title: Clearance of carbon nanotubes in the human respiratory tract-a theoretical approach.
abstract: BACKGROUND
  Theoretical knowledge of carbon nanotube clearance in the human respiratory tract represents an essential contribution to the risk assessment of artificial airborne nanomaterials. Thus, single phases of nanotube clearance were simulated with the help of a theoretical model.
  
  
  METHODS
  In this study, clearance of single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) was simulated by using a validated mathematical approach that includes all clearance mechanisms known hitherto. Fast mucociliary clearance is approximated by a steady-state steady-flow mucus model, whereas slow clearance mechanisms are modeled by definition of related clearance half-times.
  
  
  RESULTS
  Clearance may be subdivided into three phases, including fast bronchial clearance (mucociliary escalator), slow bronchial clearance (particle uptake by airway macrophages, transcytosis), and alveolar clearance (phagocytosis by alveolar macrophages, endocytosis by alveolar epithelium). According to the clearance model used in this study, mucociliary clearance is completed within the first 24 h after exposure, whereas slow bronchial clearance is characterized by a half-time of 5 d. Alveolar clearance is marked by half-times >100 d. As a result of their different deposition patterns, SWCNT and MWCNT show some discrepancies with regard to their clearance insofar as long SWCNT reside significantly longer in the lungs than MWCNT. This circumstance is among other expressed by higher 24-h, 10-d, and 100-d retentions computed for SWCNT compared to MWCNT.
  
  
  CONCLUSIONS
  Due to their partly high residence times in distal lung regions, carbon nanotubes may bear the potential to act as triggers of inflammatory reactions or fibrotic modifications of the lung structure. Further they may also induce malignant transformations of lung cells, resulting in the development of lung tumours.
authors.affiliation: Brunnleitenweg 41, A-5061 Elsbethen, Salzburg, Austria.
authors.forename: Robert
authors.initials: R
authors.lastname: Sturm
keywords: Clearance model; clearance mechanisms; deposition; multi-walled carbon nanotubes (MWCNT); single-walled carbon nanotubes (SWCNT)