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October 7, 2009

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Noise- Safety in numbers?

David Leonard outlines the problems of relying on the Single Number Rating (SNR) system for determining the effectiveness of hearing protection, and suggests other methods to use in the development of a successful hearing conservation programme.

These days our hearing is
arguably under greater threat than at any other time in history. The proliferation of mobile phones, MP3 players, super-loud car stereos, home cinema systems, and the other ‘essential’ elements of modern life mean that this vital sense is literally under attack on a daily basis. That the general population has such a blithe attitude towards hearing damage is possibly because unlike, say, smoking, the wider consequences of hearing loss are not considered, or widely publicised.

Some 8.7 million people in the UK — or just under 15 per cent of the population — are classed as deaf or hard-of-hearing.1 In the United States, the situation is no better, with work-related hearing loss reported as one of the most common occupational diseases. It is estimated that more than 30 million US workers are exposed to hazardous noise on a daily basis, and more than 10 million suffer from noise-induced hearing loss.2

A report issued earlier this year by the HSE entitled Real World Use and Performance of Hearing Protection,3 noted that although the availability of appropriate hearing protection devices (HPDs) in the UK is good, “possibly only 60 per cent of workers supposedly using hearing protection are, in fact, protected”. One of the main reasons why hearing protection fails in the ‘real world’ (and this applies particularly to ear plugs) is incorrect fitting. Other reasons include lack of compatibility with other personal protective equipment, deterioration of products, and abuse.

There is also the problem that, sometimes, the protection simply isn’t used. Common objections are that the HPD is “uncomfortable”, or that communication with fellow workers is hindered, or that it in some way inhibits the ability to work safely in a given environment. Road workers, for example, often use the maxim “better deaf than dead” to explain why they do not use HPDs. A reason commonly given in industry is “my machine sounds different”. While this is true a typical machine operator will adjust to the new sounds after around seven days.

Other, more frivolous objections — which I repeat here only to highlight and debunk a couple of myths — are: “I don’t need an HPD, I am used to the noise”; “I have already lost some of my hearing, what’s the point in wearing it”; “I’ll hurt my ear drums if I insert a plug too deeply”; and I don’t want an ear infection”. All nonsense!

Singled out

The way in which to address the above issues is to implement a hearing conservation programme, but this is not as straightforward as it should be, because of the problems associated with the Single Number Rating (SNR) system, de-rating schemes for HPDs, and HPD fitting and fit training.

SNR was adopted by the ISO (International Organisation for Standardisation) in 1994 as a simple method for estimating attenuation — the extent to which hearing protectors reduce sound. But the SNR is simply an estimate of the protection an HPD can offer, based on population averages; it is not a measurement of actual protection. The number printed on the side of a typical pack of earplugs is the level of attenuation achievable by 84 per cent of users when the HPD is properly fitted. SNR is therefore a theoretical number, so the level of protection an individual worker will achieve with a particular earplug in the field remains unknown.

This weakness of the SNR system is recognised and, consequently, many countries have devised their own HPD de-rating recommendations (but again, these rules apply generally to a population and not an individual person). In Germany, for example, the BG Technical Committee recommends subtracting 3dB from the ‘advertised’ attenuation rating for custom-moulded earplugs, 5dB from earmuffs, and 9dB from all other earplugs. Some experts in the UK advise subtracting 4dB from the SNR, while in the US, 7dB is a common de-rating.

But this de-rating is largely meaningless in practice because of the aforementioned main reason for failure of hearing protection: incorrect fitting. In other words, the attenuation achieved by an individual HPD wearer greatly depends on whether or not he or she has fitted it and is wearing it correctly.

This was illustrated by a recent study in the field4 to determine the infinite variability in effectiveness of HPDs when relying only on SNR and de-rating, and to identify factors that contribute to good earplug fit and, hence, good attenuation in use. The study was conducted among 104 workers at eight different facilities. The participants were not pre-screened, and were tested during a standard shift with the earplugs that they routinely wear on the job, with no modifications. The tested earplugs were from four different manufacturers, and workers received no training as part of the test. They were simply asked to insert the earplugs as they normally would, and no feedback or correction was offered if they failed to fit the earplug correctly.

