In reality, the testing labs don't really burn the test subjects they give them just enough UVR to cause the skin to turn barely red. They work either by absorbing the UVR or by reflecting it. Either way, if used properly, they can keep a lot of UVR from reaching you. Organic sunscreens have been the mainstay of commercially available products for years. It does not mean that they are naturally occurring and, in fact, they are not.
Organic sunscreens are relatively complex molecules that are to some degree absorbed into the skin.
PABA became so well known because a significant number of people turned out to be allergic to it. It is probably the one most associated with adverse reactions. Physical sunscreens are actually microscopic solid pieces of sunscreen. Zinc oxide and Titanium dioxide are the two that are commonly used. Most people associate zinc oxide with the white thick paste lifeguards used to use.