| Oh Hobo was asking me about this a while back but I finally settled on how I handle the way the Mist works:
Every hour you are exposed to it, it makes an attack vs. your Fort Defense. The + To Hit on this is based on the intensity of the Mist.
If it "hits" you then you make a Saving Throw or lose 1 point of Constitution.
If your Constituation is below half its normal value then you also make a second Saving Throw. If you fail that then you gain some kind of mutation.
The exact nature of these mutations are determined by me but I've got my WFRP books handy for that kind of thing. It is possible to gain some mutations that actually help you (like Mist Resistance) but ALL of them come with aspects that change your appearance and not for the better. | 
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