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Skin Cancers and Diet
Part 2: Skin cancers — And Ultraviolet radiation
All types of skin cancer are attributed to exposure to the ultraviolet (UV) part of the spectrum of sunlight. UV is classified as three distinct wavebands: A, B and C. They are all believed to contribute to the development of skin cancer.
- UVA rays constitute between 90% and 95% of the ultraviolet light that reaches the earth as it is not absorbed by the ozone layer. UVA light penetrates furthest into the skin and is involved in the initial stages of sun-tanning. UVA tends to suppress the immune function and is implicated in premature aging of the skin. [i]
- UVB rays are partially absorbed by the ozone layer and by the atmosphere. They do not penetrate the skin as far as the UVA rays but are the primary cause of sunburn. UVB is blamed for cataract formation. But UVB also our primary source of Vitamin D.
- UVC rays are the most harmful, but are almost completely absorbed by the ozone layer.
How strong is the evidence linking exposure to sunlight with melanoma?
During the 1980s and early 1990s more than a dozen studies compared histories of sunburn in patients with melanoma and people without it. The most complete data on melanoma and sunburn came in 6 studies from Australia, Europe and North America.[ii] They found the sunlight/melanoma link was unconvincing; while there was a suggestion of an association, the effect was modest; and they emphasised that brief periods of exposure seemed more risky than constant exposure.
Other clinicians agree. Pointing out that melanoma can be found on ovaries, occurs less frequently on sun-exposed areas, that there is 5 times more melanoma in Scotland on the feet than on the hands, that in Japan 40% of melanomas on the feet are on the soles of the feet, and that there is 10 times more melanoma in Orkney and Shetland than in the Mediterranean islands, Karnauchow writes: 'The simplistic idea of a sun/melanoma relationship is based more on a belief than science. . . . As with other neoplasms [cancers], the cause of melanoma remains an enigma and most probably the sun has little, if anything, to do with it.'[iii]
Newcastle dermatology professor, Sam Shuster, isn't convinced either. He states that the main reason for the supposed increase in melanomas was a change in diagnostic beliefs: lesions previously regarded as benign became classified first as dubious then as malignant. 'Melanomas are being invented, not found,' he says, 'Exposure to screening and pigmented lesion clinics is a greater cause of melanoma than sun exposure.'[iv]
Dr Anne Kricker and colleagues, looking at studies into skin cancer other than malignant melanoma and exposure to sunlight, also say that the evidence linking skin cancers with sun exposure is weak. They note that most studies have not found statistically significant positive associations, while the few that have lacked empirical evidence that sun exposure was the cause.[v]
The sunscreen connection
But why has there been such an enormous increase in skin cancer recently?
The Australian experience might provide the first clue. Queensland doctors have vigorously promoted the use of sunscreens for many years and, today, Queensland has more cases of melanoma per head of population than any other place in the world.
The numbers of cases of melanoma have risen especially steeply since the mid-1970s. The two principal strategies for reduction of risk of skin cancers during this period were sun avoidance and use of chemical sunscreens. Rising trends in the incidence of and mortality from melanoma have continued since the 1970s and 1980s, when sunscreens with high sun protection factors became widely used.
Significantly, the rises in malignant melanoma followed the rising use of sunscreens.
Drs Cedric and Frank Garland of the University of California are the foremost opponents of the use of chemical sunscreens. They point out that the greatest rises in melanoma are in countries where chemical sunscreens have been heavily promoted,[vi] and add that, while sunscreens do protect against sunburn, there is no scientific proof that they protect against melanoma or basal cell carcinoma in humans. Indeed, the Garland brothers strongly believe that the increased use of chemical sunscreens is the primary cause of the skin cancer epidemic. Recent studies by them have shown a higher rate of melanoma among men who regularly use sunscreens and a higher rate of basal cell carcinoma among women using sunscreens.[vii] This was confirmed by another study group who found that 'always users' of sunscreens had 3.7 times as many malignant melanomas as those 'never using'. The reasons why chemical sunscreens may be dangerous are several:
- Chemical sunscreens do not stop UVA rays. UVA penetrates deeper into the skin where it is strongly absorbed by the melanocytes which are involved not only in the production of the skin-tanning pigment, melanin, but also in the formation of melanoma.20 UVA rays also have a depressing effect on the immune system.[viii]
- More importantly, however, may be the fact that most chemical sunscreens contain up to 5% of benzophenone or its derivatives, oxybenzone or benzophenone-3, as their active ingredient. Benzophenone, used in industrial processes to initiate chemical reactions and promote cross-linking,[ix] is one of the most powerful free radical generators known to man. Moreover, benzophenone is activated by ultraviolet light.
- Harvard Medical School researchers also discovered that psoralen, another UV-activated free radical generator, is an extremely efficient carcinogen. The rate of SCC among patients with psoriasis, who had been repeatedly treated with UVA light after an application of psoralen to their skin, was 83 times higher than among the general population.[x] This added weight to a study in 1991-2, in which scientists at the European Organisation for Research and Treatment of Cancer (EORTC) found that regular use of sunscreens increased cancer risk by 50% but sunscreens containing psoralen multiplied the risk by 228%. They also showed that in people with a poor ability to tan, psoralen users had almost four-and-a-half times the risk of melanoma compared to regular sunscreen users.[xi]
References
[i]. Fitzpatrick TB, Haynes HA. Photosensitivity and other reactions to light. In Harrison's Principles of Internal Medicine. 7th ed, McGraw-Hill, 1974, 281-84.
[ii]. Marks R, Whiteman D. Sunburn and melanoma: how strong is the evidence? BMJ 1994; 308: 75-6.
[iii]. Karnauchow PN. Melanoma and sun exposure. Lancet 1995; 346: 915.
[iv]. Shuster S. Melanoma and sun exposure. Lancet 1995; 346: 1224.
[v]. Kricker A, et al. Sun exposure and non-melanocytic skin cancer. Cancer Causes and Controls 1994; 5: 367-392.
[vi]. Garland CF, et al. Could sunscreens increase melanoma risk?
Am J Publ Hlth 1992; 82: 614-15.
[vii]. Garland CF, et al. Effect of sunscreens on UV radiation-induced enhancement of melanoma growth in mice.
J Natl Cancer Inst 1994; 86: 798-801
[viii]. Fuller CJ, et al. Effect of beta-carotene supplementation on photosuppression of delayed-type hypersensitivity in normal young men. Am J Clin Nutr 1992; 56: 684-90.
[ix]. Kirk-Othmer. Encyclopedia of Chemical Technology. 1981; 13: 367-68.
[x]. Stern RS, Laid N. The carcinogenic risk of treatments for severe psoriasis. Cancer 1994; 73: 2759-64.
[xi]. Autier P, et al. Melanoma and use of sunscreens: an EORTC case-control study in Germany, Belgium and France. Int J Cancer 1995; 61: 749-55.
