UVB filter mostly used to stabilise avobenzone. Effective, but can break down to benzophenone in the bottle and occasionally causes photoallergy.
Octocrylene absorbs UVB and short-wave UVA2 and, just as important, it stabilises avobenzone so the UVA protection lasts in sunlight. It also helps dissolve other filters.
Two things put it under scrutiny. Lab work found that it slowly degrades to benzophenone (a suspected carcinogen) in the bottle, rising as products age. And it is a known, if uncommon, cause of photoallergic rashes.
Its UV absorption and stabilising role are well established in formulation testing and SPF measurement. It is absorbed into the blood (around 7 ng/mL in the FDA trial), above the screening threshold. Benzophenone formation comes from one lab group's analysis of products and has been disputed in published letters; no study has measured health effects in users. FDA lists it as needing more data; the EU caps it at 10% (9% in propellant sprays) and requires benzophenone to be kept at trace level.
Sixteen octocrylene-containing sunscreens from the EU and US contained benzophenone at an average 39 mg/kg, rising to 75 mg/kg after an accelerated-ageing protocol; a product without octocrylene had none. Shows a degradation pathway in the bottle, not a measured harm to users; the paper drew published critical letters.
Photocontact allergy to octocrylene has been reported increasingly since 2003, mainly in adults previously sensitised to the anti-inflammatory gel ketoprofen; plain contact allergy is rarer and seen mostly in children using octocrylene sunscreens.
48 healthy adults applied one of four sunscreens to 75% of the body, up to 4 times a day for 4 days. Avobenzone, oxybenzone, octocrylene, homosalate, octisalate and octinoxate all exceeded the FDA's 0.5 ng/mL screening threshold after a single application; oxybenzone reached about 258 ng/mL from lotion. The authors state this does not mean people should stop using sunscreen.