How Sunscreen Protects Skin

Sunscreen works through one of two basic strategies, and most bottles on store shelves rely on a blend of both. The first strategy uses chemical filters, ingredients that absorb ultraviolet radiation before it can reach living skin cells. These molecules are specifically shaped to soak up the energy carried by UV light, and in absorbing that energy, the molecules briefly shift into a higher-energy state before releasing the absorbed energy back out as a small, harmless amount of heat. The second strategy uses physical filters, usually mineral particles like zinc oxide or titanium dioxide, which sit on top of the skin and work mainly by reflecting and scattering UV rays away from the skin's surface rather than absorbing them. Both strategies target two separate bands of ultraviolet light, called UVA and UVB, since each causes different kinds of damage: UVB is the primary cause of sunburn and plays a major role in skin cancer risk, while UVA penetrates more deeply into the skin and is more associated with premature aging and wrinkling. A sunscreen labeled "broad spectrum" specifically means it has been tested to protect against both UVA and UVB, not just one. The SPF number on a bottle, meanwhile, measures protection specifically against UVB and indicates how much longer protected skin can be exposed before burning compared to unprotected skin, though it says nothing on its own about how well a product blocks UVA.