Guide to OSHA Noise Exposure Limits

Permissible Exposure Limit and Action Level

OSHA's permissible exposure limit (PEL) for occupational noise is 90 dBA as an 8-hour time-weighted average (TWA). This is the legal maximum. Exposures at or above 90 dBA TWA require engineering controls, administrative controls, or hearing protection to bring the effective exposure below the limit. OSHA uses a 5 dB exchange rate, meaning that every 5 dB increase in noise level cuts the allowable exposure time in half: 95 dBA is permitted for 4 hours, 100 dBA for 2 hours, 105 dBA for 1 hour, and so on.

The action level is 85 dBA TWA. Exposures at or above the action level but below the PEL do not require engineering controls, but they do trigger the hearing conservation program requirements: baseline and annual audiometric testing, hearing protection availability, employee training, and recordkeeping. Many employers voluntarily adopt 85 dBA as their internal PEL because NIOSH and most occupational health professionals consider 90 dBA inadequate to prevent hearing loss over a working career.

The NIOSH Recommended Exposure Limit (REL) is 85 dBA TWA with a stricter 3 dB exchange rate. Under the NIOSH criteria, 88 dBA is permitted for 4 hours (vs. OSHA's 4 hours at 95 dBA). The 3 dB rate is based on the equal-energy principle, as it reflects the actual physics of sound energy accumulation in the ear. Most international standards (ISO, EU) also use the 3 dB exchange rate.

Noise Dose and TWA Calculation

Most workers are not exposed to a single steady noise level for an entire shift. They move between tasks, equipment cycles on and off, and break periods reduce exposure. The noise dose formula handles these mixed exposures by summing the fraction of allowable time consumed at each noise level:

D = (C₁/T₁ + C₂/T₂ + ... + C_n/T_n) × 100

Where C is the actual duration at a given noise level and T is the maximum allowable duration at that level. A dose of 100% equals the PEL. Under OSHA's 5 dB exchange rate, the allowable time T at any level L is: T = 8 / 2^(L-90)/5 hours.

The TWA converts the dose back to an equivalent steady-state level: TWA = 16.61 × log₁₀(D/100) + 90 dBA. A dose of 50% equals a TWA of 85 dBA (the action level). A dose of 100% equals 90 dBA (the PEL). A dose of 200% equals 95 dBA.

When performing noise surveys, take representative measurements at each task or work area using an integrating sound level meter set to A-weighting and slow response. Record the noise level and the time spent at each level. The calculator handles the dose summation and TWA conversion automatically.

Hearing Conservation Program Requirements

When any employee's noise exposure reaches or exceeds the 85 dBA TWA action level, OSHA requires the employer to implement a hearing conservation program. The program has five mandatory elements: monitoring, audiometric testing, hearing protection, training, and recordkeeping.

Monitoring: Conduct noise surveys whenever changes in production, process, equipment, or controls may increase exposures. Personal dosimetry is preferred over area monitoring because it captures the actual exposure of mobile workers. Repeat monitoring when conditions change significantly.

Audiometric testing: Establish a baseline audiogram within 6 months of first exposure at or above the action level (or within 1 year if a mobile testing van is used, provided the employee wears HPDs for the interim). Conduct annual audiograms thereafter. Compare annual results to the baseline to detect standard threshold shifts (STS), defined as an average shift of 10 dB or more at 2000, 3000, and 4000 Hz in either ear. An STS triggers follow-up action including HPD refitting, retesting, and potential medical referral.

Training: Annual training must cover the effects of noise on hearing, the purpose and proper use of hearing protection, and the purpose and procedures of audiometric testing. Training must be repeated annually and updated when HPD types change.

Hearing Protection Selection and NRR Derating

Hearing protection devices (HPDs) are rated by their Noise Reduction Rating (NRR), a laboratory-measured value printed on the packaging. However, laboratory NRR values consistently overstate the protection achieved in real-world use because lab testing is performed under ideal fit conditions that workers rarely replicate. OSHA recommends derating the NRR to estimate actual field attenuation.

OSHA derating method: Subtract 7 dB from the NRR, then divide by 2 for A-weighted measurements (the most common). Effective exposure = Measured dBA − (NRR − 7) / 2. For example, a worker in 100 dBA wearing earmuffs with NRR 25: effective exposure = 100 − (25 − 7) / 2 = 100 − 9 = 91 dBA.

NIOSH derating method is more aggressive: multiply the NRR by 0.75 for earmuffs, 0.50 for formable (foam) earplugs, and 0.30 for all other earplugs. The NIOSH approach better reflects real-world attenuation, especially for earplugs that are frequently inserted incorrectly.

For high-noise environments above 100 dBA, consider dual protection, meaning earplugs plus earmuffs worn simultaneously. The combined NRR is approximately the higher-rated device's NRR plus 5 dB (not the sum of both NRRs). Dual protection is essential for workers near impact tools, large presses, and jet engine maintenance.