COLIPA has published guidelines for evaluating a sunscreen product’s water resistance.

The SPF is determined before (static SPF evaluation) and after (wet SPF evaluation) the immersion in water of an area of skin on which the sunscreen product has been applied. The immersion must occur in a recirculating-water pool in which the water temperature is constant at between 27 °C and 31 °C. For a product to be declared “water resistant”, the wet SPF is measured after two immersions, each 20 minutes long (a total of 40 minutes), separated by a 15-minute interval of air-drying time.Meanwhile, to be declared “very water resistant”, the number of twenty-minute immersions is 4 (for a total of 80 minutes). Based on these guidelines, a sunscreen product can be declared “water resistant” or “very water resistant” if the 95% CI of the mean static SPF falls within ± 17% of the SPF value, and if the unilateral 90% CI for the mean percentage of water resistance retention (%WWR-d) is ≥ 50%.

The in vitro sun protection factor is calculated by means of spectrophotometric measurements of the diffuse transmittance of a sample substrate, depending on wavelength within the ultraviolet spectrum.

Water resistance tests for WO emulsions made with SESAMULS© WO (INCI = Polyglyceryl-6 Pentaoleate, Sesamum Indicum Seed Oil, Malic Acid) were conducted at the University of Siena School of Specialisation in Cosmetic Science and Technology using a Labsphere UV-2000S UV Transmittance Analyzer spectrophotometer and PMMA slides. The quantity of product applied for the determination was 0.75 mg/cm2. The SPF determinations were performed both at the time of application and following the substrate’s immersion in water.

DEHYMULS PGPH (INCI= Polyglyceryl-2 Dipolyhydroxystearate), a WO emulsifier produced by BASF and considered a market “leader”, was used as a baseline for comparison.