Editorials

Sunscreens, suntans, and skin cancer

Sunlight causes sunburn, photoaging, and skin cancer,¹ the price many people seem prepared to pay for a tan. Yet recent studies suggest that tanning is a direct consequence of damage to DNA caused by ultraviolet radiation.² Even seemingly trivial sun exposure, enough just to turn the skin red, causes considerable damage to both keratinocytes³ and melanocytes and is sufficient for many cells in the skin to sustain lethal doses of ultraviolet radiation. Patients with xeroderma pigmentosum, who are unable to repair this damage, develop skin cancer in the first 5-10 years of life, but in normal individuals the accumulation of genetic alterations sufficient to cause skin cancer may take five to six decades. However, despite the efficiency of our DNA repair machinery, the rising incidence of both melanoma and non-melanoma skin cancer in fair skinned populations suggests that in many cases DNA repair is not able to keep pace with 20th century lifestyle.

In recognition of the economic and public health concerns, the government's strategy document, Health of the Nation, has as one of its major targets the reversal of the year on year increase in melanoma and non-melanoma skin cancer by 2005. The key to achieving this target is public health education, to increase awareness of the dangers of sun exposure, even in Britain, and to promote a healthier, less "sun seeking" lifestyle. The slogan of Britain's Health Education Authority, "Shift to the Shade," emphasises that the safest way to reduce sun exposure is by changing behaviour. The use of sunscreens to prevent skin cancer is more controversial, not least because by preventing sunburn they may encourage longer exposure to the sun.

Sunscreens work mainly by blocking solar ultraviolet B wavelengths (295-320 nm), which (despite accounting for less than 5% of solar ultraviolet radiation) are primarily responsible for sunburn. There is increasing evidence that ultraviolet A (320-400 nm) can also damage the skin, and most sunscreens in Britain now contain good ultraviolet A blocking agents, providing "broad spectrum" cover. A sunscreen's ability to prevent sunburn is designated by its "sun protection factor" (SPF), which is defined as the ratio of the minimum erythemal dose of simulated sunlight on protected skin compared with unprotected skin. In reality, it is widely perceived as how many times longer it takes to obtain a mild sunburn, the skin's early warning system, while wearing a sunscreen. But we know little about the precise relation between sunburn and skin cancer. Although sunscreens are highly effective at preventing sunburn, there is concern that they may be less effective at preventing skin cancer.

Studies in hairless albino mice have shown that, as with sunburn, ultraviolet B is the main cause of non-melanoma skin cancer.⁴ This suggests that sunscreens used to prevent sunburn might also prevent, or at least reduce the incidence of, non-melanoma skin cancer. Evidence to support this comes from mouse experiments⁵ as well as preliminary human sunscreen studies.⁶ These show that the incidence of malignant and premalignant lesions is indeed reduced by the use of sunscreens, although the limited evidence suggests that the "cancer protection factor" of sunscreens may be lower than their "sunburn protection factor."

The complex relation between sunscreens and skin cancer is further illustrated by reports of an increased risk of malignant melanoma in sunscreen users.^{7 8} Confounders apart, one possible explanation is that some sunscreens fail to provide adequate protection from ultraviolet A. Studies on fish models,⁹ supported by limited evidence that sunbeds are a risk factor for melanoma,¹⁰ suggest that ultraviolet A wavelengths may be particularly important in the induction of this tumour type. The sunscreens used in the malignant melanoma studies were almost certainly primarily ultraviolet B blocking agents: although most British sunscreens now contain effective ultraviolet A blocking agents, this is not always the case in other countries. Sunscreens containing only ultraviolet B blocks would protect against sunburn and therefore enable greater exposure to ultraviolet A than would otherwise be possible to obtain. Another possibility is that sunscreens may afford insufficient protection against the immunosuppressant effects of ultraviolet radiation.¹¹ This immunosuppression is thought to have an important role in the promotion of non-melanoma skin cancer and may also be involved in melanoma.

Whatever the potential shortcomings of sunscreens in preventing skin cancer, they do provide an important way of reducing cumulative exposure to solar ultraviolet radiation if they are used in conjunction with other photoprotective measures. However, all photoprotection is lost if sunscreens are used to prolong sun exposure to the maximum achievable by the sun protection factor; so that, for example, sunburn develops over five hours instead of 30 minutes. Thus the marketing of sunscreens as a safe way to stay out in the sun longer may be misleading. Indeed, animal¹² and human studies^{7 8} suggest that this approach may actually increase the risk of both melanoma and non-melanoma skin cancer for the same daily exposure dose.

A recent survey by Britain's Health Education Authority showed that 57% of the public had bought or used sunscreens in the past two to three years. Most people claimed an interest in the photoprotective aspects of sunscreens against sunburn (80%) and skin cancer (53%) rather than their use to get a good tan (35%).

Along with the ongoing trend for buying products with higher sun protection factors, this suggests that the public increasingly regards sunscreens as health rather than cosmetic products. Though this is certainly encouraging, the role of sunscreens in preventing skin cancer still needs clarifying.

Senior lecturer in dermatology Senior lecturer in photobiology Department of Photobiology, St John's Institute of Dermatology, St Thomas' Hospital, London SE1 7EH

J M Mcgregor, A R Young 

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