The water resistance of sunscreen and day-care products.

The water resistance of 10 sunscreen products and three day-care products containing ultraviolet filters has been assessed using an in vitro method based on transmission spectrometry. The sunscreen products which claimed to be 'water resistant' or 'waterproof' had significantly better substantivity than products which made no claim concerning water resistance. However, there was no significant difference between the substantivities of the 'water resistant' and 'waterproof' products, suggesting that it may be unnecessary to differentiate between these two categories on product labelling. Consumers would find choosing a sunscreen product more straightforward if sunscreens were simply labelled either 'water resistant' or 'not water resistant'. The day-care products were readily washed off, losing almost all photoprotection after one 20-min immersion. The water resistance of four products was measured using water temperatures of 20 degrees, 25 degrees and 30 degrees C. In three products water temperature had no effect on substantivity, although one product washed off more readily with increasing temperature.