Photodermatol Photoimmunol Photomed 1997: 13: 186-188 Printed in Denmark . All rights reserved

Copyright 0 Munksgaard 1997

Photodermatology Photoimmunology & Vhotomedidne

ISSN 090S-4383

How well are sunscreen users protected? Stokes R, Diffey B. How well are sunscreen users protected? Photodermatol Photoimmunol Photomed 1997: 13: 186-188. 0 Munksgaard, 1997.

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 thick- ness. The protection provided by differing quantities of sunscreens con- taining 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 ap- plication, coupled with overexposure to the sun, might partially explain why sunscreen use has been reported to be a risk factor in melanoma.

The photoprotection provided by a sunscreen product is assessed in terms of its sun protection factor (SPF). Sunscreen SPFs are generally meas- ured by in vivo assay and defined as the ratio of the ultraviolet (UV) dose required to cause minimal erythema in protected skin to that required for un- protected skin.

Internationally agreed procedures (1, 2) define protected skin as that to which a 2 mg/cm2 layer of sunscreen has been applied. (This is equivalent to a volume of 2 pl/cm2 assuming the sunscreen has a density of 1 g/cm3). Yet a number of studies have shown that typical thicknesses applied in practice are much less than this (3-5). It is not unexpected therefore that the average protection achieved is less than the nominal SPF (6). In a previous study (7), we found that most subjects chose to apply about two-thirds the quantity of a sunscreen in which the sole active ingredient was a physical agent, such as titanium dioxide (Ti02), compared with sunscreens containing organic chemical filters. We now report how the application of different amounts of sunscreens with varying concentrations of titanium dioxide affects the SPE

Material and methods isolation of epidermis

Skin was taken from the underside of female breast during the operation of breast reduction. The skin

R. Stokes, B. Diffey Regional Medical Physics Department, Dryburn Hospital, Durham, UK

Key words: sunscreen; application thickness; excised epidermis; sun protection factor

Brian Diffey. Regional Medical Physics Department, Dryburn Hospital, Durham DH1 5TW, UK Accepted for publication October 21, 1997

(approximately 10x4 cm) was received within 1 day of surgical operation, and these strips were cut into squares of approximately 4x4 cm. The samples of skin were placed in a water bath at 60°C for 45 s (8). On removal from the water bath, the epidermis was gently separated from the dermis by careful peeling. Epidermal sheets were stored in physiological saline at 4°C until required, which was normally within 5 days. Sheets of epidermis can be stored at 4°C for several weeks without loss of barrier function (8).

Sunscreen products

Three sunscreen products were used, each contain- ing titanium dioxide as the sole active ingredient at concentrations of 4.4% 6.9% and 7.8%, respec- tively. The products were commercially available from the same manufacturer and had quoted SPFs of 8, 15 and 25.

Experimental technique

A piece of epidermis (2x2 cm) was placed over a circular aperture of diameter 1.5 cm cut into an aluminium holder. The holder was positioned so that the circular aperture was directly over the Te- flon input optics of an Optronic model 742 spec- troradiometer controlled by a Hewlett Packard HP85 microcomputer. Radiation from a 75 W

186

How well are sunscreen users protected?

where E(h) is the spectral irradiance of sunlight expected for a clear sky at noon in midsummer for a latitude of 40"N (solar altitude 70°), and ~ ( h ) is the effectiveness of radiation of wavelength h nm in producing delayed erythema in human skin (10).

For each epidermal sample, increasing amounts of sunscreen were applied so that the SPF could be measured for application thicknesses of 0.65, 1.3, 2, 3, 4 and 8 mg/cm2. Measurements were made on samples of epidermis from seven subjects.

Table 1. SPFs obtained using the sunscreen of nominal SPF 8 containing 4.4% TiOp on seven samples (A-G) of epidermis

Thickness (mg/cm2) A B C D E F G Mean+SD

0.65 2.7 2.4 3.4 2.5 2.7 2.5 2.8 2.720.3 1.3 5.1 4.4 5.8 5.1 5.0 4.2 4.6 4.920.5 2.0 7.0 11.2 11.3 7.4 9.3 8.6 9.9 9.251.7 3.0 19.6 21.0 24.5 15.1 21.4 20.0 25.0 20.923.3 4.0 29.6 37.0 39.9 31.8 31.3 36.6 39.8 35.1?4.2 8.0 122 125 146 108 148 162 173 141223

Table 2. SPFs obtained using the sunscreen of nominal SPF 15 containing 6.9% Ti02 on seven samples (A-G) of epidermis

