Adding that spooky fog effect for you Halloween project or haunted house is pretty easy. Most fog machines use a water based fog fluid that is safe and produces many different fog effect depending on the fog machine. Most fog machines plug into your standard 120V power outlet and are very easy to use.
You can choose from your standard fog machines that can blast out fog in a certain area or cover larger areas with fog pretty quickly or Low-lying fog machines that uses regular or dry ICE to cool the fog down so it will spill out slowly and lay close to the floor or ground and give you that spooky cloud fog effect over you scary grave yard - but remember wind can effect these effect if used outdoors
The bigger the area you want to cover, the more powerful of a fog machine you will need, so keep that in mind when shopping for a fog machine for Halloween or for special effects!