It is the consensus of medical opinion that the risks of skin cancer, including malignant melanoma, are increased by exposure to the
ultraviolet component of sunlight. This view is the basis of the public health campaign which urges people to protect themselves from the sun using high-factor sunscreens whenever they expose themselves. The reason for the campaign is that the incidence of melanoma in the UK has increased steadily since the 1930s.
But is this campaign well-founded? Melanoma is in fact more common in indoor than in outdoor workers, in contrast to other types of skin cancer (J of Royal College of Physicians, April 1987). The simplest explanation of this would be that UV exposure actually reduces the risk of melanoma. Nevertheless, scientists argue that the increase in melanoma is "probably" due to intermittent high exposure to sunlight rather than chronic exposure. This argument is supported by the observation that only since the 1920s have people wanted suntans; before that, a pale skin was a sign of higher social class, and only outdoor workers were suntanned. This social trend seems to coincide with the serious increase in melanoma over the same period.
What is the evidence? Sorahan and Grimley carried out a study (Br J Cancer, 1985; 52: 765-769) of 58 cases of malignant melanoma to test whether the disease was associated with "taking holidays abroad in a hot climate" and other possible factors. They found a statistically significant association with the type of skin that burns easily, bouts of painful sunburn and a number of moles but no such association with intense, intermittent exposure during holidays.
This finding is not mentioned in the summary of their paper; clearly they are embarrassed by their failure to confirm the consensus view. Scientists reacted to this finding, not by questioning the consensus, but by inserting the word "probably" into all their assertions about the etiology of melanoma.
Scientists forget that the human body evolved in sunny conditions, long before we started wearing clothes or living in houses.
An alternative hypothesis is that the effect of UV on the body is non-linear; that very high and low exposures can both be harmful, and that moderate exposure is healthy. This implies that the effect of UV is complex, with both helpful and harmful components. Some helpful effects are well known: UV is essential to the synthesis of vitamin D in the body, but most scientists see this as relatively unimportant, because vitamin D can also be obtained from the diet. Helpful effects of sunlight on cancer have not been much explored.
One scientist who did consider this possibility was Dr S Peller, who analyzed all cancer deaths in the US Navy in the years 1929-1946 (American Journal of Medical Science, 1937; 194: 326-33). He found that Navy personnel on active service had three times the skin cancer mortality of the civil US population of comparable ages, but only 56 per cent of the total death rate from cancer. He saw an obvious connection with the greater amount of sunlight to which men serving on the Navy at that time were exposed. This study strongly suggests that UV does have a duel effect, with the helpful component predominating in internal cancers, and the harmful component in skin cancers.
If this is true, the current health education campaign may be misguided. If people avoid the sun and use high-factor sunscreens, the effect may be to increase the incidence of other fatal cancers.
How did connections between increased UV exposure and increased incidence of melanoma come about? The genesis of the campaign was the melanoma research by Dr Rona MacKie in Scotland, and its welcome by members of the British Photodermatology Group. Dermatologists see patients with many skin complaints that are clearly related to UV exposure, though some of these same complaints also respond to treatment with narrow-range UV radiation, either UVA or UVB. Dermatologists are not concerned with internal cancers; their interest in the body is literally skin-deep. Dr MacKie's 1987 paper was guarded about the etiology of melanoma, but recommended sunscreens for people with the type of skin that burns easily, mainly because some melanoma patients had a history of bad sunburn. The paper was issued as a report of the Royal College of Physicians.
The cause was then taken up by the Cancer Research Campaign. Their pamphlet is based quite closely on Dr MacKie's paper, even saying "we do not know whether prolonged exposure to UVA is linked with melanoma", but it puts across a much stronger message. It recommends sunscreens without reservation, adding the assertion "although UVA does not burn the skin, it can do hidden damage inside it".
Most doctors and scientists know little about skin cancer, and have
not read any of the original papers. Scientific knowledge of the etiology of skin cancer is incomplete. If there is a consensus, it has not come about through the impersonal dissemination of scientific data, but through an essentially political process doctors have been persuaded to adopt the opinions of a small group of dermatologists as proven facts. The Cancer Research Campaign has committed extensive resources to promote this view, and has incidentally gained much useful publicity for itself.
The danger is that assumptions about the harmful effects of UV are now limiting the scope by narrowing the range of questions that people ask, and blocking other avenues which could be fruitful. Do we know so much about cancer that we can afford to limit our research in this way?