How is it that some physical sunscreens having eg. 10% zinc oxide & 7% titanium dioxide are SPF 50, while others having eg. 19% zinc oxide & 4% titanium dioxide, are SPF 35 or lower?

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Shouldn’t the higher concentration of sunscreen actives have a higher SPF level? How does this work?

In: Chemistry

Only if the active stays put on your skin.

The rest of the formula is just as important. If it absorbs into your skin a different way then it may not last as long or reflect a different amount of UV light or protect a different depth of skin.