Vibration

Introduction

Exposure to vibration is a more than a century-old risk factor that is still present in various sectors ^{[1] [2]}. Depending on its direction and the body part affected vibration has various health effects that affect the worker’s ability to work. In the final stage the majority of the diseases are incurable, thus prevention is essential ^[3]. European legislation regulates vibration at work in Council Directive 2002/44/EC. Exposure measurements and medical tests are guided by standards ^[4]. Physical characteristics of vibration, risk assessment and measurements methods, objective and subjective classification of health effects, common vibration induced diseases and prevention thereof are detailed below.

Characteristics of vibration

Mechanical vibration is the transmission of oscillating physical energy in solid material that rises during the operation of machines ^[5]. It is a dynamic phenomenon with varying intensity in time. Thus vibration can be characterised by its:

• course in time (stable, periodically changing, transitory, randomly changing, shock type excitation, impulse/impact)
• magnitude
• frequency spectrum

The magnitude of vibration can be characterised by the displacement, velocity and acceleration, these three parameters are correlated. Due to the nature of mechanical systems (inertia), significant displacement amplitude is achieved only on lower frequencies. Velocity is proportional to the energy that causes vibration. Measuring acceleration is important in case the assessment is broadened towards the higher frequencies. Measurement of peak and effective mean values are available for the above three variables. Root-mean-square is measured in whole-body vibration ^[6].

Health effects of vibration

Above a certain level the vibration transferred can harm the human body in different ways. Short term exposure to intense vibration can lead to discomfort and, thus, decreased productivity ^[1][7]. Chronic exposure can initially lead to functional and afterwards organic aberrations: changes in the body functions or even the structure of the tissues that cause impairment. “Coarse" vibrations, which are of mostly low frequency and high amplitude, harm predominantly the bones and joints. High frequency and low amplitude “smooth" vibrations are damaging mainly the soft tissues. In general, impact devices are more dangerous then those with rotating mechanism. Besides the physical characteristics of vibration, the severity of damage is dependent on other factors, such as: noise, wet, cold, ergonomics, and work methods ^[7][8].

Based on the place of entry of vibration into the human body, local (segmental) and whole-body vibration can be distinguished. Local vibration occurs predominantly in the hands and arms, as the worker mainly operates tools/machines with his/her hands. This is termed hand-arm vibration (HAV). The source can be the vibrating hand tool, or the object of work that is processed by a vibrating machine (e.g., use of pedestal grinder) ^[1][7][8]. The source of whole-body vibration (WBV) is the platform on which the worker is positioned and transmits the vibration of a nearby vibrating machine. It enters the human body via the lower extremities or the gluteal region ^[1][7][9]. Combined exposure is the simultaneous presence of HAV and WBV. An example of combined exposure is a tractor driving or the operation of heavy equipment; whereby, the driver is seated in the vibrating machine (WBV), while steering and manipulating the handles that are vibrating too (HAV).

Health effects of local vibration (mainly HAV)

Vibration is not a physiological stimulus to the human body (it is uncommon in the natural environment) and it mainly acts as repeated minute injuries (microtrauma) via the energy dissipated in the organ(s) ^[8]. The pathophysiology is still not completely understood. Vibration injuries are characteristic but not specific: there is no such disorder that can be seen only in vibration and not because of other cause. Thus diagnosis cannot be based solely on the clinical picture. The most typical, but still not specific symptoms of HAV syndrome are Raynaud-phenomenon and the avascular necroses of the bones. The final combined pain and disability in the upper limb can be significant ^[10] and correlated to the severity of the sensory component or the frequency of blanching attacks ^[11]. In the verification of an occupational disease it is essential to prove appropriate exposure and to rule out other diseases with similar symptoms (e.g., diabetes, frostbite, autoimmune diseases) ^[7][12].

Vascular

Vibration white finger starts with feeling cold in the hands. Typical sign is the so-called Raynaud-phenomenon ^[7]. The Raynaud-phenomenon begins with an increased ability to spasms in the small arteries, which temporarily and suddenly decreases the blood flow of the affected region. There may be three phases ^[7]. The fingers: (i) go white and cold (blanching resulting from spasm of the small blood vessels); (ii) turn into blue (from the lack of oxygen due to blocked circulation); and (iii) become bright red (the blood vessels open up and the return of the blood flow). Tingling, throbbing and pain may accompany the phenomenon.

