On April 24, 2013, the National Institute for Occupational Safety and Health (NIOSH) released Current Intelligence Bulletin 65: Occupational Exposure to Carbon Nanotubes and Nanofibers, which includes a proposed recommended exposure limit (REL) that is significantly lower than that in the 2010 draft. NIOSH issues Current Intelligence Bulletins (CIB) to disseminate new scientific information about occupational hazards. CIB 65 reviews animal and other toxicological data relevant to assessing the potential non-malignant adverse respiratory effects of carbon nanotubes and carbon nanofibers; provides a quantitative risk assessment based on animal dose-response data; proposes a REL of 1.0 microgram per cubic meter (µg/m3) of elemental carbon as a respirable mass 8-hour time-weighted average concentration; and describes strategies for controlling workplace exposures and implementing a medical surveillance program. NIOSH notes that in the 2010 draft of this CIB, it indicated that “risks could occur with exposures less than 1 μg/m3 but that the analytic limit of quantification was 7 μg/m3.” Based on subsequent improvements in sampling and analytic methods, NIOSH states that, in the final CIB, it is now recommending an exposure limit at the current analytical limit of quantification of 1 μg/m3. According to NIOSH, the REL “is expected to reduce the risk for pulmonary inflammation and fibrosis. However, because of some residual risk at the REL and uncertainty concerning chronic health effects, including whether some types of [carbon nanotubes] may be carcinogenic, continued efforts should be made to reduce exposures as much as possible.” NIOSH notes that, just prior to the release of CIB 65, it reported preliminary findings from a new laboratory study in which mice were exposed by inhalation to multi-walled carbon nanotubes. NIOSH states that the results of the study indicate that multi-walled carbon nanotubes can increase the risk of cancer in mice exposed to a known carcinogen. According to the CIB, NIOSH is conducting additional research to learn more about worker exposures and the potential occupational health risks associated with exposure to multi-walled carbon nanotubes and other types of carbon nanotubes and carbon nanofibers. NIOSH states that, as results from its research become available, it will reassess its recommendations and make appropriate revisions as needed.