Fog and haze differ in that fog is a thick, opaque effect that lasts a short time, while haze is a thin, translucent effect that lasts a long time. Fog is used as a special effect, whereas haze is used for lighting/atmosphere enhancement.
Whether created by nature or machine, fog consists of liquid droplets suspended in the air. Fog machines create fog by vaporizing fog fluid – that is, they convert the fog fluid from a liquid form to an aerosol form. Most accomplish this by forcing the fluid at high pressure through a heated pipe.
Bursts of fog are typically used for special effects in live and filmed productions. They’re useful for anything from causing a cigar to “smoke” in a small ashtray, to obscuring a huge battlefield. Fog is a smoke-like effect; however, it’s not smoke, since smoke consists of solid particles rather than liquid droplets. (Also, smoke is usually created by burning, and although fog machines do heat the fog fluid to vaporize it, they don’t burn it.)
Fog can also be made to stay low to the ground, for a “walking on clouds” effect. This is usually accomplished by ducting the fog through a cold chamber to cool it. Typically, fog fluid for creating fast-dissipating fog is used, so that the fog will evaporate before it warms and begins to rise. Some machines make low fog by mixing a cryogenic liquid (such as liquid nitrogen) or solid (such as dry ice) with hot water.
Like fog, haze consists of liquid droplets, but the drops are very fine and are distributed evenly over a large area to form a mist. Some hazers vaporize fluid by forcing it through a heater, and others vaporize it using high air pressure.
The primary use of haze is to make light beams visible. Since light reflects off the droplets, you will see light travelling through the air that you ordinarily would not see. Haze is also used to create a misty atmosphere.
Fog fluids are generally made of water-soluble glycols, such as propylene glycol. These glycols have been used in industry for decades and health data is available. There is an ANSI standard that defines the limit of glycol that a healthy adult can safely breathe (ANSI E1.5 – 2003 Entertainment Technology – Theatrical Fog Made with Aqueous Solutions of Di- and Trihydric Alcohols). Still, you should always use only the smallest amount of fog necessary to produce the required effect.
Fog machines that use cryogenic products to cool the fog add gases like carbon-dioxide and nitrogen to the air, so care must be taken to avoid build up of these gases to toxic levels or to levels that might cause oxygen deficiency.
Haze fluids are generally either water-soluble glycols or highly refined oils. The safe level for oil is different than that for glycols, but is still much higher than the amount typically used for haze effects.
For more information, I would suggest reading Introduction to Atmospheric Effects published by the Entertainment Services & Technology Association (www.esta.org), and consult the various fog manufacturers’ catalogs and specification sheets. The ANSI standard referenced above is also available on the ESTA website, for free, in the Technical Standards Program section.