Toyota Prius Owner's Manual page 344

3-1. Using the air conditioning system and defogger

Switching between outside air and recirculated air modes
Press
The modes switches between
and
the button is pressed.
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PRIUS_OM_OM47A19E_(EE)
.
(introduces air from outside the vehicle) modes each time
Mainly air flows to the feet.
Air flows to the feet and the
windshield defogger operates.
(recycles air inside the vehicle)