in dB HL) for the right ear:

1. Calculate the unadjusted average hearing threshold shift at 2k, 3k, and 4k Hz.
2. Calculate the age-corrected threshold shift using the OSHA Appendix F values.
3. Determine whether an OSHA Standard Threshold Shift (STS) has occurred.

Solution Steps:

1. Calculate unadjusted average shift:

   • Shift at 2000 Hz: 20 - 10 = 10 dB
   • Shift at 3000 Hz: 30 - 15 = 15 dB
   • Shift at 4000 Hz: 35 - 15 = 20 dB $\text{Unadjusted Shift} = \frac{10 + 15 + 20}{3} = \frac{45}{3} = \mathbf{15.0\text{ dB}}$ (Unadjusted shift of 15.0 dB ≥ 10.0 dB meets initial STS criteria).

2. Apply OSHA Appendix F Presbycusis Age Correction:

   • Age correction factor (Age 48 value minus Age 40 value):
     • At 2000 Hz: 5 - 3 = 2 dB
     • At 3000 Hz: 11 - 6 = 5 dB
     • At 4000 Hz: 17 - 9 = 8 dB
   • Age-corrected annual thresholds:
     • At 2000 Hz: 20 - 2 = 18 dB → Corrected shift = 18 - 10 = 8 dB
     • At 3000 Hz: 30 - 5 = 25 dB → Corrected shift = 25 - 15 = 10 dB
     • At 4000 Hz: 35 - 8 = 27 dB → Corrected shift = 27 - 15 = 12 dB $\text{Age-Corrected Average Shift} = \frac{8 + 10 + 12}{3} = \frac{30}{3} = \mathbf{10.0\text{ dB}}$

3. Determine STS Status:

   • The age-corrected average shift is exactly 10.0 dB. Because OSHA defines an STS as an average shift ≥ 10.0 dB, a confirmed Standard Threshold Shift has occurred.

Result: Even after applying full OSHA Appendix F age corrections, the employee exhibits an STS of 10.0 dB in the right ear, triggering mandatory written notification within 21 days and HPD refitting/retraining.