Sunscreen is a filter on the surface of skin. Mineral formulas (zinc oxide, titanium dioxide) scatter and reflect UV. Chemical filters (avobenzone, octinoxate, octocrylene, and others) absorb UV and dump the energy as heat. Many bottles mix both.

Mineral vs chemical is a feel, not a morality play

Mineral screens work as soon as they are on. They can leave a white cast, though newer micronized lots blend better. Chemical screens usually rub in clear and need about 15–30 minutes to sit before you walk into open sun. Some chemical filters, especially avobenzone, can break down over a long bright day—which is one reason reapplication is not optional.

The FDA monograph treats both classes as over-the-counter drugs in the U.S. Pick the one you will actually wear thick enough. A bottle left in the bag does nothing.

SPF is the UVB slice

UVB is the burn band and the SPF test. UVA goes deeper, drives tanning and aging, and passes ordinary window glass. Broad-spectrum is the label that means both. SPF 30 ≈ 97% UVB blocked in the lab; SPF 50 ≈ 98%. The leftover percent is why you still tan through “full” sunscreen.

The filter only works at the test thickness

Lab SPF uses 2 mg/cm²—about a shot-glass for an adult body. Dermatology groups keep repeating that because most people use a third to a half. Water-resistant 40- or 80-minute labels were timed in a pool protocol, not a surf day with a towel.

More on the clock: how long sunscreen lasts. More on the digit: what SPF stands for.