ALN about NRR - A Little Noise about Noise Reduction Ratings Presented at the 2007 “Tidewater LS AIHA Fall PDC” on Nov 16, 2007 by Christopher D. Jones, MS, CIH NAVMEDCEN Portsmouth, IH Department
BACKGROUND • Think back, way back, to your early career education in industrial hygiene or safety, to the time you first heard about "the hierarchy of controls"
BACKGROUND • Remember them? • Engineering controls; then • Administrative controls; and finally • Personal Protective Equipment • We are all pretty comfortable with this method of prioritization to respond to occupational stressors in our workplace when we are assessing personnel exposures to odds and ends
BACKGROUND • For some reason, it is second nature for us to recommend an engineering control for paint mist before we jump on a respirator ….
BACKGROUND • But we forget all about this when we are dealing with noise; put plugs in their ears or muffs on their head is often choice #1, not choice #3
BACKGROUND • If we put someone in a half mask respirator for protection against dust, what is the assigned protection factor for that mask/filter combination? • Ten - regardless of respirator manufacturer • If, on the other hand, you put earplugs inside the ear canal of an employee, what is the "assigned protection factor" for that plug? • It depends on who made the plug
WHAT'S AN NRR? • The EPA requires that HPDs be labeled with their noise reduction rating, the NRR; a single number rating of the hearing protection; the higher the NRR, the higher the attenuation for a specific ideal situation ("The Occupational Environment - Its Evaluation and Control", aka., the "White Book", AIHA, 1997) • Two kinds, A = the one on the box; and B = a subject/fit NRR, a special one, hard to find • Test frequencies = 125, 250, 500, 1000, 2000, 3150, 4000, 6300 and 8000 Hz
WHAT'S AN NRR? • The NRR doesn't provide an accurate estimate of the true attenuation provided to the wearer, because it can be affected by fit, insertion technique, and the noise level in the work environment • Since the NRR isn't a solid number to hang your hat on, how do you use it? • "Derate" it - adjust it in some fashion to better reflect a "real world" level of protection that the HPD provides
DERATING AN NRR - the OSHA way • Adjust or derate the NRR, then compare the new # to your measured noise exposure • In Appendix B of OSHA's general industry noise standard, 29 CFR 1910.95, OSHA recommends a method to derate the NRR to determine the adequacy of selected HPD: • Estimated Exposure (dBA) = TWA (dBA) - (NRR -7) • If using dBC values, Estimated dBA = TWA (dBC) - NRR • Example: TWA = 100 dBA; NRR = 19 • X = 100 - (19 - 7) = 88 dBA
DERATING AN NRR - the OSHA way • How do you derate double hearing protection? • OSHA says 5 dB can be added to the NRR of the most protective device • Example: Noise TWA = 100 dBA; NRR = 19 for a muff, 30 for a plug; the assigned NRR for this combo is 30 + 5 = 35 • Estimated Exposure (dBA) = TWA (dBA) - (NRR -7) • X = 100 - (35 - 7) = 72 dBA
DERATING AN NRR - the OSHA way, Part II • OSHA directs inspectors to take into consideration the relative performance of HPDs; derate the NRR by 50% (E.A.R Log 20, "The Naked Truth About NRRs", Cabot Safety, 1993; also found on OSHA's web site at www.osha.gov/dts/osta/otm/noise/hcp/ attenuation_estimation.html) • This derating has no relationship to the just mentioned 7 dB correction spelled out in 1910.95; this derating adjusts NRRs to "real world" performance, whereas the 7 dB correction adjusts the NRR to a dBA measurement
DERATING AN NRR - the OSHA way, Part II • So here's how OSHA inspectors derate HPDs to determine the relative performance of HPDs; they cut it in half! • Estimated Exposure (dBA) = TWA (dBA) - [(NRR -7) X 50%] • Example: TWA = 100 dBA; NRR = 19 • Estim. Exposure = 100 - [(19 - 7) X 50%] = 94 dBA • The relative performance of an NRR of 19 is 6
DERATING AN NRR - the NIOSH way • NIOSH has an even more exciting way to determine a "true" NRR; the NRR is adjusted by the type of protection you are getting ready to wear (see "Criteria for a Recommended Standard, Occupational Noise Exposure, Revised Criteria 1998 " NIOSH, 1998) • Earmuffs = subtract 25% from the MFR NRR • Formable plugs = subtract 50% from the MFR NRR • All other plugs = subtract 70% from the MFR NRR
DERATING AN NRR - the NIOSH way • So here's how NIOSH asks you to derate HPDs • Effective A-weighted noise level (ENL, dBA) = dBA - (derated NRR -7) • Example ENL: TWA = 100 dBA; NRR = 19 • Muffs = 100 - [(19 X .75 = 14) - 7] = 93 dBA • Form. plugs = 100 - [(19 X .50 = 10) - 7] = 97 dBA • Other plugs = 100 - [(19 X .30 = 6) - 7] = 101 dBA ? • Whoa, it got louder???
REAL WORLD CYPHERIN' WITH DERATED NRRs • We (NAVMEDCEN Portsmouth IH Dept) have started collecting noise dosimetry data and paying more attention to the brand of HPD in use by the sampled individual • We can take our measured TWA (dBA), compare it to the derated NRR, and calculate a "corrected TWA" • This will help us make determinations about HPD effectiveness; might cause us to pursue new brands of HPDs instead of the brands currently in use
REAL WORLD CYPHERIN' WITH DERATED NRRs • There are multiple OELs in our IH world • OSHA = 90 dBA, 5 dB exchange rate • ACGIH = 85 dBA, 3 dB exchange rate • DOD = 85 dBA, 3 dB exchange rate • US Navy = 84 dBA, 4 dB exchange rate • We measured five TWAs at one facility (an aircraft squadron) and three were > Navy OEL or 84 dBA; after derating, two of the exposures were still above the Navy OEL • Let's look ….
REAL WORLD CYPHERIN' WITH DERATED NRRs • Adjusted TWAs after derating MFR NRRs
DERATING AN NRR - the super cool 21st Century way • You can have an NRR that is specific to you and a particular insert plug - personalized just for you • Takes into consideration the way you inserted the plug, the shape of your ear canal, and your hearing thresholds at the testing frequencies • A new hardware/software combo, "Fit Check" from Michael and Associates (michaelassociates.com) allows you to insert plugs (or wear muffs), put on some headphones, and get an audiometric test while wearing those plugs; results given as a personal attenuation rating (PAR);
DERATING AN NRR - the super cool 21st Century way • Three plugs, one head (mine), four ways of looking at them; PAR = personal attenuation rating
WHAT DOES THIS ALL MEAN? • We probably ought to take a closer look at the brands and types of HPDs that are being worn in our facilities; an NRR in single digits is not doing anyone any favors • If you routinely use insert plugs, grab moldable plugs; they usually have an NRR of around 30 • If you use the OSHA 1910.95 Appendix B protocol, (NRR -7), then a 30 is a 23 • If you use the OSHA 50% derating protocol, [(NRR -7) X 50%], then a 30 is a 12
WHAT DOES THIS ALL MEAN? • If turtles could fly and we all had budgets that are appropriate for what we do … we'd determine PARS for each of our employees so we could give them the best plug possible • Just three grand puts the Fit Check system in your hand! • Perfect plugs prevent poor performance!
CONCLUSION • Any questions? Now go forth and measure, brothers and sisters …