Rating protection: Research refines in vitro SPF calculation

Key Points

Currently, U.S.-based manufacturers of sunscreens determine SPF in vivo by irradiating the skin of 20 volunteers (European testing requires 10) with and without sunscreen with UV radiation that simulates sunlight, says Uli Osterwalder, M.S., a chemical engineer and scientific adviser, suncare, with Basel, Switzerland-based Ciba Inc.

However, he says that for ethical reasons, "Humans should not be used as a radiometer. This is why in vivo SPF measurement will eventually be replaced by in vitro measurement."

However, Mr. Osterwalder tells Dermatology Times, "One difficulty with this process is that not all sunscreens are completely photostable during the duration of their use. Since most sunscreens are not completely photostable, they lose protection power over time."

Therefore, the level of sun protection a photolabile product provides actually begins higher than the labeled SPF, but ends up lower than the product's purported SPF when sun exposure lasts until the sunburn threshold is reached, he says.

"So far," Mr. Osterwalder says, "in vitro measurement did not take this dynamic into account."

To address this shortcoming, Mr. Osterwalder and a team of colleagues compared two existing in vitro methods that factor in photostability.

"They are both based on the same idea - to measure over time (and thus, UV dose) how much erythema-generating UV radiation accumulates through the sunscreen. This is the same principle by which sunscreens are measured on human skin," he says.

Researchers referred to these methods as Method I (Stanfield JW. "In Vitro Techniques in Sunscreen Development" Nadim Shaath (Ed.), Taylor & Francis, Boca Raton, Fla., 2005) and Method II (Wloka M, et al. Proc Int Sun Protection Conference, London, June 8-9, 2005; Mueller S, et al. Proc Int Sun Protection Conference, June 6-7, 2007, London).

Both methods allow determination of the in vitro SPF after transmission of exactly one minimal erythemal dose (MED), Mr. Osterwalder says.

The study's objective was to determine whether the two methods show equivalent results, and to investigate the influence of various light sources on in vitro SPF results.

To test the calculation methods, investigators applied them to the following commercially available sunscreens:

Researchers applied these products - approximately 1 mg/cm² - to roughened polymethylmethacrylate (PMMA) plates. They used a 150 W xenon arc solar simulator with a 1 mm thick UVC blocking filter, a 1 mm thick visible/infrared blocking UG-11 filter, a heat rejecting dichroic mirror and an 8 mm liquid light guide (Model 16S, Solar Light Company, Philadelphia).