Photo-stability is still a problem for some sunscreen products according to a recent study from scientists in Italy.
The researchers, based at the University of Padova, Italy, tested 25 commercially available sunscreens with the aim of investigating a new in vitro test using Teflon (PTFE) as the substrate material.
PTFE has never been used as a substrate material for in vitro sunscreen testing, according to the scientists who tested it against the industry favorite for the job – PMMA plates.
In the course of the investigation the team looked at the protection factor provided by the sunscreens, and how these values compared to the published SPF figures on the label, as well as the photo-stability of the products.
Some products still photo-unstable
In terms of the stability of the products, the researchers found that four of the sunscreens were unstable when radiated with a solar simulator.
All of the unstable four contained Avobenzone (butylmethoxydibenzoylmethane), well known to be photo-unstable, according to the researchers.
They said that photo-stability is not indicated on the bottle and that any product that claims to give broadband protection should continue to do so even after sun exposure.
“When buying a sunscreen, the consumer should automatically receive a photo-stable product,” wrote the scientists.
Tests on photo-stability were performed using both PTFE and PMMA plates, and the two materials brought in the same results, confirming the choice of the material as a suitable substrate.
New material for in vitro tests
Evaluation of the protection factor provided by the sunscreens also supported the use of the new material as results showed a good correlation between the in vitro SPF determined by the new method and that provided by the manufacturer.
The researchers conclude: “PTFE is a useful material for rapid screening of sunscreen photo-protection. It requires no particular preparation, is readily available and is inexpensive.”
However, the team did note that like other in vitro substrates it is not a substitute for photo-testing in human volunteers when establishing product efficacy claims.
Source: Journal of Dermatological Science
Issue 52, pages 193-204
Sunscreen tests: Correspondence between in vitro data and values reported by the manufacturers
Denis Garoli, Maria Guglielmina Pelizzo, Bianca Bernardini, Piergiorgio Nicolosi, Mauro Alaibac