There are three types of UV radiation: UV-A, UV-B, and UV-C. UV-C is completely filtered out by the earth's atmosphere (another reason to love the ozone layer and stop making holes in it!), but UV-A and UV-B make it down to us earth dwellers. UV-B is responsible for sunburns, and while UV-A doesn't have an obvious immediate effect, it does cause damage, like premature aging and skin cancer.
Next: SPF stands for "sun protection factor." It's a general marker for how long you can be out in the sun without burning. This does mean it only takes UV-B radiation into consideration, however, so you need to make sure that your sunscreen also deals with UV-A rays, as well.
Finally: as with all things, sunscreen is a result of ongoing experimentation, studies, and observations. A lot of this information is simply as much as we know right now. I do encourage you to do some research on your own on issues that are important to you - I've included a few starter links at the bottom of this post.
Now. A common question: what is the difference between physical and chemical formulas?
Physical chemicals are mineral formulas; that is, they are titanium dioxide or zinc oxide. These are inorganic compounds (no carbon involved), and they work by reflecting or scattering UV radiation; this means that it bounces off, and doesn't get absorbed by your skin (think: lots of tiny mirrors spread all over your skin). This is where the term sunblock comes from, and also explains the white cast you get from physical sunscreens - visible light is also reflected off of the particles.
|"Nomenclature" = naming system. We use a lot of prefixes.|
Since they're mineral compounds, they are not absorbed by your skin, and are therefore usually safer for those prone to skin sensitivities (the "nano-" ones are controversial right now, though - they might just be small enough to penetrate). They can clog pores, so make sure you use a good cleanser. These formulas tend to be thicker and maybe harder to rub in. However, in general, these compounds are stable (they don't break down easily), and zinc oxide is broad-spectrum - that is, protects against both UV-A and UV-B radiation. This is really important.
|Common chemical sunscreen.|
Chemical formulas, on the other hand, act as sunscreens by absorbing UV radiation and releasing it as heat, instead. Common ingredients include PABA (para-aminobenzoic acid), avobenzone, octisalate, oxybenzone. In general, since these are not chemically inert (they do readily absorb UV radiation, after all), they can eventually break down and can also be absorbed by the skin, which is the reason that they are somewhat controversial and still widely studied. Certain ones also do protect against UV-A radiation - a quick Google search of an ingredient will tell you exactly what it does.
Hope this clears up any confusion or questions you may have had about the all-important issue of sun protection! There is actually a lot more to talk about, but I feel like this is already information overload (it is for me, anyway). One last thing I have to mention: sunscreen ratings and common ingredients actually vary greatly depending on where in the world you are - different countries have different rules about what qualifies as a sunscreen, and what is considered safe to use.
With that said, I'm opening this up for questions and comments as always. I love hearing from you, so please don't hesitate to let me know what you think!
(If you missed it, here's the introduction to the "Beauty of Science" series.)
Smith, Janice. Organic Chemistry, 3rd ed. McGraw-Hill, 2006.
"How to Pick the Best Sunscreen," Bailey, Cynthia. http://www.drbaileyskincare.com/blog/how-to-pick-the-best-sunscreen/. Dr. Cynthia Bailey Skincare. 2010.
"How Sunscreen Works," Helmenstine, Anne Marie. http://chemistry.about.com/od/howthingsworkfaqs/f/sunscreen.htm. About.com Chemistry.