Occupational Noise Exposure

After workplace injuries, occupational noise exposure is the most common risk factor in the workplace (WHO Europe, 2017). For hundreds of years we have known that time spent working in noisy workplaces may lead to Noise-Induced Hearing Loss (NIHL). But despite this, occupational noise-induced hearing loss is the most prevalent occupational disease in the world.

While scientists and physicians have addressed NIHL over the centuries, a Glaswegian surgeon, Thomas Barr, is credited with one of the first controlled epidemiological studies into the effects of occupational noise. Barr examined the link between his patients’ work and their deafness, and in 1886 he reported that c. 75% of boilermakers had difficulty in hearing. He also identified the critical elements of NIHL and also the need for hearing protection, the earlier forms of which were rolled from cotton wool. While Barr noted the characteristic loss at high frequencies, significant progress in the formulation of risk criteria was not made until after World War II. However, major political and scientific barriers gave rise to widespread reluctance in regulating noise exposure. Initially uncertainties in the understanding of variations in individual susceptibility made it impractical to specify the criteria necessary to protect every worker’s ears throughout their working lifetime.

Notwithstanding, the many scientific and political challenges, and carried on the tide of sweeping societal and workplace change, over 60 years ago, criteria were established to ‘preserve hearing at the frequencies important for good speech recognition’ (Burns and Littler, 1960). These criteria were formally endorsed in the ‘Wilson Report’ (HMSO, 1963) which recommended that the UK Ministry for Labour should publicise information pertaining to the danger of noise and the need to reduce exposure. This was achieved with the publication of ‘Noise and the Worker’ (HMSO, 1963). This latter publication encouraged employers to address noise exposure in the workplace; and in Common Law 1963 is, thus, regarded as the definitive timeframe from which ‘guilty knowledge’ applies.

Statutory controls

Amongst workers and populations who are subject to similar exposures, some will exhibit extensive hearing loss while others will demonstrate negligible effects. While undoubtedly the individual differences (in NIHL) can be partly attributed to differences in humans’ vulnerability to the effects of noise; it is also attributed to actual differences in personal exposure. This is because all workers in an industrial setting (or even within a particular department or job) do not necessarily acquire the same exposure.

Natural variations in individual susceptibility were some of the factors which initially distracted and deferred the legislators from imposing statutory obligations on employers to protect their employees’ hearing. However, legislation has evolved throughout the EU and internationally which initially was targeted at protecting people with ‘average sensitivities’. Thus, the ‘original’ EU Directive (European Union Council, 1986) provided for minimum statutory obligations when the daily noise exposure level (L_{EX, 8h}) exceeded 85 dBA. The 2003 Directive (2003/10/EC) provided even more stringent controls than the 1986 Directive and some of these take effect when workers’ L_{EX, 8h} exceeds 80 dBA.

The cornerstone of the EU’s occupational safety legislation (Directive 89/391/EEC) emphasises the need to eliminate risk. Furthermore, the 1989 Machinery Directive and subsequent amendments (89/392/EEC and 2006/42/EC) have ensured that since 1993 most machinery supplied in the EU has fulfilled essential health and safety requirements.

The ‘Machinery Directive’ sets out procedures and standards that manufacturers must conform to and only conforming machinery is permitted to carry the CE mark. The provisions are designed to ensure that the accidents and injuries caused by the use of machinery can be reduced. The EU emphasis, therefore, is on inherently safe design and construction of machinery and on proper installation and maintenance. This works in harmony with the ‘Physical Agents (Noise) Directive’ (2003/10/EC) with the former, providing a commercial incentive for equipment designers and manufacturers to produce quieter or ‘silenced’ alternatives.

The manufacturer’s approach to preventing risks due to noise emissions must take account of the principles of safety integration.  

The information to be provided in the noise emission declaration includes three different noise emission quantities:

• The A-weighted emission sound pressure level, L_pA, produced by the machinery at its workstation(s)
• The peak C-weighted instantaneous sound pressure value
• The A-weighted sound power level, L_WA.

Measurements of noise emissions should be carried out under conditions which are reproducible and representative of the foreseeable conditions of use of the machinery.

Hierarchy of Control

For many years practitioners have applied the unmistakable adage: ‘prevention is better than cure’ and avoiding noise ‘at source’ is the obvious and preferred option. However, statutory obligations (which are widely ignored) effectively enshrine the maxim in EU law. 

Directive 89/391/EEC ‘on the introduction of measures to encourage improvements in the health and safety of workers’ (generally referred to as the Framework Directive) provides a well-defined hierarchy for implementing preventative measures, with risk avoidance assuming top priority. An example of a hierarchical control system for construction noise is presented below.

