I would take the concept that they experienced a workplace injury and weren't informed of the scope of danger that it may pose.
Sure everyone has the right to refuse to do a job but changing jobs requires the resources to do so. If you have some type of leverage such as a house or bills to pay then thats an issue. The other problem is how do you perform this moderation?
Possibly you could restrict the amount of time someone is exposed to this and then change people out once they have reached some type of exposure but thats primarily speculative.