UVA is long-wave ultraviolet rays, divided into UVA-1 (360-400nm) and UVA-2 (320-360nm), accounting for about 10% to 20%. UVA exists as long as it is during the day, and can penetrate most clouds and glass, reaching directly Dermal layer of skin. Therefore, even if it is cloudy and rainy, UVA rays will not be reduced. It can reduce the content of water-binding hyaluronic acid in the skin, dry the skin, accelerate the formation of melanin, and darken the skin color. After reaching the dermis layer of the skin, it can destroy the collagen fibers and elastic fibers in the dermis layer of the skin, causing wrinkles and wrinkles on the skin. Aging is also an important cause of skin cancer.
The UVA protection index is generally expressed as PPD or PA. PPD is a multiple used by European and American systems to extend the time the skin is tanned by UVA. Generally, after the skin is exposed to ultraviolet UVA for 2 hours, the skin will still continue to tan. Using PPD2 sunscreen can effectively protect UVA for 2 hours. By analogy, it is a protection factor that is consistent with the actual prevention of dark spots. PA is used by the Japanese system to reflect the effect of UVA tanning. It is expressed by the number of plus signs: PA+ = PPD 2-4, PA++ = PPD 4-8, PA+++ = PPD>8. In other words, PA+ can protect UVA for 2 hours to 4 hours, and so on for each additional plus sign.
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