Sunscreen Science: How SPF Works and What Really Protects Your Skin

When you put on sunscreen, a topical product designed to absorb or reflect ultraviolet radiation to prevent skin damage. Also known as solar cream, it's not just for beach days—it's a daily shield against premature aging and skin cancer. Most people think SPF 30 is enough, but sunscreen science shows that how you apply it matters more than the number on the bottle. A study from the Journal of the American Academy of Dermatology found that 75% of people apply less than half the amount needed to get the labeled SPF protection. That means your SPF 50 might be working like SPF 15—and that’s not enough.

Not all UV rays are the same. UVA, long-wave radiation that penetrates deep into the skin and causes aging and DNA damage is present all day, even through clouds and windows. UVB, short-wave radiation that causes sunburn and directly damages skin cells is strongest between 10 a.m. and 4 p.m. That’s why you need broad-spectrum sunscreen, a formula that protects against both UVA and UVB rays. Only broad-spectrum products reduce your risk of skin cancer, not just sunburn. Zinc oxide and titanium dioxide are physical blockers that sit on top of your skin and reflect UV light. Chemical filters like avobenzone and octocrylene absorb UV rays and convert them into heat. Both work—but only if they’re applied properly and reapplied every two hours, or after swimming or sweating.

High SPF numbers don’t mean you can stay out longer. SPF 30 blocks about 97% of UVB rays. SPF 50 blocks 98%. SPF 100 blocks 99%. The difference is tiny, but the false sense of security is huge. People who use SPF 100 often skip reapplying, skip wearing hats, and skip seeking shade—all things that actually matter more than the number. And don’t forget: sunscreen doesn’t protect against all skin damage. Blue light from screens and pollution also contribute to skin aging, and most sunscreens don’t cover those.

What you see on the shelf isn’t always what’s in the bottle. The FDA doesn’t require third-party testing for sunscreen claims, so some products labeled "water-resistant" lose most of their protection after 40 minutes in water. Some "mineral" sunscreens still contain chemical filters. And many sprays don’t deliver enough product to be effective—you’d need to spray for 15 seconds per body part to get the right amount, which most people don’t do.

Here’s what actually works: use a broad-spectrum SPF 30 or higher, apply a full shot glass worth for your whole body, reapply every two hours, and combine it with shade, clothing, and hats. It’s not about buying the most expensive bottle—it’s about using the right one, the right way, every day. The posts below break down real-world comparisons of top sunscreen formulas, explain what ingredients to look for (and avoid), and show you how to layer sunscreen with your other skincare products without breaking the bank or clogging your pores. You’ll find out why some sunscreens cause breakouts, which ones work under makeup, and how to pick one that actually stays on during workouts or beach days.