However, as the above attacks are repeated, structural histological changes can be observed: overgrowth (hypertrophy) of muscles in the blood vessels and scarring (fibrosis) under the lining cells. These changes thicken the walls of the small arteries, which become rigid and may get even blocked. There are debates whether primarily Raynaud-phenomenon is the disorder of the blood vessels or of the nerves that innervate the vessels ^[7][12].

Specific localisation of the symptoms may be different according to different sorts of vibration exposures in the workplaces. Symmetrical exposure (i.e., jackhammer, rammer) usually causes symmetrical signs, while in unilateral exposure (e.g., pneumatic drill) signs are predominantly localised only on the hand exposed ^[3]. Physical examination may be negative in the beginning as there can be no identifiable alterations during the medical check in the incipient stage. Later the hands may be permanently cold and look livid (bluish colour), and Raynaud-phenomenon attacks can develop when they are exposed to low temperature. Ulceration or gangrene (trophic changes) due to loss of blood flow is extremely rare. In medical diagnosis the following tests are common: Allen’s and refill tests, nail compression, Allen manoeuvre, Adson's sign. Witnessing a Raynaud-phenomenon by the physician is decisive and the cold provocation test is a useful method to achieve a straightforward positive result. Unfortunately its sensitivity is low (around 50%) and in certain diseases it may be contraindicated (e.g., heart diseases, uncontrolled hypertension). Whole-body cooling of the patient is more effective (e.g., wintertime walk without gloves) ^[13]. The duration of a provoked Raynaud-phenomenon is short and it must be well-documented (localisation on the digits, type of discoloration), because it helps in the judgment of severity and in the follow-ups ^[7][12]. However, the grading of severity is not standardised ^[14].

While the Stockhom Workshop Scale ^[15] has a staging system for the vascular component relying on the attack frequency (Table 1), Griffin ^[7] introduced a classification of whitening of digits after the cold provocation test (Figure 1).

Table 1. Stockholm Hand-arm Vibration Syndrome Classification System - Vascular Component.

Source: Mason & Poole ^[16]

Neurological

Disorders of the peripheral nerves (peripheral neuropathies) may be caused by insufficient blood supply, while direct vibration injury to the nerves has been shown as well ^{[8] [17]}. Neuropathy starts with sensory symptoms followed by motor symptoms. Vibration damage of the peripheral nerves of the upper limb can take two forms ^[17][3]:

• upper limb polyneuropathy, which is diffuse and is getting more pronounced in the distal parts; and
• tunnel-syndromes, which are localised injuries of certain peripheral nerves.

The tunnel syndromes are named according to the anatomical region affected: thoracic outlet syndrome (chest), cubital (elbow), and carpal tunnel syndromes (hands). The initial complaints of peripheral neuropathies are various extent of numbness, "pins and needles" in the upper limb that are more pronounced in rest and at night, and can disturb sleep. Altered sensation (paraesthesia) may also occur during work. Sensory symptoms may be accompanied by motor injury, for example: weakness (decreased grip strength, dropping objects). Medical diagnosis is made by clinical neurological and electrophysiological tests, including: measurement of nerve conduction and sensation threshold limit, and tests with tuning forks ^[7][8][12]. The staging of nerve symptoms is made according to the Stockholm Workshop Scale (table 2)[25], which seems to be not closely correlated with the nerve conduction measurements ^[18]. As common polyneuropathies usually affect all limbs, the lower extremities are also tested to exclude neuropathies of non-vibration origin ^[8].

Table 2. Stockholm Hand-arm Vibration Syndrome Classification System - Sensorineural Component

Source: Gemne et al., ^[15]

Musculoskeletal

The injury of the bones and joints is mainly due to the vibration microtrauma, but may have components from harmed blood circulation and innervation. Damage to the musculoskeletal system decreases with distance from the vibration entry point. Various forms of bone tissue death (avascular carpal bone necroses, osteochondritis dissecans), structural aberrations and degenerative disorders may be the underlying pathology of pain, weakness, disability and swelling. However, there are cases when the worker is free from complaint despite having detectable alterations. Medical diagnosis starts with the physical examination, but the radiological tests are decisive (plain x-ray, computed tomography and magnetic resonance imaging).

