Sunscreen myths, checked against the evidence

Do you really have to reapply sunscreen every two hours?

Short answer: only when you're actually out in the sun. Most sunscreen "rules" are half-right — here's the honest version.

Sunscreen is the single most proven step in skincare, so it deserves better than folklore. Here are the common myths, checked.

• "Reapply every two hours." True — but outdoors. Sunscreen rubs, sweats, and breaks down while you're in the sun, so the rule is every two hours outside, and after swimming, sweating, or toweling off [1]. It was never meant for your desk. The habit with real proof is simply daily use: in a randomized trial that followed adults for 4.5 years, daily users aged 24% less than occasional users [2].
• "SPF 50 is barely better than SPF 30." SPF 30 blocks about 97% of burning UV and SPF 50 about 98% — but flip it around and SPF 30 lets one burning ray in 30 through while SPF 50 lets one in 50, roughly 40% less UV. And since everyone applies less than the tested amount (about a teaspoon for the face, per AAD), the higher number buys margin. Applying enough beats every other tip here [1].
• "PA++++ is just marketing." PA is a real rating. SPF measures only UVB (the burning rays); PA measures UVA (the deeper rays that age skin and cause dark spots without a sunburn). More plus signs, more UVA protection. If aging or pigment is your concern, PA — or "broad spectrum" in the US — matters.
• "Tinted sunscreen is just makeup." The tint is iron oxides, which block visible light (the ordinary light you see by). That's irrelevant for burning, but for stubborn dark patches — especially in deeper skin tones — visible light drives pigment, and tinted mineral sunscreens have trial support for blocking it [3].
• "Sunscreen indoors is pointless." Glass stops burning UVB but passes plenty of UVA. Far from a window, you're fine; by a sunny window or in a car, the morning layer earns its place.
• "You can't make vitamin D with sunscreen on." Real-world studies keep finding normal vitamin D in sunscreen users — partly because nobody applies it perfectly.

What to actually do: a generous teaspoon (for the face) of broad-spectrum SPF 30–50 every morning, reapply when you're genuinely out in the sun or after water and heavy sweat, and choose tinted if dark spots are your fight [2].

One honest caveat: a lot of this depends on how much you apply, which almost nobody measures. The evidence is strongest for daily use and using enough — the rest is sensible calibration, not ironclad law.