Human beings have been vibration exposed for thousands of years to nonhuman‐generated cyclic or repetitive loading since the beginning of the use of tools, boats, airplanes, railway travel, or other transport methods using animals or platforms that could be dragged or placed on runners/skids, rollers, or wheels. Impact and vibration (acceleration) have been of increasing interest since the early 1900s. As early as 1918, there was intense interest in the medical effects of power tool vibration exposure on workers by the famous occupational medicine pioneer Dr. Alice Hamilton.¹

OCCUPATIONAL SETTING

Vibration is the periodic motion of a body in alternately opposite directions from a position of rest. Vibration is present in most work settings where mechanical equipment is used. When vibration interacts with the human body, the coupling pathway in which it enters and moves through the body defines its path of travel. There are two major types of vibration that have human health concerns. Whole‐body vibration (WBV) affects the entire body and is usually transmitted in a sitting or standing position from a vibrating seat or platform. Segmental or hand–arm vibration (HAV) affects one or both upper extremities and is usually transmitted to the hand and arm only from a motorized hand tool. WBV is generated by motor vehicle operation, including cars, trucks, buses, trains, marine craft, construction and agricultural (tractors, threshers, or combined) equipment, and heavy manufacturing equipment such as looms, large machine tools, and presses. HAV is generated by any powered hand tool, including chippers, jackhammers, chainsaws, trimmers and blowers, nut‐tightening guns, polishers, grinders, and rivet guns. These tools have widespread use in industry and may be electric, pneumatic, hydraulic, or combustion engine powered. Workers using any powered hand tools have potential exposure.

In most cases, WBV and HAV exposures are distinctly separable. In a few cases there are crossover situations where simultaneously both intense WBV and HAV exposures occur simultaneously. For example,

• When a “pavement breaker tool” is operated at arm’s length away from the operator’s body, only the fingers, hands, and arms are involved, and this is clearly HAV exposure; but when this same tool is operated differently and the operator now also leans into this tool while working and places it against the stomach, in an attempt to damp the vibration, it results in HAV and WBV exposure because the vibration has a second pathway coupling it to the body (hands and stomach) with the distinct possibility of injury or disease to the operator’s fingers and hands (HAV) and/or the omentum (WBV).
• The off‐road operation of a motorcycle, resulting in intense HAV exposure to the hands via vibration coupled from the handlebars and simultaneously intense WBV exposure entering the spine via the operator’s seat from the engine, ground conditions, and vehicle speed.

The health consequences and effects of WBV and HAV exposures are mostly distinctly different as are the WBV and HAV protective standards, so they will be discussed separately in this chapter. The basic physics and vibration engineering terminology are the same for both WBV and HAV.

OCCUPATIONAL VIBRATION MEASUREMENTS

Vibration is a physical agent, expressed in terms of motion (acceleration), time, and frequency. It is not practical or acceptable to determine a dose of vibration that an individual receives inside the body as this would require an invasive measurement method such as fixing a motion sensor directly to a point of interest in or on the skeleton. Instead, surrogate motion information is used, measured at an interface between the individual and a vibration source.

Vibratory motion is by definition a mathematical “vector quantity” which simply means that it is described by both a direction and a magnitude. At each measurement point, the total motion is described using six possible vector directions; three so‐called linear directions (up–down, side to side, front to back) with their magnitudes and three rotational directions (pitch, yaw, and roll) with their magnitudes. In most human vibration work, only the linear directions are measured, reported, and compared to health/safety standards.

Figure 4.1 shows the internationally accepted “biodynamic coordinate system” used for head to toe, or whole‐body vibration (WBV), and similarly for segmental or hand–arm vibration (HAV). For WBV the sternum is the reference point of the measurements. The vibration intensity or magnitude quantity of choice for WBV (and HAV) vibration is “frequency‐weighted acceleration (ms⁻²), root mean squared (rms).” For WBV, by definition, motion in the “Z” direction is head to toe; the “Y” direction is side to side (shoulder to shoulder); and the “X” direction is front to rear. Each of these three linear directions and corresponding acceleration magnitudes is separately measured, digitally stored, and reported, and these data are used for exposure calculations.

For HAV measurements, the third metacarpal is the reference measurement point (Figure 4.2). The defined motion in the “Z” direction (axis) is along the long bones of the forearm; the “Y” direction is motion across the knuckles; and the “X” direction is motion through the palm. Many times it is not practical to place measurement accelerometers on the third metacarpal; thus measurements are obtained directly from the vibrating tool handle as the operator works and grips the handle. This tool handle frame of reference is called the “basicentric coordinate system.” The reason for using these types of WBV and HAV coordinate systems is to establish uniform measurement methods worldwide and a means of easily comparing measurements to health and safety standards.

In practice, three‐axis (triaxial) vibration measurements are simultaneously and individually obtained and recorded on separate channels (on a computer‐based data acquisition system), and all triaxial data are saved. In both WBV and HAV measurements, what is usually sought is the total impinging vibration or the so‐called weighted vector sum of all three measurement axes. Once this quantity has been determined, it is next compared to the appropriate health and safety standard(s) to determine if and by how much said standards have been exceeded. If these results indicate that the appropriate standard has been exceeded, then vibration controls are most likely needed.

