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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. Study Objectives. - PowerPoint PPT Presentation
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Office of Research and DevelopmentNational Exposure Research Laboratory
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Measurement of Chrysotile Fiber Retention Efficiencies on MCE
Filters to Support Exposure Assessments
Daniel A. Vallero, U.S. EPA/NERL, RTP, NCJohn R. Kominsky, EQM, Cincinnati, OHMichael E. Beard, RTI International, RTP, NC.Owen Crankshaw, RTI International, RTP, NC.
2Office of Research and DevelopmentNational Exposure Research Laboratory
Study Objectives
Post-preparation fiber retention
• Compare collection efficiency of chrysotile asbestos on 0.45 µm 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
3Office of Research and DevelopmentNational Exposure Research Laboratory
Study Objectives
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
4Office of Research and DevelopmentNational Exposure Research Laboratory
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.
5Office of Research and DevelopmentNational Exposure Research Laboratory
Study Design – Collection Efficiency
Batch
Filter Pore Size/Loading and Number Samples
0.45 µm 0.8 µm
Low High Low High
1 12 - 6 -
2 - 6 - 12
3 6 - 12 -
4 - 12 - -
Total 18 18 18 18
6Office of Research and DevelopmentNational Exposure Research Laboratory
Study Design – Etching Time
Filter
Loading
0.45 µm Pore MCE Filters
Plasma Etching Time (min) and
Number of Samples
2 4 8 16
High 12 12 12 12
7Office of Research and DevelopmentNational Exposure Research Laboratory
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
8Office of Research and DevelopmentNational Exposure Research Laboratory
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
9Office of Research and DevelopmentNational Exposure Research Laboratory
Mean Fiber Density (s/mm2) by loading and fiber length (µm)
Batch
Low Filter Loading High Filter Loading
≥0.5-5 >5-10 >10 ≥0.5-5 >5-10 >10
0.45 µm pore size
1 237 63 21 - - -
2 - - - 585 284 89
3 316 71 21 - - -
4 - - - 1200 235 66
0.8 µm pore size
1 194 60 19 - - -
2 - - - 429 225 88
3 287 78 22 - - -
4 - - - 960 261 71
10Office of Research and DevelopmentNational Exposure Research Laboratory
Asbestos Fiber Density w/ Variable Etching Time
0
200
400
600
800
1000
1200
1400
1600
1800
2 4 8 16
Etch Time, Minutes
Co
nc
en
tra
tio
n,
Fib
ers
/mm
2
>0.5 = Total>0.5-5>5-10>10
11Office of Research and DevelopmentNational Exposure Research Laboratory
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 length was 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.
12Office of Research and DevelopmentNational Exposure Research Laboratory
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.
13Office of Research and DevelopmentNational Exposure Research Laboratory
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.
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