Do you mean while you are driving? The car is designed to constantly flow air in from the cowl, out through the floor vents into the cabin, and then out through the vents in the rear door jams. It happens naturally as you are moving. As far as I know, it is supposed to keep fresh air flowing through the cabin, so you don't get asphyxiated. I've had three GM cars and they've all worked the same way. In my father's '79 Corvette, the blower motor turns on with the ignition, so air is ALWAYS being pumped through the cabin, as long as the car is running.
Other manufacturers handle it differently, however. In both my wife's Corolla, and my father's Plymouth Voyager, you can seal the cabin up completely if you wish, but in the owner's manual, they recommend doing that only if absolutely necessary, and only for short periods of time.