How do I know who is badly injured and is or is not likely to survive? Even if I was medically qualified I'm not on the scene and in a position to judge. My job is (or was) to do the best I possibly can to assist based on the advice of the guys getting their boots dirty. And I will do that up to my own personal limits, which may or may not coincide with anybody else's. None of us says "Oh goody, here's fog, let's go fly!" But if it comes across our path, we should make the best judgment we can.
A safety point for those who have not been there - the helicopter creates its own bad weather - if you are forced to stop, and turn round you will find that the air has been stirrred up behind you to make your trip home likely worse, so the go decision should not be taken lightly.