Reading a sun-protection label properly.

The number on a sunscreen label gets more attention than almost anything else on it, and it’s the part most people understand least. SPF 30 versus SPF 50 becomes the entire decision, when it’s genuinely one part of a longer, more useful sentence the label is trying to say.

SPF Measures One Thing, Not Everything

SPF — Sun Protection Factor — measures protection against UVB rays specifically, the wavelength primarily responsible for sunburn. SPF 30 filters approximately 97% of UVB rays; SPF 50 filters approximately 98%. The jump from 30 to 50 is real but smaller than the doubled number suggests, and neither number says anything at all about UVA protection on its own.

This matters because UVA and UVB affect skin differently. UVB is the more immediate, visible aggressor — the one responsible for sunburn. UVA penetrates more deeply, contributes significantly to premature aging, and passes through window glass and cloud cover in ways UVB largely doesn’t. A product can carry an impressively high SPF number and still offer only modest UVA protection, unless it’s specifically formulated and labeled to address both.

What “Broad Spectrum” Is Actually Claiming

“Broad spectrum” is the phrase doing the real work on a well-formulated label. It indicates the product has been tested and shown to protect against both UVA and UVB, not UVB alone. In several regulatory regions, a product without adequate UVA protection isn’t legally permitted to use this phrase, which makes it a meaningfully more reliable signal than the SPF number by itself.

SPF answers how strong. Broad spectrum answers how complete. Both questions matter, and they aren’t the same question.

In the EU and UK, look additionally for a UVA circle logo, which indicates the UVA protection meets a minimum ratio relative to the SPF. It’s a small detail, easy to miss, and considerably more informative than the large number beside it.

How Much and How Often, Realistically

Laboratory SPF testing applies roughly 2mg of product per square centimeter of skin — markedly more than most people apply in practice. Real-world application, at a thinner layer, typically delivers meaningfully less than the labeled protection. This is less a flaw in the product than a gap between how sunscreen is tested and how it’s actually used.

Reapplication matters more than the initial number. Sweat, water, and simple surface friction from clothing or touch reduce a product’s effectiveness over the course of a day, regardless of how high the original SPF was. A consistently reapplied SPF 30 will generally outperform a once-applied SPF 50 by the middle of an afternoon outdoors.

Reading Past the Front of the Bottle

The most informative sunscreen labels are the least dramatic ones: a specific SPF number, the words “broad spectrum,” a water-resistance duration if relevant, and an ingredient list that isn’t trying to also make five other claims at once. The rest — the number itself included — is only useful in the context of those other few words around it.