Talc fits the framework of poorly soluble low-toxicity particles - implications for hazard classification.

An extensive toxicology database demonstrates talc exhibits the defining characteristics of poorly soluble, low-toxicity particles (PSLTs), including slow dissolution in biological fluids; macrophage-mediated clearance as the key determinant of lung residence time; significant inflammatory and proliferative responses only at lung burdens that overload normal particle clearance capacity; and, lung tumors in rats-but not in mice or hamsters-under conditions of excessive overload. This profile mirrors that documented for other PSLTs, such as titanium dioxide, carbon black, and diesel soot. Consistent with other PSLTs, talc has not been associated with increased lung cancer risk in humans, even after prolonged occupational exposure. Collectively, these observations demonstrate adverse rat lung responses to talc and other PSLTs are a consequence of lung overload rather than intrinsic toxicity, and the tumor response is unique to rats. Accordingly, hazard classification and risk assessment for PSLTs should explicitly consider the overload-dependent nature of responses. In the absence of evidence for intrinsic carcinogenic activity, lung tumors occurring only in rats and only under conditions of lung overload should not be interpreted as evidence of a human cancer hazard. Despite the well-established understanding of overload-driven PSLT responses in rats, recent IARC and ECHA/RAC evaluations interpreted rat lung tumors in the NTP study as evidence of inherent carcinogenicity. These assessments did not rigorously account for the PSLT properties of talc, the extensive evidence on lung particle overload, or that tumors occurred only at lung burdens greatly exceeding normal clearance capacity. Consequently, interpretations of the NTP findings conflict with the current state of the science.