Sometimes these studies are not "true", i.e., it takes 1000 times the exposure to cause anything "substantial" to happen.
So the question should be....If he worked someplace else, would his condition have gotten worse. It's a yes or no...There is no maybe.
Wouldn't wish it on anyone...but...
True... most people are genetically predisposed to something one way or another. It just takes the right trigger.