The OSHA Technical Manual states that when combinations of HPDs are used, the total noise reduction will be the greater of the two NRR plus 5 dB.

Hearing Protectors On the Job Site

Employers should not focus only on the NRR when selecting protectors.

One difficulty with hearing protective devices (HPDs) is selecting the right one.  Careful forethought is required to select HPDs that will help ensure that workers obtain maximum effective protection. Effective protection is a combination of adequate noise attenuation and actual wear time. While this article focuses on HPDs, safety professionals must remember all personal protective equipment is the choice of last resort. Where feasible, engineering and administrative controls—using technology and planning to control or eliminate employee exposure—are always the preferred methods and required by U.S. OSHA.

1. How can I tell if the HPD provides enough protection?
For OSHA compliance, assuming the employee does not have a standard threshold shift and is not under the requirements of 29 CFR 1910.95(j)(3), HPDs must provide at least enough protection to reduce the employee’s noise exposure at or below 90 decibels A-weighted (dBA). Each HPD on the market has been laboratory tested and received a Noise Reduction Rating (NRR). The NRR is printed on the packaging or is available through the manufacturer's product literature. OSHA's Hearing Conservation Amendment states that the following formula will be used to determine whether hearing protectors are adequate (assuming a baseline audiogram has been completed and no standard threshold shift has been documented):

[TWA (dBA)]  -  (NRR - 7) ≤  90 dBA       (1)

TWA = Employee 8 hour Time Weighted Average exposure to noise

NRR = Noise Reduction Rating. Determined by the hearing protector manufacturer.

7 = constant modifying factor to account for noise frequency uncertainty

For example, if the worker's 8 hour TWA exposure is 97 dBA and she is using an ear plug with an NRR of 19, the employee's "protected" exposure will be:

97 - (19 - 7) =  97 - 12  =  85 dBA       (2)

This is below 90 dBA, the HPD is adequate.

OSHA and many safety and health professionals, however, recommend using a modified formula:

[TWA] - (NRR – 7)    ≤  90 dBA      (3)

                                                                     2

The 2 is a safety factor to account for the fact that the laboratory-determined NRR is seldom achieved in the workplace due to improper usage of the HPD by workers.

In the above example:             97 - (19 - 7)   =  91 dBA   (4)

                                                        2

According to the recommended formula, this protection would be considered inadequate.

The same formulas are used regardless of the type of HPD (e.g., ear muffs, formable ear plugs, pre-molded ear plugs).

Using a little algebra, we can calculate what the required NRR would be:

                             90 dBA* =    97 -  (NRR- 7)             NRR = 21                                                                                                       2                       

* This number can be set to any level the employer wishes to protect the employee to, but it must be less than 90 dBA and, in some cases as noted above, less than or equal to 85 dBA.

Two other methods of evaluating HPD attenuation that the safety professional may wish to consider are:

  • The National Institute for Occupational Safety and Health in its criteria document suggests derating ear muffs by 25 percent, "slow recovery" formable ear plugs by 50 percent, and all other ear plugs by 70 percent.
  • Fit-testing of hearing protection. Similar in concept to a quantitative respirator fit test, there are commercially available devices to determine the individual employee’s Personal Attenuation Rating (PAR).

While safety professionals may consider these two methods to further evaluate their HPD selection, formula (1) is what OSHA requires for compliance with 29 CFR 1910.95.

2. So, if I want to be really safe, I should get hearing protectors with the highest NRR available?
Not necessarily. Employers should not focus only on the NRR when selecting protectors. Several studies have indicated that interference with speech communication and comfort are two critical factors when it comes to employees wearing hearing protection. Usually, it is better to get an HPD with a lower noise rating that still provides adequate protection but is more comfortable for the employee. European Union Guidelines (Stand No. EN 458:2004) suggest an optimal “protected level” of 75-80 dBA, with an acceptable range of 70-85 dBA. 

3. Which are better to use: ear muffs or ear plugs?
To a great extent, this is a personal decision. Whichever HPD the employee prefers to wear is usually the best one. However, you should keep in mind the advantages and disadvantages of each.

Muffs:

Advantages

  • Easier to see that the HPD is being worn
  • Easier to put on properly
  • Can be equipped with radios for improved communication in high-noise areas
  • Avoids some employees' concerns about "sticking something in my ear"

Disadvantages

  • Heavier and hotter than plugs
  • More expensive
  • Requires regular inspection and maintenance of muff spring band, shell, and sealing pads
  • Safety glasses' temple bars may interfere with muff/head seal and can reduce the noise attenuation by up to 10 decibels
  • Long hair may interfere with muff/head seal

Plugs:

Advantages

  • Lighter, cooler
  • Glasses and long hair do not affect performance, are immaterial
  • Generally have higher noise reduction rating than muffs
  • Wide variety of types and sizes

Disadvantages

  • More difficult to check for compliance
  • Can be more difficult to properly insert
  • Can be uncomfortable for some employees
  • If reusable plugs are used, they require regular washing
  • In construction, 29 CFR 1926.101, require fitting by competent person

4. If I use a muff and plug combination, can I add the two NRR together to determine the total NRR?
No. The OSHA Technical Manual states that when combinations of HPDs are used, the total noise reduction will be the greater of the two NRR plus 5 dB.  Therefore if you have a muff with NRR of 26 and a plug with NRR of 29, the total protection would be:

                                               (29 - 7) + 5 = 27 dB NRR

                                     

                                         29  = higher NRR of the two HPDs

7 = modifying factor for noise frequency uncertainty

5 = additional allowance for second HPD

This has to do with skull bone conducting the noise, bypassing the HPD and reaching the inner ear.

Conclusion
Noise exposure is a significant problem at many work sites.  With careful forethought on HPD selection and proper use of HPDs, the risk of noise-induced hearing loss in workers can be greatly reduced.

References
1. CFR 29 Part 1910.95, Occupational Noise Exposure
2. CFR 29 Part 1926.101, Hearing Protection
3. Criteria for a Recommended Standard Occupational Noise Exposure – Revised Criteria 1998 DHHS (NIOSH) Publication 98-126

This article originally appeared in the September 2015 issue of Occupational Health & Safety.

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