Thickness (mg/cm2) A B C D E F G MeankSD

0.65 3.0 3.8 3.7 4.3 4.1 3.1 2.8 3.550.6 1.3 5.9 7.5 6.6 7.2 9.2 8.0 5.9 7.251.2 2.0 15.3 14.3 9.8 13.9 13.4 12.7 11.3 13.051.9 3.0 22.4 29.4 21.2 30.9 33.6 26.1 31.0 27.824.7 4.0 59.8 61.0 49.9 71.1 64.1 44.4 60.0 5929 8.0 252 271 289 229 338 414 173 281578

Table 3. SPFs obtained using the sunscreen of nominal SPF 25 containing 7.8% Ti02 on seven samples (A-G) of epidermis

Thickness (mg/cm2) A B C D E F G MeankSD

0.65 4.5 5.0 5.5 4.8 5.9 3.6 4.4 4.8-tO.8 1.3 9.6 10.6 10.4 9.1 9.3 7.7 10.7 9.651.1 2.0 24.4 23.2 26.0 17.6 26.1 22.1 19.0 22.6?3.3 3.0 42.9 39.9 56.9 44.8 63.3 37.9 57.0 49210 4.0 62.5 54.0 101 92.9 89.1 71.3 100 82219 8.0 375 301 538 522 567 308 286 4142124

xenon arc lamp (filtered by a Schott UG5 filter) was directed onto the epidermis via a light guide, and the photocurrent recorded from 290 to 400 nm in steps of 5 nm. A micropipette was then used to dispense the required amount of sunscreen onto the epidermis. The sunscreen was 'spotted' at sev- eral positions on the epidermis and a light, circular rubbing motion with a gloved finger used to give as uniform a layer as possible. The sunscreen was allowed to dry for 15 min and the photocurrent again measured in 5 nm steps between 290 and 400 nm. For each wavelength, the ratio of the photo- current recorded before and after application of the sunscreen was calculated; this gives the mono- chromatic protection factors, PF(h), which were used in the following expression (9) to give the SPF:

400 z E(V E ( V

z E(V @)@F(V

290 SPF = 4oo

290

Results

Tables 1 to 3 summarise the SPFs measured on each of the products. The mean SPFs for an appli- cation thickness of 2 mg/cm2 given in the final col- umn of Tables 1 to 3 are in close agreement with those claimed by the manufacturer. Furthermore the coefficients of variation for each product are generally smaller than those expected from in vivo assay of products of similar SPF (11), indicating that in vitro assay using excised human epidermis as a substrate is a reliable and reproducible tech- nique.

Discussion

Spectral transmission measurements were used to determine the SPFs provided by sunscreens con- taining varying amounts of the same active in- gredient, Ti02, when applied to excised human epidermis. It can be seen from the mean SPFs that application thickness had a very significant effect on the photoprotection provided by the sun- screens. The SPFs at 1.3 mg/cm2, typical of that applied by many users (3), were approximately 50% of those at 2 mg/cm2 for all products. This is in reasonable agreement with results obtained by others (4, 6), who found that a 1 mg/cm2 layer of sunscreen gave an SPF that was approximately 50% of that for a 2 mg/cm2 layer. At 0.65 mg/cm2, close to the mean application thickness found by Bech-Thomsen & Wulf (9, the protection was only 20-30% of the nominal SPE

Sunscreen applied at a surface density of 2 mg/ cm2 is equivalent to a layer of 20 pm on a flat plane (assuming a density of 1 g/cm3). For most sun- screens, the extinction coefficient at the wavelength of maximum absorption is in the range 0.1 to 1 pm-I. The expected SPFs if the skin surface was flat would then be in the range lo2 to 1020. Clearly SPFs observed in practice are considerably less than this, the reason being that when sunscreen is applied to the skin surface it tends to accumulate in the sulci, leaving the epidermal ridges relatively unprotected (12, 13). The maximum extension from ridge to valley on most skin surfaces is

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around 50-100 pm (14, 15); and it can be estimated that once the application thickness exceeds about 3 mg/cm2, sunscreen should form a continuous layer over the whole skin surface. If this happens, the SPF would be expected to increase dramatically. Yet the results we obtained for application thick- nesses of 4 and even 8 mg/cm’ do not support this. The reason, we believe, is that small areas of skin are left with a reduced, or no, sunscreen coverage even when applied thickly.

In conclusion, we found that application thick- ness has a significant effect on SPF, with most users probably achieving a mean SPF of between 20-50% of that expected from the product label as a result of common application thicknesses in the range 0.5 to 1.3 mg/cm2. Compounded with this is the likely variability of protection over the skin surface due to application technique (16, 17). It is likely that sunscreen users are unaware of the fact that many of them are protected to a degree equivalent to about one-third of the SPF quoted on the bottle.

Acknowledgements This study was funded by the Department of Health. The views expressed are those of the authors and not necessarily those of the Department of Health. We thank Mr. R. B. Berry, Consul- tant Plastic Surgeon, for providing excised skin.

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