Most modern passenger cars have disc brakes on the front wheels, which unlike drum brakes are not sealed off to the ambient air. During braking, there is wear to both the rotor and the pads.

This wear process generates particles, which may become airborne. In field tests it is difficult to distinguish these particles from others in the surrounding environment. It may be preferable to use laboratory test stands where the cleanness of the surrounding air can be controlled. The validity of these test stands has to be verified by comparison with field tests and therefore a test series has been conducted. These tests were performed in Stockholm, Sweden, in urban traffic.

Low metallic type brake pads and gray cast iron rotors were tested. The results indicate that this test methodology can be used to study the number and mass concentrations as well as size distributions of particles generated from car disc brakes. Overall, the measured mean particle number and mass diameters of the airborne particles were 0.39 μm and 1.5 μm, respectively.