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Sunlight is composed of a spectrum of various
visible and invisible typesole radiation with wavelengths extending
from 280-3000 nm. Radiation emitted by the sun is partly absorbed
by the ozone layer, by clouds and by air pollution.
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UVA rays penetrate into
the connective tissue and are the primary cause of immunosuppression
and chronic light-induced damage such as premature skin ageing. They
are also responsible for the formation of free radicals and for phototoxic
as well as photoallergic reactions.
UVB rays penetrate into the deepest cell layers of the epidermis and
are the main cause of sunburn, DNA-damage and the development of skin
cancer. Warming infrared rays and the rays of visible light play no
role in these effects. |
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The most significant with
respect to pathological damage to the skin are the invisible UVA and
UVB rays, which stimulate the body's own photoprotective mechanisms
in the different skin layers. According to leading photobiologists
the short-wave UVC radiation, which has strongly erythematogenic and
carcinogenic effects on human skin, is completely absorbed in the
stratosphere and atmosphere and never reaches the earth's surface. |
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The different parts of
the solar spectrum reach the earth in different ways, some directly
and some indirectly after being scattered in the different layers
of the earth's atmosphere. The scattering is dependent on the wavelength:
the shorter the wavelength, the greater the scattering. Only small
doses of sunshine are needed to have positive effects on the human
organism. The damage from UV radiation is also lessened with less
exposure.
UV rays and their effect on the skin
As wavelength increases, there is a corresponding percentage increase
in both the amount of penetrating radiation and the depth of penetration.
At 300 nm in the UVB region, 10 percent of the radiation still
reaches the basal cell layer of the skin.
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Sunburn |
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Distribution of Langerhans cells (dark-coloured)
A without irradiation
B 2 days after sun-simulator irradiation |
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The highly erythematogenic
UVB rays are the main cause of DNA damage and the most common acute,
light-induced damage, sunburn (erythema solare). They are additionally
responsible for epidermal changes in the context of chronic, light-induced
damage.
The skin burdening UVB portion of sunlight is dependent on geographical
factors such as latitude and altitude (alpine, coastal regions etc.),
as well as on the time of day and the level of air pollution. The
thinning of the ozone layer in particular has led to a rise in UVB
radiation.
UVB radiation acts as a local immunosuppressant by damaging the Langerhans`
cells, which are responsible for antigen presentation in the epidermis.
Langerhans` cells react to UV radiation by leaving the epidermis.
Immunological studies on persons subjected to extended UVB irradiation
show additional, systemic immunosuppression. It is assumed that immunosuppressive
elements of the keratinocytes in the epidermis are released to the
immune system.
UVA rays penetrate deeply into the connective
tissue and cause changes in the dermis which can result in irreversible
long turn damage, e.g. premature skin-ageing (photoageing).
In addition, they are primarily responsible for the formation of cell-damaging
free radicals.
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Aged Skin
Photoageing: premature skin ageing primarily
due to chronic UV irradiation. |
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Further
dermatological implications of UVA radiation:
The most important trigger of sun allergy - polymorphous
light eruption (PLE) is the most common sun-induced skin disease.
The formation of free radicals through UV radiation is considered
one of the causes of this disease, which occurs mostly in young women.
Photoallergic
and phototoxic skin reactions
Photoallergic and phototoxic reactions may be triggered by chemical
substances including certain active ingredients of sunscreen and skincare
products under the influence of sunlight or by the use of certain
medicines while staying in the sun.
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Formation
of free radicals (oxidants)
UV radiation, particularly UVA, triggers skin damage through the formation
of free radicals. Free radicals are chemical compounds with free electrons
that are highly reactive. Free radicals damage the cells of the epidermis
and dermis. The cumulation of these processes, over years, gradually
leads to the signs of chronic light-induced damage. They are also
considered triggers for polymorphic light eruption (PLE).
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Action of radical scavengers
A Free radicals
B Radical scavengers |
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