There are two types of sunscreen, “chemical” and “physical” blockers.
Physical blockers are made of zinc oxide and titanium dioxide. These are small fragments of minerals that reflect the light, thus are protective against both UVA and UVB radiation.
Chemical blockers are composed of several different chemicals that each absorbs various wavelengths of UVA, UVB rays, or a combination of both. These chemicals soak into the skin, thus need to be applied 20-30 minutes prior to turning the light energy into vibrational or “kinetic” energy, then release that energy as heat energy into the skin.
As the molecules absorb light energy and vibrate, they gradually degrade or fall apart – therefore they need to be applied frequently. Some safety concerns have been raised about the safety of chemical sunblockers.
The Environmental Working Group (EWG) raised concerns about oxybenzone causing cancer after long term use. Although this may represent a theoretical concern for infants that have never been exposed to sunscreens before, for adults, we have been exposed to oxybenzone for years, thus “the horse is out of the barn” and the benefits of skin cancer prevention greatly outweighs any minimal risk of using oxybenzone.
If you have a small child, you may want to consider using only physical blockers to avoid long term exposure to chemical sunscreens.