Sunlight, sunscreens, health and melanoma (update)
Summary
Rates of skin cancer are increasing worldwide. While skin cancer is most common in Australia and New Zealand, many other traditionally less sunny countries are also now experiencing increases in this disease. In this article Dr Val Doherty provides an overview of the effects of sunlight on health, the risk factors for skin cancer and the effectiveness of sunscreens and other methods of protecting skin from exposure to sunlight.
Key Points
- Melanoma has been the most rapidly increasing Caucasian malignancy over the past 30 years; the highest incidences are in Australia and New Zealand. Melanoma incidence is expected to increase by 75% over the next decade in Scotland – the highest predicted rate for any solid cancer.
- The relation between sun exposure and melanoma is well known, but not clear-cut. The level of sun exposure in childhood and adolescence may be particularly important.
- Individuals who freckle and sunburn easily and who have light eye colours are at higher risk.
- Ultraviolet radiation (UVR) is of two types. Ultraviolet A (UVA) is more constant throughout the day and year than ultraviolet B (UVB), penetrates skin deeply and contributes to skin ageing. UVB is more intense in summer and in the noontime sun, and causes sunburn.
- Sun protection factors (SPF) quoted on sunscreens mainly apply to UVB. In the last couple of years, SPFs for UVA have become more common.
- There is evidence that sunscreens prevent squamous skin cancer, but not melanomas. There is no evidence to support the suggestion that sunscreen use increases melanoma risk.
Declaration of interests: No conflict of interests declared
Cutaneous melanoma remains one of the most fascinating and challenging conditions to treat. Worldwide incidence rates have risen more rapidly than that of any other malignancy in Caucasian populations over the past three decades.
The relationship between the sun and melanoma has long been recognised. Sunny countries, most notably Australia and New Zealand, have the highest age-adjusted incidence rates (55 and 56/100,000 respectively) in the world.1 The lifetime risk of developing melanoma in Australia is 1 in 25 for men and 1 in 34 for women. Throughout Australia there is a clear inverse relationship between incidence and latitude.
Melanoma and sun exposure
Unlike other forms of skin cancer, the relation with sun exposure is not a clear-cut one. In non-melanoma skin cancer (basal cell and squamous cell cancers), the risk is, generally, greatest in those with the highest cumulative sun exposure over a lifetime and most frequent on exposed sites, for example the head and neck. With melanoma, it seems that intermittent sun exposure, particularly that received in childhood and adolescence, is most relevant. Melanoma occurs more frequently in individuals with poor sun tolerance, that is those who freckle and burn as opposed to tan, and this is often associated with lighter eye colours. The Celtic skin type is frequently encountered in Scotland, where cutaneous melanoma has an incidence of 12.96/100,000 for women and 10.93/100,000 for men.2
The Scottish Melanoma Group has clinical, pathological and follow-up data on all cutaneous melanomas diagnosed since 1979 and remains an active multidisciplinary group involved in audit and research. This data forms one of a very small number of population-based melanoma databases in the world. Much of the work on melanoma in Scotland has been modelled on the Australian experience, and this holds true particularly for preventative and educational ventures.3
The Queensland Melanoma Project has been running since the early 1970s and has involved both public and professional education about the early recognition and prevention of melanoma. The key message as regards sun behaviour is encapsulated in the project’s slogan – Slip (into the shade), Slap (on a hat), Slop (on sunscreen). The importance of this venture is so widely recognised that there is now legislation in place to ensure the central registration of all melanomas and the provision of adequate shade in schools and workplaces in Australia.
Sunscreens
Clearly the situation in Scotland is different in some ways to that in Australia. The number of melanoma patients is not as great, and the climate is radically different. However, the message of sensible sun exposure has been widely agreed by health professionals and educationalists. Yet, as is often the case with health messages, there has been diversity of opinion, particularly with regard to sunscreen use. Sunscreens, in the form of creams and lotions, have been available for more than 40 years. The range available has dramatically increased in the past ten years, with improved recognition of different types of ultraviolet radiation (UVR) and their role in skin cancer development.
In animals, sunscreen use can reduce the formation of squamous cell cancers of the skin; in humans, it can limit development of actinic keratoses, which are a recognised precursor of squamous cell cancers. There is no evidence as yet that sunscreen use prevents melanoma or basal cell cancer in humans.
