But where do these numbers come from, and what do they mean to todayâ€™s hearing conservationist?
The NRR legacyWhile HPD selection and administration are part of OSHAâ€™s mandate under 29CFR1910.95, the actual evaluation of the devices has long fallen under the jurisdiction of the Environmental Protection Agency. The Office of Noise Abatement and Control (ONAC) within EPA has held statutory responsibility for HPD evaluation since the late 1970s. Unfortunately, ONAC has been inactive due to lack of funding since the early 1980s â€” thus there has been no ability to consider changes to the hearing protector rule. The evaluation process is frozen in time, requiring HPD manufacturers to evaluate their products using an ANSI standard from 1974 (ANSI S3.19-1974).
The good news is that, while the EPA is locked into the 1974 standard, the standards community itself has continued to review, revise and develop HPD evaluation standards. The most recent revision, ANSI S12.6-1997, reflects updates to laboratory evaluation standards. It includes an updated version of the lab-based, experimenter-fit procedure called Method A, where HPD is tested with significant experimenter interface. It also provides a subject-fit procedure called Method B, where interaction between the test subject and the experimenter is severely limited, providing a better indication of how HPD will perform in practice.
All of the current procedures use real ear attenuation at threshold (REAT) procedures, where the test subject is administered hearing tests with and without the HPD in place. The difference with HPD in and out, with some statistical corrections, provides the basis for the NRR.
Lab & field differencesA series of field studies compiled by Elliott Berger of AEARO Co. indicates that HPD do not perform in the field like they do in the lab. There are likely a number of reasons for this:
In essence, a higher NRR does not necessarily mean more protection in the field. The NRR is simply unreliable to determine the amount of protection provided by HPD in actual practice.
Deratingâ€™s downsideWhen evaluating HPD for sufficiency (i.e. is there enough protection to reduce the workerâ€™s net exposure to 85 dB TWA or less?) using A-weighted noise measurements, OSHA simply corrects the NRR by 7 dB and does not derate. The 7 dB correction results from the NRR evaluation process being conducted using C-weighted noise. If C-weighted noise exposure measurements are available, the NRR can be used for this application at face value.
However, OSHA has another application of the NRR. Under CPL2-2.35A, an employer may balance the cost and efficacy of a hearing conservation program against noise reduction via engineering controls. For this application, assuming the use of A-weighted noise measurements, OSHA requires the 7 dB correction to be applied first, then the remainder NRR be divided in half. In practice, the â€œ50 percent deratingâ€ is commonly applied to all aspects of HPD administration, as managing NRR differently for two applications has proven complex and problematic.
The unforeseen downside of derating is that HPD manufacturers are under pressure to improve the NRR of their devices to keep up with competition. If only allowed â€œcreditâ€ for half the NRR, most buyers will default to selecting the device with the highest possible rating. This can result in most HPD providing too much attenuation for most noise exposures if the devices are used properly in the field. If they are not used properly, they donâ€™t perform in the field as they did in the lab, and the vicious cycle starts again.
NIOSH weighed in on the HPD evaluation process in its 1998 Criteria Document. They suggest a variable derating system to account for differences among types of HPD, with a 25 percent derating for earmuffs (which are harder to misuse); 50 percent for formable/foam earplugs; and 70 percent for all other earplugs. This complex approach to managing an HPD program can be simplified by using the new NIOSH Hearing Protector Compendium Web site (http://www.cdc.gov/niosh/topics/noise/). The compendium factors in the NIOSH derating system when selecting HPD by noise exposure.
Industryâ€™s responseMany HPD manufacturers are addressing these issues by making new classes of products. Examples include:
SIDEBAR: Keys to hearing loss prevention
- Appropriate selection of HPD for the noise environment;
- Consideration of worker ergonomics;
- Fitting HPD to individuals;
- Paying close attention to hearing conservation audiometric findings to detect trends of hearing loss early in the process.