In car air circulation settings

Under most conditions, outside air is used. It is a automatic setting when you turn on the heater or ac. Max A/C switches to recirculate and recools the inside air to cool things down faster. Here in the midwest, I sometimes switch to recirculate when traveling past hog confinement units because of the nasty odor. It shuts off the outside air and you breathe just the cabin air. (for a mile or two)