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Hearing Protectors

Assessment of the real protective properties

The importance of the proper assessment of PPE characteristics

The test methods and the specifications developed to assess the performances of the PPE shall in particular be representative of:

  • The real risks that the PPE is intended to protect the user against
  • All foreseeable conditions of use, which can affect the efficiency and comfort of PPE

In practice the assessment in laboratory of the efficiency and comfort of PPE remains, in many cases, rather theoretical. As a matter of fact, the standardised test methods adopted and used as basis for this assessment are very often empirical and simplistic. This is often the way followed by standardisers to get repeatable, reproducible and not too expensive tests.

Consequently, PPE having satisfied all the tests in the laboratory can sometimes appear in real conditions of use, less efficient and comfortable than expected. These differences in appreciation can appear when the real qualities of the PPE are difficult to assess objectively in the laboratory. This applies to all PPE, when their performances are closely linked to the morphological characteristics of the end-users and to the difficulty to identify the constrains related to foreseeable conditions of use. This is the case for safety aspects, such as the air tightness of respiratory protective devices and the noise attenuation of hearing protectors. This is also the case for the assessment of characteristics like practicability, thermal comfort, impairment of sensory perception of PPE. To reduce these discrepancies as much as possible, it is always advisable to try to correlate the results obtained in the laboratory with the reality of the work place. This could be done by conducting assessments in real situations of use of PPE being worn by the operators while carrying out their normal tasks.

Many studies have been carried out in particular by Occupational Health & Safety Institutes, testing laboratories and universities around the world (BGIA, FIOH, INRS, INSHT, CIOP, TNO, NIOSH, Technical University of Denmark, Kent state, Harvard, Washington) in particular on respiratory protective devices, hearing protectors, safety footwear, gloves, protective clothing, kneepads, eye protectors. They sometimes have shown very big differences between the theoretical performances evaluated in laboratories and in real condition of use.

"PPE having satisfied all the tests in the laboratory can sometimes appear in real conditions of use, less efficient and comfortable than expected"

?D in dB According to Frequencies (Hz)
earing Protectors 500 1000 2000 4000 8000 ?d
Ear-plugs A -7.5 -3.4 -3.1 -8.0 -7.4 -5.9
Ear-plugs B -7.5 -1.1 -8.1 -11.7 -15.2 -8.7
Ear-plugs C -21.0 -14.4 -6.5 -9.4 -15.2 -13.3
Ear-plugs D -12.0 -6.3 -5.9 -9.7 -12.6 -9.3
Ear-muffs E -5.6 -2.7 -6.8 -4.3 -4.7 -4.8
Ear-muffs F -2.5 -6.1 -4.1 -1.5 -9.4 -2.3
Ear-muffs G +0.5 -4.3 -7.7 -11.5 -5.5 -5.7
Ear-muffs H -11.9 -6.5 -4.0 -0.2 -3.6 -5.2
Ear-muffs I -4.9 -1.9 -5.7 -4.4 -5.8 -4.5

Note: ?d is the arithmetical mean value according to frequency (Hz)

Typical example of hearing protective devices

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The noise attenuation of hearing protectors is evaluated in Europe, according to the standard EN24869-1: 1993 on the basis of measurement conducted with 16 test subjects 1 . They shall be preliminary trained in the wearing of the protective device and its adjustment. They shall be also familiar with audiometry and their audiogram must be repeatable (less than 6dB of variation).

In practice, the workers are not always well informed about how to adjust the hearing protection device. Even when they are informed, the fitting of the protective device, in particular of ear-plugs, tends to evolve, to move, resulting in a lesser degree of tightness, therefore a decrease in the protection performance. Regarding this matter, it is worth noting that it is not always easy to re-adjust the ear-plug if he/she perceives that the ear-plug is less tight than it ought to be due to bad adjustment.

That is the reason why attenuation values found at work places and with untrained subjects, reported for many years in various publications are appreciably lower than those measured in laboratories 6-19 .

To illustrate the importance of these discrepancies and the need to not sacrifice the estimated protection of the users, only for the sake of simplicity and of economical aspects I would like to give these three typical examples of study results.

  • A study (PFEIFFER, 1989) on noise attenuation characteristics of hearing protectors was carried out by BGIA in Germany in a series of enterprises of different industrial sectors. The results (Table 1) show that the effective attenuation values for ear-plugs were distinctly below what could be expected according to the type test in laboratory 6 . Mean differences come up to 13.3 dB for foam plugs and 5.9 to 8.7 dB for fibreglass wool. It would be possible to considerably decrease these differences if the workers were instructed on how to correctly use the ear- plugs and how to survey their own wearing behaviour. With a mean value of 4.5 dB differences for ear-muffs are distinctly smaller. Control measurements of already worn and new ear-muffs prove decreased protective efficiently to be mainly a consequence of ageing and wear. This could be avoided by guaranteeing regular maintenance (situation of cushions and liners) and limit the usable life
  • A bibliographical study published by NIOSH in 1998, shows similar differences7. The summing up of the results, that is detailed in the NIOSH report are given in the table 2. They correspond to 20 independent studies carried out by 7 different countries with 2,900 subjects on 30 different models of HPD in two different testing conditions: real working-world and with trained subjects in laboratory in accordance with the old standards ANSI S12.6-1984 and 1974. The biggest differences can be observed with fibreglass and premoulded ear-plugs where the effective attenuation is respectively 70% and 84% lower than the one evaluated in laboratory with trained subjects. These results demonstrated that relying on the manufacturer’s instructions based on not enough realistic methods of measurement may be of little value in estimating the effective protection a worker obtains under conditions of real use
  • The last example is a similar bibliographical investigation carried out by INRS on 6 other studies 8-13 that shows also big differences between laboratory and near real-world measurements: 13.1 to 17.5 dB for ear-plugs and 6.5 to 7.4 dB for ear-muffs (Table 3)