Chronic vibration exposure or a sudden shock of the carpal bones (lunatum, naviculare) in the hand or the elbow may result the death of the bone tissue, which is rare but typical to vibration. The neighbouring area and joint may become painful and secondarily injured ^[19]. Arthritis of the elbow, the shoulder and their surroundings are mainly due to the unbalanced heavy physical load and less to the vibration. X-rays may reveal the fatigue fracture of the spinous processes in the lower neck and the upper thorax, which have no clinical relevance and do not cause pain or disability ^[19].

Health effects of whole body vibration

Subjective

Acute effects cause discomfort: nausea, vomiting, loss of balance may appear after work. Fatigue, dizziness, headache, sleep disorders might develop ^[9]. Oscillation of organs due to vibration has an effect on body functioning. Vibration around 1 Hz affects the sense of balance. Vibration in the range of 3-6 Hz affects organs in the thorax and abdomen. Vibration in the range of 20-30 Hz targets the head, neck and shoulders. The resonance point of the eyes is in the range of 60-90 Hz, while the system of the mandible and the skull is sensitive to vibration of 100-200 Hz. Such vibrations can affect visual acuity. Vibration of vehicles can be close to the resonance of the spinal column. Whole body vibration of higher frequencies can lead to gynaecological complaints among women ^[9].

Musculoskeletal

Very intensive acceleration (over 10 m/s2) may cause acute injury of the spine (fracture of the spinous process) ^[7]. Degeneration of the intervertebral discs is more common, more severe and its onset is earlier in workers exposed to whole-body vibration than in the non-exposed population. Decades of vibration microtrauma of the intervertebral discs and the vertebrae may cause early degenerative spine disorders (spondylosis, discopathy) with disability and resulting in pain during exercise. The most affected segments are in the lower lumbar region: the 4th and 5th lumbar and the upper sacrum. These musculoskeletal disorders are not specific to vibration at all and are quite common in the average population. Furthermore it must be noted that the correlation between the radiological extent of these degenerative disorders and the complaints is rather weak ^[20]. In a survey low back pain was found to occur when the worker was exposed to combined events; especially shock/jerking WBV events, plus awkward body posture when seated ^[21].

Vibration sources and occupations at risk

In Europe 33% of men and 10% of women, a total of 48.6 million workers, reported to be regularly exposed to vibrations ^[2]. Construction, manufacturing and mining, agriculture, fishing and forestry and electricity, gas and water supply are the most affected ^[1].

Local vibration

Five million Europeans are estimated to work in hand-arm vibration exposure and ISSA reported 1.7-3.6% having developed occupational disease ^[8]. Vibration may be necessary for the equipment (impact tools) or may be an undesired by-product (rotating mechanisms). In the former case vibration hazard cannot be eliminated, while in the later there can be plenty of room for technical intervention. HAV can be present during work with hand-held power tools (e.g., powered by compressed air, combustion engine, electricity) or operation of machines where the handles or the object of work transmit the vibration (e.g., hand guided machines and hand-fed tools with rotating mechanism). Foot vibration is rare ^[22].

Whole body vibration

Twenty percent of the population may be exposed to whole body vibration of whom occupationally exposed is in the range of one million in Europe ^[1]. Examples of machines that can post WBV include vehicles and heavy equipment. Some example occupations at risk include drivers and machine operators.

Risk assessment

Directive 2002/44/EC sets the minimum health and safety requirements regarding the exposure of workers to the risks arising from mechanical vibration. It stipulates the employer to assess and, if necessary, have the vibration levels measured to which the workers are exposed. The methodologies to be used are set in its annex. Special attention should be paid by the employer to ^[4]:

• the level, type and duration of exposure (including intermittent vibration or repeated shocks);
• the exposure limit values and the exposure action values;
• any particularly sensitive health and safety risk;
• any indirect effects from interactions;
• information provided by the manufacturers of work equipment;
• the existence of vibration decreasing replacement equipment;
• the extension of exposure to whole-body vibration;
• specific working conditions such as low temperatures;
• appropriate information from thehealth surveillance.

