A fog machine is a device which uses a superheated mixture of glycol and water to generate a supply of fog-like smoke. A DJ working at a nightclub might use a fog machine to enhance the lighting display behind and above the dancers. Movie set designers routinely use a commercial-grade fog machine to create an ominous atmosphere during a nighttime scene. Every Halloween, amateur party planners pull out a fog machine 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.

A commercial fog machines works on a very different premise. When oil on a stove overheats, the result is often a smoky fog. Vegetable oil would not make a promising ingredient in a fog machine, however. Through experimentation, inventors settled on a mix of water and glycol as the safest catalyst for smoke (fog) generation. A typical fog machine has a small reservoir or tank which holds a supply of the 'fog juice'. An electric pump draws the liquid over a heating coil inside the fog machine itself. When the atomized fog juice becomes overheated, the result is an odorless white smoke. A nozzle on the front of the fog machine directs this smoke into the outside air.

The smoke generated from a fog machine 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 a fog 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 fog machine to cool the smoke quickly. This 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. 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 few inches in front to catch the vapor. Try to keep your fog machine a few feet away from visitors, since the smoke comes out hot for a few seconds.