Skin and Sun

Sunlight is energy, and it is essential for our lives. The sun activates our metabolism, stimulates the immune system and the production of hormones, and promotes our well-being. Consequently, light is used as a therapy for both the skin and the psyche (heliotherapy). Whether sunlight is healing or harmful depends solely on the right dose of ultraviolet rays.

Excessive exposure to sunlight causes sunburn as well as long-term skin damage and premature ageing of the skin. Acute and chronic damage to the skin from exposure to sunlight (photopathy) weakens the immune system and increases the risk of skin cancer. So it is essential to allow the skin to gradually acclimatise to the sun. This should be done over a period of about 3 to 4 weeks, with gradual tanning of the epidermis (outer skin) and thickening of the outer horny layer.

One of the positive effects of sunlight is the production of vitamin D by the body, though this is already achieved with very small doses of light, on a level far below that needed for a tan. Sunlight also activates the immune system and promotes a general feeling of physical well-being. This is why ultraviolet (UV) light is used for treating depression in the autumn and winter months (seasonal affective disorder – or SAD). This treatment is also effective without any tanning of the skin. However, a slight suntan can help make us feel happier and also raise our self-esteem.

The good mood that most of us feel in sunshine, and indeed in good weather in general, can be explained by the activation of the pineal gland, a small gland located in the brain. This responds to light and emits signals to the body that make us feel happier and arouse our energy and the urge to do things. With sunshine and blue sky, people tend to become positively invigorated.

The worsening depletion of the ozone layer – not only above the Antarctic – will lead to a marked increase in skin disorders, skin cancer and allergies of the respiratory organs. The ozone layer high up in the stratosphere protects us from too much UV radiation. Where the ozone layer is intact, it filters out virtually all the UVC rays from the sun, and also significantly reduces the amount of UVA and UVB radiation that reaches the earth. Of the sun’s energy that actually reaches the surface of the earth, about 65 % is infra-red radiation, 30 % visible light, and only about 5 % UVA and UVB radiation. It is practically only the ultraviolet light that affects our skin.

The ultraviolet radiation from the sun is made up of rays with 3 wavelengths: UVA light (with a wavelength of 320 to 400 nanometres (nm)), UVB light (with a wavelength of 280 to 320 nm), and UVC light (with a wavelength of 100 to 280 nm). These three kinds of radiation affect the skin in different ways.

UVA radiation has the longest wavelength, penetrates through to the dermis and causes browning of the skin within hours. Excessive UVA exposure leads to premature ageing of the skin by destroying the elastic and collagen fibres. UVA light also encourages the formation of skin cancer. One of the main reasons for this is that it weakens the skin’s immune defences. UVA radiation penetrates to a depth of about 1 mm.

UVB radiation is what causes sunburn and also delayed tanning. By directly damaging the genetic substance (DNA) in the cells of the epidermis, UVB is a major factor in the formation of skin cancer and its pre-stages. UVB radiation penetrates to a depth of about 0.5 mm.

The intensity of the UVA radiation varies only slightly over the year, reaching a modest peak in spring. UVB radiation, on the other hand, changes strongly in the course of the year, and depends on the time of day and year (height of the sun) and also on geographical latitude, i.e. the distance from the equator. The weather (cloud cover), reflection, e.g. from water, snow or sand, and altitude naturally also play a role.

UVC radiation is largely prevented by the ozone layer in the stratosphere from reaching the earth’s surface and therefore is not currently important.