In order to assess the amount of airborne particulate matter (PM) attributable to vehicle disk brakes, a system was devised for collecting brake wear debris on a laboratory brake dynamometer. The brake dynamometer test hardware was enclosed and vented through a duct in which the airflow was controlled to ensure isokinetic sampling. Two brake dynamometer simulations were implemented: urban driving (low velocity, low g) and the Auto Motor und Sport (AMS, high velocity, high g). These test procedures were performed repeatedly on the brake system hardware of vehicles utilizing three different friction material types: low-metallic, semi-metallic, and

non-asbestos organic (NAO). Airborne brake wear was collected on filters and via other airborne PM sampling techniques. Larger, non-airborne wear debris was collected from the wheel, below the brake, and brushed off the hardware. Considering the effect of the wheel, 50-

70% of the collected wear debris was airborne PM. In the urban driving test, the collected wear debris accounted for more than 90% of the total wear (rotor and lining mass loss). In addition, the airborne mass scaled with the total wear. Airborne fractions from the low-metallic and semi-metallic linings were 5 and 1.5 times higher (respectively) than the NAO lining. For the AMS test, the wear per stop was 30 times higher, and the wear debris collection efficiency was less than 50%. Uncollected combustion products and other volatiles are likely responsible for the poor mass balance during the AMS test.