A complete assessment of exposure to vibration requires the measurement of vibration acceleration in meters per second squared (m/s2). Vibration exposure direction is also important and is measured in a well-defined directions. Vibration frequencies and duration of exposure are also determined. How hard a person grips a tool affects the amount of vibrational energy entering the hands; therefore, hand-grip force is another important factor in the exposure assessment.
The amount of exposure is determined by measuring acceleration in the units of m/s2. Most regulating jurisdictions and standard agencies use acceleration as a measure of vibration exposure for the following reasons:
Health research data tells us that the degree of harm is related to the magnitude of acceleration.
A typical vibration measurement system includes a device to sense the vibration (accelerometer), and an instrument to measure the level of vibration. Today a number of industries are making vibration measuring instruments that look like sound level meters. This equipment also has settings for measuring frequency, a frequency-weighting network, and a display such as a meter, printer or recorder.
The accelerometer produces an electrical signal. The size of this signal is proportional to the acceleration applied to it. The frequency-weighting network mimics the human sensitivity to vibration of different frequencies. The use of weighting networks gives a single number as a measure of vibration exposure and is expressed as the frequency-weighted vibration exposure in metres per second squared (m/s2), units of acceleration.
The frequency-weighting network for hand-arm vibration is given in the International Organization for Standardization (ISO) standard ISO 5349. Human hand is not equally sensitive to vibration energy at all frequencies. The sensitivity is the highest around 8-16 Hz (Hertz or cycles per second). Measuring equipment takes this fact into account by using a weighting network. The gain is assigned a value of 1 for vibration frequencies to which the hand-arm system has the highest sensitivity. The dashed lines in Figure 1 represent the filter tolerances in the weighting network.
Protecting workers from the effects of vibration usually requires a combination of appropriate tool selection, the use of appropriate vibration-absorbing materials (in gloves, for example), good work practices, and education programs.
Tools can be designed or mounted in ways that help reduce the vibration level. For example, using anti-vibration chain saws reduces acceleration levels by a factor of about 10. These types of chain saws must be well maintained. Maintenance must include periodic replacement of shock absorbers. Some pneumatic tool companies manufacture anti-vibration tools such as anti-vibration pneumatic chipping hammers, pavement breakers and vibration-damped pneumatic riveting guns.
Conventional protective gloves (e.g., cotton, leather), commonly used by workers, do not reduce the vibration that is transferred to workers' hands when they are using vibrating tools or equipment. Anti-vibration gloves are made using a layer of viscoelastic material. Actual measurements have shown that such gloves have limited effectiveness in absorbing low-frequency vibration, the major contributor to vibration-related disorders. Therefore, they offer little protection against developing vibration-induced white finger syndrome. However, gloves do provide protection from typical industrial hazards (e.g., cuts, abrasions) and from cold temperatures that, in turn, may reduce the initial sensation of white finger attacks.
Along with using anti-vibration tools and gloves, workers can reduce the risk of hand-arm vibration syndrome (HAVS) by following work practices:
Training programs are an effective means of heightening the awareness of HAVS in the workplace. Training should include proper use and maintain vibrating tools to avoid unnecessary exposure to vibration. Vibrating machines and equipment often produce loud noise as well. Therefore, training and education in controlling vibration should also address concerns about noise control.
The following precautions help to reduce whole-body vibration exposure:
The vibration control design is an intricate engineering problem and must be set up by qualified professionals. Many factors specific to the individual work station govern the choice of the vibration isolation material and the machine mounting methods.
Many Canadian jurisdictions do not have regulations concerning vibration exposure. However, it is prudent to reduce the level of exposure as much as practical since vibration causes ill health effects. It is possible to do this by engineering controls, the use of protective equipment and safe work practices. The design of vibration-damped equipment and engine mountings are the most effective engineering methods of controlling vibration exposure.
In the absence of formal regulations, Canadian agencies often use the Threshold Limit Values (TLVs) and guidelines recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). These TLVs are based on the recommendations of the International Organization for Standardization (ISO).
