Occupational noise-induced hearing loss is a hearing impairment resulting from exposure to excessive noise at work. The degree of hearing loss is generally cumulative, increasing with both the length of time exposed and the level of noise. Once acquired, the damage to employees’ hearing is irreversible. Fortunately, however, workplace hearing loss is almost entirely preventable.
So how can employers prevent occupational noise-induced hearing loss in their workplaces?
The Model Code of Practice: Managing Noise and Preventing Hearing Loss at Work, published by Safe Work Australia in 2012, is the best place to start. It has been developed to provide practical guidance to persons conducting a business or undertaking on how noise affects hearing, how to identify and assess exposure to noise and how to control health and safety risks arising from hazardous noise.
With the incidence of compensated noise-induced hearing loss on the rise – the rate increased from 491 claims per million employees to 523 over the nine-year period 2000–01 to 2008–09 after a 5 year period of stabilisation – it is certainly a timely issue to address.
Indeed, along with a number of other work-related disorders, noise-induced hearing loss has been identified as a national priority in the new Australian Work Health and Safety Strategy 2012-22. The prioritised disorders have been chosen based on the severity of consequences for workers, the number of workers estimated to be affected, and the existence of known prevention options.
The benefits of managing the risks related to noise are clear: workers will be protected from hearing loss, the conditions for effective communication will be improved, and a less stressful and more productive work environment will be created.
Find out how to manage the risks in the following excerpt from the Model Code of Practice: Managing Noise and Preventing Hearing Loss at Work or download the full code in PDF format:
2. Noise and Its Effect on Health and Safety
2.1 How does hearing loss occur?
Hazardous noise affects the functioning of the inner ear, which may cause temporary hearing loss. After a period of time away from noise, hearing may be restored. With further exposure to hazardous noise, the ear will gradually lose its ability to recover and the hearing loss will become permanent.
Permanent hearing loss can also occur suddenly if a person is exposed to very loud impact or explosive sounds. This type of damage is known as acoustic trauma.
Permanent hearing loss results from the destruction of hair cells in the inner ear. These cells cannot be replaced or repaired by any presently known medical treatments or technology.
Usually, hazardous noise first affects the ability to hear high-frequency (high-pitched) sounds. This means that even though a person can still hear some sounds, conversation will start to sound ‘muffled’ and a person may find it difficult to understand what is being said.
Communication difficulties occur especially when there are competing background noises. Modern hearing aids may improve the ability to hear speech but they are unable to completely restore the clarity of the full hearing function.
Workers exposed to hazardous noise may also experience tinnitus, which could become permanent. When severe, it may disrupt sleep, reduce concentration, make people extremely irritable and lead to depression.
The degree of hearing loss that occurs is dependent on how loud the noise is, how long someone is exposed to it and, to some extent, individual susceptibility. The frequency or pitch can also have some effect on hearing loss, since high-pitched sounds are more damaging than low-pitched ones.
Exposure to a number of common industrial chemicals and some medications can also cause hearing loss or exacerbate the effects of noise on hearing. These substances are called ototoxic substances.
Ototoxic substances absorbed into the bloodstream may damage the cochlea in the inner ear and/or the auditory pathways to the brain, leading to hearing loss and tinnitus. Hearing loss is more likely if exposure is to a combination of substances or a combination of the substance and noise.
There is also some evidence that exposure to hand transmitted vibrations can exacerbate the effects of noise on hearing.
Further information on these other causes of hearing loss is provided in Appendix A.
2.2 How much noise is too much?
Whether the exposure standard of 85 dB(A) averaged over eight hours is exceeded depends on the level of noise involved and how long workers are exposed to it.
Peak noise levels greater than 140 dB(C) usually occur with impact or explosive noise such as sledge-hammering or a gun shot. Any exposure above this peak can create almost instant damage to hearing.
Decibels are not like normal numbers. They can’t be added or subtracted in the normal way. The decibel scale is logarithmic. On this scale, an increase of 3 dB therefore represents a doubling or twice as much sound energy. This means that the length of time a worker could be exposed to the noise is reduced by half for every 3 dB increase in noise level if the same noise energy is to be received.
Table 1 below demonstrates the length of time a person without hearing protectors can be exposed before the standard is exceeded.
|Table 1: Equivalent Noise ExposuresLAeq,8h = 85 dB(A)|
|Noise Level dB(A)||Exposure Time|
1 The adjustment factor for extended workshifts shown in Table 3 of this Code is taken into account.
Essentially, a worker who is exposed to 85 dB(A) for 8 hours receives the same noise energy as someone exposed to 88 dB(A) for 4 hours, with the balance of the day in a very quiet environment. In both cases the exposure standard is not being exceeded. However, being exposed to 88 dB(A) for more than 4 hours would mean that the standard is exceeded. Similarly, if a worker is using a machine that generates 121 dB(A) then the exposure standard would be exceeded after only 7.2 seconds.
There is a big range in different people’s susceptibility to hearing loss from noise. Research shows that 8-hour average daily noise exposure levels below 75 dB(A) or instantaneous peak noise levels below 130 dB(C) are unlikely to cause hearing loss. With progressively increasing levels, the risk becomes greater.
The WHS Regulations set the exposure standard for noise at an LAeq,8h of 85 dB(A) and a peak noise level at 140 dB(C), which protects most but not all people. Therefore, workplace noise should be kept lower than the exposure standard for noise if reasonably practicable.
2.3 Other effects of noise
Noise at levels that do not damage hearing can have other adverse health effects. This can arise when noise chronically interferes with concentration and communication. Persistent noise stress can increase the risk of fatigue and cardiovascular disorders including high blood pressure and heart disease.
Although safe levels to guard against these effects have not yet been fully determined, as a guide, the risk of adverse health effects can be minimised by keeping noise levels below:
50 dB(A) where work is being carried out that requires high concentration or effortless conversation
70 dB(A) where more routine work is being carried out that requires speed or attentiveness or where it is important to carry on conversations.
These levels include the noise from other work being carried out within the workplace.
To work safely, workers must be able to hear warning signals above any other noise (ambient noise) at the workplace. For reversing alarms on mobile plant, the guidance in ISO:9533: 2010 Earth-moving machinery – Machine-mounted audible travel alarms and forward horns – Test methods and performance criteria should be followed. This requires the noise level of the alarm at potential reception points to be at least as high as the noise from the engine under high idle.
For other situations, the levels needed are higher – at least 65 dB(A) and more than 15 dB(A) greater than the ambient noise level at any position in the signal reception area. More detailed guidance on assessing the audibility of warning signals can be found in ISO 7731:2003 Ergonomics – Danger signals for public and work areas – Auditory danger signals.