The expression of vibration magnitude or intensity is “acceleration” expressed in units of meters/second/second root mean squared or ms⁻² rms as, for example, 10 ms⁻² rms, 20 ms⁻² rms, etc. worldwide; in the United States we also use the familiar “gravitational g term for acceleration,” where 1 g = 9.81 meters/second/second or ms⁻². Acceleration is defined as the time rate of change of velocity. Vibration frequency is expressed as hertz (Hz), defined as a complete vibration cycle occurring in a 1 second time span.

A unique concern is a vibration characteristic called “resonant or natural frequency vibration.” Resonance or vibration at a resonant frequency is an undesirable condition when input vibration impinges on a structure or mechanism that in turn triggers an uncontrolled, undesirable, internally amplified, and exacerbated response from the structure or mechanism. This is of concern in occupational vibration because humans can respond at their human resonant frequencies when impinging vibration is uncontrollably amplified and exacerbated by the human body, depending on the impinging vibrations direction, magnitude, frequency, and exposure duration time. Human response to vibration at these resonant frequencies represents the “Achilles heel” of human vulnerability, impairment, and disease.

Because of resonance and other less well‐understood factors, human response to vibration is not linear, meaning that our bodies do not respond to each vibration frequency the same way. At resonance, these special frequencies of vibration can unleash a large involuntary response from the body, sometimes with small triggering inputs, as compared to the results of nonresonant frequencies of vibration, being triggered with the same vibration intensities. Vibration axis/direction, exposure time, frequency, and intensity all matter. To take these factors into account when determining a dose–response relationship, we use a mathematical calculation called a “frequency weighting function,” whose purpose is to mimic and attempt to characterize human WBV response across a wide frequency range or bandwidth and similarly use another weighting function for HAV response.

Another measure used to analyze vibration is Crest Factor, which is defined as the ratio of peak acceleration value divided by its corresponding root‐mean‐squared acceleration value. Since acceleration units are both in the numerator and denominator, the result is expressed as a pure number. This is of particular concern in WBV when vehicles are driven on bumpy roads or when trains experience “slack action” due to changes in their car couplers.

Most vibration measurements of WBV and HAV workplace environments contain complex vibration consisting of multiple vibration frequencies occurring simultaneously. Currently, a very few vibration standards data analysis require that a Fourier analysis be performed for each of the three axes of data. This analysis mathematically dissects and identifies each of the vibration frequencies from its complex overall vibration revealing its total elemental contents and its respective total amplitude contribution to the original overall complex vibration. Fourier analysis is a mathematical decomposition technique that is analogous to the well‐known electro‐optical “spectroscopy” technique used in chemical analysis when examining an unknown complex compound by determining and revealing the compound’s elemental composition and the respective concentration for each element contained in the complex compound.

In this chapter the terms jolt, impact, and mechanical shock are used synonymously. “Shock is a somewhat loosely defined aspect of vibration wherein the excitation is non‐periodic, e.g., in the form of a pulse, a step, or transient vibration” rather than repeating itself over and over again. “The word ‘shock’ implies a degree of suddenness and severity.”²

OCCUPATIONAL VIBRATION GUIDELINES USED IN THE UNITED STATES

Standards relating to human exposure to vibration have been evolving since the early 1970s. With the exception of the European Union’s Directive 2002 where WBV and HAV standards both are binding law in member states, WBV and HAV standards used in the United States are voluntary nonbinding consensus standards/guidelines which are periodically reviewed and updated as new information becomes available.³ Currently there are no official Occupational Safety and Health Administration (OSHA) vibration standards. However, OSHA can and does cite organizations for excessive WBV and/or HAV exposures under the general duty clause of the Occupational Safety and Health Act.

Currently four whole‐body vibration standards/guidelines are used in the United States:

• ACGIH standard for WBV,⁴ ISO 2631 WBV standard,⁵ ANSI S3.18‐2002/ISO 2631‐1:1997, Nationally Adopted International Standard,⁶ and the EU Physical Agents Directive 2002.³

Three hand–arm vibration standards/guidelines are used in the United States:

• ACGIH standard for HAV,⁷ ANSI S2.70‐2006,⁸ and the National Institute for Occupational Safety and Health (NIOSH) Publication #89‐106 criteria document for a standard for HAV.⁹ There is also a 2014 antivibration/reduced HAV glove testing certification standard ISO 10819¹⁰ used worldwide including the United States. The following discussion contains the basic elements of the occupational vibration standards.

Whole‐body vibration standards

All WBV standards are generally used as follows:

1. Triaxial WBV acceleration measurements are made, digitized, and stored as separate channels in memory.
2. Signals from each of the three axes are weighted.
3. A weighted vector sum is calculated resulting in a single weighted total value. This is what is sought.
4. With knowledge of the worker’s daily WBV exposure time, this resultant value is compared to the exposure range limits: from 0.5 to 1.15 ms⁻² rms (for an 8‐hour daily exposure).
5. If the calculated value for an 8‐hour exposure is:

   ≤ 0.5 ms⁻² rms, then the conditions require no treatment,

   > 0.5 ms⁻² rms but ≤ 1.15 ms⁻² rms, conditions must be treated, and

   > 1.15 ms⁻² rms, exposure must be reduced.

