The National Institute for Occupational Safety and Health (NIOSH) has published a chapter in the fifth edition of the NIOSH Manual of Analytical Methods (NMAM) entitled “Analysis of Carbon Nanotubes and Nanofibers on Mixed Cellulose Ester Filters by Transmission Electron Microscopy.”  The draft chapter provides standardized approaches for the analysis of carbon nanoparticles.  The chapter provides detailed guidance on effective means to perform transmission electron microscopic analysis on carbon nanotubes and nanofibers that are sampled from occupational atmospheres.  The chapter notes that, with respect to quantification, it is important to recognize the limitations of a particle counting method.  According to the chapter, because of the heterogeneity of carbon nanotube powders, particles in the same size bin can vary greatly in their properties.  Complex structures of varying mass, density, shape, and composition (e.g., amorphous carbon, residual catalyst) can fall into the same size bin, and dissimilar particles (e.g., cluster and matrix) in a given bin may have different toxicological effects.  Thus, the chapter states, “even for a given material, particle envelope size alone may not be an adequate risk indicator.”  Given the heterogeneity and variety of the carbon nanotube/carbon nanofiber products to which workers are exposed, “it is important to apply several methods to assess worker exposure, through inhalation and other routes.”