Asbestos brake emissions

Abstract

An enclosed brake-testing machine was built to determine the importance of asbestos emissions from automobile brakes. Higher emissions resulted from disc brakes than from drum brakes due to the smaller surface area contacting the wheel in the former case. Median emissions from one disc brake were 3.3 mg particulate/stop (55% airborne, 12% sedimentary, 34% brake-entrained), 0.6 mg C/stop as hydrocarbons, 0.7 mg C/stop as CO 2, and 0.1 mg C/stop as CO. Higher emissions were observed with increases in velocity and deceleration rate. The mass median aerodynamic diameter of the emitted particles was 3.1 μm. Although the original brake material was about 50% asbestos, the emitted particles averaged only 0.029% asbestos. Therefore, it appears that over 99.9% of the mass of original asbestos fibers was broken down into nonfibrous magnesium silicates. In addition, asbestos fibers which are on the order of millimeters in length in the brake had a median length of 0.5 μm in the emitted debris. Based on the mass emission results and a stopping rate of 1.2 stops/km, it can be estimated that 2.6 μg of asbestos/km/vehicle is emitted into the air, 0.76 μg/km settles to the roadway, and 2.2 μg/km is entrained in the wheel. Although brake emissions are responsible for a minor fraction of ambient asbestos levels, there may be a noticeable increase in asbestos levels near high-braking areas such as tollbooths.