ACGIH WBV STANDARD

The early 1970s saw the introduction of the world’s first occupational WBV standard: the international standard ISO 2631.^4,11 The limits used were similar in the 1978 and 1985 versions and were the “gold standard” and were used extensively worldwide until 1997 when the ISO chose to significantly change and modify its standard to its current version.^5,12,13 Some strongly believed this early version of ISO 2631 should continue to be used because it was developed from some of the best and most pristine research conducted in this field to date. Eventually ACGIH agreed and adopted the best parts as their own.¹⁴ Graphs of the values from the ACGIH WBV standard are given in Figures 4.3 and 4.4. In order to use this standard, all measured WBV acceleration per axis must first be Fourier analyzed and converted into spectra where Figure 4.3 is used to evaluate WBV measured spectra in the vertical “Z” direction. Figure 4.4 is used separately, twice, first to evaluate spectra in the side‐to‐side “Y” direction and separately again in the front‐to‐rear “X” direction.

Referring to Figure 4.3, the horizontal axis shows vibration frequency in so‐called 1/3 octave bands, extending from 1 to 80 Hz. The vertical axis of Figure 4.3 gives a measure of the vibration “intensity” magnitude in root‐mean‐squared (rms) acceleration in either “g’s” or meters per second per second, where 1 g = 9.81 ms⁻². A “family” or set of parallel “U”‐shaped daily exposure time curves is also shown in the graph. These U‐shaped exposure curves are the frequency weighting function for Z axis WBV, where the trough of the curves is 4–8 Hz in which are found the resonant frequencies, and greatest sensitivities, mentioned above in the Z direction. What Figure 4.3 really shows are vibration intensity (acceleration) limits as function of vibration frequency and daily worker exposure time for the “Z” axis. Figure 4.4 shows the same type of information for the “Y” and “X” axes where 1–2 Hz are the resonant frequencies. If one or more axis acceleration exceeds the standard, then the entire standard has been exceeded, and appropriate vibration control action is necessary.

ISO 2631 WBV STANDARD (ISO 2631‐1:1997; 2010), (ISO 2631‐2:2003; 2003), (ISO 2631‐4:2001; 2001), (ISO 2631‐5:2004; 2004)

ISO 2631 is a multipart standard related to human exposure to whole‐body vibration that is promulgated and periodically updated by the International Standards Organization of Geneva, Switzerland. The latest version of ISO 2631 used in the United States includes several parts that address measurement and evaluation of human exposure to whole‐body vibration. Current versions address possible health, comfort, perception, and motion sickness effects of vibration in general (2631‐1)⁵; occupant comfort and annoyance in buildings (2631‐2)¹⁵; passenger and crew comfort in fixed‐guideway transport systems, such as trains (2631‐4)¹⁶; and adverse health effects on the lumbar spine in conditions containing multiple shocks (2631‐5).¹⁷

EUROPEAN UNION (EU) PHYSICAL AGENTS (VIBRATION) DIRECTIVE 2002

The EU Physical Agents Directive is legally binding in all EU member states.³ The EU Physical Agents Directive established two levels of WBV exposure evaluation criteria:

• The daily exposure action value (DEAV) = 0.5 ms⁻² rms for an 8 hours/day WBV exposure
• The daily exposure limit value (DELV) = 1.15 ms⁻² rms for an 8 hours/day WBV exposure

When the DEAV 8 hours/day WBV exposure total weighted rms acceleration value is less than 0.5 ms⁻² rms, it is considered a safe level. If the DEAV = 0.5 ms⁻² rms or greater, then WBV corrective action is required to start reducing exposure levels.

When the DELV 8 hours/day WBV exposure total weighted rms acceleration limit value of 1.15 ms⁻² rms or greater is reached, then all efforts must be made to reduce exposure levels even if the WBV source must be used for less than 8 hours/day, or added control measures must be used, all in an effort to reduce human WBV daily exposure to acceptable levels.

In practice, triaxial WBV acceleration measurements are made, usually at or near the point where vibration enters the body, for example, for seated postures, using a special rubber “pie plate” circular disk, containing the triaxial accelerometer buried at its center. This instrumented disk is placed between the seat top cushion and the operator’s buttocks. Measurements are conducted under actual work conditions, digitized, converted to rms values, weighted, and stored in memory. The resultant total weighted rms acceleration value is calculated and compared to the DEAV and DELV EU values just described, and a determination is made as to the need for any corrective vibration control measures.

Fourier spectrum analysis is not needed to use this EU standard. If such analysis is later needed in order to correct a problem and reduce the WBV, then the so‐called stored raw acceleration data for each vibration axis can be revisited with more analysis.

