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October 25, 2006

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Another quiet day at the office

Measuring worker exposure to noise, and the damage it does, can be very difficult in some industries. Here, Peter Zymanczyk looks at how studies into hearing loss among firefighters have revealed the sources of the damage and how best to combat it.

Workers in some industries are exposed to noise at varying volumes and durations in an inconsistent pattern throughout the working day. Measuring and assessing this type of noise exposure is not as straightforward as it would be in a factory. Things can become difficult for the safety practitioner when cases of noise-induced hearing loss (NIHL) are diagnosed and, while the hearing loss appears to be occupationally related, all the information from the workplace suggests there is no noise over exposure.

This article uses the Fire and Rescue Services (FRS) in the United Kingdom and the United States of America as a case study to illustrate the difficulty in assessing noise in these circumstances. In spite of structured shift patterns, programmed routines and planned tasks – such as training and fire prevention duties – the requirement for the FRS to make a swift response to an emergency call means there is no ‘standard’ working day.

How big is the problem?

It is difficult to quantify just how great a problem NIHL is. In the UK, before the recent extension of the Disability Discrimination Act 1995 to the emergency services, firefighters have always required hearing of the highest standard. There have not been any recently published studies into hearing loss within the UK FRS, but anecdotal evidence from FRS occupational health practitioners (OHPs) suggests that some hearing loss is a common characteristic in firefighters.

In the USA, the view that loss of hearing is just a natural consequence of being a firefighter has resulted in a number of studies into firefighter hearing loss, particularly from R.L. Tubbs.1 Lifestyle factors were considered, but these failed to identify any consistent element that would explain the level of hearing loss. It is now accepted, however, that loss of hearing may result from a single exposure to a very brief impulse noise or explosion, but such traumatic losses are rare.

Studies have consistently recommended that hearing conservation programmes (HCPs) be implemented, including audiometry, noise assessments, engineering controls and ear defenders. Tubbs suggests three possible mechanisms might be at work to damage firefighters’ hearing:2

– That noise exposure at the same time as exposure to chemicals acts synergistically to cause greater than anticipated damage to hearing;

– That exposure to higher frequency noise might be more damaging to hearing than anticipated; and

– That the long quiet periods at fire stations disrupted by short periods of loud noise from sirens and vehicle engines, etc. may be more damaging than exposure to constant noise levels.

Chemical damage

The possible interaction between noise and chemicals is worth deeper examination, especially as many studies of firefighter health associate hearing loss with noise, and pulmonary and cardiac dysfunction with exposure to fire smoke and related toxins. Currently the combined effect exposure to chemicals and noise will have on hearing remains poorly understood. An increase in the use of plastics has increased the range of chemical exposures from fire smoke, but firefighters recognise the hazard presented by the cocktail of chemicals present in fire smoke, and protect themselves from the risk of acute asphyxiation or poisoning, or chronic lung damage, by wearing breathing apparatus (BA).3

In fire situations, carbon monoxide (CO) is possibly the most common gas that firefighters encounter. While chemical asphyxiants (including CO) do not seem to affect the auditory system of laboratory animals by themselves, they have been shown to potentiate NIHL.

Even under intermittent noise exposure with long quiet periods, CO exposure could produce unexpectedly large, permanent threshold shifts. This might be relevant, as firefighters often do not wear BA during overhaul (damping down) after a fire, and while contaminant levels are lower during these activities, they are not absent. However, in general, chemicals only appear to enhance the effect noise has on hearing; the focus therefore should remain on controlling noise exposure.

Exposure

The second and third mechanisms suggested could be found in any environment where the pattern of noise exposure is inconsistent. However, they are a direct challenge to the principle of equal energy: the direct relationship between the sound pressure level (SPL) and exposure duration. According to this principle, the damaging effect of a SPL of 90dB(A) for eight hours is equal to 99dB(A) for one hour, or 87dB(A) for 16 hours, irrespective of the frequency of the noise, or pattern of exposure (see table 1 above).

Tubbs is not alone in challenging this principle: the American College of Occupational and Environmental Medicine observes: “Measures to estimate the health effects of such intermittent noise are lacking.”4

Recent background noise measurements at UK fire stations fell between 45 and 68 dB(A), but results from firefighters’ personal noise dosemeters (PNDMs) for an eight-hour reference period were evenly distributed between 77 and 92 dB(A) before any allowance is made for the over-estimation of noise exposure between 2 and 7 dB (often noted for PNDMs due to knocks, etc). While subtracting up to 7 dB from these results will bring them within the range described in the USA studies, it still leaves a discrepancy between background noise and recorded noise exposure levels, suggesting that there are some periods within the working day when noise levels are raised.

An examination into the working day identified a number of noise sources and tasks that have a wide variation both in SPL and exposure duration. This includes sirens, automatic fire alarms, casualty extrication tasks and engines/generators running. Many of these exposures are of short duration, between 5 and 45 minutes, in an otherwise ‘quiet’ but unpredictable working day.

