Noise Dose and TWA Calculator

Noise dose is the percentage of a worker’s allowable daily noise exposure that they actually received. It is calculated by dividing the time spent at each sound level by the time allowed at that level and adding the fractions together. Under OSHA, a dose of 100 percent equals the permissible exposure limit of 90 dBA as an 8-hour time-weighted average, and a dose of 50 percent equals the 85 dBA action level.

Under OSHA’s 5 dB exchange rate, TWA = 16.61 x log10(D / 100) + 90, where D is the dose in percent. Under the NIOSH 3 dB exchange rate, TWA = 10 x log10(D / 100) + 85. Both formulas convert a dose from any shift length into the equivalent constant 8-hour exposure.

The OSHA action level is an 8-hour TWA of 85 dBA, which equals a dose of 50 percent. Employers must implement a hearing conservation program under 29 CFR 1910.95(c) for employees exposed at or above the action level, including exposure monitoring, annual audiometric testing, hearing protectors at no cost, annual training, and recordkeeping.

OSHA uses different integration thresholds for the two determinations. Sound levels below 90 dBA are not counted when comparing exposure to the PEL, while all sound levels from 80 dBA through 130 dBA are counted when determining whether the action level and hearing conservation program apply. Both use a 90 dBA criterion and a 5 dB exchange rate.

The NIOSH recommended exposure limit is 85 dBA as an 8-hour TWA with a 3 dB exchange rate, so allowed time is cut in half for every 3 dB increase instead of every 5 dB. It is more protective than the OSHA PEL and is widely used as a best-practice target, but it is a recommendation rather than an enforceable OSHA limit.

Appendix B to 1910.95 estimates protected exposure for A-weighted measurements by subtracting 7 dB from the Noise Reduction Rating (NRR) and subtracting the result from the TWA. Because real-world attenuation is usually lower than the laboratory rating, OSHA guidance applies an additional 50 percent derating when evaluating protector effectiveness. Hearing protection must not be used in place of feasible engineering or administrative controls when exposure exceeds the PEL.

Use it as a screening tool. It estimates dose from sound level meter readings and estimated task times, which is useful for planning and for spotting likely overexposures. OSHA requires monitoring that is representative of each employee’s exposure, and personal noise dosimetry is the most reliable way to measure exposure for workers who move between areas or tasks.