An air door or air curtain is a machine used for separating two spaces from each other. The machine is mainly used at doors into buildings. The most common setup for air curtains is a downward-facing blower fan placed over an opening. Normally this opening is an door to a building The air curtain keeps flying insects out by making turbulence. It also helps keep outside air out, avoiding cold winds by mixing-in warm air from the air curtain. The fan must be powerful enough to generate a jet of air that can reach the floor.