WBV standard limitations

Jolt/impact or repetitive mechanical shock has emerged as an important problem. In the past, the standards/guidelines have tried to cope with jolt/impact by comparing its frequency‐weighted peak to the frequency‐weighted root mean square of the signal of which it is a part over a period of time. This ratio is known as the crest factor.¹⁸ The current standards/guidelines emphasize that if the crest factor exceeds 9, then the exposed individual’s risk is underestimated.⁵ Jolt/impact can result from a combination of sinusoidal signals and are not fully addressed by the current standards. When Cohen, Wasserman, and Hornung studied human response to single versus combined sinusoidal whole‐body vibration signals, it was clear that, for the same root‐mean‐squared acceleration levels, people (healthy firefighters) were more sensitive to the combined vibration.¹⁹ The above standards are based on the assumption that workers at many jobs are exposed to sinusoidal vibration, but workers are also exposed to nonsinusoidal vibration. For these jobs, the above standards do not fully address and likely underestimate the risk. To address these issues, research by Fethke et al.²⁰, Morrison et al.²¹, Robinson²², Sandover²³, Gant et al.²⁴, and others has led to ISO and ANSI standard development efforts for exposure to nonsinusoidal vibration and repetitive mechanical jolt/impact.¹⁷ Exposure to seated, whole‐body, or nonsinusoidal vibration and jolt/impact or repetitive mechanical shock conditions can be found in, but are not limited to, the marine, agricultural, rail‐guided, rail‐constrained, forklift, mining/quarrying, over‐the‐highway trucking, and heavy equipment vehicle environments. In a recent “prospective study of professional drivers, measures of internal spinal load” via ISO/WD 2631‐5²⁵ “were better predictors of the occurrence of sciatic pain than the measures of daily vibration exposure established by the EU Directive (2002).³ Herniated lumbar disc, lumbar trauma and physical work load were also associated with sciatic pain.”²⁶

Hand–arm vibration standards

Most hand–arm vibration standards are focused at minimizing the probability that vibration white fingers (i.e., Raynaud’s phenomenon) would occur.²⁷

ACGIH HAND–ARM VIBRATION STANDARD

The first HAV standard to be introduced in the United States was the ACGIH standard for hand–arm vibration in 1984 and used thru 2013.²⁸ Weighted triaxial acceleration measurements are obtained over a 1/3 octave band vibration frequency range of 5.6–1250 Hz. A radically modified version of their old HAV standard was introduced in 2014, which emphasizes the need to determine overall total HAV‐weighted rms acceleration from separately weighted totals for each of the three linear axes. A total weighted vector sum is next calculated with a numerical result, which should not exceed 5 ms⁻² for a worker using the HAV tested tool for 8 hours/day.⁷ If the total weighted overall rms acceleration has exceeded this limit, then protective measures must be established to reduce HAV exposure; as appropriate, this can also include operating said tool for less than 8 hours/day as calculated.

ANSI HAND‐ARM VIBRATION STANDARD

A second HAV standard was promulgated in the United States in 1986 (ANSI HAV document S3.34, 1986) which was extensively used from 1986 to 2006.⁸ This document is no longer used and has been replaced with a simpler‐to‐use and better standard in current use (ANSI S2.70, 2006). Figure 4.5 graphically identifies the HAV weighting function which is retained and used in virtually all HAV standards in use worldwide today and used for all three measurement axes. The vibration frequency range extends from 5.6 to 1250 Hz. Vibration gain is given in the vertical axis. The elbow shape of this weighting curve reveals that the emphasis from potential HAV‐damaging vibration appears to be at the lower vibration frequencies, 5.6–16 Hz, and as frequencies increase, from 16 to 1250 Hz, the vibration damage potential is lessened. This assumed that frequency dependency is under investigation.

In 2002 the EU issued a directive/law to all its member countries and included WBV and HAV limits.³ The HAV limits were established for “total weighted rms acceleration vector sum” at two levels for power tool operators who are exposed to 8 hours/day HAV. The Daily Exposure Action Value (DEAV) = 2.5 ms⁻². If the total weighted vector sum is <2.5 ms⁻², then the tool is safe and allowed to be operated for the full 8 hours/day; if the total weighted sum greater than 2.5 ms⁻², but less than 5.0 ms⁻², this falls in a cautionary range where HAV disease can occur and the employer needs to begin measures to reduce HAV exposures; the daily exposure limit value (DELV) = 5.0 ms⁻² for an 8 hours/day; if the total weighted sum equals or is greater than 5 ms⁻² where disease is highly likely to occur, then the employer must significantly reduce these high HAV exposures, not use the tested tool, or use it in a very restricted time limited way. The EU holds the employer responsible, not the employee. As a result of this EU directive, power tool manufacturers worldwide are developing new and better low vibration power tools.

In 2006, ANSI decided to help create a “level playing field” by using the EU HAV criteria worldwide and replaced its old S3.34 HAV standard with S2.70, adopting the EU HAV 8 hours/day exposure criteria for both DEAV = 2.5 ms⁻² rms and DELV = 5 ms⁻² rms. In 2014 ACGIH decided to adopt only the EU’s DELV maximum exposure criteria = 5 ms⁻² rms for 8 hours/day for HAV exposures.^7,8,29

