Use of direct versus indirect preparation data for assessing risk associated with airborne exposures at asbestos-contaminated sites

Our plain-English summary of Use of direct versus indirect preparation data for assessing risk associated with airborne exposures at asbestos-contaminated sites, by M.P. Goldade, W.P. O'Brien (Journal of Occupational and Environmental Hygiene, 2014).

The study

• Authors: M.P. Goldade, W.P. O'Brien
• Journal: Journal of Occupational and Environmental Hygiene
• Published: 2014;11(2):67-76
• Read it: PubMed · Publisher

Abstract

At asbestos-contaminated sites, exposure assessment requires measurement of airborne asbestos concentrations; however, the choice of preparation steps employed in the analysis has been debated vigorously among members of the asbestos exposure and risk assessment communities for many years. This study finds that the choice of preparation technique used in estimating airborne amphibole asbestos exposures for risk assessment is generally not a significant source of uncertainty. Conventionally, the indirect preparation method has been less preferred by some because it is purported to result in false elevations in airborne asbestos concentrations, when compared to direct analysis of air filters. However, airborne asbestos sampling in non-occupational settings is challenging because non-asbestos particles can interfere with the asbestos measurements, sometimes necessitating analysis via indirect preparation. To evaluate whether exposure concentrations derived from direct versus indirect preparation techniques differed significantly, paired measurements of airborne Libby-type amphibole, prepared using both techniques, were compared. For the evaluation, 31 paired direct and indirect preparations originating from the same air filters were analyzed for Libby-type amphibole using transmission electron microscopy. On average, the total Libby-type amphibole airborne exposure concentration was 3.3 times higher for indirect preparation analysis than for its paired direct preparation analysis (standard deviation = 4.1), a difference which is not statistically significant (p = 0.12, two-tailed, Wilcoxon signed rank test). The results suggest that the magnitude of the difference may be larger for shorter particles. Overall, neither preparation technique (direct or indirect) preferentially generates more precise and unbiased data for airborne Libby-type amphibole concentration estimates. The indirect preparation method is reasonable for estimating Libby-type amphibole exposure and may be necessary given the challenges of sampling in environmental settings. Relative to the larger context of uncertainties inherent in the risk assessment process, uncertainties associated with the use of airborne Libby-type amphibole exposure measurements derived from indirect preparation analysis are low. Use of exposure measurements generated by either direct or indirect preparation analyses is reasonable to estimate Libby-type Amphibole exposures in a risk assessment.

In plain English

This study compared two laboratory methods of preparing air filters for counting Libby-type amphibole (LA) asbestos by transmission electron microscope. Of 77 archived breathing-zone air samples, 31 meeting the study criteria, including at least 100 LA structures per square millimeter on direct preparation, were randomly selected. They came from activities around Libby, Montana that disturbed LA-contaminated dust, soil or other source materials; 26 were collected outdoors. Each filter had already been analyzed by direct preparation and was then re-analyzed by indirect preparation. For total LA, concentrations averaged 3.3 times higher with indirect preparation, a difference that was not statistically significant (p = 0.12). For particles 5 micrometers or longer the ratio was 1.9. For particles shorter than 5 micrometers the difference was statistically significant, with a ratio of 12 calculated without 2 of the 31 samples, which had non-detects. In filter-by-filter comparisons, indirect results were sometimes statistically higher, sometimes statistically lower and sometimes not statistically different. Two indirect analyses unexpectedly recorded no particles. The authors concluded that data from either method may be used in LA exposure assessments for risk assessment at the Libby Asbestos Superfund Site.

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: Retrospective laboratory methods comparison of randomly selected archived air filters, each analyzed by direct preparation and then by indirect preparation, using transmission electron microscopy
• Who was studied: 31 of 77 archived breathing-zone air samples from EPA investigation and cleanup monitoring in the Libby, Montana area. Selection was restricted to filters with a direct-preparation LA loading of at least 100 structures per square millimeter and at least half of the original filter remaining. 26 were collected outdoors and 5 indoors.
• Exposure: Airborne Libby-type amphibole (LA) asbestos in breathing-zone samples taken during activities that disturbed LA-contaminated dust, soil or other source materials
• Main results:
  • Across all 31 filters combined, the indirect-to-direct ratio of mean LA concentration was 3.3 for total LA and 1.9 for particles 5 micrometers or longer
  • The difference between methods for total LA concentration was not statistically significant (p = 0.12)
  • For LA particles shorter than 5 micrometers, the difference was statistically significant (p less than 0.05), with indirect concentrations on average 12 times the direct concentrations
  • The ratio of 12 for particles shorter than 5 micrometers left out 2 of the 31 samples, which had non-detects, because their ratio would have meant dividing by zero
  • In filter-by-filter comparisons for total LA or LA 5 micrometers or longer, indirect preparation gave statistically higher concentrations in 32% to 45% of cases, statistically lower concentrations in 23% to 29%, and concentrations not statistically different in 32% to 39%
  • For particles shorter than 5 micrometers, indirect results were statistically higher in 45% of filter pairs, statistically lower in 3%, and not statistically different in 52%
  • On average, 2.2 times more LA particles were recorded with indirect preparation than with direct preparation
  • Two indirect analyses unexpectedly recorded no LA particles
  • The authors considered the factor-of-12 increase for particles shorter than 5 micrometers to be low-to-moderate uncertainty
• The authors' conclusion: The authors concluded that data from either direct or indirect preparation may be used in LA exposure assessments intended for risk assessment at the Libby Asbestos Superfund Site
• Limitations the authors state:
  • The archived filters had already been analyzed by direct preparation, so cassette washing, a step usually part of indirect preparation, was not done
  • Samples were not ashed before dilution; 26 of the 31 samples were collected to monitor outdoor exposures
  • Only half of each original filter was available for indirect preparation, which halved analytical sensitivity
  • The cause of the two indirect analyses with no particles was uncertain; the authors suggested over-dilution
  • Counting rules that exclude very small particles may have introduced analyst bias in identifying smaller particles

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Libby vermiculite mine, Montana

All asbestos mines

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Amphibole Asbestos, Amphibole Group, Transmission Electron Microscopy (TEM).

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