There are many sources of vibration in the occupational environment and from a workplace perspective we are generally concerned about the potential risk to human health. Exposure in the workplace is assessed in terms of hand-arm vibration and whole-body vibration. In addition, legal controls and criteria for workplace exposure are addressed in terms of these phenomena.
Vibration may be transmitted to the human body through the part in contact with the vibrating surface: the handle of a machine, the surface of a piece of equipment, or the seat of a mobile machine.
Hand-arm vibration (HAV) is vibration which is transmitted from work processes and equipment into workers’ hands and arms. It can be caused by operating hand-held power tools, such as pneumatic wrenches, angle grinders, road breakers, and hand-guided equipment.
Whole-body vibration (WBV) has been described as a shaking or jolting of the human body through a supporting surface (usually a seat or the floor). The vibration can potentially enter the body via a number of routes and potentially affect organs and/or human health.
Examples of WBV include exposure arising when driving or riding on a vehicle over rough terrain, operating earthmoving machines, or standing on a structure attached to a large, powerful, fixed machine which is vibrating.
Regular and long term exposure to WBV is linked to lower back pain and industries associated with WBV include agriculture, mining and construction. However, emphasis is given to HAV throughout the remainder of this article.
Vibration –basic concepts
A vibrating object moves back and forth from its normal stationary position. The number of cycles that a vibrating object completes in one second is an important characteristic called frequency. The unit of frequency is hertz (Hz) and one hertz equals one cycle per second.
Vibration magnitude can be measured using any one of three different quantities: Displacement, Velocity or Acceleration. Typically, acceleration is used particularly when dealing with occupational vibration. A typical vibration measurement system includes an accelerometer and an instrument to measure the level of vibration in accordance with ISO 5349 (with the instrumentation based on ISO 8041).
A transducer is firstly required to convert the movement of the vibrating body into a voltage which is proportional to the motion of the vibrating body. The transducer (accelerometer) will predominantly utilize piezoelectric crystals to produce an electrical voltage when subjected to an applied force. This is proportional to the applied force and the voltage is subsequently assessed and processed by the vibration meter. The measurement system incorporates the accelerometer and the standardised frequency-weighting to give a single number measurement (expressed as the frequency-weighted vibration exposure).
There have been major developments in Micro-Electro-Mechanical Systems (MEMS) and this technology has been used to develop ‘wearable personal vibration exposure meters’. Compliance with ISO 5349 and ISO 8041 is not generally claimed by the manufacturers of these meters but nonetheless studies have established their relevance in assessing and managing HAV risk.
Measuring and Assessing HAV Exposure
Many factors need to be considered when measuring vibration. The mounting of the accelerometer is very important and can often raise challenges. There are a number of methods available for mounting accelerometers, but the most important consideration is that the accelerometer is securely fixed to the surface of the vibrating body.
Typically, the measurements are taken with the accelerometer primarily fixed onto the tool handle and located close to the hand position. Ideally, the accelerometer should be located at the middle of the gripping zone (e.g. halfway along the width of the hand when gripping a power tool handle). It is at this location that the most representative evaluation of the vibration entering the hand is obtained. However, it is often not practicable to locate transducers exactly at this point (e.g. where the transducers could interfere with the normal grip used by the operator). For many measurements, the accelerometers are mounted either side of the hand or on the underside of the tool handle adjacent to the middle of the hand.
The mounting of accelerometers on a power tool or hand-held workpiece can be intrusive and will have some effect on how the operator works.
The mounting of the transducers should be arranged so that the operator can work as normally as possible. It is important, prior to measurements, to observe how a power tool or hand-held workpiece is held, to identify the best location and orientation of the accelerometer.
In some cases, an accelerometer will be mounted on a workpiece (e.g. in the case of a workpiece being worked on at a grinding wheel, belt sander, or belt grinder) close to where the workpiece is held. This is because in these cases, the employee’s hand is exposed to the vibration by holding the workpiece (which is applied to a vibrating source) and he/she has no direct contact with any tool handle.
Triaxial accelerometers measure the vibration in the three axes (x, y and z) simultaneously. In HAV surveys, the accelerometer is typically mounted using a band clamp or ‘Jubilee clip’ and representative measurements are taken of the potential vibrational energy transferred to the operator’s fingers/hands. A schematic of the survey/assessment technique is presented in Figure 1.
It is important to note that HAV measurements should be undertaken in accordance with ISO 5349. In addition, regard should be had to best practice guidance and there needs to be some awareness of the uncertainty of the measurement data as well as the extrapolation of results. Vibration levels from identical tools have been shown to fluctuate and many factors can influence the survey results. Variables such as the age and the condition of the tool, the type of attachment/accessories, the material being worked on, and the experience/skill of the tool operator, will potentially influence the results. The grip and feed forces applied by the operator through the hands to the tool or the workpiece are also notable variables.
