Haze machines, or haze generators (commonly referred to as hazers), are effects machines similar to fog machines, designed to produce an unobtrusive, homogeneous clouds suspended in the air intended primarily to make light beams visible or create a subtle diffusion. Unlike theatrical fog, which is typically intended to be dense and/or opaque, haze is generally very light and subtle. These properties allows a venue to be filled with haze prior to or during an event without creating an overtly distracting cloud. Haze typically has a substantially longer persistence ("hang time") than conventional theatrical fog. While conventional fog will hang in the air for several minutes, a haze effect filling the same volume of space can last upwards of an hour to several hours or more, depending on the size of the venue and the amount of ventilation. The fluid used in haze machines to generate the effects are either oil or water-based. Although both formulations of fluid are referred to as haze fluid, the different formulations are neither compatible nor interchangeable. Glycol/water haze fluid is sometimes referred to as "water based haze" for the purposes of disambiguation.