The NTP chronic inhalation study does not support an inherent lung cancer hazard of talc: Implications of lung particle overload and maximum tolerated dose

Interpretation of inhalation carcinogenicity studies for poorly soluble particles requires explicit consideration of lung particle overload in relation to maximum tolerated dose (MTD). In the NTP talc inhalation study, lung tumors occurred in female rats following chronic exposure to 18 mg/m 3 . Detailed analysis of lung burden data shows that both 6 and 18 mg/m 3 exposures resulted in lung overload, with lung volume burdens at 18 mg/m 3 exceeding 10 μL/g lung, a level associated with clearance cessation. Together with significant lung pathology and impaired lung function, these findings provide clear evidence that the MTD was exceeded. Analyses of relationships between particle burden, lung cancer, and neutrophilic inflammation across poorly soluble, low-toxicity particles indicate that rat lung tumors following talc exposure arise as a non–material-specific consequence of overload. In recent evaluations, ECHA misinterpreted and discounted the NTP lung burden data, and IARC did not consider lung overload or relevant exposure conditions. As a result, both interpreted these findings as evidence of inherent carcinogenicity. Mechanistically, the rat lung cancer response to talc reflects an inflammation-driven pathway in which excessive lung burdens lead to sustained inflammation and epithelial proliferation. Tumors occur only under conditions exceeding the MTD and represent a non–material-specific, overload-driven effect rather than an inherent carcinogenic hazard. Accordingly, the use of these findings for hazard classification is not scientifically justified.