How does sunscreen actually work?

Two categories of filter do the work. Mineral sunscreens use zinc oxide or titanium dioxide, which sit on the skin surface and both scatter and absorb ultraviolet radiation. Chemical filters such as avobenzone and octocrylene absorb UV photons and dissipate the energy as a small amount of heat. Both are effective, and the popular framing of mineral as a physical shield and chemical as absorbing is a simplification, since mineral filters absorb a substantial share of what they block. Mineral tends to be gentler on sensitive skin and leaves more visible cast; chemical tends to be cosmetically lighter.

SPF is where most misreading happens. It measures protection against UVB, the wavelengths that cause sunburn, and says nothing about UVA, which penetrates deeper and drives aging and skin cancer risk, so broad spectrum on the label is the term that matters. The scale is also non-linear and misleading: SPF 30 blocks about 97 percent of UVB and SPF 50 about 98, so doubling the number does not double protection. Far more consequential than the number is quantity and reapplication, since studies consistently find people apply roughly a quarter to half the amount used in laboratory testing, which means real-world protection is a fraction of what the bottle claims. About an ounce for the body, every two hours, and immediately after swimming or heavy sweating. Water resistant is a regulated claim limited to 40 or 80 minutes; waterproof is not permitted on labels.