Asbestos exposures of mechanics performing clutch service on motor vehicles

Our plain-English summary of Asbestos exposures of mechanics performing clutch service on motor vehicles, by H.J. Cohen, D.R. Van Orden (Journal of Occupational and Environmental Hygiene, 2008).

The study

• Authors: H.J. Cohen, D.R. Van Orden
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2008;5(3):148-56
• Cited by: 15 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

A study was conducted to assess historical asbestos exposures of mechanics performing clutch service on motor vehicles. For most of the 20th century, friction components used in brakes and manual transmission clutches contained approximately 25-60% chrysotile asbestos. Since the late 1960s, asbestos exposure assessment studies conducted on mechanics performing brake service have frequently reported levels below the current OSHA permissible exposure limit (PEL) of 0.1 fiber/cc (flcc). Although there is a robust asbestos exposure data set for mechanics performing brake service, there are almost no data for mechanics removing and replacing clutches in manual transmission vehicles. Personal and area airborne asbestos samples were collected during the removal of asbestos-containing clutches from 15 manual transmissions obtained from salvage facilities by an experienced mechanic. Clutch plates and debris were analyzed for asbestos using EPA and ISO published analytical methods. More than 100 personal and area air samples were collected and analyzed for asbestos fibers using NIOSH methods 7400 and 7402. A separate study involved a telephone survey of 16 automotive mechanics who began work prior to 1975. The mechanics were asked about the duration, frequency, and methods used to perform clutch service. Wear debris in the bell housing surrounding clutches had an average of 0.1% chrysotile asbestos by weight, a value consistent with similar reports of brake debris. Asbestos air sampling data collected averaged 0.047 flcc. Mechanics participating in the telephone survey indicated that clutch service was performed infrequently, the entire clutch assembly was normally replaced, and there was no need to otherwise handle the asbestos-containing clutch plates. These mechanics also confirmed that wet methods were most frequently used to clean debris from the bell housing. Combining the asbestos exposure that occurred when mechanics performed clutch service, along with the duration and frequency of this task, the incremental contribution of this task to mechanics' 8-hr time-weighted average (TWA) asbestos exposures was 0.0016 flcc. Using the range of data inputs that were obtained, the authors calculated a range of TWA exposures of 3.75 x 10(-5) flcc to 0.03 flcc. The mean value of 0.0016 flcc is below background levels of asbestos that have been reported in garages during this time and below the current OSHA PEL of 0.1 flcc.

In plain English

This study estimated mechanics' asbestos exposure from clutch service. Of 15 salvage-yard vehicles made 1948–1975, an experienced mechanic removed clutches from 11 at a test facility; plates and debris from the other 4 were removed at a salvage facility. Chrysotile was 35–55% by weight in 14 of 15 clutch plates; bell housing debris from 12 of those 14 vehicles averaged 0.10% chrysotile, 5 below detection. During most removals, 46 personal samples on the mechanic averaged 0.047 fibers per cubic centimeter (f/cc); 58 area samples averaged 0.013 f/cc. In a telephone survey, 16 mechanics who began before 1975 reported clutch jobs about once every 3 weeks; 15 always replaced the whole clutch assembly. Combining these, the authors calculated that clutch work added 0.0016 f/cc to a mechanic's 8-hour average exposure (range 3.75 times 10 to the minus 5 to 0.03 f/cc). The authors concluded this was below the current OSHA limit of 0.1 f/cc. They noted that only removal was studied, and rusted salvage vehicles needed more aggressive handling, which might have raised measured levels.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Exposure assessment study with two parts. Clutches were removed with personal and area air sampling, and clutch plates and debris were analyzed for asbestos. A separate telephone survey of mechanics was also conducted. The results were combined into an estimated 8-hour time-weighted average exposure.
• Who was studied: 15 manual-transmission vehicles from salvage facilities, manufactured 1948–1975: 7 passenger cars, 4 light-duty trucks, 2 passenger vans, 1 school bus and 1 dump truck. A professional mechanic removed engine/transmission units and clutches from 11 vehicles at a test facility in a suburb of Pittsburgh, Pennsylvania. Clutch plates and debris from the other 4 were removed on site at a salvage facility. Separately, 16 mechanics from several US states who began work between the 1950s and 1974 were surveyed by telephone.
• Exposure: Chrysotile asbestos in clutch plate friction material and in bell housing wear debris. It was studied during removal of clutches, not installation of new ones. The mechanic wore coveralls and no respirator.
• Main results:
  • Asbestos was found in the clutch plates of 14 of the 15 vehicles, at 35–55% chrysotile by weight (mean 43%).
  • Bell housing debris was analyzed for 12 of the 14 vehicles with asbestos-containing clutch plates. Debris from the other two was not collected.
  • In those 12 debris samples, chrysotile ranged from below detection (less than 0.01% by weight) to 0.31% by weight, with a mean of 0.10%. No other form of asbestos was identified.
  • Five of the 12 debris samples were below 0.01%. Half the detection limit was used for these samples in calculating the mean.
  • Personal air samples on the mechanic (46 samples, from 11 vehicles) ranged from 0.015 to 0.13 f/cc, with a mean of 0.047 f/cc.
  • Area air samples (58 samples, from 14 vehicles) ranged from less than 0.002 to 0.03 f/cc, with a mean of 0.013 f/cc.
  • Surveyed mechanics reported clutch service about once every 3 weeks, averaging about 4 hours per job (range 1–10 hours). 15 of 16 always replaced the entire clutch assembly.
  • The authors calculated an incremental 8-hour time-weighted average exposure from clutch service of 0.0016 f/cc. The calculation was 0.05 f/cc times 4.2 hours times 0.06 jobs per day, divided by 8 hours.
  • Using the ranges of their inputs, the authors calculated 8-hour time-weighted average exposures from 3.75 times 10 to the power of minus 5 f/cc to 0.03 f/cc.
• The authors' conclusion: The authors concluded that clutch wear debris had a very low percentage of chrysotile, because it converted at high temperatures to non-fibrous forms. They concluded that debris from clutch removal therefore should not have created a hazardous exposure. They also concluded that measured airborne levels were below the current OSHA limit of 0.1 f/cc and comparable to brake service data. The estimated incremental exposure of 0.0016 f/cc was, in their words, nearly two orders of magnitude below that limit and comparable to background levels reported for garages.
• Limitations the authors state:
  • Salvage-yard vehicles were harder to disassemble and needed more aggressive handling. This might have generated higher asbestos levels than routine clutch servicing.
  • The exposure estimate depends on the quality and accuracy of the input data.
  • The authors had no reliable published data to compare with the clutch service frequency reported by the surveyed mechanics.
  • Survey selection may have produced a higher clutch service frequency than the true value.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Chrysotile, Permissible Exposure Limit (PEL), NIOSH, Air Sampling, Time-Weighted Average (TWA).

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