Even with technological and manufacturing improvements and regular attention to noise control at source, Hearing Protection Devices (HPDs) will need to be utilised by many workers for the foreseeable future. This is in recognition of industry’s inability to readily source ‘quiet’ equipment and tools and also recognises the practical constraints which apply to retrofitting. However, the hierarchy of control measures must be followed to ensure the safety and health of employees. Thus, the elimination of the source of noise is the most effective way to prevent risks to employees and should always be considered when new work equipment is being selected, purchased or commissioned or when workplaces are being planned. Where possible a “low noise” procurement policy will need to be adopted to prevent or control noise. Such policies can target ‘problem’ areas and activities and set specific limits to be achieved.

Notwithstanding the “Framework Directive” (89/391/EEC) provisions, there is widespread recognition that in many workplaces hearing protection will continue to be an essential component of the ‘practicable’ control mechanisms. However, there is a tendency for some employers to bypass the hierarchy of control and/or to over rely on hearing protection without fully addressing the necessary requirements for an effective Hearing Conservation Programme. This is recognised as a failing which contributes significantly to the alarming incidence of NIHL. Some employers misinterpret their Duty of Care and/or think that by providing hearing protectors that they are somehow absolved from further obligations. However, in circumstances where hearing protection is necessary it is both naïve and negligent for any employer to expect hearing protection to succeed without considerable effort.

For hearing protection to be successful, it is essential that the employer understands and applies all of the requisite parts of the programme. Effective hearing protection requires considerable effort and attention to detail and a commitment (by both employer and employee).

“the training obligations are explicit and challenging but nonetheless must be rigorously pursued”

Employees need to know when and where they need to wear hearing protection and it needs to be recognised that good hearing protector performance is critically dependent on the user. Often the importance of workers’ hearing is rarely appreciated until their occupational exposure diminishes it irreparably. Furthermore, in many cases the risk of hearing loss is poorly understood and for this reason occupational noise training should always be given the necessary attention it warrants. The training obligations are explicit and challenging but nonetheless must be rigorously pursued in fulfilment of the employers’ Duty of Care. Combined with effective risk assessment, good training practices are recognised as key requirements and many employers do not fulfil their basic obligations.

In terms of the training needs of employees, it is essential that occupational noise training should always address the following issues:

• The nature of the risks resulting from exposure to noise
• The organisational and technical measures taken to eliminate noise at source and/or reduce it to a minimum
• The relevant exposure limit values and exposure action values
• The findings of the assessment and measurements of noise and an explanation of their significance and the potential risks
• The correct use of hearing protectors
• Why and how to detect and report signs of hearing damage
• The circumstances in which health surveillance is made available (audiometric testing)
• Safe working practices to minimise exposure to noise

The forgoing details have not been extracted from an idealised training manual or from a research paper. They have in fact been extracted (almost verbatim) from Regulation 130 of the Safety, Health and Welfare at Work (General Application) Regulations 2007. In essence, these obligations arise under Directive 2003/10/EC. The stated training issues constitute the absolute minimum content for the training which must be provided to any employee whose L_{EX, 8h} equals or exceeds 80 dBA (or the lower exposure action value). 

European standard 458:2016¹ provides guidance on the additional type of training required for employees who need to use hearing protectors. The standard emphasises the importance of the fit of the protector on the protection achieved and also addresses the audibility of speech and/or warming signals/alerting sounds. In addition, the standard recognises the need for the regular training and motivation of employees and notes that the performance of hearing protection is subject to natural variability amongst workers. It is perhaps this variability alongside the known natural variation in sensitivity to noise which accounts for much of the NIHL arising in the 21st century.

Optimising hearing protection

Like any form of PPE, hearing protectors are often regarded as the ‘last line of defence’ and as a control mechanism their success is impeded by many factors. Extensive research has identified that the selection and fitting of HPD are limiting factors and many studies have concluded that the most common reason for failing to wear protection was discomfort.

International studies report that employees inconsistently use protectors when exposed to high noise levels, with workers’ reports indicating that a minority wore the protectors all the time when necessary or noise exposed. Many protectors can provide 30 dB attention if they are worn properly at all times that the employee is in a hazardous area. However, it is often not appreciated that if hearing protection is worn for 75% of the time (that it is needed), the maximum protection which can be provided by the protector is 6 dB. 

Despite their limitations, hearing protection devices (HPDs) can provide a potentially effective and reliable means to protect workers’ hearing. However, different circumstances require different protectors and for any HPD to be effective, regard must be had to numerous factors in the selection process, including:

• The work environment
• Essential work-related communication
• The potential attenuation of the HPD
• The actual exposure conditions (intensity and frequency of noise)
• Compatibility with other PPE
• How the HPD will be used
• Special user groups and medical factors
• Conformity with relevant requirements for incorporated electronics

The correctness of fit, duration of use, ease of use, and user acceptability are hugely critical factors. Despite this, decisions on selecting and purchasing HPDs are generally based on two key criteria – unit cost and noise reduction rating. 

