In the aftermath of the large spill of crude 4-methylcyclohexylmethanol (MCHM) into the Elk River in 2014, there were reports of a strong licorice-smelling odor and reports of respiratory illness/cough suggestive of chemical-induced adverse airway effects. Levels of MCHM in the air were predicted to be as high as 0.39 ppm after 10 minutes of showering, suggesting that inhalation of MCHM vapors was a potential route of chemical exposure. NTP conducted prenatal developmental toxicity and 5-day toxicogenomic oral gavage studies that showed minimal MCHM-induced adverse effects; however, no in vivo inhalation studies for MCHM have been performed to date. In this study, we utilized an organotypic human air-liquid interface (ALI) culture model (EpiAirwayTM) derived from a single donor to determine if there was any potential for MCHM-induced airway toxicity in vitro following acute exposure to MCHM vapors for 6 hours, which might warrant future inhalation toxicity testing in vivo. ALI tissues were exposed to estimated nominal vapor concentrations of 35 or 350 ppm MCHM (pure or crude) for 6 hours using vapor cups (and calculations based on Henry’s law constants derived by Sain et al.). In this model, 350 ppm was selected as the top dose because 350–400 ppm was estimated to be the highest achievable nominal vapor concentration for exposures based on the maximum solubility (17.6 mM) of pure MCHM in water. Cell viability (using an MTT assay) and evidence of a MCHM-induced pro-inflammatory response in culture media (using interleukin (IL)-1 receptor activation and multiplex cytokine/chemokine assays) were measured at 24 hours. Acute exposure to MCHM vapors for 6 hours at these concentrations, which were ~90- to 900-fold greater than human exposure levels as predicted while showering, induced no airway cytotoxicity or pro-inflammatory response. In contrast, acute exposure of ALI tissues for 1.5 hours to very high vapor concentrations (estimated >150,000 ppm) derived from pure or crude 7 M MCHM as positive controls induced a potent cytotoxic and pro-inflammatory response at 24 hours. Based on these preliminary data using the EpiAirwayTM ALI model derived from a single donor, acute exposure of human airway epithelial tissues to MCHM vapors for 6 hours was not toxic in vitro at concentrations relevant to human inhalation exposure (<0.50 ppm).
National Toxicology Program. 2018. NTP Research Report on the Preliminary Evaluation of 4-Methylcyclohexylmethanol in an In Vitro Human Airway Model. NTP RR 7. Research Triangle Park, NC: National Toxicology Program (7): 1-19. https://doi.org/10.22427/NTP-RR-7