Research Roundup on Transdermal Absorption of Sunscreen Ingredients
Traditional sunscreen research:
A study of 24 people who used four different sunscreens – 2 sprays, a lotion, and a cream – applied to 75% of their body, 4 times per day. This went on for four days.
The FDA guidelines are that the active ingredients in sunscreen shouldn’t be absorbed at more than 0.5 ng/mL.
The results showed that the active ingredients were all absorbed and found in the bloodstream at levels way above 0.5 ng/mL by the end of day one. Oxybenzone levels varied from 169 – 209 ng/mL. The other three active ingredients (avobenzone, octocrylene, and ecamsule) ranged from around 2ng/mL to over 20 ng/mL. By the end of day 4, concentrations were even higher.
Neurotoxic effect of active ingredients in sunscreen products, a contemporary review
Review article in the journal Toxicology Reports from 2017 discussing the known and unknown effects on neurotoxicity from several common sunscreen ingredients. A lot of the studies included in the review use a skin penetration of 5% for some of the organic compounds. Many of these compounds are metabolized (broken down in the body) and excreted eventually. Interestingly, the compounds in sunscreen also are found in breast milk samples, and because they are lipophilic, they can easily cross the blood brain barrier.
Most of the studies of neurotoxicity of sunscreen ingredients crossing into the brain were done on animals.
Nanoparticles (Titanium oxide, zinc oxide):
Frontiers in Bioengineering: The Toxicity Phenomenon and the Related Occurrence in Metal and Metal Oxide Nanoparticles: A Brief Review From the Biomedical Perspective (July 2020)
This is a big review of studies involving nanoparticle toxicity. Nanoparticles in sunscreen are usually titanium oxide or zinc oxide. The research shows that the size of the nanoparticles is important for being absorbed through the skin and transported throughout the body. Zinc oxide nanoparticles are toxic at high concentrations, but these concentrations in animal studies would be hard to obtain with sunscreen.
My takeaway: There are many factors that go into nanoparticles – shape, size, structure – that impact the effect in vivo. But with more studies on each of those factors, nanoparticles may be a safer alternative.