Comparison of different measures for hand–arm vibration exposure
Introduction
The term `hand–arm vibration syndrome' (HAVS) is used collectively for the different symptoms associated with manual work involving vibrating power tools. These symptoms, which include vascular, bone, musculoskeletal and neurological disorders, have also been recognized as an important preventable occupational disease (for a review see Gemne et al., 1993). However, a great deal of uncertainty still surrounds the mechanisms behind the development of vibration injuries. The association between generated disturbances and the characteristics of the vibration stimulus has also proved to be complex and it has not yet been discovered which characteristics of the vibration are responsible for the detrimental effects (Griffin, 1990).
At present, risk assessment of hand-transmitted vibration syndrome is based on the international standard ISO 5349 (ISO, 1986). The standard specifies general methods for measuring and reporting hand-transmitted vibration exposure. According to the standard, measurements of the vibration should be done as close as possible to the surface which is in contact with the hand. Measurements should be done within the frequency range of, at least, 5 Hz to 1500 Hz in three mutually orthogonal directions and expressed in terms of frequency-weighted acceleration (SI-unit: m/s2). The frequency-weighting network should conform with the characteristics specified in ISO 8041 (ISO, 1985), i.e. an attenuation of 0 dB from 6 to 16 Hz and 6 dB per octave for frequencies above 16 up to 1250 Hz. The frequency-weighted acceleration concept assumes that the harmful effects of acceleration are independent of frequency between 6.3 and 16 Hz but progressively decrease with higher frequencies. All assessments should be based upon the component with the largest acceleration magnitude. According to the Annex to the standard it has been thought possible to develop an exposure–response relationship between vibration exposure, expressed in terms of frequency-weighted acceleration, and the early stages of the vibration syndrome in the form of finger blanching. Since several investigations have produced results that disagree with the risk predicted by this exposure–response relationship, the validity of the model in ISO 5349 has been questioned (Gemne et al., 1993).
Another view of the relationship between risk and measured acceleration has been discussed by NIOSH in its publication for hand–arm vibration `Criteria for Recommended Standard' (NIOSH, 1989). This document proposes that frequency-weighting is not to be used as the basis for prevention and that the measurement range should be extended to 5000 Hz. The frequency-unweighted concept assumes that the magnitude of pathophysiologic effects from exposure to vibration is proportional to the acceleration and is frequency independent (Pelmear et al., 1989).
Absorption of vibration energy in the human hand and arm has also been claimed to have a good correlation with vibration injuries (Lidström, 1977). The quantity of energy per unit time (power) to which the hand and arm system is exposed can be estimated from measurements of the transmitted force and the velocity (Burström and Lundström, 1994). This quantity can be divided into two components — one real and one imaginary. The real component reflects the energy absorbed by the system and the imaginary component reflects the energy-storing part (Burström and Lundström, 1994). The average transferred energy can therefore be expressed in the frequency domain within the cross-spectrum. Since the cross-spectrum is complex the coincident spectrum describes the energy absorption (Burström and Lundström, 1994). The quantity of absorbed energy is not only influenced by vibration intensity but also by several other factors, such as frequency, transmission direction, grip and feed forces, hand–arm postures, and individual factors (Burström, 1994).
The assessment of vibration exposure is primarily based on the daily exposure. It is important, therefore, to base estimates of total daily exposure times on appropriate representative samples for various operating conditions and exposure durations and their intermittency. The total time during which vibration is transmitted to the hand during a working period is not believed to exceeded 4 hours (ISO, 1986) and this is, therefore, used as a basis for assessment. In accordance with ISO 5349, the total daily exposure can be obtained by using the different frequency-weighted accelerations for the tools and the exposure times for these tools. In order to facilitate comparisons between different durations of exposure, the ISO standard 5349 proposes that the daily exposure be expressed in terms of an energy-equivalent frequency-weighted acceleration for a period of 4 hours. If the daily duration of exposure is not 4 hours, the energy-equivalent acceleration for a period of 4 hours can be calculated by the integration of the square of the frequency-weighted acceleration over the whole of the daily exposure. The time-dose concept assumes that the daily exposure time required to produce symptoms is inversely proportional to the square of the acceleration.
The probability of developing symptoms of HAVS increases with the number of years of vibration exposure (Nelson and Griffin, 1993; Tominaga, 1995). Therefore, energy-equivalent acceleration for the whole lifetime as well as accumulated vibration exposure could be alternative techniques for dealing with the influence of exposure time.
In the literature, there is a lack of investigations that have compared different methods for vibration measurements and estimates of exposure time.
Against this background, the aim of the present investigation is therefore to measure the vibration acceleration and absorption of vibration energy. The measurement of the acceleration should be done frequency-weighted and frequency-unweighted in accordance with both ISO 5349 and NIOSH (1989). Furthermore, the aim is to determine the exposure time by both subjective assessments and objective measurements. Finally, energy-equivalent accelerations and accumulated vibration load should be calculated from these data.
Section snippets
Subjects
The study base was a cohort of 74 workers employed as platers at a company which produces paper and pulp-mill machinery. The platers in the company are highly skilled workers and have all completed the training required to become a licensed plater plus an additional trainee year. The work task consisted mainly of welding, plating and grinding on iron and stainless steel. The work is very varied and each component is produced in small numbers. The criteria for inclusion in the platers category
Vibration intensity
The mean magnitudes of the measured vibration exposure with the three different techniques as a function of the frequency (1/3-octave band) are illustrated in Fig. 2. The figure shows the vibration exposures in the dominant direction for the two most frequently used tools, namely an angle grinder and a hammer.
As can be seen, there is some agreement between the different measurement methods. Compared with ISO 5349, the NIOSH suggestion for measurements takes higher frequencies into more
Discussion
The platers' exposure to vibration has its origin in the use of grinders and hammers. These two types of tools correspond to about 90% of the total daily use of hand-held tools. In the present study, a relatively large deviation between the vibration levels could be found among these tools. The reasons for this are among other things the condition of the machine, grinding wheel, type of work, type of material, etc.
The results reveal great differences between the three different methods for
Acknowledgements
The financial support of the Swedish Work Environmental Fund is gratefully acknowledged. Special thanks to Ms Sonya Hörnqwist Bylund and Ms Ruth-Marie Mathsson for their technical assistance.
References (17)
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