Researchers funded by the Danish EPA added eight commercially available nanoparticles (six titanium dioxide, two zinc oxide) at 2.5 wt% to a chemical-filter base sunscreen and applied it to a reconstructed human epidermis and to two in vivo skin models. Zinc was found beneath the in vitro tissue at roughly 2,400 to 2,700 µg/L against about 25 to 37 µg/L in controls, but no zinc oxide particles were seen in the tissue. The authors conclude that zinc crosses as ions. Titanium was not distinguishable from controls in vitro and was found only in the outer stratum corneum in vivo.
Key numbers
- EpiDerm, medium beneath the tissue after 20 h (Table 11, PDF p. 51), mean ± SD of three tissues: zinc 2744.67 ± 308.80 µg/L (uncoated 18 nm ZnO) and 2439.00 ± 905.38 µg/L (coated 20 nm ZnO), against 25.00 ± 1.73 (negative control) and 37.00 ± 4.36 µg/L (sunscreen control). Titanium 0.47 to 2.03 µg/L in all groups; these are upper-bound estimates because calcium interferes, and none differed from control.
- Mouse ear biopsies (Table 15, PDF p. 64): titanium 3.2 to 121.56 mg/kg dry weight in treated ears, stated as 0.0011 to 0.12 % of the applied titanium dioxide. Controls ranged from below the printed limits to 4.49 mg/kg. The authors attribute the titanium to incomplete wash-off.
- Human skin xenografts (Table 16, PDF p. 71): titanium below 0.11 to 2.51 mg/kg dry weight.
- Particle size: most test sunscreens had weight-based D50 around 200 nm.
Methods (brief)
Nanoparticles were pre-dispersed in polyhydroxystearic acid and C12-C15 alkyl benzoate and mixed into the base sunscreen at 2.5 wt% (lower than the 20 to 25 % used commercially, for dispersion reasons). EpiDerm tissues were exposed for 20 h; mouse ears received three applications with or without TPA-induced inflammation; human skin xenografts on mice received three applications. Titanium and zinc were measured by ICP-MS after pressure digestion, with TEM and histology.
Evidence fitness
The report supports the conclusion that zinc from zinc oxide nanoparticles reaches the medium beneath a reconstructed epidermis, most likely as ions, and that titanium dioxide particles were not seen below the stratum corneum. It measured titanium and zinc only. It does not address the lead or cadmium carried by the oxides. The report notes children as users and damaged or sunburnt skin as open questions.
Limitations
Low nanoparticle loading, one base formulation, small animal numbers. The detection limit is given as 1 µg/L in the methods and 1 µg/mL later; censored table cells fall below the printed table detection limits; and the coatings of samples E and F differ between Table 1 and the in vivo methods.
Related evidence
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