TOXICOKINETICS AND EFFECTS OF FIBROUS AND NONFIBROUS PARTICLES

Chronic inhalation of fibrous and nonfibrous particles by rats at high concentrations results in lung tumor formation if the particles are poorly soluble in the lung. Even rather benign nonfibrous particles such as TiO 2 produce this result. One significant change during a chronic inhalation exposure of poorly soluble particles of low cytotoxicity (PSP) is an impairment of normal clearance mechanisms in the alveolar region of the lung in rats, resulting in a continued buildup to high lung burdens accompanied by chronic alveolar inflammation, fibrosis, and mutational events. Since these are obviously high-dose effects, questions about their extrapolation to humans exposed to much lower concentrations have been raised. Results of key studies reported for chronic inhalation of PSP in rats indicate that mechanisms of PSP-induced lung tumors at high doses do not operate at low dose levels. Furthermore, the existence of two thresholds can be postulated: One is a dosimetric threshold for the endpoint alveolar macrophage-mediated clearance, which is related to lung particle overload. The other is a mechanistic threshold for the endpoint mutation, which is determined by the level of antioxidant defenses to counter-balance reactive oxidant species released by activated inflammatory cells. A no-observed-adverse-effect level (NOAEL) could therefore be based on avoiding alteration of the toxicokinetic of the particles such that the lung burdens stay below the dosimetric threshold. The suggestion that PSP-associated organic compounds (e.g., diesel particulate matter) contribute to the lung tumor responses in rats observed in chronic inhalation studies is not supported by experimental data from in vivo studies. It can be concluded that high-dose rat lung tumors due to PSP should not be used for low-dose extrapolations, and no significant contribution to human lung cancer risk can be predicted from levels of PSP below lung overload. With respect to the pulmonary toxicokinetics of inhaled fibrous particles, the biopersistence of long fibers (>20 µm) which cannot be phagocytized by alveolar macrophages is a key parameter related to long-term carcinogenic effects. Long fibers with a very low biopersistence should not be considered as carcinogenic. Since the clearance kinetics of fibers can generally be described by a biphasic or multiphasic pattern - fast initial and slow final phase - it is essential that the slow phase of the retention kinetics of fibers longer than 20 µm is considered in a biopersistence assay. Based on the results of such assay, fibers can be classified into one of two categories: a biopersistent fiber that cannot be dissolved in the lung within an acceptable time period; or a biosoluble fiber when even long nonphagocytizable fibers will be disappearing rapidly from the lung. However, in addition to biopersistence, it should be mandatory to evaluate fiber toxicity in an appropriate assay relative to a fiber whose long-term effects are well known. Moreover, for organic fibers it is likely that different rules may have to be established for characterization of their toxic and carcinogenic potential.