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Reisetter et al. (1) confuse environmental carbonaceous particles with commercially produced carbon black (CB), a form of pure carbon (typically >97%) (McCunney et al. (2)). “Black carbon,” a term that refers to environmental soot and to which the authors were most likely referring, has clear physicochemical differences from carbon black (Watson and Valberg (3)). Unbound CB is not found “in many environmental exposures” or “a primary component of air pollution.” As a binder in rubber products, CB is tightly bound in a matrix, and abrasion particles do not release unbound CB into the environment. The authors state that CB forms “aggregates” in cell culture media, but in fact manufactured CB is already in the aggregate form (Gray and Muranko (4)).
The authors' claim that the 30 μg/cm2 “dose” of CB is within the mass range observed in occupational and environmental settings is incorrect. If one assumes a human alveolar surface area of approximately 100 m2 (Gehr et al. (5)) and 0.05 alveolar deposition fraction, then the dose of 30 μg/cm2 converts to a human alveolar dose of 1500 mg. This value equates to 21 mg/kg body weight, significantly higher than the 0.12 mg/kg cited as a possible daily dose of CB in the working environment. Thus, the CB concentrations used in the study are much higher than CB concentrations in occupational settings.
The authors' observation that CB, and not TiO2, nanoparticles caused toxic responses is not universally observed. Other studies have shown that TiO2 nanoparticles can elicit greater toxic responses than CB (Sager and Castranova (6)).