The sun protection factor (SPF) you see on the label of sunscreens is misleading. It's not a measure of total sun protection but of protection against sunburn from UVB light. The longer light waves in the ultraviolet A (UVA) part of the spectrum actually penetrate deeper into the skin and may even cause more harm than sunburn-producing UVB. Moreover, the main part of the UVA spectrum passes through glass, and UVB doesn't, so you may be exposed to UVA in a car and indoors.
Sunscreens that claim to block both UVA and UVB light are increasingly popular. But currently there's little FDA regulation of those claims. If companies can show their lotions absorb or block even just a little bit of UVA light (and they are free to design the tests) they're allowed to label them as "broad spectrum" sunscreens and even as sunblocks. Some dermatologists reserve the term sunblock for lotions with ingredients that physically block light from penetrating the skin, such as zinc oxide or titanium dioxide. The FDA has proposed regulations that would go further and ban the term altogether. But those regulations have languished in the proposal stage for six years, and there's no immediate prospect of them becoming law soon.
So if you're looking for UVA and UVB protection, don't pay too much attention to the front of the bottle. Inspect the fine print on the back. The active ingredients with the best reputation for fending off UVA light are avobenzone (Parsol 1789), titanium dioxide, and zinc oxide.
But it's not quite that simple. Some researchers see a problem with avobenzone. In experimental situations it breaks down in sunlight — not a good attribute for a sunscreen ingredient.
Therefore, the lotions containing zinc oxide and titanium dioxide may be the best bet. The particles are very fine, so they're not like the white zinc oxide that lifeguards paint their noses with. Still, some find these lotions gritty.
Regardless of which sunscreen you use, slather the stuff on. People rarely use enough to get the lotion's full protection.
July 2005 Update