Measurement of Chrysotile Fiber Retention Efficiencies on MCE Filters to Support Exposure Assessments

1. Measurement of Chrysotile Fiber Retention Efficiencies on MCE Filters to Support Exposure Assessments Daniel A. Vallero, U.S. EPA/NERL, RTP, NC John R. Kominsky, EQM,Cincinnati, OH Michael E. Beard, RTI International, RTP, NC. Owen Crankshaw, RTI International, RTP, NC.

2. Study Objectives Post-preparation fiber retention • Compare collection efficiency of chrysotile asbestos on 0.45µm and 0.8 µm pore MCE filters for: • Structures ≥ 0.5 µm and < 5 µm long • Structures ≥ 5 µm and < 10 µm long • Structures ≥ 10 µm long SEM 0.8 µm pore MCE, 8,000X (top) TEM 0.4 µm pore PC, 16,000X

3. Etching Time Effect of plasma etching time (2, 4, 8, and 16 minutes) using a 0.45 um MCE filter on chrysotile concentrations of: Structures ≥ 0.5 µm and < 5 µm long Structures ≥ 5 µm and < 10 µm long Structures ≥ 10 µm long Preparation of a mixed cellulose ester filter for TEM specimen preparation. (Adapted from Chatfield, 1985.) Collected fibers Study Objectives

4. Why Chrysotile? • Appropriate for these study objectives due to its finely fibrous nature. • Of all asbestos fiber types, chrysotile most likely to penetrate the tortuous matrix of MCE filter material, • Thus optimizes ability to see differences in asbestos collection efficiency, due to: • MCE pore size (larger pore sizes equate to greater potential penetration of fibers into the matrix) and, • Differential plasma etching time. • Amphibole asbestos fibers, with their larger average diameter and length, are less likely to penetrate the MCE matrix. • Chrysotile asbestos is by far most commonly seen asbestos type on air filters (such as from remediation sites). • Thus, it best reflects real-world situations. • Allows results for chrysotile asbestos to give an indication of filter effectiveness for numerous fiber types.

5. Study Design – Collection Efficiency

6. Study Design – Etching Time

7. Analytical Methodology • ISO 10312:1995 TEM Specimen Preparation • DMF collapsing procedure • Collapsed filter plasma etched for 8 min (ISO 10312) and additional times either 2, 4, or 16 min. • TEM Measurement Strategy • Aspect ratio 3:1 • Size categories (length) • ≥0.5 to 5µm • >5 to 10 µm • > 10 µm

8. QC/QA • MCE Filters (0.45 and 0.8 µm pore size) • Independent QC lab monitored loaded filters for asbestos concentration and intra-batch uniformity. • Lot blanks • Laboratory blanks • Duplicate Analyses • Verified Counts

9. Mean Fiber Density (s/mm2) by loading and fiber length (µm)

10. Asbestos Fiber Density w/ Variable Etching Time

11. Conclusions – Collection Efficiency • Post-preparation fiber retention of 0.45 µm pore size MCE filter was higher than 0.8 µm MCE for chrysotile asbestos ≥0.5<5 µm length (p=0.01). • Post-preparation fiber retention of 0.45 µm and 0.8 µm pore size MCE filters for chrysotile asbestos >5 µm lengthwas not different (p>0.05). • Monitoring should be conducted on 0.45 µm pore size MCE filters for exposure risk models that suggest a more significant role for asbestos fibers < 5 µm length.

12. Conclusions – Etching Time • The mean concentration of chrysotile asbestos fibers (>0.5<5 µm length) increased with etching time. • Doubling the etching time(>0.5<5 µm length) increased the number of exposed fibers by an average of 13% to the total asbestos concentration within the concentration range tested. • Plasma etching time showed no effect observed on chrysotile asbestos fibers >5 µm.

13. Disclaimer: The U.S. Environmental Protection Agency through the Office of Research and Development funded and managed the research described here. Although this work was reviewed by EPA and approved for publication, it may not necessarily reflect official Agency policy.