Sunlight Is Responsible for the Development of Skin Cancer
Sunlight is a natural source of well being for a large number of people and an essential component of leisure activities. In addition, sunlight stimulates a variety of physiologic processes in the skin, including vitamin-D synthesis which is an important compound in bone metabolism. However, even in northern latitudes, very short sun exposure suffice for maintaining vitamin-D at normal levels. With respect to medical concerns, the ultraviolet (UV-) spectrum of sunlight is the most relevant, in particular the UV-B range of the spectrum with its shorter wavelength, which is responsible for the development (initiation) and growth proliferation (promotion) of skin cancer.
A large part of the UV-radiation is filtered by the ozone layer. In particular in the morning (until 11 am) and in the afternoon (after 3 pm) large parts of the spectrum will be filtered. On the other hand, the concentration of skin cancer inducing UV radiation is particular high in the time between 11 am and 3 pm. During these times any unprotected sun exposure should be avoided. Apparent protective filters, including the cover of a planket or the shadow of a tree, still allow between 80 percent and 50 percent of the light to pass, and are no reliable protection against sunlight.
Effects on the skin
The shorter wavelengths of UV-B radiation typically induce a sunburn and are the most effective in inducing an erythema (redness, sunburn) on the skin. The longer wavelengths of UVA radiation are less potent in inducing an erythema, and typically induce UV-tanning. UV-A radiation also penetrates deeper into the skin, and thereby damages the elastic fibers in the connective tissue. The consequences are increased wrinkeling, premature skin aging and pigment alterations (freckling, mottled pigmentation).
For a long time, UV-B radiation was considered the sole risk factor for the development of skin cancer by introducing genetic alterations into human DNA. In recent years however, evidence has been increased that UVA, by mechanisms including photooxidation and formation of free oxygen radicals may significantly contribute to cutaneous tumor formation. Besides, UV-A radiation is the major source of a variety of adverse skin reactions, often misinterpreted as ‚sun allergies’ or sun hypersensitivity.
Another aspect that is commonly neglected is the effect of UV-radiation on the weakening of the skin’s immune system. UV-radiation promotes bacterial, viral or fungal skin infections and significantly enhances the growth of skin cancer cells.
Types of Skin Alteration as a Result of Sun Exposure
Following moderate sun exposure several cutaneous repair mechanims are aimed at eliminating the arising molecular defects. A sunburn is the clinical visible sign of an acute overdosage of UVB radiation, resulting in an injury of large number of skin cells and a decompensation of inherent repair mechanisms.
Premature Aging (photoaging) of the Skin
Besides an induction of malignant skin tumors, UV-radiation also induces photoaging of the skin with increased wrinkeling, decreased elasticity (solar elastosis), and formation of small, red dilated blood vessels (telangiectasias) resulting in a ‚weather-beaten’, leather-like appearance of the sun exposed skin sites.
Generally reffered to as polymorphic light eruption (PLE), this condition may be induced by UVA and/or UVB radiation. Affected individuals present with erythema, hives, intense pruritus (itching) and papular/nodular eruptions on exposed skin sites following the first sun-exposures in early spring or summer.
The term mallorca acne or acne aestivalis (from latin aestivalis: summer) is an acneiform eruption of the pilosebaceous unit. This acne variant develops as an reaction of the skin to UV-radiation. Hereby, a number of small skin-colored to erythematous papules/nodules arise in the area of hair follicles, favoring the shoulders and upper back.
These characteristic inflammatory nodules are a result of of long-standing sun exposure and possibly occlusive effects of sunscreen products with a high lipid/oil content. While the clinical acneiform appearance has given rise to the name of this condition, the typical comedones or pustules of acne vulgaris are generally absent.