Broad-spectrum SPFSunscreen · SPF
Sun protection against both UVA and UVB: the best-evidenced skin habit.
Broad spectrum means a sunscreen protects against both UVB and UVA. The SPF number describes only the UVB side, which is the radiation that burns you, so a high SPF without broad spectrum protection leaves the deeper ageing wavelengths largely unaddressed.
UVA matters because it penetrates further, degrades collagen and elastin, passes through window glass, and varies far less by season and time of day than UVB does. It is the wavelength band most responsible for photoageing, and it does not announce itself with a burn.
The labelling is regionally inconsistent, which is genuinely confusing. In the UK and Europe a UVA logo in a circle indicates UVA protection at least a third of the labelled SPF, and a star rating gives more detail. Elsewhere the words broad spectrum carry the meaning. The number on the front means the same thing everywhere; the UVA claim does not.
SPF numbers are also non-linear and widely misread. SPF 30 filters roughly 97% of UVB and SPF 50 roughly 98%, so the jump from 30 to 50 is small in filtration and useful mainly as insurance against the real world problem, which is that almost nobody applies enough.
Application is where the protection is actually lost. Laboratory SPF is measured at a thickness most people never use, so realistic application delivers a fraction of the labelled figure. Applying generously and reapplying after a couple of hours outdoors, or after swimming and towelling, matters more than the number chosen.
The evidence that it prevents ageing, not just burning, comes from a randomised trial: daily sunscreen use produced no detectable increase in skin ageing over four and a half years compared with discretionary use.
In practice, choose broad spectrum, apply more than feels necessary, reapply, and treat shade and clothing as the primary defence with sunscreen covering what they miss.