title: Carbon Nanotube Emissions from Arc Discharge Production: Classification of Particle Types with Electron Microscopy and Comparison with Direct Reading Techniques.
abstract: BACKGROUND
  An increased production and use of carbon nanotubes (CNTs) is occurring worldwide. In parallel, a growing concern is emerging on the adverse effects the unintentional inhalation of CNTs can have on humans. There is currently a debate regarding which exposure metrics and measurement strategies are the most relevant to investigate workplace exposures to CNTs. This study investigated workplace CNT emissions using a combination of time-integrated filter sampling for scanning electron microscopy (SEM) and direct reading aerosol instruments (DRIs).
  
  
  METHODS
  Field measurements were performed during small-scale manufacturing of multiwalled carbon nanotubes using the arc discharge technique. Measurements with highly time- and size-resolved DRI techniques were carried out both in the emission and background (far-field) zones. Novel classifications and counting criteria were set up for the SEM method. Three classes of CNT-containing particles were defined: type 1: particles with aspect ratio length:width >3:1 (fibrous particles); type 2: particles without fibre characteristics but with high CNT content; and type 3: particles with visible embedded CNTs.
  
  
  RESULTS
  Offline sampling using SEM showed emissions of CNT-containing particles in 5 out of 11 work tasks. The particles were classified into the three classes, of which type 1, fibrous CNT particles contributed 37%. The concentration of all CNT-containing particles and the occurrence of the particle classes varied strongly between work tasks. Based on the emission measurements, it was assessed that more than 85% of the exposure originated from open handling of CNT powder during the Sieving, mechanical work-up, and packaging work task. The DRI measurements provided complementary information, which combined with SEM provided information on: (i) the background adjusted emission concentration from each work task in different particle size ranges, (ii) identification of the key procedures in each work task that lead to emission peaks, (iii) identification of emission events that affect the background, thereby leading to far-field exposure risks for workers other than the operator of the work task, and (iv) the fraction of particles emitted from each source that contains CNTs.
  
  
  CONCLUSIONS
  There is an urgent need for a standardized/harmonized method for electron microscopy (EM) analysis of CNTs. The SEM method developed in this study can form the basis for such a harmonized protocol for the counting of CNTs. The size-resolved DRI techniques are commonly not specific enough to selective analysis of CNT-containing particles and thus cannot yet replace offline time-integrated filter sampling followed by SEM. A combination of EM and DRI techniques offers the most complete characterization of workplace emissions of CNTs today.
authors.affiliation: 1.Solid State Physics, Lund University, SE-22100 Lund, Sweden; 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden; linus.ludvigsson@ftf.lth.se.
authors.forename: Linus
authors.initials: L
authors.lastname: Ludvigsson
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Christina
authors.initials: C
authors.lastname: Isaxon
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Patrik T
authors.initials: PT
authors.lastname: Nilsson
authors.affiliation: 3.Occupational and Environmental Medicine, Lund University, SE-22100 Lund, Sweden;
authors.forename: Hakan
authors.initials: H
authors.lastname: Tinnerberg
authors.affiliation: 1.Solid State Physics, Lund University, SE-22100 Lund, Sweden;
authors.forename: Maria E
authors.initials: ME
authors.lastname: Messing
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Jenny
authors.initials: J
authors.lastname: Rissler
authors.affiliation: 4.National Institute of Occupational Health, P.O. Box 8149 Dep, 0033 Oslo, Norway.
authors.forename: Vidar
authors.initials: V
authors.lastname: Skaug
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Anders
authors.initials: A
authors.lastname: Gudmundsson
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Mats
authors.initials: M
authors.lastname: Bohgard
authors.affiliation: 3.Occupational and Environmental Medicine, Lund University, SE-22100 Lund, Sweden;
authors.forename: Maria
authors.initials: M
authors.lastname: Hedmer
authors.affiliation: 2.Ergonomics and Aerosol Technology, Lund University, SE-22100 Lund, Sweden;
authors.forename: Joakim
authors.initials: J
authors.lastname: Pagels
mesh_terms: D000336:Aerosols / Q000032:analysis; D000395:Air Pollutants, Occupational / Q000032:analysis*; D004784:Environmental Monitoring / Q000379:methods*; D006801:Humans; D019570:Inhalation Exposure / Q000032:analysis; D008855:Microscopy, Electron, Scanning / Q000379:methods; D037742:Nanotubes, Carbon / Q000032:analysis*; D016273:Occupational Exposure / Q000032:analysis*; D010316:Particle Size
chemical_list: D000336:Aerosols; D000395:Air Pollutants, Occupational; D037742:Nanotubes, Carbon
keywords: APS; NOAA; arc discharge; carbon nanotubes; counting rules; direct reading instruments; workplace exposure