Did you know ultraviolet light/ UV light absorber is considered a food preservative? PABA, Para-amino benzoic acid,  vitamin B (although technically not a vitamin, the body makes it). prevents light from reacting and creating harmful by product toxins. Three other categories, color's, stabiliser's, antioxidant's, and antimicrobial's.
Do not take PABA when taking sulfa drugs. Defeats the purpose as sulfa fools microbes into trying it instead of PABA and they die. Microbe's require PABA to live.
Yes it is used to block UV rays and must be applied topically. I am wondering if this is where the fear to not use PABA in sunscreen is coming from. Skin, PABA, and microbes? Bacteria require an acidic environment and PABA is acidic. Well the microbiologists will know. I would watch the expiry date on sunscreens though.
Bacteria plus PABA equals folic acid in the intestines. And if you use lots of sunscreen during the summer and tanning... it blocks the bodies ability to produce vitamin D.