If they exceed the 'action level' at any point you have to take 'action'! This includes the annual medical surveillance questionnaire (not necessarily a medical - it depends on the review of the forms by a competent person). A question I'd ask is how do they/you know they're exceeding the action level - it's notoriously difficult to measure accurately. Not saying they don't exceed it, more just intrigued that they can be that accurate about the number of times they exceed it in a year.

The whole point of health surveillance is to ensure the health of workers which in turn will reduce the likelihood of workers becoming ill which in turn will reduce any claims made against the company.

Racer and Chris's - you are absolutely right, health surveillance should generally be in place as an early indication to identify that if your preventative controls are working effectively and yes as a responsible employer this can help defend any potential claims. My point I was making is that the OP asks the question 'Vibration: when to do medical surveillance?' - two things are clear, he knows that two workers are overexposed and he does not conduct medical surveillance. The fact vibration equipment is being used, staff are over exposed, and no surveillance is being carried out can only increases the changes of a claim IMHO.