Fog machines and fog juice are the most common. The basic mechanism is simple -- the fog juice is heated to create smoke. When you overheat oil on the stove and create a lot of smoke, you are doing approximately the same thing. Cooking oil has a tendency to get gummy and smell bad, however. Fog machines use glycerin or glycol mixed with water.

According to a Seattle Times article, "The fog is created using propylene glycol and triethylene glycol, mixed with 20 percent water." One thing that is not clear is whether this fog has any side effects on people's lungs. It seems like it is a problem for asthmatics, but nothing has been proven conclusively for the general population.

Fog machines are used to create spooky effects in plays such as Dracula, A Christmas Carol, and Macbeth. Fog machines are also frequently used in Halloween displays such as haunted houses or forests.

Fog machines are also useful for industrial purposes, such as visualising the flow of air, such as over a surface in a wind tunnel or testing filters in air conditioning. Fire fighters also use fog machines in their training.

Because of the reflective and refractive qualities of the smoke created by fog machines, they are often used in nightclubs to enhance the effects of lighting and laser arrangements.

Haze machines, (also called haze generators or hazers), are taking over the job of fog generators for highlighting lighting or laser effects. Haze machines are often seen with bands and bigger venues when a lighting effect may need to be visible in the air. Haze machines use the same fluids to create fogs and were included on studies of health effects from exposure to theatrical smoke and fog.