The sun is the source of all life on Earth. Heat, light and warmth from the solar furnace sustain us all. There is a price to pay, however: the cumulative effects of invisible rays on our skin.
In parts of the world where the sun is very strong the local people usually avoid exposure, particularly when the sun is high in the sky. For example, people in tropical Africa do not stay out of doors for long if they can help it. Many of them feel uncomfortable in the sun's glare, complaining of feeling unbearably hot, possibly because of the vast heat production by the free radicals formed.
Health effects of sunlight
A whole travel industry was founded on people's desire to get away from more miserable climates to the sun, and to acquire a tan at almost any cost. Yet this very sunlight is arguably damaging to the skin in the short term and a significant health hazard in the long term - the actual benefits of sun exposure are few indeed.
Daily routine exposure to the sun - even in very small doses and in more cloudy climates -can lead to long-term effects that increase our apparent age.
The part of the sunlight that causes most damage to the skin is called ultraviolet radiation (UVR). For convention and completly arbitarly, the UV spectrum has been divided into UVA,UVB and UVC (see diagram).
Some exposure to ultraviolet B is necessary, since it is essential for vitamin D production in the skin. The amount required is tiny, however, and 15 minutes a day is probably sufficient even in cloudy countries. Occasional exposure to visible sunlight is believed to enhance psychological well-being. For many years the treatment of skin disorders such as psoriasis has relied on deliberate controlled exposure to solar simulated sunlight and to UVA lamps.
In spite of these benefits, however, the UV radiation from the sun is the environmental factor that is overall most damaging to the skin. People who have not been over-exposed to the sun for many years will tend to have pale and unmarked skin; even those with significant pigment will look paler than their kin. Sun worshippers will look very different.
Sun tanning and sunburn
Sun tanning is a response to the damaging effects of ultraviolet radiation. In people with pale skins sunlight stimulates the melanocytes to increase melanin pigment production, and also increases the transfer of melanosomes to keratinocytes. This melanocyte response to sunlight results in tanning, and by dramatically increasing melanin production provide an immediate and important defence for the nuclei of the skin cells.
Initially, acutely sun-damaged skin develops a thickened epidermis. This is caused by faster cell renewal, which is part of the immediate defence mechanism of the skin. The epidermis will return to normal provided the skin is not repeatedly over-exposed.
As nearly everyone knows, acute over-exposure to the sun results in sunburn. Intense redness is produced by increased blood flow due to the release of chemicals by damaged cells.
Three types of ultraviolet radiation
- Ultraviolet C (UVC, 100-290 nm) are the shortest and most energetic portion of the UV spectrum. These highly energetic wavelengths are the most dangerous in terms of the damage it can inflict on living material. The important wavelengths in the UVC are removed within the atmosphere, mainly by absorption in the ozone layer and not reach the earth's surface in any quantity.
- Ultraviolet B (UVB, 290-320nm) is the most damaging part of UVR that we encounter. It is currently thought to generate most of the photodamage to skin, though not all. UVB are wavelengths mostly blocked by dense clouds, closely woven clothing and glass window panes. Significant amounts are transmitted from blue sky in the middle of the day in summer. It is less dangerous when the sun is low in the sky, at high latitude in winter, and in early mornings and late evenings in summer.
- Ultraviolet A (UVA, 320-400nm) is about 1000 times less damaging to the skin than UVB as measured by sunburn (Erythema) or damage to cell DNA. On the other hand, 20 times more UVA than UVB reaches the earth in the middle of a summer's day. It is not greatly affected by absorption and scattering in the atmosphere when the sun is low in the sky, and is now known to contribute significantly to the total exposure at moderate levels throughout the whole day and year. UVA penetrates deeper into the skin and leads to deeper damage than UVB does. It penetrates cloud cover, light clothing and untinted glass relatively easily, and may induce a degree of continuing skin damage over long periods, even when UVR exposure is not obvious.
The spectrum of ultraviolet radiation
The depth of penetration of the skin by UV radiation of different wavelengths: UVB mainly affects the epidermis, while UVA penetrates deeper into the dermis.
How skin pigmentation (represented by green coloration) develops during a holiday in a sun resort.
Sun tan is a defence against the sun which arises as a result of UV induced skin damage. These two forearms reflect
the difference in daily exposure to the sun over two weeks.
The epidermis, particularly the stratum corneum, is not able to control water effectively. Fortunately this can be rapidly remedied, especially if a good-quality moisturizer is used and further exposure avoided.
