Ultraviolet light from the sun encompasses a range of wavelengths, all the way from 280 nm to 400 nm. The shorter and more energetic wavelengths from 280 to 320 nm are referred to as UV-B, while the longer less energetic wavelengths are termed UV-A. All ultraviolet rays are energetic enough to break bonds in molecules, meaning that they can disrupt the structure of DNA and trigger cancer. UV-B rays are mostly absorbed by molecules near the surface of the skin and cause the damage we associate with a sunburn, while UV-A rays, which unlike UV-B can pass through glass, are more penetrating and cause aging of the skin. For proper sun protection both UV-A and UV-B must be blocked. Sunscreens generally contain organic molecules that filter out the shorter UV-B rays. These compounds, such as octylmethoxycinnamate, one of the most common sunscreens, absorb UV-B waves and reemit the energy as harmless infrared light, essentially heat. There are also compounds, avobenzone being an example, that absorb UV-A. But the best protection against ultraviolet light is to not allow the rays to penetrate the skin at all.
And that’s where zinc oxide and titanium dioxide come into the picture. Not only do they prevent penetration, but unlike some of the organic compounds in sun protection products, they do not break down on prolonged exposure to ultraviolet light. Zinc oxide and titanium dioxide prevent UV rays from penetrating by scattering them. But the problem with the metal oxides is that they also scatter visible light, which means they appear white when applied to the skin. However, the extent to which particles scatter visible light depends on the size of the particles. If they are smaller than about 0.2 microns, there is virtually no scattering of visible light, while the scattering of ultraviolet is unaffected. Modern technology can control particle size, and makes possible the production of titanium dioxide or zinc oxide containing lotions that are transparent. Incorporating dimethicone, a type of silicone, into the formula keeps the small particles uniformly dispersed and prevents them from clumping into larger aggregates that would then appear white. This is an important advance given that people are more likely to apply transparent lotions than cosmetically unappealing white ones.
It is important to realize that the sun protection factor, or SPF, refers only to UV-B. No matter how high the number, it does not indicate protection against UV-A. In any case, the ultra high numbers are more of a marketing gimmick than anything else. An SPF of 15 already filters out 93% of UV-B, and that rises to 97% for an SPF of 30. There is really no need to go to higher numbers. It is important, though, to use the lotion properly, applying a handful to cover the whole body roughly twenty minutes before exposure. How safe are sunblocks and sunscreens? One can always conjure up scary scenarios by referring to laboratory studies demonstrating that some ingredients have estrogenic potential, or can yield damaging free radicals. As with so many other issues, it is a question of balancing risks versus benefits. The risks of sun protection products are theoretical whereas damage by ultraviolet light is based on hard evidence.