How this instrument works
Both major US noise standards work the same way in principle: pick a reference level and duration, then halve the permitted exposure time for every fixed jump in decibels above it — an exchange rate. Where they differ is how aggressively they halve. OSHA references 90 dBA for 8 hours and uses a 5 dB exchange rate, halving the allowed time every 5 dB louder. NIOSH references 85 dBA for 8 hours and uses a 3 dB exchange rate, halving every 3 dB — a noticeably faster falloff for the same rise in loudness.
That 3 dB figure is not an arbitrary tightening. Sound energy actually doubles with every 3 dB increase — it is the physically accurate exchange rate for how acoustic energy scales, which is what actually drives damage to the inner ear over time. OSHA's 5 dB rate is more permissive than the underlying physics would suggest, a legacy of decisions made when the original standard was set decades ago, but it remains the legally enforceable limit for US workplaces regardless of what NIOSH considers more scientifically accurate.
NIOSH is a research and recommending body, not a regulator — its 85 dBA, 3 dB standard is a recommended exposure limit, not a legal one. OSHA is the enforcement agency, and 29 CFR 1910.95 is the actual regulation employers must follow. The gap between the two exists because OSHA's rule has not been updated to match current acoustic science, even though NIOSH published its more conservative recommendation decades ago.
- Enter the Noise level in dBA, as measured by a sound level meter at the location in question.
- Choose the Standard — OSHA (90 dBA reference, 5 dB exchange) or NIOSH (85 dBA reference, 3 dB exchange).
- Read the permissible exposure time in hours; the working block shows the halving calculation.
- Use OSHA's figure for legal compliance and NIOSH's figure for a more conservative, hearing-protective target.
Worked example — 95 dBA under both standards
At 95 dBA under OSHA's rule: 95 is 5 dB above the 90 dBA reference, one full exchange-rate step, so the permitted time is 8 ÷ 2^1 = 4 hours. Push the same OSHA rule to 100 dBA — two steps of 5 dB above reference — and the allowed time halves again: 8 ÷ 2^2 = 2 hours.
Now run that identical 95 dBA through NIOSH's rule instead: 95 is 10 dB above the 85 dBA reference, which is 10 ÷ 3 = 3.33 exchange-rate steps, so the permitted time is 8 ÷ 2^3.33 ≈ 0.794 hours — under 48 minutes, less than a fifth of what OSHA's rule allows at the identical noise level. The same 95 dBA reading produces a legally compliant workday under one rule and a rule-breaking one within under an hour under the other.
Questions
Why do OSHA and NIOSH allow such different exposure times at the same noise level?
Because they use different exchange rates for the same jump in decibels. OSHA halves the permitted time every 5 dB above its 90 dBA reference; NIOSH halves it every 3 dB above its 85 dBA reference. The 3 dB rate tracks how sound energy physically doubles, which NIOSH treats as the scientifically accurate basis for hearing damage. OSHA's 5 dB rate is more permissive and reflects decisions made when the regulation was originally written, not an update to match current acoustic research.
Which standard is legally binding in US workplaces?
OSHA's, under 29 CFR 1910.95. NIOSH is a research and recommendation body within CDC — its 85 dBA, 3 dB exchange-rate limit is a recommended exposure limit intended to better protect hearing, but employers are only legally required to meet OSHA's more permissive 90 dBA, 5 dB rule.
What does the exchange rate actually mean in practice?
It is the decibel increase that cuts the permitted exposure time in half. A 5 dB exchange rate (OSHA) means every 5 dB louder halves the allowed hours; a 3 dB exchange rate (NIOSH) does the same at a faster pace, since 3 dB corresponds to sound energy actually doubling, a shorter step than OSHA's rule uses for the same halving.
Why is NIOSH's exchange rate considered more scientifically accurate?
Because 3 dB is the point where sound energy itself doubles, which is the physical quantity actually driving damage to the structures of the inner ear over cumulative exposure. OSHA's 5 dB rate is based on older assumptions about how loudness, rather than raw acoustic energy, translates into risk, which is why NIOSH's more conservative figure is generally viewed as the better predictor of hearing damage even though it carries no legal force on its own.
Where do these two standards come from?
OSHA's rule is published as 29 CFR 1910.95, Occupational Noise Exposure, the enforceable federal regulation. NIOSH's rule comes from its 1998 publication, 'Criteria for a Recommended Standard: Occupational Noise Exposure,' DHHS/NIOSH Publication No. 98-126, which reaffirmed and updated the 85 dBA, 3 dB recommendation after reviewing the underlying research.
References
- OSHA — 29 CFR 1910.95, Occupational Noise Exposure
- NIOSH — Criteria for a Recommended Standard: Occupational Noise Exposure, Revised Criteria 1998 (Publication No. 98-126)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.