The Noise Reduction Rating (NRR) printed on every earmuff and earplug is a lab number — measured in a controlled environment by trained subjects with perfect fit. In the field, with sweat, beard stubble, hard-hat interference, and the 11th hour of a shift, you do not get the lab number. OSHA knows this and prescribes a derating formula.
The OSHA formula (single protector)
For a single hearing protector worn alone:
Effective protection (dBA) = (NRR − 7) ÷ 2
That "− 7" accounts for the difference between OSHA's A-weighted measurement and NRR's C-weighted lab measurement. The "÷ 2" accounts for real-world fit variability.
Worked example
NRR 33 muff (highest available class):
- Lab: 33 dB reduction
- OSHA-derated: (33 − 7) ÷ 2 = 13 dB
So an 100 dB(A) saw becomes 87 dB(A) at the ear — still over OSHA's 85 dB action level for an 8-hour shift.
Dual protection
For plugs + muffs together, OSHA allows adding 5 dB to the higher of the two NRRs (NIOSH stops at 5 dB additional, regardless).
When NRR 26 is plenty
For impact tools (drills, chippers) where the noise floor is 92-95 dB(A), a derated NRR 26 plug gives you ~10 dB at-ear, dropping to 82-85 dB(A). Below action level. NRR 33 muffs are necessary for sustained 100+ dB(A) exposure (saws, planers, plasma cutters).