Finally, because of the high prevalence of HAV‐related disease worldwide, special reduced vibration called “Anti‐Vibration” (A/V) gloves have been designed and marketed worldwide. There was no uniform standard prior to 1996 certifying the efficacy of these products. The international A/V glove standard ISO 10819 was developed in 1996 and updated in 2013.^30,31 This standard has been used in the United States and elsewhere for many years; it is a difficult standard to meet but does serve the purpose of determining which products are most effective at reducing overall HAV exposure. The hallmark of this standard is to use, test, and certify only full‐finger protected gloves which provide the best overall protection of fingers, palms, and hands and not test gloves where the fingers are exposed, because irreversible hand–arm vibration syndrome (HAVS) virtually always begins at the fingers working its way toward the palm.²⁷

Limitations

The NIOSH HAV criteria for an HAV standard #89‐106⁹ is old and different from ACGIH or ANSI documents. This was an interim, and rather more limited standard, since NIOSH has not chosen a maximum permissible acceleration value for HAV. It relies instead on medical monitoring and engineering controls and an extended high frequency range cutoff of 5000 Hz instead of 1200 Hz.^32–34

The NIOSH document was developed in 1989 and is not often used. When it is cited, it is usually when NIOSH research discusses and compares HAV‐unweighted acceleration data to identical weighted acceleration data, because a major current effort of NIOSH HAV research is to verify or challenge the current aforementioned HAV weighting functions adequacy and correctness, so that NIOSH can propose modifications if necessary. The heart of WBV and HAV measurements is the accuracy of the respective weighting functions of HAV and WBV, which form a critical link between vibration exposure dose and human response to vibration impinging daily. In the United States only NIOSH has the necessary legal mandate and resources to perform this very difficult and expensive task to protect workers.^32,35

WHOLE‐BODY VIBRATION AND LOW BACK PROBLEMS

Low back problems are a significant disabling health issue. Numerous epidemiologic studies have shown an association between back trouble and whole‐body vibration.^36–41 Hoogendoorn et al.⁴² and Bovenzi and Hulshof⁴³ reviewed the literature associating mechanical factors with back trouble and reported that there were sufficient properly executed studies to conclude that low back trouble is associated with WBV exposure. Demonstrating an association of WBV exposure with low back pain (LBP) is more challenging. A comprehensive review assessing the quality of the evidence conducted by the Evidence‐Based Practice Workers’ Compensation Board of British Columbia in 2002 and updated in 2008 identified methodological limitations, inconsistencies, and bias in the literature regarding a causal relationship between WBV and LBP. The reviews concluded that the available evidence did not satisfy the Bradford Hill criteria for causation of LBP.^44,45 This indicates that studies to date have not demonstrated that WBV causes LBP, although it does not exclude the possibility of causation. Low back pain is a common nonoccupational health problem, and workers who are exposed to WBV are also exposed to many other workplace hazards, making it difficult to sort out the cause of symptoms.

WBV: impact as a sudden and unexpected load

If we consider that an impact event can be considered a suddenly, and in many cases an unexpectedly applied load, then we can apply another area of the literature to the understanding of the body’s response to impact. Impact loading can come from many sources such as load shifting, slips, trips, stepping off a unexpected curb, mechanical slop in seats, sloshing of liquid in a tank trailer, and slack action in train or truck couplings (Figures 4.6, 4.7, and 4.8). The trunk musculature around the lumbar region responds differently to sudden loads depending on whether or not the load is expected. Whether standing or sitting, the pelvis acts as a foundation for the spine. The orientation of the pelvis is affected by leg muscles located below the pelvis along with trunk muscles located above the pelvis. Sudden events such as slips, trips, and falls affect actions of the leg and trunk muscles. In 1981, Manning and Shannon⁸⁹ and, in 1984, Manning et al.⁹⁰ showed that slip events are considered first events that lead to back injuries and expressed concern that this was a neglected research area in back injury etiology. Marras et al. showed that sudden, unexpected loads applied at the hands lead to large overcompensations in the back muscles, suggesting that the hazard is the body’s excessive reaction or overcompensation to the applied load.⁹¹ Mannion et al. predicted that disk overloading occurs as a result of sudden, unexpected loading.⁹² These papers show that sudden loads and sudden movements of the body can place the back at risk of injury, as they each describe conditions that require the back muscles to respond rapidly to an imposed load or movement.

Responses to sudden loads applied at the hands are not necessarily symmetrical either. Pelvic orientation,⁹³ load application location,⁹⁴ and fatigue and hand dominance⁹⁵ can all affect the symmetry of the trunk muscle response.

PREVENTION

Low back pain can have a variety of occupational and nonoccupational causes. Many industrial risk factors can be modified to reduce the rate of back disorders. Workers in occupations where vibration is present are also frequently exposed to lifting, pulling, or pushing, for example, truck drivers who also load and unload trucks. Regardless of the role of WBV in precipitating injury, many of the preventive measures that may mitigate vibration may also reduce other hazards.

Engineering redesign may include using vibration damping techniques (i.e., converting vibration into a small amount of heat due to the deformation of a viscoelastic material) and/or vibration isolation (i.e., intentionally mismatching the vibration pathway between the vibrating source and the worker receiving this vibration).

Epidemiologic data are supported by laboratory studies of spine changes that might produce back problems—for example, lumbar disk flattening, disk fiber strain and height increase, and intradiscal pressure. It is apparent from WBV data that the human spinal system has a characteristic response to vibration in a seated posture. Resonances occur at uniform frequencies for all of the subjects. The first vertical resonance occurs within a band of 4.5–5.5 Hz.

