Non-invasive in vivo determination of UVA efficacy of sunscreens using diffuse reflectance spectroscopy.
BACKGROUND
Evaluation of sunscreen efficacy is most relevant when measured on the surface it is meant to protect, namely on human skin in vivo. Application of any material to the surface of the skin alters its optical properties. Diffuse reflectance spectroscopy (DRS) is a non-invasive technique to measure changes in the optical properties of the skin decoupled from its biological responses following sunscreen application.


METHODS
This study compared measurements of UVA efficacy of oxybenzone and avobenzone at different concentrations (0-5%) using DRS, human phototest and an in vitro technique. Twenty subjects were enrolled for each product measured by DRS and 10 different subjects were enrolled for each product measured by human phototest. Six areas 5 cm x 10 cm were outlined on each subject's back. DRS measurements were performed on four subsites within each area before and 20 min after sunscreen application. UVA efficacy for each concentration of product was calculated from the measured transmission spectrum of a given product convoluted with the spectrum of a Xenon light source adequately filtered to obtain the UVA spectrum from 320 to 400 nm and the erythema action spectrum. Phototesting was performed using the same light source and persistent pigment darkening as the biological endpoint. Measurements were made with sunscreen coverage of 2 mg/cm2. In vitro measurements were performed using an Optometrics instrument.


RESULTS
All three techniques showed a linear response between calculated UVA efficacy and product concentration.


CONCLUSIONS
This study showed that DRS is a rapid and reproducible method to calculate UVA efficacy of sunscreen materials and that its results correlate closely with those obtained by human phototesting.
Gillies
R
R
Wellman Laboratories of Photomedicine, Massachusetts General Hospital, Boston, MA 02114, USA.
Moyal
D
D
Forestier
S
S
Kollias
N
N
D006801:Humans; D010777:Photochemistry; D011835:Radiation Protection; D012867:Skin / Q000528:radiation effects; D013057:Spectrum Analysis / Q000379:methods; D013471:Sunburn / Q000517:prevention & control*; D013473:Sunscreening Agents / Q000737:chemistry*; D014466:Ultraviolet Rays
D013473:Sunscreening Agents