A fog machine (also called a smoke machine) is a device which emits a dense vapor that appears similar to fog or smoke. This artificial fog or smoke is known as theatrical smoke and fog within the entertainment industry. Most fog machines create the fog by either vaporizing a water and glycol-based or glycerine-based fluid or a mineral-oil-based fog via atomization. For glycol-based fogs, the fluid (fog juice) is injected into a heated block, and evaporates quickly. The resulting pressure forces the vapor out of the exit. Upon coming into contact with cool outside air the vapor forms a fog.
Some fog machines use dry ice to produce a fog of carbon dioxide.
Although often believed to be safe to breathe, exposure to mineral oil, glycol and glycerin-based theatrical fogs produced from fog or haze machines have been associated with adverse short and long-term effects on respiratory health in exposed performers and crew members, especially those with respiratory problems (such as asthma). One should therefore avoid breathing generated smoke or fog It is safer to add smoke effects digitally in postproduction of films than to use theatrical smokes on set. Glycol-based fog is associated with headaches, dizziness, drowsiness and tiredness in those exposed. These results would be expected given that glycols share some chemical similarities to alcohols and given the small size of fog particles making it easy for them to enter the bloodstream. Symptoms of wheezing and chest tightness were associated with longterm exposure to theatrical smoke and fogs.
extracted from wikipedia
You can purchase special oils that can scent your smoke this aids as a great air freshner and impresses your guests! it makes good sense to clean your smoke machine from the inside out - Details anyone I can't remember! Be aware that smoke machines have a tendency to set of fire alarms use less output or have something to disperse the smoke better i.e. a fan postioned above the smoke machine
You need to be aware that sometimes the fog machine can spit small amounts of hot fluid from the end of the machine which could cause injury to anyone underneath it. Some machines are fitted with a drip tray to minimize the risk.
Take care when emitting large amounts of fog/smoke as if you can't see the guests then they can't see you and probably can't see the floor and other obstacles!