Dissolution of the metal sensitizers Ni, Be, Cr in artificial sweat to improve estimates of dermal bioaccessibility

Abstract

Dermal exposure to sensitizing metals is a serious occupational and public health problem. The usual approach to dermal exposure assessment is to process samples by chemical methods that use reactants to digest the metal particles and quantify the mass. In the case of dermal exposure assessment, these reactants are not representative of the skin surface film liquids and hence, may overestimate bioaccessibility. We hypothesize that the amount and form of sensitizer on a sample that leaches in a biological fluid, as can be estimated using artificial sweat, may be a more relevant metric for assessing health risks. Beryllium metal (Be), nickel metal (Ni), and chromium carbide (Cr3C2) particles were characterized and masses of sensitizing ions were measured using established reactant-assisted digestion procedures and extraction in artificial sweat under physiologically relevant conditions. Chromium ions released into artificial sweat were speciated to understand valence states. The ratios of the fraction of metal dissolved in artificial sweat relative to that dissolved by chemical-specific reactants were 1/2 (Be), 1/108 (Ni), and 1/2500 (Cr). The divalent Be and Ni cations were stable in artificial sweat over time (did not precipitate) whereas hexavalent chromium [Cr(vi)] ions decayed over time. Further analysis using speciated isotope dilution mass spectrometry revealed that the decay of Cr(vi) was accompanied by the formation of Cr(iii) in the sweat model. Use of reactant-assisted analytical chemistry to quantify amounts of metal sensitizers on samples could overestimate biologically relevant exposure. In addition to mass, the valence state also influences penetration through the outer stratum corneum of the skin and is an important consideration when assessing exposure to complex sensitizers such as Cr which have multiple valence states with differing penetration efficiencies.

Penetration of Zinc into Human Skin after Topical Application of Nano Zinc Oxide Used in Commercial Sunscreen Formulations

“…4,5 Human skin is irrigated by sweat, which is an electrolyte solution containing many components such as lactic acid, urea, and sodium chloride that may interact with metal oxides such as ZnO NPs. 6 The pH of sweat is slightly acidic with typical skin pH values ranging from 4.6 to 5 7 and is an important aspect to consider when it comes to metal oxides and their dissolution into constituent ions. Zn 2+ solubility increases with decreasing pH in various aqueous environments 8 including artificial sweat.…”

Comparison of Synthetic Sweat and Influence of Sebum in the Permeation of Bioaccessible Metal(loid)s from Contaminated Soils through a Synthetic Skin Membrane

“…The dermal bioaccessible fraction of a chemical is the amount that is dissolved in sweat and is available for penetration through the skin. 5,18 This concentration can be used in conjunction with the permeation data to calculate bioavailability. 5 To estimate the dermal absorption of contaminants from aqueous media, the USEPA 19 proposes the water approach methodology.…”

“…5,18 This concentration can be used in conjunction with the permeation data to calculate bioavailability. 5 To estimate the dermal absorption of contaminants from aqueous media, the USEPA 19 proposes the water approach methodology. It assumes contact with contaminated water and aims to calculate the dermally absorbed dose using the migration rate of a chemical through the skin.…”

“…Humans can be exposed to toxic metal­(loid)­s in various environmental media, including soils, via the dermal pathway. The majority of scientific studies have focused on oral bioaccessibility of pollutants (ingestion pathway), and more recently on in vitro test development and application for assessing inhalation bioavailability − and in vitro dermal bioavailability. − …”

Metal Nanomaterials: Immune Effects and Implications of Physicochemical Properties on Sensitization, Elicitation, and Augmentation of Allergic Disease

“…In addition to promoting the release of larger quantities of bioavailable metal ions from parent materials, the biochemical characteristics of sweat can also facilitate alterations in metal ion valence state (596). Since the passage of Cr ions through the epidermis is known to be dependent on oxidation state, these interactions may, therefore, greatly contribute to the biological activity of metal nanomaterials (597).…”