Occupational Exposure to Carbon Nanotubes and Nanofibers

the BMDL, NOAEL, or LOAEL is the estimatedpoint of departure (POD) for extrapolation belowthe range of the data. This low dose extrapolationmay be risk-based (e.g., from the 10% BMDL) bylinear or nonlinear modeling (depending on themode of action information); or in noncancer riskassessment, the POD may be adjusted downwardto account for uncertainty in the extrapolationfrom animals to humans and other factors (SectionA.6.3.3).The risk estimates based on the subchronic andshort-term animal studies (Section A.3) generallyindicate that risks less than 10% would be associatedwith working lifetime exposures below1 µg/m 3 (8-hr TWA). This is estimated by eitherlinear or model-based low-dose extrapolation ofthe rat BMDL estimates (Section A.3.3). Althoughan acceptably low level of risk for early-stage pulmonaryeffects has not been established, some ofthe working lifetime excess risk estimates at 1 µg/m 3(8-hr TWA) are less than 10% (e.g., approximately0.5% to 16% for the slight/mild, grade 2) lung effectsin the rat subchronic inhalation studies; 95%UCL estimates) (Table A–8). Given the uncertaintyabout the early-stage lung effects and the shapeof the dose-response relationship, the actual riskcould be much lower, even zero (if exposures arebelow an effect threshold). Alternatively, adjustingthe POD downward using standard uncertaintyfactors, also generally results in estimates of