Personal exposure to vibration arises as a result of the combination of the time a vibrating tool (or item of equipment) is used per day and the typical vibration (acceleration) level of the surface in contact with the exposed person. The risk assessment is based on the measured (frequency-weighted acceleration value) for the most exposed hand. Where tools are held in both hands, we measure them separately and take the highest result, i.e. the one with the greatest exposure is used for the risk assessment.
The duration of exposure or ‘trigger time’ is the time the hands and arms are actually exposed to the vibration from the tool or work piece. The ‘trigger time’ is often much shorter than the overall time on the job and can be overestimated by workers. The method used for estimating trigger times often depends on whether the tool usage is continuous or intermittent. In many cases, tool setting up time and preparation time will mean that an employee has a trigger time lower than the total time he/she uses it. Trigger times can be estimated or measured by a variety of techniques such as a controlled assessment using a stopwatch, the use of a dedicated data logger linked to power tool usage or by the analysis of video recordings.
In order to calculate the A(8) values, the results of the HAV measurements need to be ‘adjusted’ to reflect the likely exposures. It is generally advocated that in order to reduce the uncertainty of vibration measurements which are designed to evaluate the vibration attributable to a specific task, that repeat measurements involving a number of different workers are taken.
According to the ISO standard, 5349-2 ‘if the purpose of the measurement is not to evaluate the vibration exposure of a specific worker, but to evaluate the exposure of a specific task, the evaluation of vibration exposure should, if possible, be based on measurements using at least three different workers. The reported result shall be the arithmetic mean of the measurements, the standard deviation should also be recorded.’
One of the key methods used to control exposure involves minimising the time spent using tools with high levels of HAV. In some facilities this can be achieved by job rotation and using rest breaks to ensure that long periods of continuous equipment use do not arise. During these ‘breaks’ employees can undertake other tasks which do not give rise to HAV exposure.
Once we measure the actual vibration levels (from the tool or process), the results can be used to determine the maximum exposure time permitted to ensure that the Exposure Action Value or Exposure Limit Value is not exceeded. Where the HAV weighted acceleration level (ahv) summed over the three measurement axes is relatively low, this will not be necessary.
HAV Risk Assessment
The risk of HAV can potentially affect people across many industries and occupations. The risks are greatly increased with the use of higher vibration equipment and with prolonged and regular use of the equipment.
One of the critical issues in HAV assessment is the level of personal exposure. It can be acceptable to use tools with relatively high levels of vibration if the duration of exposure to that vibration is relatively low. A detailed review of exposure times is generally undertaken, and job rotation schedules are amended (where necessary) to ensure that no employee’s exposure is permitted to exceed the Exposure Limit Value (5 m/s2) and/or the Exposure Action Value (2.5 m/s2).
Throughout Europe, the principal legislation concerning the regulation and control of vibration exposure is set out in Directive 2002/44/EC, which is referred to as the ‘Physical Agents (Vibration) Directive’.
The Directive provides general obligations to consult with employees regarding the vibration exposure levels, to assess the vibration exposure levels and to eliminate or minimise health risks at source. In addition, a series of obligations become applicable at the Exposure Action Value and Exposure Limit Value which are provided for HAV as shown in Table 1.
Daily exposure, A(8) is the quantity of mechanical vibration to which a worker is exposed during a working day, normalised to an 8-hour reference period, which takes account of the magnitude and duration of the vibration.
The Exposure Action Value can be simply defined as the level of daily exposure for any employee which, if exceeded, requires specified action to be taken to reduce risk.
Exposure Limit Value, on the other hand means the level of daily exposure for any employee which must not be exceeded.
While reference should be made to the detailed requirements of the Directive (and/or any associated regulations), a summary of the key obligations which arise is provided in Table 2.
In effect, higher exposure levels pose greater risk and the obligations on the Employer should be commensurate with the level of risk.
The Machinery Safety Directive, 2006/42/EC requires manufacturers to consider safety in the design of plant and machinery and that vibration emission is reduced to as low as possible. According to the Directive: ‘Machinery must be designed and constructed in such a way that risks resulting from vibrations produced by the machinery are reduced to the lowest level, taking account of technical progress and the availability of means of reducing vibration, in particular at source’.
The requirement to take account of ‘progress and the availability of means of reducing vibration’ is important as it requires the designers and manufactures to adapt their equipment in line with technological development. This Directive (2006/42/EC) establishes the overall duty of care of manufacturers to design plant and equipment to minimise vibration emission levels. The manufacturer is required to declare values measured according to an ISO test code (if one exists) or to describe the test procedure used. If the HAV value is less than 2.5 m/s2 (based on the Exposure Action Value in Directive 2002/44/EC) for example, there should be a statement to say that the threshold has not been exceeded. If the value is greater, then the measured value must be quoted along with the uncertainty of measurement.
