Amy Holmes and colleagues applied zinc oxide at 10% by weight in capric/caprylic triglycerides and in aqueous suspensions at pH 6 and pH 9 to heat-separated human epidermis for up to 48 hours. The copy is the ACS Just Accepted manuscript, not the final paginated article. Zinc oxide particles did not penetrate intact viable epidermis. Labile zinc fluorescence increased, and the manuscript describes the highest signal with the pH 6 vehicle. A general remark that sunscreens often contain 5 to 25% zinc oxide by weight is not a measurement from this experiment.
Key numbers
The formulation used here is 10% wt zinc oxide. The membrane area was 1.30 cm2, and the skin surface was dosed at 10 mg/cm2 of each preparation. That figure is an applied dose. For a pH 9 buffer, viable-epidermis pH was 6.80 ± 0.17 before and 6.17 ± 0.29 after (mean ± SD, n = 3). For a pH 6 buffer, viable-epidermis pH was 6.83 ± 0.42 before and 5.10 ± 0.20 after (mean ± SD, n = 3). Those are pH readings, not zinc concentrations. Stratum corneum pH pairs are in the same paragraph and are not repeated as metal concentrations.
Methods (brief)
Heat-separated human epidermis, multiphoton and zinc-ion fluorescence, up to 48 hours.
Evidence fitness
The paper supports a penetration statement about these three vehicles. It does not report contaminant metals in a market sample of sunscreens.
Limitations
Just Accepted text. The 10 mg/cm2 figure is the applied dose. No mass-per-mass zinc concentration in viable epidermis was transcribed.
Related evidence
Update history
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