Ultraviolet radiation has two components of major relevance for skin cancer: ultraviolet B (UVB) (290–320 nm) and ultraviolet A (UVA) (320–400 nm). UVA penetrates more deeply into the skin and is of more constant intensity throughout the year and at different times of the day. It is known to cause many of the features of chronic sun damage, including skin ageing. UVB is often recognised as the main cause of sunburn (erythema), and is at its peak around noon and in the summer months. Originally, sunscreens were directed at reducing UVB penetration into the skin. Traditional sunscreens carry a sun protection factor (SPF) grade, which largely describes UVB protection by reducing/preventing erythema. The often-advised SPF 15 will filter out 93.3% of UVB. This, however, is based on tests in which 2 mg/cm2 of sunscreen is applied to all skin. In reality, most people apply 0.5–1 mg/cm2. Thus most actual use equates to a lower SPF.
In addition, high SPF sunscreen use confers an ability to spend longer in the sun without burning compared to unprotected skin. Concern was raised that this might encourage sun-sensitive individuals to spend more time in the sun, and thus risk greater exposure to UVA- and to UVB-related damage other than erythema. More recently, sunscreens have been developed to reflect/absorb both UVB and UVA. An increasing number of sunscreens now quote protection factors for both types of UVR.
Health benefits of sun exposure
Another concern raised around the Australian-style modification of sun behaviour was the loss of other health benefits from sun exposure. One of these benefits is the manufacture of vitamin D in the skin. Vitamin D levels have been associated with variations in the incidence of various internal malignancies, with conflicting results. It has been estimated that between 9% and 40% of Americans are deficient in vitamin D, varying with age, gender and ethnicity. It is also recognised that the average American diet contains minimal vitamin D and as a result most people need to manufacture vitamin D in the skin to prevent osteomalacia or rickets. It has been argued that sunscreen application virtually abolishes vitamin D manufacture and thus sunscreen users, along with sunlight-deprived individuals, should double their dietary intake of vitamin D. This assumes that sunscreen is used all the time and applied at a thickness to give full activity. Generally, as mentioned above, this is not the case, so most people will have enough inadvertent sun exposure to manufacture some vitamin D and those at greatest risk can be advised on dietary supplementation.
Sunscreen use and melanoma risk
A number of publications have suggested that sunscreen use may increase the risk of melanoma. These have been, in the main, retrospective questionnaires about sun protection use in melanoma patients compared with controls. A few studies reported increased numbers of melanomas among sunscreen users, which was an obvious cause for concern. Other similar studies reported either a protective benefit of sunscreen or no difference between the two groups. One obvious problem has been that several of the studies failed to control for skin type. Thus, sun-sensitive individuals tend to use sunscreens and are also at increased risk of melanoma. The problem now seems to have been solved in that two recent meta-analyses4 5 have found no association between melanoma and sunscreen use. This should reassure patients, particularly those with an increased risk of melanoma, of the wisdom of continuing to protect their skin adequately from the sun.
It is also advisable not to place too much reliance on sunscreen use as a sole method of reducing sun overexposure, and to remember to spend some of the time in the shade and/or covered with close woven clothing.
Conclusion
In the future it is likely that a greater understanding of the role of UVR in melanoma will emerge, which may allow for the development of more specifically protective, acceptable sunscreens for all. In the meantime, while the rates of melanoma and other skin cancers continue to increase and while our understanding of the aetiology remains unclear, it seems wise to continue to recommend the sensible sun protection measures as advised by our Queensland colleagues, though adapted to be appropriate for different climates.
References
- Lens MB, Dawes M. Global perspectives of contemporary trends of cutaneous melanoma. Brit J Dermatol 2004; 150:179–85.
- MacKie RM, Bray C, Vestey J et al. Melanoma incidence and mortality in Scotland 1979–2003. Brit J Cancer 2007; 96(11):1772–7.
- SIGN guideline no. 72. Cutaneous melanoma. Edinburgh: SIGN; 2003.
- Dennis LK, Beane Freeman LE, Van Beek MJ. Sunscreen use and the risk of melanoma: a quantitative review. Ann Intern Med 2003; 139:966–78.
- Huncharek M, Kupelnick B. Use of topical sunscreens and the risk of malignant melanoma. Am J Public Health 2002; 92:1173–7.
