Sunscreen keeps UV from reaching your skin cells. Mineral formulas reflect and scatter rays off the surface. Chemical formulas absorb UV and release it as heat. Many products combine both.

Mineral (physical) sunscreen

Zinc oxide and titanium dioxide sit on top of your skin like tiny mirrors. UV hits the particles and bounces away. It works the moment you put it on—no wait time. The tradeoff is a white cast on some skin tones, though newer formulas blend better.

Chemical sunscreen

Ingredients like avobenzone, octinoxate, and octocrylene absorb UV photons and convert the energy to heat, which your skin releases. These feel lighter and rub in clear, but need 15–30 minutes to bind before sun exposure. Some filters (especially avobenzone) can break down over a long day in direct sun.

UVA vs. UVB

UVB causes sunburn and is the main driver of SPF ratings. UVA penetrates deeper, contributes to aging and skin cancer, and passes through most window glass. You want broad-spectrum coverage for both. SPF 30 blocks about 97% of UVB; SPF 50 blocks about 98%.

Application is half the equation

Lab SPF tests use 2 mg per cm² of skin—roughly one ounce (a shot glass) for your whole body, half a teaspoon for your face. Most people use far less, so real-world protection is often half the label number or worse. Reapply every two hours, or right after swimming or sweating. Water-resistant labels (40 or 80 minutes) help but aren't a pass to skip reapplication.

Pick broad-spectrum SPF 30 or higher, apply enough that you're not stingy about it, and reapply on a timer. The chemistry only works if there's enough of it on your skin.