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

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Overview

This Environmental Science: Processes & Impacts paper compares reactant-assisted digestion (“total” or “soluble” mass) vs artificial-sweat extraction (bioaccessible mass) for three metal sensitizers - beryllium metal, nickel metal, and chromium carbide (Cr3C2) - and uses speciated isotope dilution mass spectrometry (SIDMS) to determine Cr-VI vs Cr-III speciation in artificial sweat over time. The work establishes that reactant-assisted digestion overstates bioaccessible (dermal) exposure to all three metals, and that Cr-VI added to artificial sweat decays into Cr-III over hours.

Key numbers

  • Bioaccessibility ratios (Table 2, fraction dissolved in sweat / fraction in chemical digestion):
    • Be: sweat 0.15 ± 0.01% vs digestion 0.29 ± 0.02% → 1/2 ratio
    • Ni: sweat 0.0013 ± 0.0003% vs digestion 0.14 ± 0.01% → 1/108 ratio
    • Cr-total: sweat 0.0024 ± 0.0008% vs digestion 6.08 ± 0.31% → 1/2500 ratio
  • Artificial sweat composition (Harvey et al. recipe): 61 constituents; highest-concentration components Na+ 30.7 mM, Cl- 23.2 mM, lactic acid 14.0 mM, urea 10.0 mM, K+ 6.1 mM, ammonium 5.3 mM; pH adjusted to 5.3 with NaOH. Skin pH range generally 4.2-6.1 (median 5.3).
  • Sweat extraction conditions: 0.050 L artificial sweat (calculated from average adult whole-body sweat output 1.08 L/h × 5% body-area fraction for hands × 1 hour); 0.05 g powder per sample; powders sealed between two filters and immersed in artificial sweat using the static dissolution technique; triplicate samples at 33, 36, 45 °C across 7 time points spanning ~12 hours.
  • Dissolution kinetics (Fig. 3, p. 345-346): Be and Cr both biphasic — initial rapid phase half-time 4-6 h for Be and 3-5 h for Cr, both followed by a slower long-term phase with half-times on the order of hundreds of days. Ni dissolution linear over time with calculated half-times on the order of several hundred days.
  • On total-mass basis (per Results section): Be <0.35%, Ni <0.003%, Cr <0.004% of total powder dissolved in artificial sweat over the test duration.
  • Cr speciation in sweat (Fig. 4 / Results): Cr-VI added as potassium dichromate at 2 or 10 µg/mL decayed over 420 minutes; decay accompanied by Cr-III formation in the sweat model (speciated isotope dilution MS using 53Cr-VI / 50Cr-III).
  • Powder characterization (Fig. 2): Be metal particle size 3.2 µm (geometric mean, GSD 1.8); BET surface area 5.6 ± 0.02 m²/g; density 1.86 ± 0.02 g/cm³. Cr3C2 cluster size 2.6 µm (GSD 1.6); surface area 1.04 ± 0.02 m²/g; density 6.97 ± 0.19 g/cm³. Ni metal cluster size 4.0 µm (GSD 1.5); surface area 7.04 ± 0.18 m²/g; density 9.46 ± 0.29 g/cm³.

Methods (brief)

Three high-purity metal sensitizer powders characterized by Auger spectroscopy (surface composition 50-75 Å outer layer), XRD (crystalline composition), TEM/SEM (morphology, particle size), helium pycnometry (density), N2 BET (specific surface area). Reactant-assisted digestion protocols: Be 0.01 M HCl (Profumo et al. 2008); Ni 0.1 M ammonium citrate/citric acid pH 4.4 sequential digestion (Zatka et al.); Cr-total NIOSH 7303 concentrated HNO3/HCl at 95 °C. Sweat extraction: artificial sweat (Harvey et al. recipe) at 33, 36, 45 °C, triplicate samples at 7 time points across 12 h. Metals quantified by ICP-OES. Cr speciation by SIDMS using 53Cr-VI/50Cr-III isotope dilution with IC-ICP-DRC-MS. Limitations stated: static (not flow-through) extraction may underestimate sweat-removal kinetics; chosen powder masses (0.05 g) exceed realistic dermal loadings to ensure analytical detection limits; static contact time of 12 h represents long workshift exposure.

Implications

  • Certification (HMTc): Direct methodological evidence that “total Cr” measurements digested by acids massively overstate dermally bioaccessible Cr (1/2500 ratio); for Ni and Be the overstatement is also large (1/108 and 1/2 respectively). Important for any HMI threshold work on children’s products where total-Cr measurements are the primary occurrence data: dermal-route risk modeling needs to apply a bioaccessibility correction or accept conservative overestimation. The Cr-VI → Cr-III decay in sweat is relevant to Part 14’s discipline around Cr-VI vs Cr discipline: skin-surface Cr-VI does not remain Cr-VI for long.
  • Courses: Teaching reference for the distinction between “total,” “soluble,” and “bioaccessible” metal in occupational/consumer dermal exposure assessment. Pairs with the OEHHA and ATSDR dermal-absorption guidance.
  • App: Not relevant to ingredient contamination_profile (no food-matrix data).

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