Epidemiological studies of vibration disease due to brush saw operation

A longitudinal study was performed to evaluate the risk for vibration disease during the period 1961-1980 among a given population of forest workers using brush saws in Japan. The vibration levels in the measurements occasionally exceeded the 60-min line proposed by the ISO/DIS 5349 in 1/3 octave bands mainly centered on 125 Hz in vertical during the sawing operations. The peak prevalence rate was 12.0% found in the cohort group beginning in 1961-1962. Thereafter, it decreased significantly and no patients with VWF were found in the cohort group beginning in 1969-1970. In the study periods, the prevalence rate was around 2, 4, 6 and 7% after 3, 7, 11 and 13 years of exposure time respectively. The incidence rate had two peaks of 2.2% in 1965 and in 1973. Among workers using brush saws, the prevalence rate and incidence rate were smaller than those of workers using various other kinds of vibrating tools. The workers used brush saws for 4 to 5 months a year during the warmer season for grass trimming and limbing. This is the main reason why the prevalence rate was so low in spite of the high acceleration levels of the brush saw. On the other hand, the latency period (6.4 +/- 3.5 years) was rather shorter than that of the chain saw. This result suggests that the individual susceptibility of workers to contract VWF may play a more important role in the case.