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, 56 (5), 542-56

Occupational Exposure Assessment in Carbon Nanotube and Nanofiber Primary and Secondary Manufacturers

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Occupational Exposure Assessment in Carbon Nanotube and Nanofiber Primary and Secondary Manufacturers

Matthew M Dahm et al. Ann Occup Hyg.

Abstract

RESEARCH SIGNIFICANCE: Toxicological evidence suggests the potential for a wide range of health effects, which could result from exposure to carbon nanotubes (CNTs) and carbon nanofibers (CNFs). The National Institute for Occupational Safety and Health (NIOSH) has proposed a recommended exposure limit (REL) for CNTs/CNFs at the respirable size fraction. The current literature is lacking exposure information, with few studies reporting results for personal breathing zone (PBZ) samples in occupational settings. To address this gap, exposure assessments were conducted at six representative sites identified as CNT/CNF primary or secondary manufacturers.

Methods: Personal and area filter-based samples were collected for both the inhalable mass concentration and the respirable mass concentration of elemental carbon (EC) as well as CNT structure count analysis by transmission electron microscopy to assess exposures. When possible, full-shift PBZ samples were collected; area samples were collected on a task-based approach.

Results: The vast majority of samples collected in this study were below the proposed REL (7 μg m(-3)). Two of the three secondary manufacturers' surveyed found concentrations above the proposed REL. None of the samples collected at primary manufacturers were found to be above the REL. Visual and microscopy-based evidence of CNTs/CNFs were found at all sites, with the highest CNT/CNF structure counts being found in samples collected at secondary manufacturing sites. The statistical correlations between the filter-based samples for the mass concentration of EC and CNT structure counts were examined. A general trend was found with a P-value of 0.01 and a corresponding Pearson correlation coefficient of 0.44.

Conclusions: CNT/CNF concentrations were above the proposed NIOSH REL for PBZ samples in two secondary manufacturing facilities that use these materials for commercial applications. These samples were collected during dry powder handling processes, such as mixing and weighing, using fairly large quantities of CNTs/CNFs.

Figures

Fig. 1
Fig. 1
EC exposures by process/task and type of sample. Box plot represents range (minimum to maximum), 25th percentile, median, and 75th percentile. Non-detect samples were graphed as half of the limit of detection. AS (also included ASC, area sample w/cyclone). Sonication included AS, ASC, and PBZ samples. *Tasks include weighing; extrusion, weighing, and batch mixer use; transferring CNFs; weighing and mixing of CNTs and CNFs. **Tasks include CNT waste collection; general office work; milling CNT composite; sieving; and spray coating; and included both AS and PBZ samples.
Fig. 2
Fig. 2
TEM images of collected PBZ samples. (A) Site A during the harvesting of MWCNTs. (B) Site C during the production and harvesting of MWCNTs. (C) Site E during extrusion, weighing, and batch mixer use with MWCNTs (EC concentration >7 μg m−3). (D) Site F during the weighing, mixing, and sonication of CNTs and CNFs (EC concentration >7 μg m−3).
Fig. 3
Fig. 3
EC exposures for all tasks by site. Box plot represents range (minimum to maximum), 25th percentile, median, and 75th percentile. Non-detect samples were graphed as half of the limit of detection. Grouped samples by site included all sampled tasks as well as all types of samples—AS; ASC, area sample cyclone; and PBZ sample; primary manufacturing Sites A–C; and secondary manufacturing Sites D–F.
Fig. 4
Fig. 4
Correlation of EC versus TEM filter-based area and PBZ samples. Non-detect samples were graphed as half of the limit of detection. All side-by-side PBZ samples and ASs were used. Included only PBZ and AS samples.

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