These studies indicate that maximum strain or stretching occurs in the seated operator’s lumbar region at the first natural frequency. In addition, back muscles are not able to protect the spine from adverse loads. At many frequencies, the muscles’ responses are so far out of phase that their forces are added to those of the stimulus. The fatigue that was found in muscles after vehicular vibration is indicative of the loads in the muscles. Thus, it would is advisable to walk around for a few minutes before bending over or lifting after vibration exposure (i.e., unloading a truck). One fuel company had their tank truck drivers take care of paperwork before handling hoses at a fuel delivery stop (P Wald, Personal communication). It would also be advisable for those exposed to prolonged vibration (i.e., long‐distance driving) to take frequent breaks including walking around for a few minutes.

The field of mechanics provides an encouraging note for attempts to improve the whole‐body vibration environment. If one considers that the damage to the spine due to vibration occurs from the work performed on the body by the kinetic energy from the vibration, two things become very clear. In the simplest form, the work performed on the body is equal to the kinetic energy applied to the body. The equation for that kinetic energy is 1/2 mv² (work = kinetic energy = 1/2 mv²), where “m” is mass and “v” is velocity. Because velocity = acceleration × time (v = at), solving the energy equation in terms of acceleration and time yields an equation where kinetic energy equals 1/2m(a²t²). Using that formulation, it is then apparent that the work performed on the body from the kinetic energy of vibration depends on the square of the vibration acceleration and the square of the vibration exposure duration (time). This is important because it means a small reduction in acceleration and/or exposure time can lead to relatively large reductions in energy absorbed by the worker. For example, a 10% reduction in either vibration acceleration or time of exposure to the vibration can result in a 19% reduction in the energy applied to the body. Reducing by 10% both the vibration acceleration and time of exposure to the vibration can result in a 34% reduction in the energy or work applied to the body. Inexpensive accommodations therefore can have a big effect.

Jolt/impact is emerging as an important factor and is a subset of sudden/unexpected load conditions. More epidemiological studies in which relevant occupational exposures are quantified are needed. The relationship between intrinsically and extrinsically applied mechanical stresses, and the accompanying hard and soft tissue deformations, both acute and chronic, still needs greater definition. It is particularly important that seated vibration exposure not be used as a “work‐hardening” treatment modality in trying to return someone with low back trouble to work.⁹⁶ It is also very important to realize that because the vibration standards/guidelines have limitations, it is critical to monitor the health and proficiency of people in the whole‐body impact/vibration environment.

In a setting where vibration measurements have been made and the data have been evaluated with regard to the appropriate standard and it is determined that vibration control is necessary, usually a series of multiple control steps are taken depending on the problem as follows⁹⁶:

1. Reduce vibration exposure by placing the worker away from vibrating surfaces by using remote controls, closed‐circuit TV monitors, etc.
2. Reduce exposure time by modifying work organization, job sharing, etc.
3. If possible, mechanically isolate the vibrating surface, machine, etc.
4. Maintain mechanisms and replace worn‐out mechanisms that contribute to production of jolt/impact and vibration.
5. In vehicles, use vibration isolating “suspended or air‐ride” seats and cabs and replace vehicle suspension systems as necessary.
6. Once changes have been made, repeat vibration measurements and compare data to WBV standards as well as previous data.
7. Ask equipment users to comment on the effectiveness of the solutions.
8. Institute a surveillance program.

HAND–ARM VIBRATION MEDICAL EFFECTS

Adverse health effects from exposure to hand–arm vibration (HAV) have been recognized since 1911 when Loriga reported “dead fingers” among the Italian miners who used pneumatic tools.⁹⁷ Such tools had been introduced into the French mines in 1839 and were being extensively used by 1890. In the United States pneumatic tools were first introduced into the limestone quarries of Bedford, Indiana, about 1886. In 1918, Dr. Alice Hamilton and her colleagues subsequently investigated the health hazard from their use.¹ Since then there have been many reports of health hazards arising from the use of handheld vibratory tools in the literature from all over the world.²⁷

It is now evident that adverse health effects can result from almost any vibrating source if the vibration is sufficiently intense over a wide frequency range for a significantly long period of time. The most important sources of HAV are pneumatically driven tools (air compressed and electrical), for example, grinders, drills, fettling tools, jackhammers, riveting guns, and chainsaws. Users of brush saws, hedge cutters, and speedway (dirt‐track) motorbike riders are also at risk.

The predominant health effect of regular HAV exposure is now known as hand–arm vibration syndrome (HAVS), a generally irreversible disease entity with the following separate peripheral components^98,99:

• Circulatory disturbances: cold‐induced vasospasm with local finger blanching “white finger”
• Sensory and motor disturbances: numbness, loss of finger coordination and dexterity, clumsiness, and inability to perform intricate tasks
• Musculoskeletal disturbances: muscle, bone, and joint disorders

The vasospasm, also known as Raynaud’s phenomenon, is precipitated by exposure to cold and/or damp conditions and sometimes vibration exposure itself. The time period between first exposure to HAV and the onset of fingertip blanching is termed the “latent interval.” It may range from 1 month to several years, depending on the intensity of vibration entering the hand and the susceptibility of the worker. The blanching is restricted initially to the tips of one or more fingers but progresses to the base of the fingers as the vibration exposure time increases. The thumbs are usually the last to be affected.