The results revealed that a third of workers achieved attenuation higher than published ratings for their earplugs, i.e. the level of protection received by the wearer is better than ‘advertised’. Another third achieved attenuation within 5dB of the advertised ratings, and so were achieving adequate protection. The remaining third achieved attenuation that was more than 5dB below the published rating, and so were not being protected adequately for the environment in which they were working.

This study reinforces the fact that no generalised rating scheme for hearing protectors can be effective without knowing how much attenuation individual workers actually attain in the real world. If, for the sake of argument, a safety professional were to supply earplugs based on the assumption that all earplugs only achieve 50 per cent of their published SNR in the field, then two thirds of their workers (as evidenced by this study) would be seriously over-protected, since they would achieve much higher protection than 50 per cent.

As a general guideline, hearing protection should bring Protected Exposure Levels (the noise level under the earplugs) to between 70 and 85dB (from ISO Guideline EN-458). If a person is over-protected, verbal communication may be hindered, warning alarms might not be heard, and, in certain hearing-critical jobs, over-protection might be just as hazardous as under-protection.

Field verification systems like that outlined above can ascertain each worker’s Personal Attenuation Rating (PAR), which can then be compared against such factors as the published SNR and the noise hazard. In this study, various personal, as well as programme factors were evaluated to determine which would contribute best to a good earplug fit among these workers. Factors evaluated included: gender, age, years working in a noisy environment, ear-canal size, familiarity with HPDs, model of product used, amount and type of training received, and enforcement.

Train to gain

What emerged most obviously from this was that the level of training had a strong bearing on how well the earplug was fitted and, therefore, the level of attenuation achieved in use. It was found that the more often a worker had received one-to-one training in the proper use of HPDs, the higher the chance of a good fit being achieved. Interestingly, the same could not be said of group training — in terms of actual attenuation in the field it appeared to make no difference at all whether a worker had attended two, five, or ten group training sessions on HPDs.

It is not clear why this should be the case but the reason may simply be that the more attention paid to the individual, the more likely they are to take the training on board. Hearing conservation training, be it group training or one-to-one, typically involves a presentation (perhaps a video), which reinforces the dangers and the consequences of hearing loss and then moves on to correct HPD selection and fitting techniques.

However, any sort of hearing conservation training must address the specific challenges presented by the workers’ environment. Within the same site different job functions may require different HPDs — reusable rather than disposable roll-down earplugs, for example. This can often be a problem if a ‘controlling mind’ selects a single HPD product to cover an entire site. It may adequately protect some but under or over-protect others, while some it just won’t suit at all and so they will not use it. Such issues are more easily addressed in a one-to-one training session, which is another reason why they are more effective.


Bridging the gap between the lab (SNR) and the field (PAR) as outlined above provides the safety professional with more ‘hard information’, on the basis of which they can construct an effective hearing conservation programme. The key elements of a successful programme are: providing a number of different types of potentially effective HPDs; one-to-one training; strict enforcement of the hearing protection policy; and continuing communication and education.
While field verification of hearing protectors can provide valuable information, it also has its limits. Using PAR information, fit-testing of HPDs may be possible but, as with respirators, fit-testing only verifies the capability of a piece of equipment in the hands of the user, and can only provide a snapshot of attenuation at the time of the test. It does not reveal the noise exposure for the worker a day later, or even an hour later.

The question of whether the fit-testing of HPDs should be governed by legislation is one for the future but with apocryphal tales of mandatory compensation pay-outs for hearing loss and the consequent hike in insurance premiums, it is, perhaps, a move that would benefit employers and employees alike.

Only action from government, in terms of education and consumer legislation, can address the threat that the growth in modern technology poses to our hearing. In the workplace, however, by bridging the gap between the laboratory and the field, safety professionals can stop relying on theoretical data and instead base their hearing conservation initiatives firmly in reality.


1    Royal National Institute for Deaf People — information_resources/ aboutdeafness/statistics
2    Noise and Hearing Loss — NIH Consens Statement Online 1990, Jan 22-24;8(1):
3    HSE (2009): Real World Use and Performance of Hearing Protection (RR720) — research/rrhtm/ rr720.htm
4    Conducted by the Howard Leight Acoustical Laboratory, San Diego, using Howard Leight by Sperian’s VeriPRO earplug fit-testing technology

David Leonard is a past chairman of the Industrial Safety Protective Equipment Manufacturers Association.

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