Previous studies have shown that people often apply less sunscreen than the recommended amount of 2 mg/cm2. Our purpose in this study was to determine objectively how photoprotection varies with application thickness. The protection provided by differing quantities of sunscreens containing varying amounts of titanium dioxide was measured in vitro using excised human epidermis as the substrate. It was found that application thickness had a significant effect on the sun protection factor (SPF), with most users probably achieving a mean SPF of between 20-50% of that expected from the product label. Underprotection due to inadequate application, coupled with overexposure to the sun, might partially explain why sunscreen use has been reported to be a risk factor in melanoma.

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.

The photostabilities of four sunscreen products have been assessed in vitro by applying sunscreen to a substrate and measuring the spectral transmission prior to, and after exposure to a source of ultraviolet (UV) radiation. Results were independent of whether an application thickness of 1 or 2 mg/cm ; 2 was used, and whether the UV source was natural sunlight or a xenon arc solar-simulator. There were significant differences, however, between results obtained on a roughened quartz substrate and those obtained on excised human epidermis. It is unlikely that any substrate will give an exact representation of the in vivo situation and, indeed, both quartz and excised human epidermis have disadvantages associated with their use. However, the ranking of the four products in terms of their photostability was the same for both substrates. This implies that transmission spectroscopy, with either a quartz or a human epidermis substrate, can be used successfully to compare the photostabilities of different sunscreen products.

A novel in vitro technique has been used to assess the water resistance of topical sunscreens. The technique is based upon transmission spectrometry using excised human epidermis as the substrate. The sun protection factors (SPFs) of four sunscreens obtained prior to water immersion and after two immersions were compared with in vivo data determined using human volunteers. In every case there was close agreement between the SPFs obtained on dry skin and on skin that had been immersed in water. Not only does the technique provide a less expensive and less time consuming alternative to in vivo testing, it also offers the possibility to study the effect that water immersion may have on the absorption spectrum of products which cannot be determined in vivo. This can give valuable information on which, if any, of the active ingredients present in sunscreens are being preferentially removed by water. In addition, the method can be easily adapted to study the effects on sunscreen substantivity of other factors such as water temperature, salinity and turbulence.

A novel ex vivo technique has been used to assess the sand/rub resistance of topical sunscreens. The technique is based on transmission spectrometry using excised human epidermis as the substrate. The sun protection factors (SPFs) of 10 sunscreen products were measured prior to, and after agitation with sand. It was found that the different products retained varying amounts of their photoprotection after agitation, ranging from 40 to 85%. This technique provides a less expensive, less time-consuming and less experimentally problematic alternative to in vivo testing.