Uniformity of sunscreen product application: a problem in testing, a problem for consumers

Area of Science:

• Dermatology
• Photoprotection
• Cosmetic Science

Background:

• Accurate sunscreen efficacy testing relies on uniform product application.
• Limited research exists on the uniformity achieved during sunscreen application.
• Understanding application variability is key to interpreting Sun Protection Factor (SPF) results.

Purpose of the Study:

• To evaluate the uniformity of sunscreen product application across different Sun Protection Factors (SPFs) and formulations.
• To assess application uniformity on various in vitro substrates using standardized techniques.
• To quantify the variability in sunscreen film thickness and its impact on SPF determination.

Main Methods:

• Five commercial sunscreen products (SPF 4-50) were tested.
• Application was performed by experienced testers on Transpore Tape, Vitro-Skin, and lambskin condom substrates.
• In vitro SPF measurements were conducted using Optometric 290 analyser and Optronic Laboratories OL754 spectroradiometer.

Main Results:

• Sunscreen application resulted in an average relative standard deviation (RSD) of 18.6% for SPF values across all substrates and methods.
• The range of RSD percentages was 10-40%, indicating significant non-uniformity.
• This variability highlights challenges in achieving consistent product film thickness.

Conclusions:

• Non-uniform sunscreen application introduces an expected uncertainty of approximately 20% in SPF testing.
• Variability in product application is a primary contributor to discrepancies in SPF test results between laboratories.
• Improving application techniques is essential for reliable and reproducible sunscreen efficacy assessments.