A vibration monitor which is configured to be releasably attached to an arm or hand of an operator during use of a power tool, the vibration monitor comprising: a vibration sensor operative to sense vibration sustained by the arm or hand of the operator when the vibration monitor is attached to the arm or hand of the operator and to provide: a first vibration signal corresponding to vibration in a first axis; a second vibration signal corresponding to vibration in a second axis; and a third vibration signal corresponding to vibration in a third axis , the first axis extending between the dorsal and palmar sides of the arm or hand, the second axis extending across the arm or hand in a direction of separation of fingers of the hand, and the third axis extending along the arm or hand; and a processor configured to receive the first, second and third vibration signals, to apply a first transformation to the first vibration signal, to apply a second transformation to the second vibration signal, and to apply a third transformation to the third vibration signal , the processor being further configured to determine vibration exposure for the operator during use of the power tool , the vibration exposure being determined in dependence on the first, second and third vibration signals as transformed by the first, second and third transformations, each of the first to third transformations providing for: a rise to a peak between 8 Hz and 20 Hz as frequency increases; a fall to a trough beyond the peak with increasing frequency; and a rise from the trough with further increasing frequency, the third transformation providing for a rise from the trough at a higher frequency than at least one of the first and second transformations . a vibration sensor operative to sense vibration sustained by the arm or hand of the operator when the vibration monitor is attached to the arm or hand of the operator and to provide: a first vibration signal corresponding to vibration in a first axis; a second vibration signal corresponding to vibration in a second axis; and a third vibration signal corresponding to vibration in a third axis , the first axis extending between the dorsal and palmar sides of the arm or hand, the second axis extending across the arm or hand in a direction of separation of fingers of the hand, and the third axis extending along the arm or hand; and a processor configured to receive the first, second and third vibration signals, to apply a first transformation to the first vibration signal, to apply a second transformation to the second vibration signal, and to apply a third transformation to the third vibration signal , the processor being further configured to determine vibration exposure for the operator during use of the power tool , the vibration exposure being determined in dependence on the first, second and third vibration signals as transformed by the first, second and third transformations, each of the first to third transformations providing for: a rise to a peak between 8 Hz and 20 Hz as frequency increases; a fall to a trough beyond the peak with increasing frequency; and a rise from the trough with further increasing frequency, the third transformation providing for a rise from the trough at a higher frequency than at least one of the first and second transformations . <EOS>