A fog machine is a device that uses a superheated mixture of glycol and water to generate a supply of fog-like smoke. A DJ working at a nightclub might use one to enhance the lighting display behind and above the dancers. Movie set designers routinely use commercial-grade machines to create an ominous atmosphere during a nighttime scene. Every Halloween, amateur party planners pull one out in order to scare their guests or simulate a graveyard experience.
The simplest form of fog machine does not involve heating elements or the water/glycol mixture called "fog juice." Instead, blocks of dry ice are dropped into waiting buckets of water, causing massive amounts of carbon dioxide and water vapor to form. This fog could be allowed to fill up a room naturally or could be coaxed along by fans. The water vapor itself is perfectly harmless, although the dry ice must be handled with gloves to prevent painful freezer burn.
Commercial fog machines work on a very different premise. When oil on a stove overheats, the result is often a smoky fog, although vegetable oil would not make a promising ingredient in a fog machine. Through experimentation, inventors settled on a mix of water and glycol as the safest catalyst for smoke (fog) generation. A typical machine has a small reservoir or tank that holds a supply of the fog juice. An electric pump draws the liquid over a heating coil inside the device, and when the atomized fog juice becomes overheated, the result is an odorless white smoke. A nozzle on the front of the machine directs this smoke into the outside air.
The generated smoke is not considered toxic, but those with pre-existing breathing troubles may want to avoid prolonged contact. Sometimes, the sight of smoke-filled air can trigger a psychosomatic reaction.
Those who invest in a fog machine for Halloween might also want to experiment with different fog effects. Fog from dry ice tends to hug the ground, but fog generated by machine may head directly into the air. This effect may work well with lighting, but it won't creep into the room menacingly. An accessory called a fog chiller can be placed on top of the device to cool the smoke quickly, which will allow it to creep along the ground like dry ice fog.
Fog can also be piped into other areas through the use of large flexible tubing and small fans, and it can also appear to come out of the ground through an irrigating pipe with a series of holes. Nothing should be attached directly to the heated nozzle of a fog machine, but tubes can be placed a short distance in front to catch the vapor. Users should try to keep the device a few feet (about 1 meter) away from visitors, since the smoke comes out hot for a few seconds.