Sunscreen stops damage by blocking UV rays from reaching and penetrating the skin. There are two types of sunscreen ingredients: mineral and chemical.
Mineral ingredients, namely zinc oxide and titanium dioxide, act as a physical barrier, reflecting the ultraviolet light off the skin. (Our eyes can detect that reflection of light, which is why these sunscreens appear to leave a white cast.) Chemical ingredients — such as avobenzone, oxybenzone and homosalate — absorb the UV rays. Both types of sunscreen can degrade or wash off over time, so it’s important to reapply every two hours, and more frequently if you’re swimming or sweating.
In general, sunscreens are more effective at stopping the shorter UVB rays than the longer UVA rays. Sun protection factor, or SPF, only refers to how well the sunscreen prevents a sunburn, meaning how well it blocks UVB.
Dr. Frey said that just a few ingredients approved for use in the United States — namely avobenzone, oxybenzone, zinc oxide and, to a lesser degree, titanium dioxide — block UVA. To make sure a sunscreen provides UVA protection, look for one with these ingredients, or check that it’s labeled “broad spectrum,” which means that it stops the longer wavelengths from penetrating. (Newer ingredients available in Europe are more effective against UVA; some are in the pipeline for review by the Food and Drug Administration.)
When it comes to choosing the best sunscreen to prevent signs of aging, Dr. Von Schuckmann advised looking for one that’s broad spectrum and SPF 50 or higher; whether it’s a chemical or mineral formulation doesn’t matter. The most important thing, she added, is that it “actually goes onto the skin every single morning.”