The car cabin air filter in your vehicle's ventilation system is used to filter dust, allergens, mold, particulates and sometimes even odors from the air entering your car's passenger compartment. The cabin air filter is used whether or not you use the air conditioner or heater in your car, as there is always air flowing into and out of the passenger compartment. Most vehicle manufacturers recommend changing the cabin air filter every 12,000 to 15,000 miles. You can check the condition of the air filter by removing it and tapping it lightly. If dust falls out, you should change it.