Methyl isothiocyanate (MITC), a soil fumigant, induced nasal tumors in Sprague Dawley rats exposed by inhalation to a high concentration (20 ppm), but not at 0.5 or 5 ppm. In an inhalation study (1 day, 5 days, or 4 weeks) with MITC, nasal lesions were observed at the tumorigenic concentration, including acute/subacute inflammation, epithelial cell degeneration/necrosis, epithelial cell proliferation (epithelial hyperplasia, increased DNA synthesis, and regeneration), olfactory sensory neuron apoptosis and loss resulting in atrophy/loss of olfactory epithelium, and/or respiratory cell metaplasia. The available toxicity data were examined using the International Programme on Chemical Safety framework and a mode of action (MOA) was proposed that included key events: 1) direct cytotoxicity in nasal mucosa because of irritation; 2) consequent regenerative cell proliferation; 3) onset and persistence of squamous cell metaplasia as an adaptive response; and 4) development of tumors. Data in support of the key events are discussed, along with possible alternative MOAs as well as human relevance of the proposed MOA. Findings are consistent with the proposed MOA and indicated that tumorigenicity of MITC exhibits exposure thresholds but is not likely to pose a carcinogenic risk to humans under normal use patterns and corresponding exposure levels.
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