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UV radiation and sun protection and sterilization

Natural daylight and many artificial light sources carry a certain amount of ultraviolet radiation. The intensity of ultraviolet radiation is generally measured in standard power units, that is, the radiation flux is measured in watts, and the irradiance is measured in watts per square meter. The calculation method of ultraviolet radiation illuminance is the same as that of light illuminance.

For ease of reference, the International Commission on Illumination (CIE) divides the ultraviolet spectrum into three ranges: from 400 to 315 nanometers is called the near ultraviolet region (UV-A; from 315 to 280 nanometers is called the mid-ultraviolet region (UV-B); from 280 to 100 nanometers is called the far ultraviolet region (UV-C).

Subcutaneous occult blood radiation When human skin is exposed to ultraviolet rays with a wavelength shorter than about 320 nanometers, it will cause subcutaneous occult bleeding, that is, redness of the skin. Although appropriate exposure to the sun has therapeutic effects, long-term exposure can cause discomfort and even dangerous consequences. The conjunctival tissue of the eye is particularly sensitive to radiation that produces "occult blood". If you are frequently exposed to this radiation, you must wear goggles.

The exact cause of redness of the skin (subcutaneous occult bleeding) is still unclear, but it has been confirmed that it is related to the wavelength of radiation. The effect of occult blood is greatest at a wavelength of 297 nanometers and is highest at 270 to 280 nanometers. It is reduced to a minimum near microns and increases again at shorter wavelengths. When discussing radiation in a certain wavelength range, its "occult blood" effect can be compared with the maximum effect at a wavelength of 297 nanometers. For convenience, a unit of occult blood effect "E-Viton" is introduced. One E-Viton is equivalent to the radiation flux that produces the same occult blood effect as 10 microwatts of 297 nanometers of radiation. It takes 2500 seconds to produce minimally detectable subcutaneous occult blood when 10,000 E-Vitons of ultraviolet light per square meter is irradiated on untanned white skin. For 297 nanometers of radiation, this amount is equivalent to a therapeutic dose of 250 joules/square meter. The dose rate (or irradiance) in this example is 0.1 watts/square meter or 1E-Viton/square centimeter, called a "Finsen".

Sterilizing radiation Ultraviolet radiation with a wavelength less than 320 nanometers can kill microorganisms. In The bactericidal effect of radiation near 260 nanometers is the greatest, and its bactericidal effect is related to the type of microorganism to a certain extent. Low-pressure mercury vapor discharge can produce strong radiation with a wavelength of 253.7 nanometers, which is close to the wavelength of maximum bactericidal effect. Therefore, this type of light source is often used for sterilization. Sometimes a bactericidal irradiance unit (GU) is also used. This unit refers to a certain amount of irradiance, which produces a bactericidal effect equivalent to 1 watt/square meter of 253.7 nanometer radiation.

Except for very low irradiance, under constant intensity radiation irradiation, the number of remaining microorganisms obeys the exponential decay law, that is, the number of remaining microorganisms can decrease by the same multiple in each successive unit time. For example, when a certain Escherichia coli is irradiated with 1GU of radiation for 15 seconds, 1% of it remains. After irradiation for another 15 seconds, only 0.01% remains, and so on. Different microorganisms have very different sensitivities to bactericidal radiation, and the bactericidal effect is also affected by environmental conditions (such as humidity).

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