Sunscreens Don't Provide the Protection They Claim
While sunscreens are generally effective at filtering out ultraviolet rays from the sun that cause sunburn (UVB rays), they don't work as well at filtering UVA rays, which penetrate the skin deeply and are more likely to cause skin cancer and wrinkles.
Misleading or confusing product labels are partly to blame for the widespread misinformation regarding sunscreens. In 1999, the Food and Drug Administration (FDA) said stricter rules for sunscreen testing and labeling would take effect in 2001, but this was put on hold indefinitely. In the fall of 2005, Congress ordered the FDA to produce the revised regulations within six months.
If you accept the premise that it is good to use sunblcok, which is typically not true, then many commercial sunblocks are not delivering what they promise.
nearly every sunblock needs to be applied every 45 minutes
clipped from: www.mercola.com