SPF testing: in vivo, in vitro and what the label can claim

Sun protection is the most regulated claim in cosmetics, and the most frequently overstated in marketing. The number on the pack is a test result with a defined method and a defined confidence interval, not a formulation target.

SPF is measured in vivo: a defined light source, a defined amount of product (2 mg/cm²), and a panel of human volunteers with skin types I–III. The result is the ratio of the minimal erythemal dose on protected versus unprotected skin. ISO 24444 is the current standard, and it is expensive — a single test runs into thousands of euros.

UVA protection is usually measured in vitro under ISO 24443, which produces a UVA-PF value and the PA+ to PA++++ rating used in Asia. The critical regulatory requirement in the EU is that the UVA protection be at least one third of the SPF; the Boots star system is a voluntary overlay on top of that.

The “+” in 50+ has a rule: the measured SPF must exceed 60 to make the claim. A formula that tests at 52 cannot be labelled 50+, however much the marketing team would like it to be.

Water resistance is a separate claim with its own test, ISO 16217, and it can only be made if the measured SPF after immersion remains above 50% of the static value. This is where most budget sunscreens fail — the filters are there, but they wash off.