Document NG0zggKgLGXdDnRVZnQBeY1e8

2. Sources The fibrous asbestos content of Detroit's ambient air is estimated to be about 1 x 10~8 grams/m3. A recent literature review by the Public Health Service states that the bulk of airborne asbestos originates from natural sources such as soil dust and locally from asbestos mines and processing plants. Limited amounts of fibrous asbestos are also emitted from the use of asbestos cement used often as spray building insulation. According to another review by the National Research Council, automotive contribution from brake-linings is believed to be negligible because asbestos fibers are destroyed by the intense heat created by the braking process (about 1400F) to a non-fibrous state. 3. Uses The total U. S. asbestos production in 1965 vas 8 x 105 tons. Cement, floor tile, paper, and other building construction materials accounted for about 90% of asbestos consumption. Friction materials - - primarily brake linings -- accounted for 2.8 x 10^ tons or 3.556 of the total asbestos production. 4. Brake Lining Baisslon Rates Attempts have been made at Ford to determine the fibrous asbestos emission rates from a laboratory brake-lining test stand. An elaborate electron, microscope technique is used to identify the tiny fibers (about 1 x 10" inches diameter). Our preliminary experiments indicate that there is very little fibrous asbestos present In brake dust. The fibers are less than 0.5 u in length and most of them are converted to a non-fibrous state by the process of braking. Our emission rate estimate is very close to the background ambient concentration levels. 5, Brake Lining Alternate Materials Alternatives for asbestos based linings are few and all have some disadvantages in terms of either performance, cost, or both. Ford has been using cermet linings (sintered metal) for heavy duty trucks and semi-met (metal strand and carbon mixture) for police cars.both containing no asbestos. Beginning with the 1971 model year , Ford has oeen supplying optional semi-met front-end brakes for police car fleets. Semi-met linings are superior in performance to conventional brake-linings (about equivalent in terms of cold-wear and noise), but the cost penalty is severe ($1.25/car. just for front-end brakes). Ford is experimenting with semi-met linings for rear wheels. However, there are some cold-stopping and wear difficulties associated with rear wheel semi-met linings in their present form and may necessitate major redesigning of the rear wheel brake systems. 8004 0505