In practice vibration is measured by accelerometers, which provide electrical signal proportional to the acceleration ^[5]. Thus during measurement the vibration acceleration values are recorded. The root-mean-square value thereof is used for the assessment of WBV vibration. For the analysis of the frequency components filters are available that help in indicating whether the limit value is exceeded or not without the knowledge of the specific components of the vibration frequencies. In the assessment of the effects of vibration on the individual measured data are in reference to (i) the activity and (ii) the daily exposure.

Under the provisions of the Machinery Directive^[23], manufacturers are required to include information on the level of vibration to which the hand-arm system of a user is potentially subjected for both hand-held and hand-guided machinery. Although no substitute for direct measurement under the actual conditions of use, such information can be of help to employers in selecting equipment and in establishing a broad impression of the likely level of any risk to health.

Concerning the effects on the human body, it is not enough to measure the momentary or proportional vibration level but the analysis of frequencies is necessary because vibration at different frequencies is absorbed in different tissues and organs ^{[5][3] [24] [25]}.

Important stages of risk assessment and management in vibration are interwoven: 1. identification of jobs with the risk of exposure 2. evaluation of modifying factors 3. estimation/measurement of exposure 4. comparison of results with the limit values 5. identification of the sources and possible control measures (taken into consideration the hierarchy of preventative measures) 6. preparation of a control plan (ranking, deadlines, responsibilities, resources) 7. implementation of the programme 8. monitoring of the progress and evaluation of the programme 9. documentation

Repeat the steps if significant risk remained, there was a change in technology/organisation, or an occupational sentinel health event was detected ^{[5] [26]}.

Different vibration exposures of an individual can be added after conversion to decibels (dB) ^[7]. In the case of exposure to various sources of vibration of different levels (e.g. work with several machines in a single shift), the overall vibration exposure (A8) of the individual may be estimated by use of calculator applications. While simple scoring systems may provide acceptable approximations, proper calculations require the professional knowledge of a competent person ^[12]. Where information on the levels and duration of vibration exposures are available then on-line tools such as that made available by the UK HSE can be of great help^[27]. As a rule of thumb, the initial target of a risk-reduction intervention is the activity/machine that accounts for the greater proportion of the exposure ^[26].

Local vibration

Risk evaluation

Several factors influence the effect of vibration exposure beyond the magnitude and duration: work technique and the working environment ^[7]. These factors should also be taken into consideration during the assessment of the vibration exposure to the hand-arm system. Noise in the working environment cause spasm of the blood vessels (vasospastic) by itself thus it enhances such effect of vibration. Wet and cold environment promotes the development of Raynaud-phenomenon and musculoskeletal disorders ^[3]. Most work with vibration exposure is carried out inawkward postures and includes work phases with unbalanced or excessive physical workload ^[1]. Impedance of the body or the arm against high frequency vibrations decreases if the muscle tone is higher in the body or the handgrip (shift of resonance towards higher frequencies). The reverse is true if the tone decreases. The handling of the tool or object is an important factor, which is influenced by the muscle strength and the experience of the worker. More vibration from the machine is absorbed in the hand-arm system if the handgrip is tighter because of better coupling. Thus tighter handgrip elevates the risk of vibration harm ^{[25][3] [24]}.

Measurement

According to Annex A of the Directive, the assessment of the level of exposure to hand-arm vibration is carried out according to the ISO standard 5349-1(2001). The magnitude of the worker’s exposure is given as a vector sum of weighted equivalent vibration acceleration: the total value of frequency weighted r.m.s. acceleration, in metres per second squared (m/s2). It is based on the calculation of the daily exposure value referring to an eight-hour reference period A(8).

The measurement:

• may include representative sampling.
• methods and apparatus used must be adapted to the particular characteristics of
  • the vibration,
  • the measuring apparatus,
  • ambient factors in accordance with ISO standard 5349-2(2001).
• must be made on each hand if the device needs to be held with both hands. The higher value determines the exposure and information for the other hand is to be given ^{[4] [28]}.

As with whole body vibration, on-line calculators can be of value where reliable data are available^[29].

