There is no evidence you should reapply sunscreen every 2 hours.

It’s incredible how many consensus guidelines dissolve when you look closely at them. If you listen to any authority on the subject of sunscreen, you will hear it endlessly repeated that you absolutely must reapply sunscreen every 2 hours while you are in the sun, and immediately after swimming, sweating, or exercising. Not only that, you’ll hear that you need to apply sunscreen before going outside, even if you put it on earlier and stayed indoors. The rationale behind this is straightforward and plausible: sunscreen’s effectiveness degrades over time, therefore prolonged sun exposure warrants topping up on protection.However, when you look closely at the origins of this guideline, and the evidence base for its instantiation in regulations and official statements, it turns out that this 2-hour rule is a baseless, circularly justified, expedient fiction. Where does the FDA’s 2 hour reapplication guideline come from?Tracing the history of the 2-hour reapplication guideline reveals an extremely shaky base of evidence.The first official sunscreen rulemaking in the US was in 1978, where they recommend: "apply sunscreen products liberally and to reapply after swimming or excess perspiration". No fixed universal time interval is mentioned, and the only basis for this recommendation is their assertion that consumers generally don’t apply enough.15 years later, in 1993, the FDA released a tentative final monograph. Again, this contained no fixed time interval, and reapplication directions are entirely activity-based.In 1999, a final monograph was released with a reapplication direction of “as needed, or after towel drying, swimming, or perspiring”(Sec. 352.52(d)(2)). No scientific justification is offered for these directions, and no “2 hour reapplication” recommendation appears. The monograph was stayed in 2001 and never took effect.The 2007 Proposed Rule: The 2-hour recommendation appearsThe first direct mention of the “2 hour” rule in the FDA regulatory record appears in