When zinc oxide and titanium dioxide are both present in a mineral sunscreen and it is applied as recommended, mineral sunscreen effectively blocks UVA and UVB rays from ever penetrating the skin, but not all of them. A sunscreen with an SPF of 30 or higher can block 97 percent of the sun’s UVB rays. A mineral sunscreen with only zinc oxide will only protect against UVA rays, and titanium dioxide helps prevent UVB ray absorption. No chemical or mineral sunscreen can block 100 percent of the sun’s UVB rays, which is why sun-safe clothing is a necessary measure to further protect the skin.