Look for the button or sign with:
1. an arrow flowing into a car, or
2. an arrow in circle within the car cabin. You want to choose this (the arrow circling within the car's cabin). This is air-recirculation within the car.
The first indicate air coming into the car's cabin. If all else fails, you can spend money to repair the intake duct seal, or spend money like some of these taiko's recommendations:
1. Gas mask
2. Scuba tank
3. My suggestion, 3M Half-face respirator with charcoal canisters/filters (you'll look pretty cool - like Storm Trooper in Star Wars).
Hope it helps.