The SPF Number Promises More Precision Than the Test Behind It Can Prove

The number on a sunscreen bottle looks like a precise measurement. SPF 30, SPF 50, SPF 100: each implies a specific, verifiable multiple of protection against sunburn. A new proposal published in the International Journal of Cosmetic Science argues that precision is largely an illusion, and that the label should stop pretending otherwise.

The paper, titled "A proposal for simplifying the labelling of sunscreens" and authored by Brian Diffey, starts from a problem regulators and manufacturers have long acknowledged but rarely advertised: the "gold standard" method for determining SPF, codified in the international standard ISO 24444, is noisy. The method requires applying sunscreen to human volunteers, irradiating their skin with controlled doses of ultraviolet light, and comparing the dose needed to produce minimal erythema (visible reddening) on protected skin against unprotected skin. Diffey's abstract states plainly that this in vivo approach "is known to be associated with appreciable uncertainty because of experimental variability."

That uncertainty matters because the SPF number is the entire basis of the claim printed on the box. A product labeled SPF 50 is understood by consumers, and by the regulatory frameworks that govern labeling, to deliver a specific, reproducible level of protection, not a number that could plausibly have come out differently depending on which panel of volunteers or which testing lab happened to run the assay. Diffey's paper does not allege fraud or mislabeling. It argues that the test underlying the number is inherently imprecise, and that in vitro alternatives, while promising, have not fully closed that gap either.

The proposal is to abandon the SPF scale altogether in favor of a binary pass or fail classification: a product either clears a defined protection threshold or it does not, communicated through a simplified symbol rather than a two or three digit figure. Diffey's phototesting method would require irradiating only a single area of sunscreen-protected skin, rather than the multiple comparative measurements the current standard demands, and would not require determining the minimal erythema dose on protected skin at all, the step he identifies as the most variable part of the existing protocol. A validated in vitro method, assessed against the same threshold, is offered as an alternative route to the same binary designation.

What the proposal would change is not the chemistry of sun protection but the shape of the claim consumers read off the shelf. An SPF number invites comparison and implies a precision that the measurement process does not reliably support, according to Diffey's account of the method's variability. A binary label, by contrast, would promise only that a threshold has been cleared, nothing more and nothing less, closing the gap between what the number on the bottle seems to guarantee and what the underlying test can actually demonstrate.