Sunscreen degradation and improvement

The headline read Scientists Warn: Sunscreen That Includes Zinc Oxide Loses Effectiveness and Becomes Toxic After 2 Hours I've learned to be suspicious of headlines, but this isn't entirely wrong.

The researchers made up their own sunscreens from the usual sorts of chemicals and exposed them to standardized fake sunlight. Some of the chemicals in sunscreen protect you by disintegrating themselves (and thus becoming ineffective), but "We were surprised to find that all five of the commercially relevant small-molecule UV-filter mixtures were mostly photostable."

But it seems that zinc oxide changes things.

The plot on the left is for ZnO micro-particles, and the one on the right for ZnO nano-particles. The blue curves are before and the orange curves are after UV exposure. The drop in absorbance at longer wavelengths (the UV-A region) is pretty dramatic. Interestingly, there's a rise in absorbance for nano-particle ZnO mixtures--so the mix gets better at blocking UV-B.

They also dosed embryonic zebrafish with the various before and after mixtures. It looks to me as though sunscreen isn't good for zebrafish, old sunscreen is worse, and defects really start to kick in with ZnO sunscreen and get really bad for old ZnO sunscreen--especially the nano-particle version. Of course the doses are huge.

They say zebrafish have "significant gene homology to humans", but I'll bet the 5-day turn-around for "more rapid screening" was the decisive reason for the choice. And everybody uses them (probably because of the quick growth).