How does sunscreen block ultraviolet light?

Sunscreen blocks ultraviolet light by putting a thin layer of molecules on your skin that absorb the rays before they reach it. There are two main families. Mineral sunscreens, also called physical sunscreens, use only inorganic compounds, zinc oxide and/or titanium dioxide, as active ingredients. These primarily work by absorbing UV rays, with some reflection and scattering also occurring. Chemical sunscreens, also called organic sunscreens, use organic molecules that work by absorbing the UV rays and releasing the heat.

The organic filters are built to take the rays in: they are often aromatic molecules with a structure that lets them absorb high-energy ultraviolet rays and release the energy as lower-energy rays, so the damaging rays do not reach the skin.

The number on the bottle is the SPF. The sun protection factor measures the fraction of sunburn-producing UV rays that reach the skin. But SPF is an imperfect measure of skin damage, because invisible damage and melanomas are also caused by ultraviolet A, which does not primarily cause reddening or pain. That is why broad-spectrum sunscreens, covering both UVA and UVB, have been recommended.

The human habits matter just as much as the chemistry. Higher SPF does not last longer, so sunscreen must be reapplied as directed, usually every two hours. And some studies have shown that people commonly apply only a quarter to a half of the recommended amount, which means the protection they actually get is much lower than the label promises.