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The NRR: What's In a Number?
When OSHA promulgated its Hearing Conservation Amendment for general industry in the early 1980s, it incorporated the Environmental Protection Agency's noise reduction rating (NRR) system for assessing hearing protector performance. The NRR is a laboratory-derived estimate of the attenuation (sound reduction) that is to be expected from a hearing protector. Shortly after the Hearing Conservation Amendment went into effect, however, it became evident that the actual amount of attenuation achieved in the workplace often fell short of the laboratory-predicted NRR.
Soon after the Hearing Conservation Amendment went into effect, OSHA issued a controversial noise control guideline intended to clarify employer responsibilities for reducing noise given the new Amendment. This compliance directive, CPL 02-02-035, Appendix A, instructs companies to take into account certain factors such as hearing protection, shifts in hearing, cost of controls, etc. when comparing the relative effectiveness of hearing protectors and engineering and/or administrative controls. In this situation, OSHA instructs employers to “apply a safety factor of 50 percent” to the NRR. Although the noise control compliance directive does not pertain to hearing protector requirements under the Hearing Conservation Amendment, OSHA strongly recommends applying a 50% correction factor to the NRR for hearing conservation purposes as well (OSHA Technical Manual Appendix IV:C. Methods for Estimating HPD Attenuation). It’s interesting to note that when MSHA promulgated its latest Occupational Noise Exposure Standard in 1999, Part 62, it did not limit mining operators to use of the NRR. Instead, MSHA took a much more flexible approach, allowing hearing protectors to be assessed according to "a scientifically accepted indicator of noise reduction value" (http://www.msha.gov/30cfr/62.0.htm).
Alternatives to the NRR and Other Laboratory Ratings
Most recently, in 2008, an alliance between OSHA, NIOSH and the National Hearing Conservation Association identified an alternative to basing hearing protection decisions on the NRR or any other laboratory rating. This group of experts recognized the limitations to relying on laboratory conditions and group statistics to predict an individual user’s hearing protector performance in the field. “The consequence of this approach is that an individual user may actually receive more but usually less attenuation than is stated on the hearing protector label.” Based on their review of research and emerging trends and technologies, the Alliance identified Individual Fit Testing as a recommended best practice for hearing conservation programs. Individual fit testing of hearing protectors is similar in concept to fit testing for respirators. Preferred methods produce a single number overall estimate of real-world attenuation for each worker; this measure is generally referred to as a Personal Attenuation Rating or PAR.
Working Through the Math
Following is an illustration of how to assess hearing protector attenuation using the laboratory-derived Noise Reduction Rating (NRR) compared to a Personal Attenuation Rating (PAR) obtained from Individual Fit Testing for a given worker. The end goal of either calculation is to ensure that the worker’s protected noise exposure has been reduced to a "safe" level, in most cases 85 dBA TWA or below.
NRR Method: Estimate worker’s protected noise exposure using the laboratory-derived noise reduction rating (NRR):
- Determine the employee’s workplace noise exposure in dBA TWA*
- Calculate the estimated "real world" noise reduction as follows:
- Start with the manufacturer's NRR for this hearing protector (NRR)
- Subtract 7 dB (if using A-weighted noise exposure values*)
- Divide in half to estimate “real world” performance
- Subtract the real world noise reduction estimate from the employee’s workplace noise exposure to approximate protected exposure:
Workplace Exposure - [(NRR - 7) ÷ 2] = Estimated Protected Exposure
PAR Method: Estimate protected noise exposure using the worker's personal attenuation rating (PAR):
- Determine the employee’s workplace noise exposure in dBA TWA
- Establish the employee’s personal attenuation rating (PAR) from individual fit testing (no need for real-world adjustments):
- Subtract PAR from the employee's workplace noise exposure to approximate protected exposure:
Workplace Exposure - PAR = Estimated Protected Exposure
*Note: If workplace noise exposure readings are available in C-weighted form (dBC TWA), there is no need to deduct 7 dB when estimating protection using the product’s NRR.
In our examples, we arrive at vastly different estimates for the same employee and same hearing protector based solely on calculation method. The NRR method suggests the hearing protector would barely achieve the target protected goal of 85 dBA TWA. The PAR Method, however, based on the worker’s own field performance indicates that protected noise exposure is well within the target goal when the hearing protector is worn correctly.
A Special Case: Dual Hearing Protection
Although most noise exposures encountered in industry today can be adequately addressed with a well-fitted earplug or earmuff, some exposures exceed the capabilities of traditional hearing protectors. When intensely loud noise cannot be controlled at the source, it may be necessary for workers to wear both earplugs and earmuffs at the same time, often referred to as “dual hearing protection” or “double hearing protection.”
Attentuation Isn't Everything
It’s true that your first step in accomplishing successful hearing protection is to identify a number of quality earplugs and earmuffs with noise reduction capabilities sufficient for the work environment. But there’s more to hearing protection than attenuation, whether measured via NRR or PAR. Employees must have the opportunity to select from a suitable variety of devices; comfort and personal preferences often prevail. Lastly, workers must be individually fitted and thoroughly trained in the proper fit, care and use of the protector.
Assistance is a click away
To help you with hearing protector attenuation estimates for your workforce, we have created a quick and easy calculator for comparing NRR and PAR values for both single and dual configurations. Request CavCom's free calculator. And contact CavCom to learn more about how our innovative products and systems can improve safety and communication in your high noise and respirator environments.