European harmonization of asbestos exposure assessment: comparing PCM, SEM, and TEM to derive conversion factors

Our plain-English summary of European harmonization of asbestos exposure assessment: comparing PCM, SEM, and TEM to derive conversion factors, by R. Franken, P. Tromp, T.K. Ervik, et al (Annals of Work Exposures and Health, 2025).

The study

• Authors: R. Franken, P. Tromp, T.K. Ervik, J. Staff, K.A. Jensen, C. Eypert-Blaison, A. Brostrøm, A. Cannizzaro, M.T. Sanchez Cabo, M.R. Bruno, A.S. Fonseca, L. Davies, P. Graff, S. Spaan
• Journal: Annals of Work Exposures and Health
• Published: 2025;69(6):575-591
• Read it: PubMed · Publisher · Free full text

Abstract

After the European ban on the use of asbestos, exposure assessment of asbestos became imperative for ensuring compliance with safety standards. However, each European country has their own legislation and requirements, including measurement strategies, analytical techniques such as the microscope used as well as occupational exposure limits (OELs). The recent EU directive (EU) 2023/2668 significantly lowered the OEL for asbestos from 100,000 fibres/m³ 8-h time-weighted average to either 2,000 fibres/m³ when counting fibres between 0.2 and 3 µm in diameter, or 10,000 fibres/m³ when counting fibres thinner than 0.2 µm and dictates a transition from optical to electron microscopy analysis by the end of 2029. This change impacts Member States that rely on phase-contrast microscopy (PCM) to quantify asbestos concentrations, prompting the need for a standardized comparison between different analytical methods. Therefore, our study investigated whether conversion factors could be developed, enabling comparison of results obtained with different analytical techniques. To achieve this, a phased approach was applied, involving a survey of measurement strategies implemented by different countries in Europe, a literature search, and analysis of in-house data to explore differences between analytical techniques. Standardized conversion factors were developed via (i) direct comparison of concentrations from analysis with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and/or PCM, (ii) a multiple linear regression model, and (iii) via log probability plots from raw data on fibre dimensions. Ten institutes from the 'Partnership for European Research in Occupational Safety and Health' (PEROSH) asbestos network participated in this study. The results showed that SEM and PCM were the most commonly used analytical techniques, with TEM also being used in 3 countries. OELs and measurement standards/protocols varied across countries, and most employed national derived standards for measurements. Conversion factors overall showed that measurements analysed by TEM resulted in higher fibre concentrations followed by PCM and SEM. Although conversion factors were developed, these were influenced by factors such as material type, applied energy, and local controls, preventing the derivation of a general conversion method.

© The Author(s) 2025. Published by Oxford University Press on behalf of the British Occupational Hygiene Society.

In plain English

Ten PEROSH network institutes compared three microscope methods for measuring airborne asbestos: phase-contrast microscopy (PCM), which cannot tell asbestos from other fibres, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). They surveyed national practices, reviewed published comparisons and pooled 40,002 workplace measurements, nearly all from France. The survey found PCM and SEM most common; 3 countries also used or accepted TEM. Across direct comparisons, conversion factors between methods ranged from 0.3 to 140. TEM/PCM factors were higher for chrysotile than amosite, and for high-energy (8.3 to 58) than low-energy work (2.3 to 11). A regression model gave higher concentrations with TEM than PCM and SEM, and a TEM-to-SEM factor of 26.8. Fibre-size data from low-energy removal work showed 30% to 45% of chrysotile fibres thinner than 0.2 µm, versus 5% to 10% for amosite; two crocidolite data sets gave 30% to 45% and 15% to 30%. The authors concluded that a generic conversion factor could not be established. They noted few data comparing SEM with other methods, and that one country supplied about 39,000 measurements.

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: Methods-comparison study with three parts: a survey of national practices, a literature search of comparison studies, and an analysis of pooled in-house workplace measurement data. The data were analysed in three ways: direct comparison, multiple linear regression, and log probability plots of fibre dimensions.
• Who was studied: 10 of the 14 institutes in the PEROSH network took part, with survey data from 10 European countries. The pooled data set held 40,002 individual personal measurements: 72 from Denmark, 15 from Norway, 25 from Spain, 216 from the United Kingdom, 272 from the Netherlands and 39,402 from France.
• Exposure: Airborne asbestos fibres measured during removal work on asbestos cement (chrysotile and/or crocidolite), insulation/fire-resistant board (amosite and/or chrysotile) and spray-on asbestos (crocidolite). Samples were analysed by PCM, SEM or TEM. PCM counts all fibres, including non-asbestos fibres.
• Main results:
  • Survey: PCM and/or SEM were mostly used, and 3 countries also employed or accepted TEM
  • Direct comparisons: conversion factors between methods varied widely, from 0.3 to 140 across all fibre types
  • Direct comparisons of TEM with PCM: conversion factors for chrysotile fibres were generally higher than for amosite fibres
  • Direct comparisons: PCM-to-TEM conversion factors were higher for high-energy activities (8.3 to 58) than for low-energy activities (2.3 to 11)
  • First regression model (explained 52% of the variance): TEM gave higher fibre concentrations than both PCM and SEM
  • First regression model: the conversion factor for TEM to SEM results was 26.8
  • First regression model: the conversion factor from SEM to PCM was 2.5, and from PCM to TEM was 10.7
  • Regression results: local exhaust ventilation and thorough wetting were associated with higher exposure estimates, contrary to expectations
  • Fibre-size plots: the authors' text gives 30% to 45% of chrysotile and crocidolite fibres thinner than 0.2 µm, compared with 5% to 10% of amosite fibres
  • Table 5, crocidolite in asbestos cement: the TNO data set (24 fibres) gave 30% to 45% of fibres thinner than 0.2 µm. The STAMI data set (92 fibres) gave 15% to 30%.
  • The fibre-size percentages come only from removal activities using low-energy (manual) tools
  • Fibre-size plots: the theoretical factor for estimating the total fibre count from WHO fibres was 2.2 (range 1.7 to 2.8) for chrysotile and crocidolite fibres, and 1.6 (range 1.3 to 2.0) for amosite fibres
• The authors' conclusion: The authors concluded that conversion factors varied greatly in all approaches, so a generic conversion factor between methods could not be established
• Limitations the authors state:
  • Not all EU member states are represented in the PEROSH network
  • Few data were available comparing SEM with PCM or SEM with TEM
  • There were too few data to derive coefficients for all interaction effects in the regression model
  • The data set was unbalanced: one country provided about 39,000 measurements
  • Asbestos content, indoor or outdoor work, and asbestos fibre types were left out of the regression model because the information was missing

Topics

Medical terms in this study, explained in our Lexicon: Asbestos Exposure, Occupational Exposure, Time-Weighted Average (TWA), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM).

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