Here's the thing: cigarette smoke can cause a similar, spooky, misty effect in photographs. It is a good thing to note that second-hand smoke from cigarettes, cigars and the like can hang around for many hours within a non-ventilated building. Often times, people are not aware and believe the room is clear from this hard-to-see mist.
Halloween Spirit Picture
Theresa E. from Chicago might have captured the presence of a spirit being on Halloween!
"Halloween Party 2006 - orbs in my pics are very common and no longer a big surprise; but, this one is very different. Does anyone agree?"
But, as far as this photo, if no one was smoking, then there is no other explanation for the spirit presence or "mist."