Written and maintained by CASRAI Editorial Board
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General product and regulatory information for laboratory and clinic staff; it is not medical advice and does not assess any individual’s hearing.
Short answer
The noise reduction rating (NRR) is the single decibel number printed on a hearing protector’s label. EPA’s labeling rule defines it as a single-number noise reduction factor, and the label ties it to use “as directed.” OSHA’s hearing conservation appendix uses it with a specific subtraction rule, which differs depending on whether the noise was measured C-weighted or A-weighted.
Definition and scope
Under 40 CFR 211.203, the NRR is “a single number noise reduction factor in decibels, determined by an empirically derived technique.” The label requirements are in 40 CFR Part 211, Subpart B, the EPA rule on hearing protector labeling. The label itself is regulated: the NRR shown in Area B cannot exceed the value computed under the rule’s calculation section, and the label carries the qualifier “(When used as directed).”
The same label section describes a range statement: “The range of Noise Reduction Ratings for existing hearing protectors is approximately 0 to 30,” followed by “(higher numbers denote greater effectiveness).” For headband devices that can be worn in more than one position, the label must show the worst-case NRR and identify the position or positions it applies to.
How the label number is derived
The EPA rule approves the test method in “American National Standards Institute Standard (ANSI STD) S3.19-1974.” The computation then works from the measured results. Figure 2, step 9 of the rule gives the final step: NRR = Step 3 minus Step 8 minus 3 dB, and values ending in .5 are rounded to the next lower whole number. The rule explains the extra 3 dB as an allowance for spectral uncertainty: an additional three decibel reduction is included when computing the NRR.
Two consequences follow directly from the regulation. First, the number is a laboratory-method figure by design, produced by a defined test procedure rather than measured in a particular workplace. Second, the label’s own wording (“when used as directed”) signals that the value assumes the protector is worn according to its instructions.
How OSHA converts an NRR into an estimated exposure
Appendix B to 29 CFR 1910.95 (“Methods for Estimating the Adequacy of Hearing Protector Attenuation”) tells employers how to use the NRR. The appendix says that hearing protector attenuation must be sufficient to reduce employee exposure to a time-weighted average (TWA) of 85 dB, and that calculated attenuation values reflect realistic values only to the extent that the protectors are properly fitted and worn. The arithmetic depends on how the noise was measured.
| How the noise was measured | What Appendix B says to do with the NRR |
|---|---|
| C-weighted dose, sound level or area level | Subtract the NRR from the C-weighted figure to obtain the estimated A-weighted TWA under the ear protector. |
| A-weighted dosimeter, sound level meter or area level (no C-weighting available) | Subtract 7 dB from the NRR, then subtract the remainder from the A-weighted figure. |
Worked illustration (hypothetical numbers)
Suppose a protector’s label shows an NRR of 25. These figures are invented to show the arithmetic only; they do not describe a real product or workplace.
- C-weighted path: a C-weighted TWA of 100 dB minus the NRR of 25 gives an estimated A-weighted TWA under the protector of 75 dB.
- A-weighted path: an A-weighted TWA of 95 dB. Subtract 7 from the NRR (25 – 7 = 18), then subtract that remainder from 95 to get 77 dB.
The 7 dB is subtracted from the NRR only on the A-weighted path. Applying it on the C-weighted path, or skipping it on the A-weighted path, produces a different answer from the appendix.
Which rule is which
| Question | Where the answer sits |
|---|---|
| What does the number on the package mean, and how must it be labeled? | EPA, 40 CFR Part 211, Subpart B (labeling). |
| How is the label number computed? | 40 CFR 211, referencing ANSI S3.19-1974 and the step 3 – step 8 – 3 dB computation. |
| How does an employer estimate exposure under the protector? | OSHA, 29 CFR 1910.95 Appendix B (C-weighted and A-weighted methods above). |
Keeping these separate matters because they answer different questions. The EPA rule governs what the label says; the OSHA appendix governs a calculation. Appendix B does not state a percentage derating of the label value; it applies the subtractions shown above.
Earmuffs and earplugs
The labeling rule is written for hearing protectors generally, and it contains specific wording for headband devices (the worst-case position requirement above). The OSHA appendix refers to “the NRR” without distinguishing a protector type. For a product-specific figure, the label on that product is the reference point.
Where it matters for labs, clinics and supply decisions
Under 29 CFR 1910.95(c)(1), a hearing conservation program is required whenever employee noise exposures equal or exceed an 8-hour TWA of 85 decibels. When a protector is chosen against a measured exposure, the Appendix B arithmetic above is the OSHA-published method for estimating the level under it, and the label NRR is its input. For how protective equipment fits into a written hazard assessment, see PPE hazard assessment and written certification, and for the general category see what lab PPE is. Clinic supply planning for hearing-related services is covered in the audiology clinic supply checklist and the occupational medicine clinic supply checklist. Exposure limits for laboratory agents are compared in PELs, TLVs and RELs.
Common misunderstandings
“The NRR is how many decibels the room drops by.”
Not directly. The appendix calculation converts the NRR into an estimated level under the protector, and that estimate depends on the measurement weighting. On the A-weighted path, 7 dB comes off the NRR first.
“Divide the NRR by two.”
This rule circulates widely, but Appendix B as published does not say to halve the NRR. Its text applies the subtract-7 step for A-weighted measurements and subtracts the NRR directly for C-weighted ones.
“A higher NRR guarantees protection.”
The label says “when used as directed,” and the OSHA appendix states that calculated values are realistic only to the extent the protector is properly fitted and worn.
FAQ
NRR rating explained: what is it in one line?
A single decibel number on the label, produced by the EPA-regulated computation (step 3 minus step 8 minus 3 dB) from an ANSI S3.19-1974 test, and valid “when used as directed.”
What is the hearing protection standard for NRR labeling?
EPA’s 40 CFR Part 211, Subpart B sets the label content and the computation, and it approves ANSI S3.19-1974 as the test method.
What does OSHA require for hearing protection?
Under 29 CFR 1910.95(j), hearing protectors must attenuate employee exposure at least to an 8-hour time-weighted average of 90 decibels, and to 85 decibels or below for employees who have had a standard threshold shift. Appendix B sets out the methods for estimating attenuation and states that attenuation must be sufficient to reduce exposure to a TWA of 85 dB. An 8-hour TWA of 85 decibels is also the exposure at which paragraph (c)(1) requires a hearing conservation program.
How is an earmuff NRR handled?
The EPA label rule covers hearing protectors including headband devices, which must show the worst-case NRR and the position it applies to. Appendix B refers to “the NRR” without distinguishing protector type, so the same subtraction rules apply to the number on an earmuff label.
Sources
- EPA hearing protector labeling, 40 CFR Part 211, Subpart B and 40 CFR 211.204-1, checked 2026-10-30.
- OSHA, 29 CFR 1910.95 Appendix B, checked 2026-10-30.
- OSHA, 29 CFR 1910.95, Occupational noise exposure (paragraphs (c)(1) and (j)), checked 2026-10-30.
