OSHA Noise Exposure Chart — Permissible Duration by dBA

What This Chart Shows

Under OSHA’s occupational noise framework, the permissible exposure limit is 90 dBA as an 8-hour TWA, using a 5 dB exchange rate. That means every 5 dB increase halves the permissible duration at a constant level. The table below is the classic duration-by-level reference (Table G-16 style) for steady noise at a single level.

Real shops are not single-level. Grinders, plasma, and compressors create varying dose — use the noise exposure calculator for combinations. Background: noise exposure guide.

OSHA Permissible Exposure Duration by dBA

Levels above the table’s upper end require specialized assessment; impulsive/impact noise has additional considerations under the standard.

Action Level Reminder

The 85 dBA 8-hour TWA action level triggers hearing conservation (monitoring, audiometric testing, training, HPDs) even when the PEL is not exceeded. Many fab shops sit between 85 and 90 dBA TWA once grinding is averaged in.

How to Use the Chart on the Floor

1. Measure representative dBA for the task (or use known equipment data).
2. If exposure is essentially continuous at one level, read allowed hours from the table.
3. If levels change, compute dose/TWA with the calculator — do not average dB arithmetically.
4. Select hearing protectors per your program and the PPE selection guide by process.

Dose — Why You Cannot Average Decibels

If a welder spends 2 hours at 95 dBA and 6 hours at 85 dBA, you do not average to 87.5 dBA. Sound energy (and OSHA dose) is logarithmic. Each segment contributes a fraction of the daily allowable dose; those fractions add. Two hours at 95 dBA alone is already 50% of the daily dose under the 5 dB table (2 hours is half of the 4-hour allowance at 95). The quieter hours still add more dose on top.

That is why a “quick grind” next to an 8-hour quiet assembly bench can push a worker over the action level even when the daily average “feels” moderate. Document tasks, use a dosimeter when the program requires it, and treat intermittent high-level work as the controlling exposure.

Typical Fab-Shop Sound Levels (Awareness Only)

Ranges vary with distance, enclosure, and tool condition. Use them to decide when to pull a meter — not as compliance measurements.

Hearing Protection and the Chart

The duration table assumes unprotected exposure. When hearing protectors are required, your program still uses measured or estimated levels to select NRR and to decide double protection. Do not “extend” Table G-16 times by subtracting NRR casually without following the employer’s derating method. For process-based PPE pairing (grinding vs welding vs machining), use the PPE selection guide.

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