Mineral sunscreens build a physical protective layer on the skin's surface, providing immediate protection against UVA and UVB rays. Synthetic sunscreens, on the other hand, are absorbed into the skin and body and take 20-30 minutes to shield skin—and typically less effective against UVB rays...

Most sunscreens offer little to no protection against UVA rays. That SPF number you see on the bottle only tells you the UVB protection factor. While UVB rays are responsible for sunburn, UVA rays penetrate to the deeper layers of skin, and is a major factor in premature aging and skin cancer. Clearly, protection from UVA rays is a serious factor to consider when purchasing sunscreen...