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

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.