The blanching is accompanied by numbness, and as the circulation to the digits returns, there is usually tingling and pain. Tingling and paresthesia may precede the onset of blanching in many subjects. These sensory symptoms and signs may be the predominant complaint in some patients, and their recognition as a distinct entity led to the revision of the Taylor–Pelmear classification for assessment of HAVS devised in 1968.¹⁰⁰ It was replaced in 1985 by the Stockholm classification based on the subjective history supported by the extensive results of a battery of clinical tests to stage the severity of disease (Tables 4.1 and 4.2).^101,102 The vascular and sensorineural symptoms and signs are evaluated separately and for both hands individually.

In advanced cases the peripheral circulation becomes very sluggish, giving a cyanotic tinge to the skin of the digits, while in the very severe cases trophic skin changes (gangrene) will appear at the fingertips. The toes may be affected if directly subjected to vibration from a local source, that is, vibrating platforms, or they may be affected by reflex spasm in subjects with severe hand symptoms. Reflex sympathetic vasoconstriction may also account for the increased severity of noise‐induced hearing loss in HAVS subjects.^103,104

In addition to tactile, vibrotactile, and thermal threshold impairment, which may vary from subject to subject, impairment of grip strength is a common symptom in longer exposed workers.^105,106 Discomfort and pain in the upper limbs is also a common complaint. Bone cysts and vacuoles, although often reported, are more likely to be caused by biodynamic and ergonomic factors.^107,108

Carpal tunnel syndrome, an entrapment neuropathy affecting the median nerve at the wrist, is often associated with HAVS.^109–111 Usually it is due to ergonomic stress factors including the constant repetitive nature of the work, grip force, and mechanical stresses, for example, torque and posture. When vibration is the primary cause of the median nerve neuropathy, the edematous reaction in the adjacent tissues and the nerve sheath compresses the central axon.¹¹² The median nerve is affected together with the ulnar in two thirds of the cases. Rarely the ulnar nerve may be affected alone.¹¹³

Whether smoking accelerates the onset of HAVS has not yet been proved conclusively, but this aggravating factor has been shown to increase the risk in several studies.^114,115

DIAGNOSIS

The diagnosis of HAVS is based on a history of HAV exposure and the exclusion of other causes of Raynaud’s phenomenon, that is, primary Raynaud’s phenomenon (Raynaud’s disease or constitutional white finger), local trauma to the digital vessels, thoracic outlet syndrome, drugs, and peripheral vascular and collagen diseases, including scleroderma. The diagnosis of HAVS is confirmed and the severity assessed by stage from the results of a battery of laboratory tests.^{113,116–118}

Vascular tests should include some or all of the following, that is, Doppler studies, plethysmography, finger systolic pressure measurement, and cold‐water provocation tests to verify that vasospasm occurs on cold exposure. Subjective sensorineural tests should include depth sense and two‐point discrimination, fingertip vibration threshold measurement, thermal hot/cold perception, and current perception threshold tests. Objective nerve conduction tests should be undertaken to confirm the presence and severity of the neuropathy which in HAVS normally affects both median and ulnar nerves. When the median nerve myelinated fibers are involved at the wrist level, there can be confusion with CTS nerve entrapment because the symptoms and signs are similar.

PATHOPHYSIOLOGY

The pathophysiology of HAVS has been well reviewed by Gemne.¹¹⁹ The basic mechanism is not yet fully understood. Due to the mechanical stimulus, specific anatomical changes occur in the digital vessels, that is, vessel wall hypertrophy and endothelial cell damage. In the initial stages there is extrusion of fluid into the tissues. This edema, together with the subsequent spasmodic ischemia from the cold‐included vasospasm, damages the mechanoreceptor nerve endings and nonmedullated fibers. Subsequently, a demyelinating neuropathy of the peripheral nerve trunks develops.

The vascular response to cold is complex because in addition to the diversity of receptor systems (adrenergic, cholinergic, purinergic, and serotonergic), there are several subtypes of specific receptors. The differential distribution and functional significance of the various receptor types is largely unknown. It is probable that the cold‐induced pathological closure of the digital arteries and end vessels is mainly mediated by alpha‐2 adrenoreceptors in the wall of the arterioles and veins. It has been demonstrated that the alpha‐2 receptors are more receptive to the cold stimulus. In HAVS it is postulated that there is selective damage of alpha‐1 receptors; hence the cold stimulus is more effective. While arterial spasm is necessary to stop the blood flow, vasospasm in the skin arterioles is essential to produce the blanching.

Cold, as well as vessel wall injury, causes platelet aggregation. The subsequent release of serotonin (5‐hydroxytryptamine (5‐HT)) promotes further release of 5‐HT from the platelets, and the increased concentration stimulates smooth muscle to contract. Besides promoting contraction, serotonin may also contribute to vasodilation by inducing the release of endothelium‐derived relaxing factor (ERDF) and prostacyclin from the endothelial cells. Acetylcholine and its agonist methacholine acting through the muscarinic receptors also release ERDF, while nitric oxide and its agonists, nitroprusside and nitroglycerine, release prostacyclin. The prostacyclin and ERDF so released, besides inhibiting platelet aggregation, stimulate the production of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) in the smooth muscle cell. The latter substances inhibit calcium utilization by the smooth muscle cells so they do not contract. A delicate balance between smooth muscle contraction and relaxation is produced by these mechanisms interacting simultaneously in the normal person.