The octave-band method of calculating attenuation in a workplace provides the most accurate estimate of the sound level under a HPD. This is because the calculation includes both the laboratory estimated attenuation of the device and the actual spectrum and level of the noise in which it will be worn. The ISO describes methods to estimate HPD performance in ISO Standard 4869-2 (ISO, 2018). Described as the ‘HML method’ and ‘SNR method’, these are regarded as ‘simplified procedures’ while the octave-band method is an “exact reference method”.

“in practice, an individual’s noise exposure can be substantially underestimated when it is computed from laboratory attenuation data”

Regardless of what methods are used to determine the overall attenuation, it is important to note that the actual attenuation provided in the workplace is substantially less than that measured in a laboratory. Laboratory measurement reflects the maximum attenuation that can be expected with a device under the most favourable conditions e.g. the device is new and in good condition, available in the correct size and is properly fitted and worn under ideal circumstances.

In reality many of the conditions of the laboratory test which are essential to obtain optimum performance are not achievable in the workplace.

Poor workplace performance arises due to a number of factors such as:

• Inadequate training
• Incorrect size or incompatibility with other PPE
• Incorrect fitting of the devices
• Slippage of the devices during use
• Deterioration of the devices and modifications by the employees that degrade the attenuation

In practice, an individual’s noise exposure can be substantially underestimated when it is computed from laboratory attenuation data. For this reason, it is recommended that the performance of the protectors is penalised by 4 dB to account for ‘real world’ performance, i.e., the noise level at the ear as computed using the manufacturers’ data, should have a ‘real-world’ factor of +4 dBA applied.

For the vast majority of applications, this 4 dB ‘penalty’ will not be problematical and most hearing protectors are likely to provide a degree of overprotection. Furthermore, caution should be exercised to avoid the selection of protectors which provide unnecessarily high attenuation. Recent guidance (HSE, 2019) suggests that reducing the nose levels at the ear to below 70 dBA is regarded as ‘over-protection’. This can cause communication difficulties and devices which overprotect have the potential to be less comfortable than protectors with lower attenuation. These critical factors (along with regulatory inertia and risk perception) are major barriers to the adoption of pragmatic and effective hearing protection practices and explain why many workers are reluctant to wear HPDs.

In theory, we should generally aim for an attenuated sound pressure level of approximately 70 to 80 dBA at the ear. However, this is merely a guideline and once employees can hear all necessary alarms and warning signals, the key factors to consider are the comfort of the HPD and the likelihood that it is worn whenever necessary. In the vast majority of cases, noise levels in workplaces will involve ambient noise levels below 95 dBA. Because of this, the fit and comfort of hearing devices as well as their practicality of use contribute more to their potential effectiveness than does their attenuation capability. While some exposure scenarios will require a highly rated HPD (or dual protection) many commonly used HPDs will potentially provide a degree of ‘over-protection’.  

Often the importance of workers’ hearing is rarely appreciated until their occupational exposure diminishes it irreparably. Furthermore, in many cases the risk of hearing loss is poorly understood and for this reason occupational noise training should always be given the necessary attention it warrants. This training is an essential component of the Hearing Conservation Programme and should be used to address the prevalent risk perception issues within industry. Training and awareness initiatives should be periodically pursued in fulfilment of the employers’ Duty of Care. In addition, an obligation to engage with employees on these issues is prescribed in law and from a practical perspective it is essential for the programme to succeed.

The European Committee for Standardization in response to technical and market developments has recently developed a series of new standards that provide a high level of safety for hearing protectors including conformity assessment methods for the integration of electronic communication facilities (CEN, 2020). These standards are required for manufacturers to demonstrate that their products comply with relevant EU legislation. However, even the optimum protectors will fail unless there is adequate engagement with the end users. For this reason, the motivation, awareness and training of employees through hearing protection programmes is paramount. Rather than been seen as a box-ticking exercise, a proactive and well-organised programme will reap benefits for employee and employer alike.

Our understanding of NIHL has significantly evolved since Barr’s seminal work in 1886. However, it is entirely reprehensible that despite all our understanding, the pernicious problem of NIHL has not been abated. NIHL can limit an individual’s ability to communicate and will often lead to social stress and depression. In addition, studies have shown that older people with mild hearing loss have a twofold increased risk of dementia while older people with severe hearing loss have a fivefold increased risk of dementia. NIHL is irreversible but it is also entirely preventable. If your employees understand the hazards of noise and how to practice good hearing health, they can protect their hearing for life. It is therefore essential to ensure that your hearing protection programme is fit for purpose and is fully complaint with all legal obligations.

References
1   European standard 458:2016. Hearing protectors. Recommendations for selection, use, care and maintenance – Guidance document.