About Noise Exposure Calculator (OSHA & NIOSH Dose / TWA)
The noise exposure calculator turns a work shift broken into periods of different sound levels into a daily noise dose and an 8-hour time-weighted average (TWA) sound level, under either of the two U.S. occupational limits: the OSHA standard of 29 CFR 1910.95, which uses a 90 dBA criterion level with a 5-dB exchange rate, or the NIOSH recommended exposure limit from Publication 98-126, which uses an 85 dBA criterion with a 3-dB exchange rate. Each period contributes the fraction C/T of the daily limit, where C is the time spent at a level and T is the reference duration allowed at that level; the dose is 100 times the sum of the fractions.
The result is graded against the regulation text: under OSHA, a dose of 50% (TWA 85 dBA) is the action level that triggers a hearing conservation program per 1910.95(c), and a dose over 100% exceeds the Table G-16 permissible exposure limit of 90 dBA for 8 hours; under NIOSH, the daily dose "shall not equal or exceed 100", so a dose of 100% or more means the 85-dBA REL is reached or exceeded. Note one difference from a real dosimeter: OSHA monitoring integrates sound levels from 80 to 130 dBA into the measurement (1910.95(d)(2)(i)), while this tool counts every period you enter, whatever its level.
How It Works
- Choose the standard: OSHA (29 CFR 1910.95 -- 90 dBA criterion, 5-dB exchange rate, the enforceable U.S. limit) or NIOSH (Publication 98-126 -- 85 dBA REL, 3-dB exchange rate, the health-based recommendation).
- Enter each exposure period as an A-weighted sound level (40-140 dBA, slow response) and its duration in hours. Up to 12 periods, totalling at most 24 hours.
- The tool computes each period's reference duration T = 8/2^((L-90)/5) hours for OSHA or T = 8/2^((L-85)/3) for NIOSH, and its dose fraction C/T.
- The daily dose is D = 100(C1/T1 + C2/T2 + ... + Cn/Tn) percent, converted to an 8-hour TWA by TWA = 16.61 log10(D/100) + 90 (OSHA App. A) or TWA = 10.0 log10(D/100) + 85 (NIOSH Sec. 1.1.3).
- The outcome badge grades the dose: below the OSHA action level (<50%), hearing conservation program required (50-100%), over the OSHA PEL (>100%); or below/at-or-above the NIOSH REL (100%). A per-period table lists the allowed time at each level.
Worked Example
A worker spends 2 hours at 90 dBA and 2 hours at 95 dBA, assessed under OSHA 1910.95. At 90 dBA the reference duration is 8 hours, so the first period contributes 2/8 = 25% of the daily dose; at 95 dBA the reference duration is 4 hours, so the second contributes 2/4 = 50%. The dose is D = 100(0.25 + 0.50) = 75%, and the TWA is 16.61 log10(75/100) + 90 = 87.9 dBA. That is below the 100% PEL but at or above the 50% action level, so a hearing conservation program is required per 1910.95(c).
Formulas
- Reference duration (allowed time at a level)
T = 8 / 2^((L - Lc) / ER)- Daily noise dose (OSHA 1910.95 App. A / NIOSH Sec. 1.1.3)
D = 100 (C1/T1 + C2/T2 + ... + Cn/Tn)- OSHA 8-hour TWA from dose (App. A Sec. II)
TWA = 16.61 log10(D/100) + 90- NIOSH 8-hour TWA from dose (Pub. 98-126 Sec. 1.1.3)
TWA = 10.0 log10(D/100) + 85
Standards & References
- OSHA 29 CFR 1910.95 -- occupational noise exposure; Table G-16 PEL (90 dBA, 8 h)
- OSHA 1910.95 App. A (mandatory) -- noise exposure computation; Table G-16a
- OSHA 1910.95(a)(2)/(c) -- action level (TWA 85 dBA / 50% dose) and hearing conservation program
- NIOSH Publication 98-126 -- Criteria for a Recommended Standard: Occupational Noise Exposure (85 dBA REL, 3-dB exchange rate)
Frequently Asked Questions
What is the difference between the OSHA and NIOSH noise limits?
OSHA 1910.95 is the enforceable U.S. legal limit: a permissible exposure limit of 90 dBA as an 8-hour TWA with a 5-dB exchange rate (every 5 dB halves the allowed time). NIOSH Publication 98-126 is the health-based recommendation: an 85 dBA REL with a 3-dB exchange rate, which matches how sound energy actually doubles and is the international consensus. The same shift can pass OSHA and exceed NIOSH -- 88 dBA for 4 hours is a 100% NIOSH dose but only about a 38% OSHA dose.
What is a noise dose and how does it relate to TWA?
The dose expresses a whole shift as a percentage of the allowed daily exposure: each period at level L for C hours contributes C/T of the limit, where T is the allowed duration at L, and the dose is 100 times the sum. The TWA converts the dose back into a single steady sound level over 8 hours: 100% dose equals a TWA at the criterion level (90 dBA OSHA, 85 dBA NIOSH), 50% OSHA dose equals a TWA of 85 dBA, and 200% OSHA dose equals 95 dBA.
What is the OSHA action level and what does it trigger?
OSHA 1910.95(a)(2) defines the action level as an 8-hour TWA of 85 decibels, equivalently a dose of fifty percent. At or above it, 1910.95(c) requires the employer to administer a continuing, effective hearing conservation program: noise monitoring, annual audiometric testing, hearing protectors made available, training, and recordkeeping. Above a 100% dose (TWA over 90 dBA), feasible engineering and administrative controls plus mandatory hearing protection are required.
How long can a worker be exposed at a given sound level?
Under OSHA the allowed time is T = 8/2^((L-90)/5) hours: 8 h at 90 dBA, 4 h at 95, 2 h at 100, 1 h at 105, and 15 min at 115 dBA. Under NIOSH it is T = 8/2^((L-85)/3): 8 h at 85 dBA, 4 h at 88, 2 h at 91, 1 h at 94 -- and only about 28 seconds at 115 dBA. The tool's per-period table lists the allowed time for every level you enter.
Why does a dosimeter reading differ from this calculator?
A dosimeter integrates the actual time-varying level over the whole shift, while this tool models the shift as a few constant-level periods. OSHA monitoring also has a defined integration range: 1910.95(d)(2)(i) requires all continuous, intermittent, and impulsive sound levels from 80 to 130 decibels to be integrated into the measurement, whereas this calculator counts every period you enter regardless of level. For compliance decisions, use measured dosimetry; use this tool for planning, task budgeting, and understanding the rules.
Does the NIOSH TWA formula use 10.0 or 9.97 as the coefficient?
NIOSH Publication 98-126 prints TWA = 10.0 x log10(D/100) + 85 in Section 1.1.3 and in its glossary, and this tool encodes exactly that. The mathematically exact coefficient for a 3-dB exchange rate is 3/log10(2) = 9.966, often quoted as 9.97, and some dosimeter standards use it; the difference is at most a few hundredths of a decibel for ordinary doses. OSHA's 16.61 is the analogous value for the 5-dB exchange rate (5/log10(2) = 16.61).