"Even when they are informed, the fitting of  ear-plugs, tends to evolve, resulting in a lesser degree of tightness, therefore a decrease in the protection performance"

Noise Attenuation on dB
Type of HPD Number of Models Number of Studies Real protection afforded to 84% of individuals Ratio = Value declared by manufacturer value
Foam ear-plug 1 15 0.49
Fibreglass ear-plug 3 10 0.26 −0.36 −0.19
Premoulded ear-plug 4 16 0.33 −0.08 −0.19 −0.007
Custom made ear-plug 3 7 0.16 −0.73 −0.33
Ear-muff 15 18 0.64 −0.35 −0.63 −0.28 −0.65 −0.47 −0.54 −1.04 −0.8 −0.75
Helmut mounted ear-muff 15 18 0.69 −0.47 −0.54 −0.66 −0.74
Ear-plug + ear-muff 1 1 0.86

The consequences for the HPD selection

Hearing protectors shall attenuate noise sufficiently to keep the worker’s “real-world” exposure (i.e., the noise exposure at the worker’s ear when hearing protectors are worn) below 85 dBA (in the USA according to OSHA regulations 29CFR1910.95) or 87dBA (in the EU according to Directive 2003/10/EC) over an 8-hour workday.

Type of hearing protective devices (number of data) Attenuation H M L SNR
ISO 4869−2 Real world ISO 4869−2 Real world ISO 4869−2 Real world ISO 4869−2 Real world
Ear-plugs user-moulded (10) Mean value 33.4 20.3 29.0 14.1 26.7 11.6 32.2 17.9
Ear-plugs pre-moulded (8) Mean value 33.4 15.9 28.3 11.3 25.8 9.8 31.8 14.8
Ear-muffs (8) Mean value 35.9 27.3 26.9 19.8 18.6 12.1 29.8 22.4

H, M, L, SNR = Attenuation values as defined in ISO 4869-2

Two ways are possible to generate valid reference attenuation data for the proper selection of HPD that could be considered to be acceptable for use in workplaces regarding the noise exposition conditions and the lawful requirements:

  • Modification of the measurement procedure in particular by using test subjects who are inexperienced in the fitting and use of hearing protectors. These measurement conditions are recognised as providing results that are representative for noise reduction obtained by group of typical users in real-world occupational settings 20 . This way of solution has been adopted since 1997 in the USA (ANSI S 12.6- 1997), 1998 in Australia and New Zealand (AS/NZS1269: 1998, 1270:1999 and 1270:2002) and more recently by ISO (ISO/TS 4869-5:2006) 1-5 . Moreover, interlaboratory reproducibility was found to be best for this “subject-fit” method in comparison with the old experimenter-fit and informed-user-fit methods 21 or at least equivalent
  • Application of derating factors to the results obtained with trained individuals. The conclusions of studies recommend significant corrections to the performances claimed by the manufacturers. OSHA: 1983 has instructed its compliance officers to derate the NRR (Noise Reduction Rate) by 50% in enforcing the engineering control provision of the OSHA noise standard. However, NIOSH on the basis of the study presented above, has recommended, if more realistic data is not available, to derate hearing protectors NRR by a factor that corresponds to the available real-world data: a. Ear-muffs: Subtract 25% from the manufacturer’s labelled NRR b. Slow-recovery formable ear-plugs: Subtract 50% from the manufacturer’s labelled NRR c. All other ear-plugs: Subtract 70% from the manufacturer’s labelled NRR In France corrections are also proposed: - 5 decibels for ear-muffs and custom moulded ear-plugs, -10 decibels for formable ear plugs and - 7 decibels for helmut mounted ear-muffs. Different derating factors are proposed by EU member states such as Germany, UK and Spain

This problem has been officially submitted for consideration to the EU Commission by the French Ministry of Labor to harmonise the national approaches and to request the revision of the existing EN ISO 4869-2 standard.

Conclusion

The example of the overestimated noise reduction of hearing protectors, which is only one typical case among others in the PPE field, shows the crucial importance of having reliable test methods to assess the real protective characteristics of the PPE. Before introducing or confirming a test method in a standard, CEN Technical committee on PPE are now officially encouraged to validate when possible its representativeness 23 . This is of particular importance for test methods related to key health and safety characteristics. The results of these studies suggest also to pay more attention to the proper selection of PPE, on the necessary preliminary information and training of the workers on the wearing and on the use of PPE.

For more information on hearing protection, please visit the ose.directory

Published: 10th Apr 2008 in Health and Safety International

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