Many epidemiological studies have implicated solar radiation as a cause of skin cancer (both melanotic and non-melanotic) in fair-skinned humans (146,147). Non-melanotic skin cancers are of two major histological types: basal cell carcinoma and squamous cell carcinoma. The risk of these cancers has generally been thought to correlate with cumulative lifetime exposure to solar radiation. Nevertheless, recent evidence suggests that the relationship is more complex. At least for basal cell carcinoma, childhood exposure may be important (148-150). These types of cancer usually develop on the
Table 5. The main effects of solar ultraviolet radiation on the health of humans
Nature of effect
Direction of effect
Effect on immunity and infection Suppression of cell-mediated immunity Increased susceptibility to infection Impairment of prophylactic immunization Activation of latent virus infections
Harmful (?) Harmful Harmful Harmful
Sufficient Sufficient Inadequate Sufficient
Effects on the eye
Acute photokeratitis and photoconjunctivitis Harmful Sufficient
Climatic droplet keratopathy Harmful Limited
Pterygium Harmful Limited
Cancer of the conjunctiva Harmful Inadequate
Lens opacity (cataract) Harmful Limited
Uveal melanoma Harmful Limited
Acute solar retinopathy Harmful Sufficient (?)
Macular degeneration Harmful Inadequate
Effects on the skin Malignant melanoma Non-melanocytic skin cancer Sunburn
Chronic sun damage Photodermatoses
Harmful Harmful Harmful Harmful Harmful
Other direct effects
Vitamin D production Beneficial
Other types of cancer Beneficial
General wellbeing Beneficial
Effects on climate, food supply, disease vectors, Probably air pollution, etc. harmful
Source: Armstrong (144).
parts of the body most often exposed to sunlight (such as the face, neck, scalp, hands and arms) (151,152).
Malignant melanoma, a cancer of the pigment-producing cells of the skin, usually develops on an already pigmented patch such as a mole (153-155). The relationship between melanoma skin cancer and ultraviolet radiation is complex. Overall, 60-90% of melanoma cases in fair-skinned populations are estimated to involve sunlight exposure (156). Repeated severe sunburn episodes in early life are considered important for the development of melanoma. The incidence of melanoma in fair-skinned populations has risen by 3-7% every year since at least the 1960s, which probably reflects a progressive increase in the average levels of personal exposure to solar radiation, owing to changes in the patterns of settlement, recreation, clothing and occupation that are unrelated to stratospheric ozone depletion (157).
Estimates have been made of how ozone depletion may affect the rate of skin cancer in certain countries. The model developed by Slaper et al. (158) takes into account the production and emission of ozone-depleting substances, the global stratospheric concentrations of chlorine, the local depletion of stratospheric ozone, the resulting increases in ultraviolet B levels and, finally, the effects on skin cancer rates. Several delay mechanisms in the effect of ozone depletion on skin cancer rates are important, such as the time taken to develop tumours. In the case of ozone depletion, the separate scenarios modelled are related to the Montreal Protocol, the international agreement restricting the production of ozone-depleting substances. Thus, full compliance with the Montreal Protocol and all its amendments and adjustments would lead to a peak in stratospheric chlorine concentration and ozone depletion by about 2000 (Fig. 6) and to a peak in skin cancer by about 2050. The latter is mainly delayed because skin cancer incidence depends on the cumulative ultraviolet B exposure.
The most recent assessment by the United Nations Environment Programme (54) has updated the above projections for total skin cancer for a "European" population living at a latitude around 45 °N. It estimates that, under the amended Montreal Protocol, an excess incidence will peak at about 5% during the third quarter of the 21st century. This means an extra 100 cases of skin cancer per million population per year from stratospheric ozone depletion. The current background rate of skin cancer is about 2000 cases of skin cancer per million population per year. If the moderate aging of the "European" population were factored into the modelling, the excess incidence would become, proportionally, a little higher. The calculations by the United Nations Environment Programme assume that behavioural and demographic risk factors do not change and that the current rate of ozone depletion and increases in exposure to ultraviolet radiation are sustained during the next several decades.
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Complete Guide to Preventing Skin Cancer. We all know enough to fear the name, just as we do the words tumor and malignant. But apart from that, most of us know very little at all about cancer, especially skin cancer in itself. If I were to ask you to tell me about skin cancer right now, what would you say? Apart from the fact that its a cancer on the skin, that is.