Monday, May 09, 2005

Stuctures v. Fibers in Brake Shoe Operations

Do brake repair operations entail epidemiologically meaningful asbestos exposures? Epidemiologically meaningful means exposure high enough to yield 3 lung cancers in 1000 exposed workers, approximately an SMR of 1.5. Physiologically meaningful exposures might be lower, an exposure increasing the rate of pleural placques on x-ray with a 20 year lag from accumulated fiber-years or structure years of exposure.

OSHA's risk assessment, based on human data, projects an attributable lung cancer rate of 6 per 1000 at the current PEL of 0.1 fibers/cc, which is a 45 year exposure of 4.5 fiber-years. BrooklynDodger guesses that 0.1 fiber/cc is about the limit of quantitation for fibers in an environment clean of other particulate. Below that, exposure would have to be measured with "structure" on TEM.

These investigators are based in Tyler, Texas, ground zero for one of the key factory studies of yesteryear. They report that at one point Chrysotile comprised 40-50% of the composition of brake linings. Unused brake linings or brake shoes with attached linings from four different sources were gently rinsed with prefiltered water that was then collected on filters for analysis by Analytical Transmission Electron Microscope. Large numbers of chrysotile asbestos containing structures, the majority of respirable size, were present in each sample. The investigators concluded that any manipulation of new asbestos containing brake components would be expected to yield free dust containing chrysotile asbestos of respirable size. That the vast majority of these fibers and particulates would not be counted as regulated fibers.

The full paper has lots of interesting pictures.

BrooklynDodger suspects the friable asbestos in the boxes is actually fallout dust from the factory making the friction product. It would be unlikely that bouncing around in the box would cause wear enough to generate the dust. The best way to protect the auto mechanics would be to reduce dust in the manufacturing plant [where ever it is, likely not in the US] and clean the product before packing.


Am J Ind Med. 2004 Dec;46(6):545-53.

Evaluation of the size and type of free particulates collected from unused asbestos-containing brake components as related to potential for respirability.

Atkinson MA, O'sullivan M, Zuber S, Dodson RF.
The University of Texas Health Center at Tyler, Tyler, Texas 75708, USA.

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