Here's why:
* UV-B: UVB radiation is the primary cause of sunburn. While it does contribute to tanning, it's much more damaging to the skin. It stimulates melanocytes (pigment-producing cells) to produce melanin (the pigment that causes tanning), but it also causes DNA damage, leading to mutations and skin cancer.
* UV-A: UVA penetrates deeper into the skin than UVB. It also stimulates melanin production. However, UVA primarily causes the oxidation of existing melanin in the skin, leading to a quicker but less lasting tan. It is also a significant contributor to premature aging of the skin. While less likely to cause sunburn than UVB, UVA still contributes to skin cancer risk, as well as increasing the formation of free radicals, which are also thought to contribute to the aging process.
Important Considerations:
* Tanning is a sign of skin damage: It's crucial to understand that any tan, regardless of how it's achieved, indicates that your skin has been damaged by UV radiation.
* Artificial tanning (tanning beds): Tanning beds primarily emit UVA radiation, which, as explained above, poses health risks. They are not a safe way to tan.
* Sunscreen: Broad-spectrum sunscreens protect against both UVA and UVB radiation. Use them liberally and reapply frequently when exposed to the sun.
In summary: UVA is more efficient at causing immediate tanning, but it also has health risks. UVB is responsible for sunburn and also has significant health risks. No amount of tanning is truly safe. Protect your skin from UV radiation to minimize the risk of skin damage, premature aging, and skin cancer.