On December 5, 2019, the National Toxicology Program (NTP) announced that the NTP Technical Report on the Toxicity Studies of 1020 Long Multiwalled Carbon Nanotubes Administered by Inhalation to Sprague Dawley (Hsd:Sprague Dawley^® SD^®) Rats and B6C3F1/N Mice is now available.  In 2003, the Rice University Center for Biological and Environmental Nanotechnology nominated nanoscale materials as a class of substances for toxicological evaluation.  The recommended studies included “[c]haracteriz[ing] the inhalation toxicology of high aspect ratio materials, such as carbon nanotubes (single-walled or multi-walled), as a function of material purity and stabilization.”  According to the abstract for the Technical Report, “[b]ecause long-term inhalation toxicity and carcinogenicity studies were being conducted on a relatively short, rigid [multiwalled carbon nanotube (MWCNT)], a representative long and thin MWCNT was selected for these studies.”  Following an evaluation of 24 different long, thin MWCNTs, NTP selected the 1020 Long Multiwalled Carbon Nanotube (L-MWNT-1020) (Sun Innovations, Fremont, CA) “based on availability, high purity (97%), and the low amount of residual nickel catalyst (0.52% by weight).”  The abstract states that 0.3 milligrams per cubic meter (mg/m³) is considered the no‑observed‑adverse‑effect level (NOAEL) for L-MWNT-1020.  The Technical Report notes that the exposure concentrations used in the study were approximately 100-fold higher than the National Institute for Occupational Safety and Health (NIOSH)-proposed recommended exposure limit (REL) for carbon nanotubes (CNT) of 1.0 micrograms per cubic meter (μg/m³) elemental carbon as a respirable mass eight-hour time-weighted average (TWA) concentration.  According to the Technical Report, “NIOSH researchers have also evaluated exposure in multiple U.S. manufacturers of CNTs and nanofibers and reported inhalable exposures of MWCNT of up to 0.4 mg/m³.”  The Technical Report states that given the long half-life and persistence of MWCNT in the lung, “even short periods of exposure at such levels would lead to chronic high-level exposures.  Therefore, although the no-observed-adverse-effect level determined in the present study (0.3 mg/m³) is approximately 300-fold higher than the recommended exposure limit, it is within the range observed in occupational settings and may be informative regarding potential health risks associated with occupational exposures to CNTs.”  The abstract includes a detailed chart summarizing the findings considered to be toxicologically relevant in rats and mice exposed to L-MWNT-1020 by inhalation for 30 days.