NRR = Not Really Relevant?
EPA is reconsidering hearing protector evaluation.
- By Lee D. Hager
- Sep 01, 2003
ADOPTION of the OSHA Hearing Conservation Amendment (29 CFR 1910.95) in the early 1980s changed forever the way employers protect workers from noise. Instead of a single focus on noise reduction to prevent hearing loss, employers were permitted to substitute the hearing conservation paradigm we know today. A hearing conservation program consisting of noise monitoring, hearing tests, hearing protection, training, and recordkeeping was assumed to provide equal protection from noise as would noise control, but at a significantly reduced cost to employers.
This change put great emphasis on hearing protection devices (HPDs) as, in practice, the first, last, and only line of defense against work-related noise-induced hearing loss. The noise reduction rating (NRR), a performance estimate of the protection offered by HPD, became part of the everyday language of the safety professional.
Although the NRR is used by OSHA to assess compliance with the Hearing Conservation Amendment, it is the administrative responsibility of the Environmental Protection Agency. EPA's Office of Noise Abatement and Control (ONAC) provides regulatory guidance in the evaluation of HPDs in 40 CFR 211, Subpart B.
Unfortunately, ONAC became a victim of governmental "rightsizing." It was maintained on EPA organizational charts but has not been allocated operating funds or staff by Congress since the early 1980s. The regulation has essentially been frozen in time for more than 20 years because there have been no resources at EPA to consider changes and updates.
EPA took the first steps toward revision of this outdated regulation in late March 2003 when it hosted a workshop in Washington, D.C., to look at new issues, technologies, and test procedures to bring 40 CFR 211 up to date.
The EPA workshop attracted a range of researchers and hearing protection manufacturers to address some fundamental questions:
- What procedure and standards should be used to evaluate HPDs? Are there new ways to evaluate HPDs that could provide better, more accurate information?
- How should attenuation or protection values be communicated? What should the label on the package say?
- Should the evaluation be a single number, or should it reflect a range of attenuation values or a categorization such as Class A for high noise, Class B for moderate noise, etc.?
- How should the regulation deal with new technology--such as electronic or sound restoration HPDs, active noise-cancellation HPDs, HPDs with variable attenuation, or new technologies in field evaluation of HPD attenuation--technologies that were not even invented when the rule was last revised?
- Can or should the United States harmonize with other governments on a global standard for HPD evaluation?
ANSI S3.19-1974, Method for the Measurement of Real-Ear Protection of Hearing Protectors and Physical Attenuation of Earmuffs, is the standard currently used and cited in 40 CFR 211. It describes a laboratory test procedure where test subjects are given hearing tests, then are tested again with the HPD in place. The difference between how they hear with and without the HPD, with some statistical corrections thrown in, should be a good reflection of how much protection the devices provide.
The difficulty is that when these S3.19 results are compared to field studies investigating how well the devices work in practice, there is a big discrepancy. The devices simply do not perform in the real world like they do in the lab. One reason for this is that in the lab, the devices are tested using "experimenter fit." The experimenter running the test inserts the devices in the test subjects' ears for optimal short-term performance, with no regard for long-term use or comfort. With experience, the experimenters have devised legitimate procedures to maximize the test results, resulting in accurate measurement of the capability of the devices but with little relationship to how they are used in practice.
New ANSI procedures have been developed with an eye to making lab results more reflective of field use. ANSI S12.6-1997(R2002), Methods for Measuring the Real-Ear Attenuation of Hearing Protectors, includes a subject-fit procedure called "Method B" in addition to an updated version of the experimenter-fit process described above. "Subject-fit" means the experimenter's interaction with the test subject is strictly limited. Test subjects are handed the HPD with the instructions supplied by the manufacturer, they insert or place the protectors themselves, and the devices are evaluated.
The table here shows a comparison between S3.19-1974, S12.6-1996, and a compilation of field studies on a range of commercially available HPDs. While Method B evaluation results are clearly closer to field results, there is still no agreement between the field studies and either of the lab protocols.
The NRR Today
HPD makers feel great pressure in today's market to develop devices with higher and higher NRR values. In part, the NRR is used as a sales tool, with the message being that "bigger is better." This is clearly not the case because 90 percent of noise exposures in U.S. industry are less than 95 dB TWA; all that is really required is about 10 to 15 dB of consistent, reliable protection.
Another part of the pressure for high NRR comes from OSHA's enforcement stance. The agency's response to the lab-to-field study comparison shown in the chart above was to put a derating system in place, where "credit" allowed for the labeled NRR was reduced to address the uncertainty reflected in their field performance in hearing conservation program application. In practice, a hearing protector with a lab NRR of 31 gets "credit" for 12 dB of protection from OSHA--the 31 from the lab, minus a 7 dB correction for "spectral uncertainty," then cutting what's left in half. NIOSH suggests a derating system in its 1998 Criteria Document, as well, with the amount of penalty varying based on the type of protector: ear muffs get a 25 percent penalty, formable or expanding foam style ear plugs 50 percent, and all others 70 percent from the labeled value.
Why the Uncertainty?
While the field studies provide useful information quantifying how well HPDs are performing for noise-exposed workers, there has been little research into the reasons for the discrepancy between the lab results and "real world" findings. Given that most workers need about 10-15 dB of protection, do they "cheat" their 32 NRR HPD to avoid overprotection and to help improve communication and warning signal detection?
Ergonomics is another significant issue. Many devices require substantial manipulation to ensure proper insertion--rolling or squeezing a device into a particular shape before placing it in the ear. If the device is not properly prepared before insertion, it may not provide optimal attenuation. Comfort can be a significant barrier to proper use. While there is currently no objective measure of HPD comfort, this could be the single most important consideration for long-term, consistent, proper use.
These topics and many more were debated and discussed at length in Washington, and discussions continue. EPA has promised ongoing activity in this area with rulemaking yet to come, but reaching consensus may be difficult among the various interests. EPA has provided the presentations form the March meeting as well as an opportunity for public comment via its electronic docket process at www.epa.gov/edocket/, docket number OAR-2003-0024.
This article originally appeared in the September 2003 issue of Occupational Health & Safety.