NIOSH vs OSHA noise exposure limits

Under 29 CFR 1910.95, OSHA's permissible exposure limit, or PEL, is 90 dBA as an 8-hour time-weighted average, treated as a full, 100 percent noise dose. The action level, 85 dBA as an 8-hour time-weighted average, or a 50 percent dose, is the point at which OSHA requires an employer to put a hearing conservation program in place: exposure monitoring, annual audiometric testing, making hearing protection available, training, and recordkeeping.

OSHA's exchange rate is 5 dB, meaning permissible exposure time is cut in half for every 5 dB increase in level. At 95 dBA, workers may be exposed for 4 hours; at 100 dBA, 2 hours; at 105 dBA, 1 hour. OSHA also sets a hard ceiling: no exposure to continuous noise above 115 dBA for any duration, and a 140 dB peak sound pressure level limit for impulse or impact noise.

NIOSH, the National Institute for Occupational Safety and Health, recommends a lower exposure limit of 85 dBA as an 8-hour time-weighted average, the same number OSHA uses only as its action level, not its enforceable limit. NIOSH also uses a 3 dB exchange rate instead of OSHA's 5 dB, on the grounds that the 3 dB rate more closely matches how equal-energy noise dose actually damages hearing.

The two differences compound. Because NIOSH's reference level is already lower and its exchange rate is steeper, permissible durations fall off much faster as level rises. At 95 dBA, where OSHA allows 4 hours, NIOSH's formula allows roughly 47 minutes; the roughly five-fold difference at that one level is entirely a product of the 5 dB versus 3 dB exchange rate, not a disagreement about where hearing damage begins.

An exchange rate, sometimes called a trading ratio, describes how much a change in level is allowed to trade off against a change in duration for the same total noise dose. A 5 dB exchange rate assumes that doubling exposure time is equivalent to raising the level by 5 dB, while a 3 dB exchange rate assumes the same doubling of time is only equivalent to a 3 dB rise. Because 3 dB roughly corresponds to a doubling of actual sound energy while 5 dB does not, NIOSH's rate is generally considered the more physically accurate representation of how noise dose accumulates, which is why NIOSH permits so much less time at any given level above its 85 dBA reference than OSHA's regulation does.

OSHA's 1910.95 remains the enforceable regulation for most workplaces in the United States; NIOSH's recommendation carries no independent enforcement authority. A hearing conservation program can, and many do, choose to follow NIOSH's stricter recommendation voluntarily even where OSHA's regulation is the only legal requirement.