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When the UV index is presented on a daily basis, it represents UV intensity around the sun's highest point in the day, called solar noon, halfway between sunrise and sunset. This typically occurs between 11:30 and 12:30, or between 12:30 and 13:30 in areas where daylight saving time is being observed.
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The time of day. In most locations, the sun's rays are strongest between approximately 10 am and 4 pm daylight saving time. [19] Cloud cover. Clouds partially block UV, but even on an overcast day, a significant percentage of the sun's damaging UV radiation can pass through clouds. [20] [21]
[17] [18] [19] UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D. [20] Time of day, time of year, geographic latitude, ground altitude, cloud cover, smog, skin melanin content, and sunscreen are among the factors that greatly affect UV intensity and vitamin D synthesis, [19 ...
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Halogen lamps with fused quartz envelopes are used as inexpensive UV light sources in the near UV range, from 400 to 300 nm, in some scientific instruments. Due to its black-body spectrum a filament light bulb is a very inefficient ultraviolet source, emitting only a fraction of a percent of its energy as UV, as explained by the black body ...
UV-B, predominantly responsible for the harmful effects of sunlight, poses the highest risk for erythema, photokeratitis, sunburn and skin cancer. [16] [17] [18] While longer UV-C wavelengths and UV-A can also cause damage, their effects are less severe than UV-B. In contrast, far-UVC has shown remarkably different results.
Theoretically, a sunscreen with a PPD rating of 10 should allow a person 10 times as much UVA exposure as would be without protection. The PPD method is an in vivo test like SPF. In addition, the European Cosmetic and Perfumery Association ( Colipa ) has introduced a method that, it is claimed, can measure this in vitro and provide parity with ...