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 [...]

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Narrated by TYPE III AUDIO.