Sun protection factor persistence on human skin during a day without physical activity or ultraviolet exposure

Recently, we showed that the sun protection factor (SPF) decreases by a constant factor to reach 55% during a day with activities. Organic sunscreens but not inorganic ones are absorbed through the skin. We wished to determine the SPF decrease caused by absorption by investigating the difference in SPF decreases between an organic and an inorganic sunscreen, assuming that the sunscreens are stable, and that the SPF decrease is time dependent if caused by absorption.

Sunscreens were used on the backs of 22 participants, who were physically inactive at 22 degrees C. SPF testing was performed 30 min, 4, and 8 h after application of 2 mg/cm(2) sunscreen. Whether cream evaporation changed the ultraviolet (UV) transmission was studied in vitro.

The SPFs of the organic and inorganic sunscreens were reduced by about 25% after 8 h. Evaporation of the cream did not cause a change in UV transmission in vitro.

A similar decrease in SPF of the organic and inorganic sunscreen was seen during 8 h without activities, and is thus not likely to be caused by absorption or evaporation from the skin. The SPF decrease after 8 h is about 55% when the participants perform activities and 25% without activities.

Registered at http://www.clinicaltrials.gov. Register name: 'Sunscreen: Persistence of Sun Protection Factor and the Influence on Vitamin D'. Register number H-B-2007-120.