The BMW cabin air filter has nothing to do with improving the performance of your vehicle but instead helps to supply the driver and passengers with cleaner air to breathe. The placement of the cabin air filters on a BMW is simple to locate. This position is just in front of the lower part of the windshield on top of the engine compartment's firewall under the hood. For many Americans, the BMW cabin air filters are a new component that has not been a standard part of vehicles in past years. Most American cars are not equipped with a cabin air filter and only have an open hole in which air from outside the passenger compartment is used by the air conditioner, heater vents and defroster. One of the reasons for the BMW engineers to introduce the cabin air filters as standard equipment on their vehicles is to help improve the air quality for the users of the car. Not only are they designed to remove dirt and debris from entering the passenger compartment but also many pollutants that are present in the air on the roadways across America. Many of the replacement cabin air filters made for BMWs are HEPA filters. These are the same type of high efficiency particle air filters that are also used in medical facilities to filter the air. To be considered a HEPA filter, the element must be able to remove 99.97% of all particles that are 3 microns or larger. In most instances the filter element is made of fiberglass that only 0.5 to 2.0 microns in thickness. They are arranged in a compressed configuration so that the distance between these small fibers is only 0.3 microns. This filter makes it easier to breathe no matter what is in the air outside your vehicle.