The large figure printed on a sunscreen package is the result of a measurement defined far more narrowly than it is usually understood. This article describes what that measurement covers, and which part of the ultraviolet spectrum falls outside it.
UVA and UVB: Two Bands, Two Damage Profiles
Diffey (2002) describes the ultraviolet radiation reaching ground level as comprising UVB (roughly 280–320 nm) and UVA (roughly 320–400 nm). UVB carries higher photon energy, is absorbed mainly in the epidermis, and is the principal cause of erythema — the reddening of skin after sun exposure.
UVA makes up a much larger share of total ground-level UV, penetrates deeper into the dermis, and produces little immediate erythema. That last characteristic is precisely the problem: cumulative UVA damage carries no immediate warning signal comparable to sunburn.
UVA is also less attenuated by cloud cover and window glass than UVB, so indoor and in-vehicle exposure is not zero as is often assumed.
What SPF Measures — and What It Does Not
ISO 24444:2019 defines SPF as the ratio between the minimal erythemal dose on protected skin and that on unprotected skin, measured on human volunteers. Because the endpoint is erythema, and erythema is driven chiefly by UVB, SPF is fundamentally a UVB-weighted figure.
The consequence: a high-SPF product may still protect poorly against UVA. This is why separate UVA scales exist, and it is not a minor technical footnote.
The relationship between SPF and the fraction of UVB blocked is also non-linear. The difference between high SPF values is far smaller than the numbers suggest, so the gap between two high ratings is not worth trading for a thinner application.
UVA Ratings: PA, ISO 24442 and Critical Wavelength
ISO 24442 specifies an in vivo method for UVA protection based on persistent pigment darkening. Results are commonly converted to the PA scale with plus signs, widely used on products in Asian markets.
In parallel, the critical wavelength method identifies the point at which cumulative absorbance reaches 90 percent; a value of 370 nm or above is the commonly used threshold for describing a product as broad spectrum. The two methods answer different questions and complement one another.
Real-World Dose Versus Test Conditions
All the measurements above are performed at a standard application density of 2 milligrams per square centimetre of skin. Numerous surveys report that real-world application is substantially below this, and because the relationship between dose and protection is non-linear, the protection achieved falls short of the labelled figure.
In other words, the largest variable is not the choice between two ratings but the amount applied and whether it is reapplied. This is a consistent finding in the literature and is rarely stated on packaging.
The above is scientific information and is not a substitute for medical diagnosis, advice or treatment.
