More realistic NRR ratings
June 1, 2008
Safety professionals selecting hearing protection devices (HPDs) have long enjoyed the apparent simplicity of a single number which could be used to differentiate products. The noise reduction rating, or NRR, was required by law to be published on the package, even though application of that number in noisy workplaces has proven to be challenging.
How can a hearing conservation program manager navigate the numbers game?
Pending changes in standards and regulations should clarify the process, the performance expectations of HPD and selection of appropriate HPD in the real noisy world in which we live and work.
Role of EPA
EPA has held legal responsibility for methods for testing and labeling hearing protectors since the early 1970s. The current HPD rating and labeling rule (40 CFR 211 Subpart B) has been in place since the early 1980s. However, the NRR HPD evaluation and labeling process mandated by the EPA has not succeeded in estimating how well HPD works for people in noise environments. This has resulted in OSHA requiring the “derating” indicated above when comparing the effect of HPD to noise control in hearing conservation programs.
In March 2003, the EPA began revising the HPD labeling rule. Revisions to key ANSI standards are forthcoming.
Getting and using the numbers
Two new or pending ANSI standards are behind the changes. HPD evaluations in the lab will change with the completion of revision to ANSI S12.6-1997 (American National Standard Methods for Measuring the Real-Ear Attenuation of Hearing Protectors). The revision is expected later this year.
ANSI S12.68-2007 (Methods of Estimating Effective A-Weighted Sound Pressure Levels When Hearing Protectors Are Worn), released in late 2007, reflects a way to use the information collected from the laboratory analysis. Put simply, S12.6 describes how to get the HPD attenuation values, and S12.68 describes what to do with the numbers.
Testing and labeling
It’s likely that changes in these key ANSI standards will lead to significant changes in how the EPA manages HPD testing and labeling:
- More realistic ratings: Efforts have been made in the standards development process for lab evaluations to better reflect real performance to reduce the likelihood of a need for derating. EPA representatives have indicated that a key reason to undertake the time-consuming regulatory process is to make the labeled HPD attenuation values more reliable, and thus less subject to after-the-fact derating. While this is a useful step forward, the variability inherent in individual attenuation values versus group-averaged data still makes individual fit testing of HPD an important consideration.
- Usable with A-weighted exposure measurements: EPA designed the original NRR to be used with C-weighted noise exposure measurements. OSHA requires, and most people collect, A-weighted noise measurements instead. The new S12.68 calculations result in protection values designed to be subtracted directly from A-weighted noise exposure levels, making them easier to use.
- Range of values: Even in the carefully controlled environment of the test lab, individuals show significant variability in HPD performance. The HPD label should reflect the expected variability in performance from person to person to lead users away from reliance on a single protection number. S12.68 addresses this by providing an expected range of performance in place of the single number based on the amount of protection that the top 20 percent and the top 80 percent of users would be expected to achieve. (See Figure 1).
- Suitable for all noise exposures: EPA has historically used a broadband “pink” noise as a surrogate for occupational noise exposure in the current NRR. The new standard takes a different approach, as S12.68 develops calculations based on how an HPD performs in a range of 100 common industrial noises collected by NIOSH. There is a secondary, more complex graphical rating in the standard to address atypical noise exposures with substantial low-frequency energy (such as boomy bass-heavy sounds). This is accomplished by looking at the difference between C- and A-weighted noise measurements, indicating how much low-frequency energy is in the exposure. This step makes sure that the S12.68 calculations are as appropriate and applicable to the real world as possible.
Revisions to S12.6, EPA rulemaking and the new S12.68 are important steps forward in helping to manage hearing loss prevention efforts. By steering away from a potentially misleading single number rating, S12.68 clearly indicates that a range of protection is to be expected.
Variability in HPD performance indicates that individual fit testing of HPD is important in order to understand how individual users are being protected against noise. In lieu of individual fit testing, however, estimation of HPD performance in the range demonstrated by the 80/20 spread will give a reasonable indication of the range of protection to be expected.
As EPA addresses its HPD labeling regulation, it is expected to adopt some of the procedures from S12.68 and from the pending revision to S12.6. EPA is required by law to manage the labeling of HPD, and is required to use the term “noise reduction rating” and “NRR” in its regulation. There will likely be a new label on HPD with new values (still called NRR) proposed by the EPA in the rulemaking process, perhaps as soon as early 2009.
The EPA is expected to hold a public hearing to officially continue the rulemaking process on HPD labeling sometime in the first half of 2008. Watch the EPA docket for more information at www.epa.gov/edocket/, docket number OAR-2003-0024 and stay tuned â€” HPD selection is about to become significantly easier and more realistic.