Suh M, Verwiel A, Proctor D. Oral and inhalation bioaccessibility of cobalt and nickel in metal alloys: A critical consideration for site-specific human health risk assessments and read across. Poster for Society of Toxicology, Virtual Annual Meeting, 2020.
For risk assessment, nickel and cobalt are evaluated as specific chemicals or pure metal because bioavailability and toxicity vary by extracelluar and intracellular ion solubility. However, nickel and cobalt in alloys, such as stainless steel, do not fit these paradigms because ion solubility may be highly limited by an impervious chromium oxide layer that forms on the surface. Hence, metal ions are not readily released from alloys into biological fluids (limited bioaccessibility and bioavailability). In vitro bioaccessibility (IVBA) or bioelution is a test method based on simulated biological fluids to estimate in vivo conditions. Arsenic and lead in soil have been tested extensively; IVBA has been used by US regulators for risk assessment and is also under consideration by ECHA for read across. However, for cobalt and nickel, limited in vitro and in vivo data are available. To expand the database of cobalt and nickel in alloy impacting environmental media, we conducted inhalation and oral IVBA studies using simulated lysosomal and gastric fluids to estimate bioaccessibility of cobalt and nickel in media from 3 facilities in the US Thirty-nine samples of soil, dust, and baghouse dust were tested. Cobalt and nickel content in the samples ranged from 16 to 14,000 mg/kg and from 16 to 45,000 mg/kg, respectively. In simulated lysosomal fluid, bioaccessibility of cobalt and nickel were 4.7% to 21.1% and 3.9% to 10.8%, respectively. In simulated gastric fluid, bioaccessibility of cobalt and nickel were 0.7% to 12.2% and 0.2% to 26%, respectively. Bioaccessibility of cobalt and nickel were inversely related to the cobalt and nickel content in the samples; the results were consistent with a previous bioaccessibility study that we conducted. These data add to the weight of evidence that metals in alloys are resistant to bioelution, have relatively lower potential for absorption as compared to the forms used in animal bioassays, which are the basis of toxicity criteria. Our studies further confirm that bioelution methods should be used for read across and in human health risk assessment.