My friend and I were in a Graveyard and happened upon a Scenario Room with a poison fog covering a dirt patch.
I had my graphics quality settings on Low, so the poison fog was completely invisible to me, and I had no idea why I was being hurt/poisoned.
Here are some pics!
Graphics on Low:
http://i.imgur.com/BXovh.jpg
Graphics on Medium/High:
http://i.imgur.com/nCzZx.jpg
And so, to be completely clear, I'm sure the poison fog should always be visible, no matter what the graphics quality setting is.
I remember that, but when on low doesnt it show white sparks