Drum brakes contained parts made up of cast iron in order to be wear-resistant. Every time they were applied on the road, these parts would heat up due to friction so asbestos was added in the brake lining to withstand the high temperatures.
Over time, the asbestos fibers found in the linings would break off due to usage. As mechanics came in direct contact with the damaged drum brakes, they might have tried to clean them by air-hosing the gathered dust instead of choosing aerosol sprays or wet brushes in order to contain the exposure. You could have been exposed if: