Proctor DM, Suh M, Dunsmore D, Verwiel A, Casteel S. 2017. Bioaccessibility and relative oral bioavailability of cobalt and nickel from metal alloys in soil and dust. Poster presented at Society of Toxicology Annual Meeting, March 15, Baltimore, MD.
The potential human health hazard posed by ingestion of metals depends on the rate and extent of absorption into the body, i.e., bioavailability. In general, oral reference doses and cancer slope factors are expressed as ingested dose rather than absorbed dose; thus, accounting for potential differences in bioavailability across various exposure media is important for site-specific risk assessments. For metals bound in a matrix resistant to acid digestion, i.e., alloys, slag, tailings, including measures of relative bioavailability (RBA) greatly improves the accuracy of risk assessment. We evaluated in vitro bioaccessibility and in vivo RBA of cobalt and nickel from soil and sidewalk dust samples in a community affected by airborne emission of chromium-enriched metal alloys. Cobalt levels in dust and soil ranged from 28.4 to 4,503 ppm and that for nickel ranged from 124 to 24,258 ppm. Test materials were sieved to less than 250 mm and tested for bioelution in simulated gastric and intestinal conditions. Measures of bioaccessibility were slightly higher by gastric extraction than intestinal, averaging 1.7% and 3.0% for nickel in dust and soil, and 4.2% and 2.4% for cobalt in soil and dust, respectively. These test materials (soil and dust) and standard reference materials—cobalt chloride and nickel sulfate—were administered daily for 14 days, twice per day, to 7 groups of juvenile swine, including one untreated control group. Daily intake doses for cobalt were up to 229 mg/kg/day and for nickel up to 1419 mg/kg/day. Nickel and cobalt were measured in blood at Day 0, 3, 7, and 14, and in liver and kidney at Day 14. Blood nickel and cobalt concentrations in animals administered soil and dust were not greater than those of untreated controls. Nickel RBA values could be only calculated from liver data and were 0.57% and 2.6% for dust and soil, respectively. Cobalt RBA values could only be calculated from kidney data and were 1.4% and 3.3% in dust and soil, respectively. These bioaccessibility and RBA data demonstrate that nickel and cobalt in soil and dust affected by metal alloys are highly resistant to dissolution and absorption in the gut and inform the quantitative site-specific risk assessment of these metals in the environment. Although RBA studies in lead and arsenic are relatively common, these are the first such studies for nickel and cobalt.