Legislation to manage HAV provides a definitive and well-practiced framework within which every worker must be protected. While the statutory limit values may be regarded as minimum standards, the universal risk assessment paradigm allows us to examine and evaluate the workplace and the appropriateness of the controls case-by-case.
Thus, the evolving nature of risk assessment and the progress of scientific method and technological advancement allow us to evaluate vibration exposures and judge whether the controls are commensurate with the degree of risk. Linkages to common law rights, commercial imperatives, (e.g. insurance) the scientific community and OSH practice significantly increase the legislation’s reach. Taking account of technical progress should not be underestimated and it requires a considered response to exposures which should be periodically assessed when there is a likelihood of significant HAV exposure.
HAV can cause a range of conditions collectively known as hand-arm vibration syndrome (HAVS), as well as specific diseases such as carpal tunnel syndrome. Identifying signs and symptoms at an early stage is important. The aims of a health surveillance programme are primarily to safeguard the health of workers (including identifying and protecting individuals at increased risk), but also to check the long-term effectiveness of control measures. One of the specific aims is to prevent workers developing a degree of HAVS that is associated with disabling loss of hand function. Health surveillance for HAVS is appropriate where a risk assessment has shown the need and it should operate alongside a programme of vibration risk control measures.
The probability of an individual developing symptoms of HAVS depends on his/her susceptibility, any pre-existing diseases and conditions, and the work-related, environmental and personal factors. The Employer must adapt the control measures to take account of any employee who is at particular risk from mechanical vibration, including those with:
• disorders of the peripheral nervous system
• disorders of the musculoskeletal system
• existing HAVS, nerve disorders or other diseases of the hands, arms, wrists or shoulders
• diseases affecting blood circulation, e.g. diabetes
• primary Raynaud’s disease or other causes of secondary Raynaud’s phenomenon
In some countries it is recommended that young people (under 18 years) do not use certain vibrating tools. In addition, special precautions may need to be addressed for pregnant workers, people who have recently undergone surgery and people with internal or external prosthetic devices (not including dentures).
The importance of training cannot be underestimated and where employees are exposed to risk from vibration, an employer must provide those employees with suitable and sufficient information, instruction and training, including:
(a) the technical and organisational measures taken in order to comply with the relevant requirements
(b) the exposure limit values and the exposure action values
(c) the results of the risk assessment and measurement of the mechanical vibration
(d) why and how to detect and report signs of injury
(e) the circumstances in which health surveillance is made available to employees and its purpose
(f) safe working practices to minimise exposure to mechanical vibration
A good training programme will encourage the cooperation of employees and should include issues such as:
• the health effects of hand-arm vibration
• sources of hand-arm vibration
• whether employees are at risk, and if so whether the risk is high or low
• the risk factors (e.g. the levels of vibration, daily exposure duration, regularity of exposure over weeks, months etc.)
• how to recognise and report symptoms
• the need for health surveillance, how it can help employees remain fit for work, how you plan to provide it, how you plan to use the results and the confidentiality of the results
You should also communicate with your employees about ways to minimise the risk including:
• changes to working practices to reduce vibration exposure
• correct selection, use and maintenance of equipment
• correct techniques for equipment use, how to reduce grip force etc.
• maintenance of good blood circulation at work by keeping warm and massaging fingers
In addition, employees need to be advised of the link between smoking, blood circulation and HAV. If exposed to HAV they should, if possible, cut down on smoking, or ideally cease it entirely.
Regulation and compliance
According to the HSE’s (UK) latest guidance (2021) where the Exposure Action Level is exceeded, health surveillance should be carried out at a pre-employment (baseline) stage, six months later and regularly thereafter, usually annually unless the risk assessment suggests a more frequent schedule is warranted. Workers who are more sensitive may have more frequent surveillance. Information should be given to those exposed on the risks, symptoms and rational for health surveillance.
While a tiered approach (5-stage) is advocated by the HSE, at its simplest level, health surveillance will involve the creation of a health record and the encouragement of self-reporting of symptoms. Self-reporting pre-supposes some knowledge of what symptoms should be reported, which in turn requires training.
The HAV risk assessment must be reviewed by the Employer where there has been a significant change in matters to which it relates, or there is another reason to believe that it is no longer valid. Following the review, the Employer shall amend the risk assessment as appropriate. In practice, this means that the Employer should review the risk assessment whenever he or she identifies any changes in availability or suitability of equipment or in work processes likely to offer reduced vibration exposure, or if there is any doubt about the effectiveness of the controls implemented.