How does sunscreen work?

Sunscreens contain two main kinds of UV filters. Mineral filters such as zinc oxide and titanium dioxide sit on the skin and absorb, scatter and reflect UV light, while chemical filters such as avobenzone absorb UV rays and release the energy as a tiny amount of heat. SPF measures protection against UVB, the main cause of sunburn: SPF 15 blocks about 93 percent of UVB, SPF 30 about 97 percent and SPF 50 about 98 percent. Broad-spectrum products also protect against UVA, which penetrates deeper and ages the skin. Most people apply too little; dermatologists recommend about 30 millilitres, roughly a shot glass, for the whole body, reapplied every two hours.