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Sunlight and Ultraviolet Radiation
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft x-rays. The “near” UV spectrum (200-400nm) is divided into three types of radiation, UV-C, UV-B, and UV-A rays, based upon wavelength:
UV-C: 200nm - 290nm
UV-B: 290nm - 320nm
UV-AII: 320nm - 340nm (short-wave UV-A)
UV-AI: 340nm - 400nm (long-wave UV-A)
Sunlight contains all three types of UV radiation. The ozone layer protects humans from UV-C radiation (which would otherwise destroy the skin) but not UV-B or UV-A radiation. This is because DNA absorbs wavelengths primarily in the UV-C range, and to a progressively lesser degree in the UV-B and UV-A ranges.
UV radiation has sufficient energy to break molecular DNA bonds and cause mutations. This is why the stratospheric ozone layer is so important. As atmospheric ozone concentrations decrease, the amount of UV-C (and UV-B) radiation reaching the earth increases, and the greater the risk for mutations, cell destruction, and subsequent cancer.
UV-B and UV-A radiation pass through the clouds and penetrate our skin producing skin cancers and photoaging; that is also why you can still get sunburned on a cloudy or hazy day. UV radiation also comes from the sunlamps and tanning beds. A person's risk of skin cancer is related to lifetime exposure to UV radiation. Most skin cancer appears after age 50, but the sun damages the skin from an early age.
When sunlight strikes our skin the shorter wavelength (290-320nm) UV-B rays are absorbed primarily by the epidermis. UV-B rays produce the early redness and burning seen during sun exposure. These rays cause direct DNA mutations, and generate oxygen free-radicals, that destroy and mutate cells and cause skin cancer. Protection against UV-B radiation is critical. All currently available sunscreens provide protection against UV-B rays. It was initially thought that if you if you prevent sunburn, you'd prevent skin cancer.
The longer wavelength (320-400nm) UV-A rays penetrate deeper into the dermis. These rays generate free radicals which damage collagen and elastin, and deplete the skin of protective antioxidants. UV-A and the aging process are intertwined. It is the long ultraviolet rays (UV-AI) that are responsible for photoaging, including wrinkles, dark blotches/dyspigmentation, freckles, a leathery texture, and loss of elasticity. UV-A rays are 20-times more prevalent than UV-B rays. Studies demonstrate that it takes relatively small amounts of repeated UV-A exposure to cause photoaging in human skin (only 9 moderate doses of UV-A are necessary before changes in the skin are evident. In the past, UV-A was thought to be less important than UV-B in the generation of sun damage and skin cancer, but it is now known that UV-A does indeed induce mutations in DNA, leading to skin cancers in both animal and human skin. Unfortunately, most currently available sunscreens don’t provide adequate protection against UV-A insults.
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Sunlight & UV rays
SPF Rating
Sunscreen Testing
Critical Wavelength
Sunscreen Ingredients
Photostability
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