Hand-arm vibration in the aetiology of hearing loss in lumberjacks

A longitudinal study of hearing loss was conducted among a group of lumberjacks in the years 1972 and 1974--8. The number of subjects increased from 72 in 1972 to 203 in 1978. They were classified according to (1) a history of vibration-induced white finger (VWF), (2) age, (3) duration of exposure, an (4) duration of ear muff usage. The hearing level at 4000 Hz was used to indicate the noise-induced permanent threshold shift (NIPTS). The lumberjacks were exposed, at their present pace of work, to noise, Leq values 96-103 dB(A), and to the vibration of a chain saw (linear acceleration 30-70 ms-2). The chain saws of the early 1960s were more hazardous, with the average noise level of 111 dB(A) and a variation acceleration of 60-180 ms-2. When classified on the basis of age, the lumberjacks with VWF had about a 10 dB greater NIPTS than subjects without VWF. NIPTS increased with the duration of exposure to chain saw noise, but with equal noise exposure the NIPTS was about 10 dB greater in lumberjacks with VWF than without VWF. With the same duration of ear protection the lumberjacks with VWF consistently had about a 10 dB greater NIPTS than those without VWF. The differences in NIPTS were statistically significant. The possible reason for more advanced NIPTS in subjects with VWF is that vibration might operate in both of these disorders through a common mechanism--that is, producing a vasoconstriction in both cochlear and digital blood vessels as a result of sympathetic nervous system activity.