Medical surveillance

The aim of this system of medical actions is to prevent vibration diseases or, with the early detection of aberrations, prevent the progression thereof by the removal from exposure and the rehabilitation of injured workers ^[30]. Although the system of occupational medical tests varies in Europe, the poor prognosis of healing and treatment calls for the periodical check-up of exposed workers. Preemployment examination may reveal existing diseases that can be worsened by vibration. During periodical checks the initial screening test are questions focusing on suspicious signs and symptoms, for example: coldness of the hands, white/blue fingers in cold, sensory disturbances, musculoskeletal pain, weakening of grip strength. Physical examinations can include the above mentioned tests of the vascular, neurological and musculoskeletal systems. In case of complaints/aberrations appropriate tests with instruments are to be made ^{[31] [32]}. When the worker reaches Stage 2 in the Stockholm Workshop Scale (premorbid state) annual check-up and in case of progression removal from the exposure is recommended. Stage 3 requires immediate removal and rehabilitation ^[16]. Termination examination at the end of employment can assure that the worker suffered no harm ^[12][3].

Whole body vibration

Risk evaluation

The whole body vibration is the most dangerous to the human health in the range of 0.5-80 Hz and in the Z-axis (feet-head). In practice – except the lying position – vibration in the Z-axis is the most important and the other two are negligible most of the time. Whole body vibration may be transmitted to the body in standing, sitting or lying, however the vibration axes are always set according to the axes of the body. Pressure within the intervertebral disc is higher when sitting, thus vibration while seated may be more detrimental ^[3].

Measurement

According to Annex B of the Directive the assessment of the level of the exposure is carried out in line with the ISO 2631-1(1997) standard: based on the calculation of daily exposure A(8) expressed as equivalent continuous acceleration over an eight-hour period. Measurements have to be assessed separately because the exposure limit values are different for the vertical and horizontal directions. The measurement ^[4]:

• may include representative sampling.
• methods and apparatus used must be adapted to the particular characteristics of
  • the vibration,
  • the measuring apparatus,
  • ambient factors

Medical surveillance

The preemployment medical examination aims to detect susceptible individuals and factors that predispose to low back pain. Questions should be asked on complaints: lumbar pain, numbness in the lower limbs. The physical examination focuses on the state of the musculoskeletal system. In case of positive findings referral to a specialist should be considered. Routine x-ray screening of the back is not recommended. The periodical check-ups aims to reveal WBV disorders or diseases that contraindicate the employment in WBV ^[33].

Prevention

Prevention of vibration is dealt with in Articles 5 and 6 of Directive 2002/44/EC. Point 1 of Article 5 requires tackle vibration risks at source. Point 2 orders the employer to implement a programme to minimise risks in case the action values are exceeded. This is to take into account:

• other working methods with less exposure to mechanical vibration;
• the choice of appropriate work equipment with the least possible vibration;
• the provision of auxiliary equipment that reduces the transmission of vibration;
• appropriate maintenance programmes;
• workplace/work station design and layout;
• adequate information and training to workers on the use of work equipment;
• limitation of the duration and intensity of the exposure;
• appropriate work schedules with adequate rest periods;
• protection of workers from cold and damp.

Point 3 states that workers shall not be exposed above the exposure limit value. Point 6 sets the principles of worker information and training ^[4][26].

Besides the physical properties of vibration the chance and severity of harm is influenced by the physiological/pathological features of the exposed individual. Thus in certain conditions and diseases working in vibration exposure needs to be avoided ^{[34] [35] [3]}:

• The body of youngsters is in development and (their especially their musculoskeletal system) can cope only with smaller load.
• Women usually has less muscle mass and strength. During pregnancy there is the risk of harming the foetus and placental abruption. After pregnancy and during breastfeeding their body is under extra strain.
• Ageing workers may already have several diseases; the rate of degenerative diseases increases and functional capacities decreases with age.

Directive 94/33/EC specially addresses young workers and Directive 92/85/EEC pregnant workers by prohibiting work in which there is a risk to health from vibration ^[34][35].

Local vibration

Article 3(1) of Directive 2002/44/EC sets exposure values for hand-arm vibration. The 8 hour time-weighted average exposure ^[4]:

• limit value is 5 m/s2;
• action value is 2.5 m/s2

Vulnerable groups

Besides the groups above, it must be taken into account that peripheral arterial diseases are more common among women. Furthermore, workers with diseases that can be specifically worsened by HAV need special attention: diseases of the vessels in the hand (narrowing of the arteries, primary Raynaud's disease) and of the peripheral nerves (neuropathies). General diseases (diabetes mellitus, alcohol misuse, smoking, TOS, carpal tunnel syndrome, thyroid diseases, etc.) that can cause the above two conditions and musculoskeletal disorders of the upper limb and the neck (congenital aberrations e.g. cervical rib) make more vulnerable to vibration harm. In these cases individual medical assessment is recommended ^[3].