The American Conference of Governmental Industrial Hygienists (ACGIH) has developed Threshold Limit Values (TLVs) for vibration exposure from hand-held tools. The exposure limits are given as frequency-weighted acceleration that represents a single number measure of the vibration exposure level. The frequency-weighting is based on a scheme recommended in the international standard ISO 5349. Vibration-measuring instruments have a frequency-weighting network as an option for vibration measurement. Table 1 lists acceleration levels and exposure durations to which, ACGIH has determined, most workers may be exposed repeatedly without severe damage to fingers. ACGIH advises that these guidelines be applied in conjunction with other protective measures including vibration control.
|Table 1 |
The ACGIH Threshold Limit Values (TLVs) for exposure
of the hand to vibration in X, Y, or Z direction*
|Total Daily Exposure |
|Maximum value of frequency weighted |
acceleration (m/s2) in any direction*
|4 to less than 8 hours||4|
|2 to less than 4 hours||6|
|1 to less than 2 hours||8|
|less than 1hour||12|
* Directions of axes in the three-dimensional system
The International Organization for Standardization (ISO) has published a method for measuring vibration and interpreting the resulting data. This 2001 standard (ISO 5349-1) also gives the set of curves shown in Figure 2 that can determine exposure levels likely to cause the first signs of white finger in workers.
The horizontal axis in Figure 2 represents vibration acceleration. This is measured as RMS (Root Mean Square) weighted acceleration in m/s2. RMS is a method of determining average for quantities that fluctuate with time about a central value. Weighting accounts for variation in human sensitivity to vibration of different frequencies. The measured value of acceleration at different frequencies is passed through a weighting filter (see Figure 1 - Representation of Vibration) to obtain a single number as an overall measure of vibration exposure. According to this frequency-weighting filter people are most sensitive to hand arm vibration in the frequency range of 1/3 octave bands with centre frequency 6.3 to 16 Hz. As frequency increases above this range the sensitivity decreases. The standard provides methods for calculating weighted RMS accelerations and equivalent acceleration values where the level of daily exposure varies with time.
Figure 2 can be used to assess the long-term effect of 4-hour per day exposures to hand-arm vibration. For example, the standard predicts that exposure to 50 m/s2 vibration acceleration will cause 50 percent of exposed workers to reach stage 1 of Raynaud's phenomenon of occupational origin in about 1.2 years. At a vibration acceleration of 5 m/s2, the standard predicts that it would take about 14 years for the same percentage of workers to reach stage 1. The data for which the curves were generated are limited, so they should not be used if exposure is greater than 50 m/s2 or if the exposure duration exceeds 25 years.
The standards and guidelines concerning whole-body vibration are designed to reduce vibration to a level where most workers can perform job tasks without discomfort.
The most widely used document on this topic is Guide for the Evaluation of Human Exposure to Whole Body Vibration (ISO 2631). These exposure guidelines have been adopted as ACGIH TLVs. The ISO standard gives three different types of exposure limits:
The reduced-comfort boundary is for the comfort of people travelling in airplanes, boats, and trains. Exceeding these exposure limits makes it difficult for passengers to eat, read or write when travelling.
The fatigue-decreased proficiency boundary is a limit for time-dependent effects that impair performance. For example, fatigue impairs performance in flying, driving and operating heavy vehicles.
The exposure limit is used to assess the maximum possible exposure allowed for whole-body vibration.
A separate set of "severe discomfort boundaries" is given for 8-hour, 2-hour and 30-minute exposures to whole body vibration in the 0.1 Hz to 0.63 Hz range. As with all standards, it is important to read and understand all the information before applying it in the workplace.
These exposure limits are given as acceleration for one third octave band frequencies and three directions of exposure - longitudinal (head <-> toe) and transverse (back <-> chest and side <-> side). The exposure limit is the lowest for frequencies between 4-8 Hz as the human body is most sensitive to whole-body vibrations at these frequencies.
It is important to remember that people vary in their susceptibility to effects of exposure to vibration so the "exposure limits" should be considered as guides in controlling exposure: they should not be considered as an upper "safe" limit of exposure or a boundary between safe and harmful levels.
Document last updated on October 21, 2008