TREATMENT AND MANAGEMENT

To reduce the frequency of blanching attacks, the central body temperature must be maintained, and cold exposure must be avoided. When possible, mitts rather than gloves should be worn. Discontinuation of smoking and all nicotine is an essential requirement because of the adverse effect of nicotine and carbon monoxide on the digital arterial system.

To attempt to reverse the pathology and seek recovery, further vibration exposure must be avoided. If avoidance of HAV is not possible, a modified work routine should be practiced to reduce vibration exposure and slow the progression of this condition. This should also include using both antivibration power tools and antivibration full‐finger protective gloves.

Recent advances in drug therapy have focused on three areas: (i) use of calcium channel antagonists to produce peripheral vasodilation, (ii) use of drugs to reduce platelet aggregation in combination with the above, and (iii) drugs to reduce blood viscosity and emboli formation. The preferred calcium channel antagonist is a slow‐release product. In the more severe cases, additional medications are often prescribed for platelet deaggregation.

The necessity for drug therapy increases with the severity of the symptoms and the age of the subject. The results are encouraging for resolution of the vascular symptoms, particularly if vibration exposure is avoided. Unfortunately drug intolerance to the earlier calcium channel antagonists caused some young patients to abandon their use prematurely. Where this has happened, the newer products should be tried. Recovery from the sensorineural effects has not been reported. It remains to be seen whether an improvement in the peripheral circulation will result in reversal of the sensorineural symptomatology. For the most part, HAVS is irreversible; therefore prevention measures are the watchword in order to minimize the effects of HAV exposure.

When patients are thought to be suffering from HAVS, their employers should be advised to assess the work situation and introduce preventive procedures. All workers should be advised of the potential vibration hazard and receive training on the need to service their tools regularly; to grip the tools as lightly as possible within the bounds of safety; to use the protective clothing equipment provided; to attend for periodic medical surveillance; and to report all signs and symptoms of HAVS as soon as they develop.

HAND–ARM VIBRATION CONTROL

As with whole‐body vibration control, once vibration measurements have been made and the data have been evaluated with regard to the appropriate standard and it is determined that vibration control is necessary, usually a series of multiple control steps are taken depending on the problem^{9,27,33–35,120,121}:

1. Use only ergonomically correct antivibration (A/V) power tools wherever possible.
2. If possible do not use materials that wrap around tool handles and claim to significantly reduce HAV exposure. Usually they increase the tool handle diameter causing the worker to grip the tool handle more forcefully, thereby compressing the material and reducing their limited usefulness. It is far better to use a well‐designed A/V tool.
3. Use only good fitting “full‐finger protected” ISO 10819 certified A/V gloves¹⁰; do not use exposed finger gloves that only protect the palm. Hand–arm vibration syndrome (HAVS) nearly always begins at the fingertips advancing toward the root; only full‐finger protected gloves offer the desired protection.
4. In factory situations, use suspended “tool balancers” to remove the weight of the tool from the operator.
5. Workers are advised to do the following as good work practices:
   1. Let the tool do the work, gripping it as lightly as possible consistent with safe tool handling.
   2. Operate the tool only when necessary and at reduced speeds if possible.
   3. Properly maintain hand tools and replace as necessary.
   4. Do not smoke, since nicotine, vibration, and cold all constrict the blood vessels.
   5. Keep your hands and body warm and dry.
   6. Consult a physician if signs of digit tingling, numbness, or blanching occur.
   7. Medical prescreening of workers is advised to minimize the risk to idiopathic primary Raynaud’s disease sufferers and others from operating hand tools that could exacerbate such a preexisting condition.
   8. Workers and others need to be made aware of this problem and its signs and symptoms.
6. Recently some vibrating tool manufacturers have begun placing warning labels on their tools and in their instruction books.
7. Use HAV standards as critical adjuncts to control measures.
8. Institute a surveillance program.

Finally, for HAV exposures, these controls may need additional engineering redesign methods using vibration damping techniques (i.e., converting vibration into a small amount of heat due to the deformation of a viscoelastic material) and/or vibration isolation (i.e., intentionally mismatching the vibration pathway between the vibrating source and the worker receiving this vibration) to be effective.

In the most significant HAVS prevention projects to date, Geiger et al.¹²² and Torelli et al.¹²³ have shown that it is possible to change the vibrating hand tool acquisition process and use the culture of a large organization, the US Department of Defense, in order to improve the well‐being of tool users and reduce their risk of long‐term, irreversible injury. Prior to this, protective antivibration hand tools were not available to the US federal workforce. The project provided a vital template for approaching such a large challenge. It made convincing arguments based on issues related to the EU Physical Agents Directive³ (binding law), avoiding injury, life‐cycle costs of tools, and making the tools available to the US federal workforce.

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