"Here’s what happened.
When I was a kid, I played outside all the time. So did all my friends. We were outside, especially during the summer, from the time we got up until it was dark. Since we played outside most all the time, as summer approached and the suns rays became more direct, we had already developed the base of tan from being outside all during the spring when it was difficult to get sunburned. Our tans protected us from the effects of the sun, blocking both UVA and UVB light.
UVB rays are those that burn the skin and the ones that drive the synthesis of vitamin D. UVA rays are those that mobilize the melanin (the pigment in the skin) and bring it to the surface. When enough melanin comes to the surface, our skin gets darker, i.e., we develop a tan. The tan then protects us from the harmful effects of the sun, allowing us to stay out all day without getting a sunburn and without getting too much UVA, which is important since excess UVA exposure is thought to be the cause of melanoma.
Although many sunscreens available today claim to block both UVB and UVA, when today’s teens were young children, virtually all of the sunscreens on the market then blocked UVB only. Which is probably the root cause of the increase in melanoma in adolescents today. Here’s what happens.
People who don’t use sunscreens and who have good sense get out of the sun when they begin to burn. Avoiding the sun limits the exposure to both UVB, the burning rays, and UVA, the melanoma-stimulating rays. When people slather on sunscreen that blocks UVB only, they can then stay out in the sun for a long time without burning. The price they pay for this is that they end up with an extremely large dose of UVA, which doesn’t cause pain but sows the seeds for later melanoma development, a fate that has in the past befallen many a vacationer to the sunny areas of the world."
http://www.proteinpower.com/drmike/supplements/sunshine-superman/#more-4165
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