Biological effects of ultraviolet light

January 14, 2021

Solar radiation is electromagnetic radiation from the sun. When solar radiation passes through the atmosphere, about half is reflected by the clouds, and the rest reaches the ground in the form of direct sunlight and scattered sunlight. Solar radiation includes visible light, infrared light, ultraviolet light, radio waves, X-rays, gamma rays, and the like. Reaching the earth's surface for the first three main wavelengths above 760 nm infrared, 760 to 390 nano meters line of sight, is less than 390 nm UV. The solar radiation reaching the ground is partially absorbed by the soil into heat and partially reflected back to the atmosphere. The reflectivity of various ground surfaces is also different, and the reflectivity of snow can be up to 80-90%.

Ultraviolet rays are classified into three categories according to their biological effects:
Ultraviolet A-stage (UV-A), with a wavelength of 320-400 nm (long-wave), has a weak biological effect, but can convert melanin in the skin into melanin by oxidation and deposit on the surface of the skin
. Melanin absorbs a variety of light and absorbs more of the short-wave radiation. The light energy absorbed by the pigment becomes heat energy, which increases the secretion of sweat, enhances local heat dissipation and protects the skin from overheating,
and prevents the light from penetrating deeply into the tissue to avoid overheating of the internal tissue.
Ultraviolet B segment (UV-B), wavelength 275 ~ 320 nm (medium wave), has a strong erythema effect and anti-caries effect. Ultraviolet light forms ergosterol and 7-dehydrocholesterol in human skin and subcutaneous tissue to
form vitamins D2 and D3. Many studies have pointed out that regardless of the prevention or treatment of rickets, the use of vitamin D alone is not as good as ultraviolet light. UV irradiation, due to the
expansion and epidermal cells of the skin capillaries is disrupted, releasing histamine and histamine-based, skin erythema.
Ultraviolet C segment (UV-C), wavelength 200 ~ 275 nm (short wave), it has a strong effect on the body cells, but also has a strong bactericidal ability. Can kill ordinary bacteria and viruses. The shorter the wavelength, the
better the bactericidal effect. The wavelength of 253.7 nanometers is the strongest. The ultraviolet wavelength in solar radiation is greater than 290 nm, so the sterilization effect is far less than that of ultraviolet light.
The ultraviolet C segment and most of the B segment in the solar radiation are absorbed by the stratospheric ozone layer, and the ultraviolet rays reaching the ground are mainly the A segment and the small portion B segment (>290 nm). The biological effect is stronger in the B segment
, and the A segment is only equivalent to 1 ‰ in the B segment. Due to the exposure of ultraviolet light, human skin begins to age from childhood, and after 20 years old, the face begins to show signs of aging, called "photoaging." In the photoaging, senile plaques and tumors are caused by segment B, and the formation of wrinkles is related to both segments A and B.

Molecular mechanism of the biological role of ultraviolet light Induced gene mutation: After 30 minutes in the summer, it can cause erythema on the skin. Segment B converts thymidine into redoxin-type dimer in reddened skin epithelial cells, which is the molecular basis for photodamage of skin by ultraviolet radiation and the initiator of skin cancer. This change causes both DNA damage and mutation of the tumor suppressor gene p53. It is found that 50%-90% of tumors in basal cell carcinoma or echinocytic carcinoma of the skin are caused by p53 mutations due to mutation of dimers in epithelial cells in the early stage of cancer.
2. Reactive oxygen: epithelial cells may produce ultraviolet radiation O2-, H2O2, OH -, -
OH and other active oxygen, the active oxygen can guanosine 8-like DNA damage, resulting in genetic variation factor.
3. Inhibition of immune response: Whitening mice can be exposed to ultraviolet light every 5-30 weeks for 20-30 weeks, and skin malignant tumors can occur. It is unsuccessful to transplant this tumor in the same line of mice, but
the B-stage ultraviolet rays one day before transplantation. After irradiation, not only the transplantation is successful but also the growth and proliferation. This indicates that ultraviolet irradiation can induce immunosuppression. The mechanism section B is an ultraviolet epidermal Langerhans cells subjected to damage, weakened immune presentation, T-cell reduction, inhibition of tumor rejection, promotion of skin cancer. Healthy people young and old can receive about 40% of immune suppression, immune suppression seen to occur in up to 95% of skin cancer patients.

The body's defense against ultraviolet light Melanin and stratum corneum: Melanin absorbs a variety of light, especially short-wave radiation, thus preventing UV rays from penetrating deep into the tissue. The incidence of black skin cancer is extremely low.
2. DNA repair function: normal human body has a certain repair function on damaged DNA. When the UV irradiation dose is not large, resulting in DNA damage repair does not exceed the scope of the body when the body can
repair the damage of DNA, which play a large role in the prevention of skin cancer. Patients with this function deficiency, such as pigmented xeroderma, have 2000 times more chance of developing skin cancer than normal people.
3. Reactive oxygen species elimination mechanism: Active oxygen formed by proper amount of ultraviolet radiation can be oxidized by non-enzymatic substances such as vitamin C, vitamin E, reduced glutathione (GSH) or superoxide dismutase
(SOD) in the body. Elimination, but excessive UV radiation to form excessive reactive oxygen exceeds the body's clearance rate, which will inevitably cause DNA damage.
4. Immune System: NK cells are both cellular immune surveillance devices and non-specific cells that attack cancer cells, which can eliminate a small number of cancer cells.

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