Our Ford Windstar has two settings for air conditioning, regular and "max." We had always assumed that "max" meant the compressor would work harder, hence using more gas, so we rarely used it. That is, until one day another Ford owner pointed out to us that "max" simply means that the vehicle re-circulates the interior air, rather than bringing in new air from outside. This is documented in the manual, but it isn't exactly intuitive; why couldn't they simply say what it does?
My truck has this feature, but it has a separate button: you can run any of the air conditioner settings with or without the air recirculating. And I was able to infer the meaning of the button without the assistance of a friend. It does make me wonder, though: if the air recirculates, wouldn't you be breathing the same stale air? How long would it be before that became a problem? I found a site demonstrating that adults inhale 8-10 liters of air per minute. (That's about 8 1/2 to 10 1/2 quarts, or a little over two gallons. Darn metric system.) That's not very much, but it could become a lot quickly: after just 10 minutes, it would be 20 gallons, and after an hour, 60. If there are two people in the car, make it 120. Surely that would have to be a significant portion of the air in a regular car (not a van or suv), so that your oxygen intake would be getting pretty thin.
Obviously, that doesn't happen. Not only doesn't it happen, there probably isn't the remotest chance of its happening, or the car companies would be risking major lawsuits (and hence wouldn't allow that option). I suppose, therefore, that cars must leak a lot of air. And that makes me wonder just how much air gets introduced into a vehicle in a given period of time, but I have no idea on that count.