An air door or air curtain is a device used to prevent air or contaminants from moving from one open space to another.

The most common use is a downward-facing blower fan mounted over an entrance to a building, or an opening between two spaces conditioned at different temperatures.

The air curtains reduce energy costs by keeping heated or cooled air in the internal building atmosphere. Efficient air curtains will save up to 80% energy looses across a doorway compared with a door without air curtain.