Summer, with its bright sun, alleviates psychological problems, creates optimism, fills up with cheering voices, gives Vitamin D in large quantities, but also creates skin problems, the largest organ of our body, with different characteristics: thick and resistant under the soles of the feet, thin and sensitive on the eyelids and supple at the joints. It consists of three tissues: superficial epidermis, intermediate dermis and underlying hypodermis.
Peculiarities of the skin
The epidermis is renewed every 28 days. The process is as follows: the keratinocytes migrate from the deeper layers to the surface, lose their nucleus, die and form the stratum corneum. The basal layer, deeper in contact with the dermis, is rich in melanocytes, branched cells or dendrites that cross the cells and reach the skin surface. The dermis or cutis nourishes the epidermis for its rich vascularization, innervation and fibers. Fibroblasts synthesize collagen and elastin, substances that support the epidermis and skin appendages.
The skin, like the chlorophyll photosynthesis of plants, has a direct relationship with the sun's rays. The skin-rays interaction is a phenomenon that is best understood by knowing the nature of sunlight.
Physical nature of sunlight
The sunlight that we see hides six types of radiation not captured by the human eye: cosmic rays, gamma rays, X-rays, ultraviolet radiation, infrared radiation and radiofrequency radiation. They are electromagnetic waves, differing in wavelength, the distance measured in manometers (nm) between two peaks. The visible light spectrum, exemplified in the colors of the rainbow from red to violet, is between 400 and 800 nm. Ultraviolet rays, called UV, are between 100 and 400 nm. Ultraviolet rays are divided into UVA, UVB, UVC.
UVA, UVB and UVC radiation
UVA radiations, 320 – 400 nm, with the longest wavelength and least energy, reach the earth's surface for 99%, penetrate more deeply than UVB, reach the dermis, second layer after the epidermis, cause pigmentation direct, i.e. the release of the melanin present in the melanocytes of the skin and are absorbed. The result is direct pigmentation, called Meyrowsky's phenomenon.
UVB radiations, 290 – 320 nm, lower wavelength and higher energy than UVA and in proportion of the 1% of ultraviolet rays, reach the epidermis, first layer of the skin, stimulate melanogenesis, production of new melanin in melanocytes, transport large amount of energy and cause erythema, nucleic acid degradation and photoaging. UVC radiations, 100 – 290 nm, lower wavelength and higher energy and incompatible with life, are absorbed in the transformation of oxygen into ozone and allow life on the earth's surface. They are used in the sterilization of surgical equipment, in large air conditioning systems to purify the air and in aqueducts to eliminate traces of organic matter.
Relationships between ultraviolet rays and geoclimatic factors
There is a direct relationship between ultraviolet rays, 99% UVA and 1% UVB, and the height of the sun, governed by the time of day. The maximum intensity is reached from 11 to 15, for the shortest journey made by the sun's rays to reach the ground. The UVBs, less affected by deflection, reflection and weakening, are more
