Scientists know relatively little about particles released into the air when a vehicle driver brakes, though evidence suggests those particles may be more harmful to health than particles exiting the tailpipe. To do the work, the team used a large lathe to spin a detached brake rotor and caliper. They then measured the electric charge of the aerosols emitted in the air and discovered the 80 percent figure. We were surprised that this has not really been studied given how common cars are in human societies.