This creates a multi-dimensional problem that is difficult to resolve. What it does suggest is that some short-duration loud noises during the working day make the major contribution to the overall noise dose, and by considering the noise exposure using the eight-hour reference period these episodes are effectively camouflaged.

Assess the damage

There are two possible approaches to assessing noise exposure in these circumstances. The first method requires that noise from all activities is measured and assessed against estimates of duration from attendance data at incidents, or actual durations of training activities. From this information, models of different working days/weeks could be constructed to enable an overall assessment of noise exposure to be made in accordance with the Control of Noise at Work Regulations 2005.

However, irrespective of how well this modelling process is done, there remains wide potential for exposure to noise in ways not considered (e.g. how would a major train crash or terrorist incident affect overall noise exposure?) This being the case, what use is modelling? The HSE has suggested that if “no single day or other period of time can be considered to be representative of noise exposure … it will be impracticable or of little use to make an accurate measurement for these workers”.5 However, no alternative method of assessment is proffered.

Tim Ward, in a 2005 SHP article discussing the Control of Noise at Work Regulations 2005, warns employers against: “Putting too much effort into getting a precise estimate of noise exposure to decide ‘which side of the line they fall’ … these regulations are concerned with controlling noise, not measuring it.”6

The second method considers a more pragmatic approach to assessing noise exposure, which takes account of the potentially disproportional effect high-frequency or short-duration noise episodes might have on hearing. Rather than trying to assess noise in relation to a reference eight-hour working day, assess each activity and its potential contribution towards the total daily noise dose. Effectively set a maximum fractional noise dose that an activity can contribute to an individual’s daily noise dose, either by actual measurement or estimate of exposure. Should this be exceeded, action must be taken.

Any limit would need to be reasonably low; a level of 30 per cent of the upper exposure limit action value is suggested as the point for intervention for firefighters. The different points at which an intervention is required can be illustrated through adapting the HSE noise exposure ready-reckoner (see table 2).7

From this table, it can be seen that noise exposure between 94 and 95 dB(A) for as few as 15 minutes in a working day is likely to require an intervention, but for a noise exposure between 85 and 86 dB(A) an intervention is not necessary until the predicted noise exposure is likely to last for two or more hours. The ready-reckoner also provides a means of ranking hazards: the higher the number, the higher the priority. In cases where different hazards generate the same number, the actual/estimated number of exposures (emergency calls in the case of the FRS) to a particular hazard can be used to further determine priority, those occurring most frequently being addressed first.

Control measures

To develop controls for noise exposure in these circumstances, engineering measures should always be attempted first, but the difficulty is that for the bulk of the working day, hearing protection will not be necessary. Management will have to assess not only the activities that will require measures to control them, but also who will require protection. In some cases it may be just the operator of a noisy tool; in other circumstances all workers in the vicinity will require protection. Another aspect of the FRS and construction industries is that the workplace is often very dynamic and there is a need for workers to be able to hear what is going on around them, particularly instructions and/or warnings. Any hearing protection would have to incorporate some form of voice attenuation and this often reduces the protection to the wearer, and means noise levels might not be reduced below the suggested 30 per cent threshold. In the case of the FRS, any debate will have to focus on whether protection from most noise sources is sufficient, if only some protection is afforded against others. If control measures can take the “edge” off noise during actual operations, will rigid enforcement of hearing protection measures during training activities be enough to prevent NIHL? Will taking these measures meet the spirit of the law, so far as is reasonably practicable, in circumstances where there is an imperative to act in order to save life and property? Finally, will this approach afford sufficient protection to prevent permanent damage?

The problem of unpredictability

In terms of the FRS, this article may stimulate a debate leading to a consistent approach to assessing noise. But in industry generally, the eight-hour reference period for noise assessment may only be of use where there are standard working practices that can be measured and assessed against a predictable and repeatable working day – in circumstances where this cannot be achieved, a standard reference period might just be concealing a problem.

References

1 Tubbs, RL (1995): ‘Noise and hearing loss in Firefighting’, Occupational Medicine, Vol 10 Pt 4

2 Tubbs, RL (1985): ‘Health hazard evaluation: The City of New York Fire Department’, National Institute for Occupational Safety and Health HHE report, 81-49-1603

3 Burgess, JL et al (2001): ‘Adverse Respiratory Effects Following Overhaul in Firefighters,’ Journal of Occupational and Environmental Medicine, Vol 43, pt. 5, pp 467-473

4 American College of Occupational and Environmental Medicine (2002): ‘Noise-induced hearing loss” – statement approved by the board of directors, 27 October 2002

5 HSE (2000): Reducing Noise at Work – Guidance on the Noise at Work Regulations 1989 (L108), HSE Books

6 Ward, T (2005): ‘Sound Advice’, SHP October, pp50-52

7 HSE (2005): Controlling Noise at Work – The Control of Noise at Work Regulations 2005 – Guidance on the Regulations, Second edition, HSE Books
 

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