Heavy equipment maintenance exposure assessment: using a time-activity model to estimate surrogate values for replacement of missing data

Our plain-English summary of Heavy equipment maintenance exposure assessment: using a time-activity model to estimate surrogate values for replacement of missing data, by F.W. Boelter, J.W. Spencer, C.E. Simmons (Journal of Occupational and Environmental Hygiene, 2007).

The study

• Authors: F.W. Boelter, J.W. Spencer, C.E. Simmons
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2007;4(7):525-37
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

This study on four pieces of heavy construction equipment was conducted to determine the concentration of airborne asbestos fibers during in-frame maintenance and repair activities, which included aggressive techniques that resulted in visible dust from work involving friction products and gaskets. Despite execution of a carefully planned sampling strategy, approximately 10% (47) of the samples collected could not be analyzed due to overloading or filter damage. To include the overloaded samples in the data analysis, surrogate values were estimated following a time-activity model. Twelve long-term personal samples, 2 short-term, 30-min personal samples, and 31 long-term area samples were modeled. Personal and area time-weighted average (TWA) data were analyzed both with and without the estimated surrogate values and compared. A total of 444 samples were collected over 9 days. Four experienced heavy equipment mechanics removed and replaced friction products and gaskets. Samples were analyzed using NIOSH Method 7400 Phase Contrast Microscopy followed by NIOSH Method 7402 Transmission Electron Microscopy. Sample data information including the surrogate values for the full-shift, TWA personal sample results ranged from 0.002 to 0.064 asbestos f/cc. Personal, short-term, 30-min sample results, including the two surrogate values, ranged from 0.038 to 0.561 asbestos f/cc. Full-shift TWA area samples, including the 31 surrogate values, ranged from 0.005 to 0.039 asbestos f/cc. Area air sample results at the end of the project were similar to levels measured before the start of the project. No fiber concentration buildup within the work area was indicated over the 9-day study. All full-shift personal and area TWA sample results were below 0.1 f/cc, and short-term 30-min personal samples were below 1.0 f/cc. Statistical results of the sample data with and without the surrogate values were consistent. Use of the time-activity model reduced the uncertainty associated with this data analysis and provided a consistent logical process for estimating surrogate values to replace missing data.

In plain English

This study measured airborne asbestos while four pieces of Caterpillar heavy construction equipment were repaired over 9 days in June 2003 in a repair facility. Four experienced mechanics removed and replaced brakes, clutch linings and gaskets. Bulk samples of the removed and replaced brake materials contained 15% to 95% chrysotile, and asbestos-positive gaskets contained 3% to 85% chrysotile. Forty-seven of 434 work-period air samples were overloaded and could not be analyzed. The authors estimated replacement values for 45 of these samples with a time-activity model and said those estimates were likely biased toward higher values. Full-shift personal averages including the estimates ranged from 0.002 to 0.064 asbestos f/cc. Thirty-minute personal samples ranged from below the detection limit to 0.561 asbestos f/cc, and 96 of 138 were below that limit. The highest value came from cutting a brake band, which the authors called an unusual practice. Full-shift area averages including the estimates ranged from 0.005 to 0.039 asbestos f/cc. The authors concluded that exposures were below 0.1 f/cc as an 8-hour average and below 1 f/cc as a 30-minute sample.

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: Field exposure assessment: personal and area air sampling during actual repair work, with a time-activity model used to estimate surrogate values for samples that could not be analyzed
• Who was studied: Four mechanics (Mechanics #1 to #4) working on four Caterpillar machines (D8 Dozer, 12E Motor Grader, 930 Wheel Loader, 955 Track Loader) in a heavy equipment repair facility over 9 days in June 2003
• Exposure: Removal and replacement of brake linings, clutch linings and gaskets. Bulk samples of the brake materials contained 15% to 95% chrysotile asbestos.
• Main results:
  • Asbestos-positive gasket materials that were removed and replaced contained 3% to 85% chrysotile
  • Brake debris collected from all four machines was nondetectable or less than 1% chrysotile asbestos
  • 47 of 434 work-period samples were overloaded with particulate matter and could not be analyzed
  • Surrogate values were modeled for 45 samples: 12 long-term personal, 2 short-term 30-min personal and 31 long-term area samples
  • Maximum full-shift personal TWA: 0.041 asbestos f/cc in measured full-shift data (AFS), 0.055 in partial-shift data (APS), 0.064 in data including surrogate values (BFS). The minimum was 0.002 for AFS and BFS.
  • Mean personal TWAs: 0.014 asbestos f/cc (AFS), 0.022 (APS), 0.018 (BFS, including surrogate values)
  • Short-term 30-min personal samples ranged from below detection limits (0.043 and 0.038 asbestos f/cc) to 0.561 asbestos f/cc
  • 96 of the 138 short-term samples were below the limit of detection
  • The highest short-term result (0.561 asbestos f/cc) came from cutting a brake band, described as an unusual practice
  • Full-shift area TWAs: minimum 0.005 for both data sets; maximum 0.022 (AFS) and 0.039 (BFS); averages 0.011 (AFS) and 0.013 (BFS) asbestos f/cc
  • No asbestos fibers were detected by TEM in the five inside background samples taken before work began
  • By the end of the study, average area air levels were essentially the same as background levels before the study
  • All TWA samples were below 0.1 f/cc and all short-term 30-min samples were below 1 f/cc
• The authors' conclusion: The authors concluded that repair and maintenance of heavy construction equipment involving asbestos-containing friction products and gaskets resulted in exposures below 0.1 f/cc as an 8-hr TWA and below 1 f/cc as a 30-min short-term sample.
• Limitations the authors state:
  • The estimated surrogate values for missing samples are likely biased toward higher exposure values
  • The time-activity model may not give consistent results for other data sets
  • Means of short-term samples were not calculated because so many results were below detection
  • Statistical analysis of short-term data used only samples above the detection limit, so only upper-bound statistics were reported

Topics

Medical terms in this study, explained in our Lexicon: Friction Products, Gasket, Personal Breathing Zone Sample, Time-Weighted Average (TWA), NIOSH Method 7400, phase contrast microscopy (PCM), NIOSH, Transmission Electron Microscopy (TEM).

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