This is the principle of the air curtain†. The local Whole Foods Market operates air curtains as invisible exterior doors. When you walk through, you feel a str

This is the principle of the air curtain†. The local Whole Foods Market operates air curtains as invisible exterior doors. When you walk through, you feel a strong down-rushing air current. Flying insects can't negotiate it. †http://en.wikipedia.org/wiki/Air_door http://en.wikipedia.org/wiki/Air_door