The dynamic behaviour of sunscreens under in-service conditions

Discussion continues over various aspects of sunscreen science: regulation, test methods, sun protection factor (SPF), labelling claims, potentially harmful components, among others. In this paper the UV transmission properties of a number of commercial sunscreens have been determined at constant sunscreen film thickness under different local UV Index conditions. The data demonstrate difficulties facing the public and the sunscreen industry as a whole, even though SPF values and other data stated on the sunscreen packaging are assumed to be correct according to standard testing methods. This work has shown that at realistic application rates the critical factors are the intensity of the incident solar radiation and the accumulated erythema UV dose transmitted over time. In one example, on 'Extreme' UV Index days, an SPF 30 sunscreen under test transmitted one minimal erythema dose (MED) of UV in only 35 min. In another example, although it should not, in theory, transmit one MED until several hours of exposure, this level was reached in 1 h by an SPF 50 sunscreen under these typical Australian summer conditions (UV Index 12) in Wollongong, NSW (34.4°S). Such properties could have severe consequnces if these sunscreens were used by individuals with Fitzpatrick Skin Type 1, organ transplant recipients or other immuno-compromised individuals.