Sunscreen combines organic and inorganic chemicals to filter the light from the sun so that less of it reaches the deeper layers of your skin. Like a screen door, some light penetrates, but not as much as if the door wasn't present. Sunblock, on the other hand, reflects or scatters the light away so that it doesn't reach the skin at all.

The reflective particles in sunblocks usually consist of zinc oxide or titanium oxide. In the past, you could tell who was using a sunblock just by looking, because the sunblock whited out the skin. Not all modern sunblocks are visible because the oxide particles are smaller, though you can still find the traditional white zinc oxide. Sunscreens usually include sunblocks as part of their active ingredients.

What Sunscreens Screen

The portion of the sunlight that is filtered or blocked is ultraviolet radiation. There are three regions of ultraviolet light.

UV-A penetrates deeply into the skin and can lead to cancer and premature skin aging.

UV-B is involved in tanning and burning of your skin.

UV-C is completely absorbed by the earth's atmosphere.

The organic molecules in sunscreen absorb the ultraviolet radiation and release it as heat.

PABA (para-aminobenzoic acid) absorbs UVB

Cinnamates absorb UVB

Benzophenones absorb UVA

Anthranilates absorb UVA and UVB

Ecamsules absorb UVA

What SPF Means

SPF stands for Sun Protection Factor. It's a number that you can use to help determine how long you can stay in the sun before getting a sunburn. Since sunburns are caused by UV-B radiation, SPF does not indicate protection from UV-A, which can cause cancer and premature aging of the skin.

Your skin has a natural SPF, partially determined by how much melanin you have, or how darkly pigmented your skin is. The SPF is a multiplication factor. If you can stay out in the sun 15 minutes before burning, using a sunscreen with an SPF of 10 would allow you to resist the burn for 10 times longer or 150 minutes.

Although the SPF only applies to UV-B, the labels of most products indicate if they offer broad-spectrum protection, which is some indication of whether or not they work against UV-A radiation. The particles in sunblock reflect both UV-A and UV-B.