http://forum.zdoom.org/viewtopic.php?f=2&t=23869
and make sure you have blood set to use particles, summon something pretty big (I used a mancubus) into the foggy half then shoot him with something that generates a lot of blood (eg the SSG) you should see the effect. It can be done with freeze mode on so that makes it easier to spot. It's not just blood that does it though, I initially spotted it with an enemy that has a railgun.