Constant exposure to sunlight causes the melanocytes to become chronically over-active, resulting in areas of excessive melanin in the skin. These form brown spots called freckles.
Overproduction of melanin in localized areas causes freckles.
Eventually, areas of damaged skin made up of increased numbers of melanocytes and increased melanin synthesis develop. These are called solar lentigines, and are the result of a lifetime of sun exposure.
A section through badly sun-damaged skin. In skin like this the epidermis thickens and the skin may become leathery.
Photoageing Is a slow process resulting from chronic exposure to solar UV. It proceeds for several decades before it becomes obvious. The degree of photoaging is determined by the skin type and by the total lifetime sun exposure. People who spend their lives almost entirely indoors may show very little sun-induced skin damage.
Moreover, the degree of damage to tissues in different regions of the body is directly proportional to the amount of sunlight received. Everyone is familiar, for example, with the sharp boundary between exposed and unexposed skin that often develops in the V-area of the neck.
In chronically sun-damaged skin the epidermis as a whole becomes thicker, and loses some of its undulations. In young people the thickness of the epidermis is between 35 and 50 micrometres. At the age of 70 in skin that has been protected it could be expected to be between 25 and 40 micrometres. In sun-damaged skin it may be much more than this.
The stratum corneum, on the other hand, does not change very much, except that the cells within it (the corneocytes) increase still more in surface area and may become even thinner. The speed at which cells are replaced slows down, and some of the functions of skin, including controlling water loss, may become less efficient. There is less elasticity and increased fragility. Skin becomes dry, flaky and less reflective of light.
As photoaging begins, the small blood capillaries in the dermis decrease in number and the remaining blood vessels become tortuous and dilated. The elastic fibres degenerate, in a process called elastosis, producing a thickened mass that replaces the collagen.
A steady accumulation of damaged elastic fibres is accompanied by deterioration of the surrounding tissues until a blue-grey tangle of swollen, degenerate fibres, quite unlike normal skin, develops in the upper part of the dermis. hese changes in the elastic fibres are not seen in skin that has always been protected, even in very old people. Curiously enough, the changes can be quite advanced before the extent of the damage becomes visible.
Section through skin taken from the arm (a 'biopsy'), showing the long-term results of collagen and elastin disorganisation - elastosis and chronic thickening of the epidermis.
Seriously photoaged skin is dry, deeply wrinkled, yellow and rough. It may be marked with darkly pigmented or whitish spots, which respectively show where levels of pigment are higher or lower than normal. With increasing sun damage small blood vessels in the dermis will become more obvious and will form the red, finely branching, spider-like marks ('broken veins') that doctors call telangiectases. These blood vessels are easily damaged, resulting in greater fragility of the skin, with the development of spots called purpura.
Loss of elastic fibres around the blood vessels of the lower lips and ears - areas especially sensitive to chronic sun damage - may result in dilated veins. On the other hand, in protected skin the vessels tend not to be so dilated or damaged.
At its worst, skin that has been over-exposed to the sun for many years looks like old leather. Constant exposure to UVR over many years can result in warty spots on the skin, called actinic keratoses. These are found most often in people with type I and type II skins, although even people with type VI skins can develop them, and then only on the areas of the skin exposed to the sun. The appearance of actinic keratoses means that the skin has received far too much sun and could develop a skin cancer eventually.
Skin Cancer occurs as non melonoma and melanoma. The common skin cancers are often seen in older people. Basal cell carcinomas, are seen on the face, especially in the area around the eyes and the nose. They represent the largest number of cases of all cancers in humans. Squamous cell carcinomas are more aggressive and invasive.They usually occur on the back of the hands, on the ears or on the edges of the lips.
Actinic keratoses and basal and squamous cell carcinomas occur mainly on exposed areas of the skin, and are the result of excessive sun exposure over many years, often commencing in childhood.
Melanomas are the best known type of skin cancer. They are increasingly recognised by the public as a potential health hazard, especially if a mole changes shape or color or begins to bleed, when they may become what is called a malignant melanoma.
Look out for these four changes in a deeply pigmented mole:
Asymmetry - has it become more irregular in shape?
Border - has it increased rapidly?
Colour - has it darkened noticeably?
Diameter - has it increased in size?
If you have a skin mark like this, and notice changes in it, see your doctor immediately.
Squamous cell carcinomas: these are aggressive cancers and can spread.
Basal cell carcinomas on the face and scalp are the commonest human cancer. They are caused by sun exposure, often commencing in childhood. This type of cancer would develop in almost all of us if UVR levels were to increase substantially.