Here are some good resources to learn about creating light fogs in Maya,
I’ve only been using the standard light fog function with a spot light, but will be testing the mental ray method shortly, as it seems to produce much more realistic results.
Light Rays In Maya & MentalRay – ( using env par ) http://vimeo.com/18319188
Create light fog using mental ray for Maya sample workflow – Link using mib_light_spot
Light Fog – ye old faithful method but with better results by controlling coneshape’s volume samples and using mental ray depth map overides – http://vimeo.com/15009370
tick “auto volume” in mental ray render features.
discussion on parti_voume – http://forums.cgsociety.org/showthread.php?f=87&t=812857
Nice tut on environment fog with parti_volume = http://www.andrewklein.net/bh/texturing_fog.html