The National Institute for Occupational Safety and Health conducted an initial, task-based comparative assessment to determine the potential for release of carbon nanofibers (CNFs) during dry material handling, wet cutting, grinding, and sanding (by machine and hand) of plastic composite material containing CNFs. Using a combination of direct-reading instruments and filter-based air sampling methods for airborne mass and transmission electron microscopy (TEM), concentrations were measured and characterized near sources of particle generation, in the breathing zone of the workers, and in the general work area. Tasks such as surface grinding of composite material and manually transferring dry CNFs produced substantial increases in particle number concentration (range = 20,000-490,000 1-cm(-3)). Concomitant increases in mass concentration were also associated with most tasks. Nearly 90% of all samples examined via TEM indicated that releases of CNFs do occur and that the potential for exposure exists. These findings also indicate that improperly designed, maintained, or installed engineering controls may not be completely effective in controlling releases. Unprotected skin exposure to CNFs was noted in two instances and indicated the need for educating workers on the need for personal protective equipment. [Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Occupational and Environmental Hygiene for the following free supplemental resource: a PDF file containing information on materials, evaluated processes, personal protective equipment, and existing ventilation/engineering controls.].