TABLE 5.

Acute to Chronic Inhalation Exposures of Rodents to CNT/CNF for Toxicity Testing. (rat as preferred species; 4–6 hrs/day, 5 d/wk; whole-body)

Acute/Subacute (14–28 days)	Subchronic (90 days)	Chronic (2 years)
to obtain hazard ID and ranking (pos/neg control?) may be preceded by i.t. instillation or 1 day inhalation with range of doses to estimate inhaled concentration with MPPD model assure and test rat respirable aerosol with range of concentr. if available use workplace or consumer exposure data to inform aerosol generation to determine concentration for 90-day; (range-finding) to collect biokinetic data for portal of entry, and possibly identification of secondary target organs, incl. pleura, and fetus to provide guidance for dose levels for mechanistic in vitro testing, incl. secondary organs post-exposure observation period desirable (~2 months)	to derive NOAEL use minimum 3 conc, including known or expected human exposure levels; both sexes optional if no effect at 50mg/m3 rodent respirable aerosol, then no need to do chronic study∗ to identify hazard: total resp.tract; pleura, cardiovascular, CNS, bone marrow identify target organs to select conc, for chronic study detailed biokinetics: retention, clearance, organ accumulation, predicting long-term effects? to perform risk assessment by extrapolation to human via dosimetric extrapolation post-exposure observation period for longer-term effects to assess progression-regression (~3 months)	to determine long latency effects (cancer); life shortening; extrapulmonary target organs 3 conc; based on 90-day or range-finding study results; include human exposure level; high dose: MTD; low dose: no significant effect to assess total respiratory tract, pleura and systemic effects, nose to alveoli; cardiovascular, CNS, bone marrow, others (reproductive?) to determine detailed biokinetics: resp.tract retention, clearance, organ accumulation to perform extrapolation to human for risk assessment post-exposure observation period up to a total study duration of 30 months (if survival of ≥20%)

^∗This suggestion is based on the fact that such high concentrations will never be reached in a repeat exposure scenario, and it would be a waste of animals (ethical) and resources ($), (Bernstein et al. 2005) to design a chronic workplace study. It should even be considered to move this suggestion to the acute/subacute category and not follow up with a subchronic study if an appropriate postexposure (~60 d) observation period is included. Even shorter-term (5 d) inhalation studies with CNT/CNF less than 10 mg/m³ or a single 6-hr. inhalation at 30 mg/m³ have induced significant effects (Ma-Hock et al. 2009; Ryman-Rasmussen et al. 2009a; b; Stapleton et al. 2012; Shvedova et al. 2008). Thus, a cutoff at 50 mg/m³ in a subchronic study is very conservative.