Noise Exposure Calculator

Noise exposure limits protect hearing. Two major standards exist: OSHA and NIOSH.

OSHA Permissible Exposure Limits (PEL): Uses a 5 dB exchange rate (doubling rate).

• 85 dB: 16 hours
• 90 dB: 8 hours (the reference level)
• 95 dB: 4 hours
• 100 dB: 2 hours
• 105 dB: 1 hour
• 110 dB: 30 minutes
• 115 dB: 15 minutes

NIOSH Recommended Exposure Limits (REL): Uses a 3 dB exchange rate (more conservative).

• 85 dB: 8 hours (the reference level)
• 88 dB: 4 hours
• 91 dB: 2 hours
• 94 dB: 1 hour
• 97 dB: 30 minutes
• 100 dB: 15 minutes

Dose formula:

• OSHA: Allowed hours = 8 / 2^((dB - 90) / 5)
• NIOSH: Allowed hours = 8 / 2^((dB - 85) / 3)
• Dose % = (Actual hours / Allowed hours) × 100

100% dose = maximum permitted exposure. Above 100% means hearing damage risk.

50% dose is the OSHA action level, not a safe harbour. A dose of 50% is an 8-hour time-weighted average of 85 dBA, and at that point 29 CFR 1910.95 requires the employer to run a hearing conservation program: noise monitoring, annual audiometric testing, training, and hearing protectors made available at no cost. Half the legal limit is where the paperwork starts, not where the risk ends. NIOSH puts its recommended limit at exactly that level for a reason.

Two ceilings the dose table does not cover.

• 115 dBA is the top of the OSHA table. No continuous exposure above it is permitted for any duration, however short the shift.
• 140 dB peak is the impulse limit under both OSHA and NIOSH. A single gunshot, a nail gun, a dropped steel plate: one event over 140 dB can cause immediate permanent damage with no dose accumulation at all. Dose arithmetic simply does not apply to impulse noise.

Converting a dose back to a TWA. Occupational records are kept as an 8-hour time-weighted average, not as a percentage, and the two convert exactly:

• OSHA (5 dB exchange): TWA = 16.61 × log10(dose ÷ 100) + 90
• NIOSH (3 dB exchange): TWA = 10 × log10(dose ÷ 100) + 85

A 100% dose is 90 dBA TWA under OSHA and 85 under NIOSH. A 50% dose is 85 dBA TWA under OSHA, which is the action level again.

Hearing protection is not worth its printed number. The Noise Reduction Rating (NRR) on the package comes from a laboratory fit that nobody achieves in the field. OSHA’s method for A-weighted measurements is to subtract 7 from the NRR first:

effective exposure = dBA - (NRR - 7)

and OSHA further recommends halving that result to estimate real-world performance. A 33 NRR earplug therefore gives (33 - 7) = 26 dB on paper and about 13 dB in practice. NIOSH derates more specifically: 25% off for earmuffs, 50% for formable foam plugs, 70% for other plugs. Fit matters far more than rating. A well-inserted cheap foam plug beats an expensive one worn half in, and doubling up plugs under muffs adds only about 5 dB over the better of the two, not the sum.

Noise adds logarithmically, and quiet time counts. Two identical machines together are 3 dB louder than one, not twice as loud. And a dose accrues across everything you do: if you spend four hours at 95 dBA and four at 85, the doses add rather than average, so the day is not “90 dBA average” in any useful sense. Working out a mixed day means computing each segment’s dose and summing them.

Why two standards that disagree? OSHA’s limits are a legal floor set decades ago and use a 5 dB exchange rate, meaning the allowed time halves for every 5 dB. NIOSH’s are health-based recommendations using a 3 dB exchange rate, which matches the actual physics: sound energy doubles every 3 dB, so 3 dB really should halve your safe time. The gap is why a workplace can be legally compliant under OSHA yet still risky by NIOSH’s stricter, more scientifically grounded view.

Decibels are logarithmic, and that’s the part people underestimate. Every 10 dB is ten times the sound energy and sounds roughly twice as loud, so 100 dB isn’t “a little more than” 90, it carries ten times the energy and burns through your daily dose ten times faster. A rock concert at 105 to 110 dB can hit your full daily limit in minutes. The damage is also cumulative and permanent: noise dose adds up across everything you do in a day, the mower, the commute, the earbuds, and the hair cells lost in the inner ear don’t grow back. That’s the real case for hearing protection, since you can’t undo it later.