Theatrical smoke and fog  Theatrical smoke and fog, also known as special effect smoke, fog or haze, is a category of atmospheric effects used in the entertainment industry. The use of fogs can be found throughout motion picture and television productions, live theatre, concerts, at nightclubs and raves, amusement and theme parks and even in video arcades and similar venues. These atmospheric effects are used for creating special effects, to make lighting and lighting effects visible, and to create a specific sense of mood or atmosphere. If beams of light are visible cutting across a room, that most likely means smoke or fog is being used. Theatrical smoke and fog are indispensable in creating visible mid-air laser effects to entertain audiences. Recently smaller, cheaper fog machines have become available to the general public, and fog effects are becoming more common in residential applications, from small house parties to Halloween and Christmas. Theatrical fog and theatrical fog machines are also becoming more prevalent in industrial applications outside of the entertainment industry, due to their ease of use, inherent portability and ruggedness. Common popular applications for theatrical fog include environmental testing (such as HVAC inspections) as well as emergency personnel and disaster response training exercises. Militaries have historically used smoke and fog to mask troop movements in training and combat, the techniques of which are technologically similar to those used in theatre and film. There are generally 4 types of fog effects used in entertainment applications: smoke, fog, haze, and "low-lying" effects. Smoke effects refers to theatrical atmospheric effects produced either by pyrotechnic materials, such as Smoke Cookies, and pre-fabricated smoke cartridges; or other, flammable substances such as incense or HVAC smoke pencils or pens. Smoke is differentiated from other atmospheric effects in that it is composed of solid particles released during combustion, rather than the liquid droplets that fog or haze are composed of. Fog is created by pumping one of a variety of different glycol or glycol/water mixtures (referred to as fog fluid) into a heat exchanger (essentially a block of metal with a resistance heating element in it) and heating until the fluid vapourises, creating a thick translucent or opaque cloud. Devices specifically manufactured for this purpose are referred to as fog machines. An obsolete method for creating theatrical fog on-stage (although the technique is still commonly used in motion pictures) is to use a device known as a thermal fogger, initially designed for distributing pesticide, which aspirates a petroleum product (typically kerosene or propane), ignites the fuel, and then mixes in air and pesticide to create a dense fog.