An aspirating smoke detector is a type of smoke detector, based on a nephelometer that is capable of detecting the presence of microscopic particles of combustion also known as (smoke) suspended in air by detecting the light scattered from the smoke particles in a sampling chamber. In the context of smoke detection, the word aspirate means to draw in, which illustrates the basis for how these smoke detectors operate. In most cases aspirating smoke detectors require a fan unit to draw in a representative sample of air from the protected area.