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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.

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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

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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

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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.

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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

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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

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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

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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

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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

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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

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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.

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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.

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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.