Technical prevention

Reduction of hand-arm vibration exposure can be achieved by the ^[36]:

• reduction of coupling intensity (reduction of the grip force by using a lighter tool);
• production of riveted joints by compression riveters or recoil-reduced rivet hammers;
• use torque screwdrivers (instead of impact wrenches);
• use of drill hammers (rather than impact drills);
• use of low-vibration pavement breakers and hammer drills;
• use of chipping hammers in stone and steel working;
• use of power chainsaws with low vibration handle
• reduction the need for vibrating tools (e.g., adhesives instead of riveting).

Organisational prevention

Organisational measures can reduce the exposure in several ways ^[1][4][26]:

• Eliminate/substitution of the tool or process that is the source of the vibration – this is the primary and most important.
• Reduction of the number of exposed workers.
• Reduction of exposure duration by dividing tasks among workers (e.g., four workers work only 2-2 hours in the exposure).
• Management of the working time by introduction of appropriate pauses (minimum 15 minutes).
• Provision of warm rest place during the pause for those working in cold environment.
• Prohibition of overtime work.
• Training and information of workers on the prevention measures.[1] [2] [38]

Personal prevention

Personal protective devices alone are not capable to prevent harm from vibration ^[37]. Anti-vibration gloves are less effective against low frequency, high amplitude vibration. However, gloves can be useful in tackling cold and wet conditions, which aggravate vibration health effects. Before procuring so-called anti-vibration gloves the technical and medical certification need to be consulted because too thick gloves can have paradox effect: the worker does not feel the tool and grips tighter. This enhances vibration transmission and also deteriorates blood circulation in the fingers ^[26]. Attention should be paid to the risk of dampening/soaking of gloves, which can have harmful effects by cooling the hands ^[3].

Evaluation of medical data

According to Directive 2002/44/EC the employer shall assess and, if necessary, measure mechanical vibration of workers ^[4]. Risk assessment shall be made based on this input. It is to be given to the specialist responsible for health surveillance, who makes the examination in line with the risk assessment. If causal relation can be proved or suspected between the exposure and a diagnosed disease then the risk assessment must be re-evaluated and the employer should do everything to minimise exposure and prevent the development of new diseases. The worker has right to know his/her medical data, exposure risks and prevention measures thereof. The employer should be informed on all relevant findings of medical surveillance, while heeding privacy of medical data ^[30].

Whole body vibration

Article 3(2) of Directive 2002/44/EC sets exposure values for whole-body vibration. The 8 hour time-weighted average exposure ^[4]:

• limit value is 1,15 m/s2. At the choice of the Member State concerned, a vibration dose value of 21 m/s1,75.
• action value is 0.5 m/s2. At the choice of the Member State concerned, a vibration dose value of 9.1 m/s1,75

Vulnerable groups

Workers with diseases that can be specifically worsened by WBV, like inflammatory and degenerative disorders of the lower limb and back with structural aberration, spine surgery are at an increased risk ^[9]. In such cases individual medical assessment is necessary.

Technical prevention

Reduction of whole-body vibration exposure can be achieved several ways ^[9]. By making the surface of the roads and rails even (repair, welding) and with the limitation of driving speed, the vibration to drivers can be decreased. The application of spring-mounted driver’s seats or cabs that are adjustable to different body weights and using only tested seats are practical in vehicles. In stationary machines and vibrating surfaces the transmission of vibrations to the human being can be reduced by appropriate vibration isolation of the machine or the workplace as a whole. In addition the machine must be isolated from all parts of the building or other machines. Modern equipments usually pose less risk to workers ^[36][26].

Organisational prevention

Measures of organisational prevention are similar to those of local vibration except for the pauses. Introduction of pauses against WBV can be successful only together with consecutive relaxation gymnastics ^[36].

Personal prevention

Vibration damping safety shoes with thick rubber soles may be beneficial ^[36].

Conclusions

The high fraction of workers exposed to vibration and the characteristics of diseases caused underline the significance of prevention, which is achievable by technical and organisational measures.