The front says Broad Spectrum SPF 50. The bottle beside it says Broad Spectrum SPF 30. Same two words. A higher sunburn number.

Those words did not scale. On a US sunscreen, Broad Spectrum means one test was passed. The SPF digit is a separate sunburn ratio—what SPF stands for. The stamp does not print a UVA protection factor, and a pass does not make that factor grow with the digit.

Where 90% of the curve sits

FDA runs the broad-spectrum test on plastic plates. The lab spreads the product, then hits it with a set dose—4 MED, an erythemal 800 J/m²—before reading the spectrum. A filter that falls apart in light can fail.

The result is a critical wavelength. Walk the absorbance curve up from 290 nm. Stop where the area already counted is 90% of the area from 290 to 400 nm. If that mean critical wavelength is 370 nm or greater, the product may say Broad Spectrum.

A pass at 370 nm does not measure a UVA protection factor. Two films can both clear it and still stop different amounts of UVA.

Pass, and the front may read “Broad Spectrum SPF” plus the sunburn number. Miss, and Broad Spectrum is not allowed there. Only Broad Spectrum SPF 15 or higher may claim a lower risk of skin cancer and early skin aging, and that line says to use the product with other sun protection measures. Miss the stamp, or sit under SPF 15, and the Skin Cancer/Skin Aging Alert is required: shown only to help prevent sunburn, not skin cancer or early skin aging.

A fraction, not a stamp

Europe’s 2006 Commission recommendation is not a statute. Before a UVA-protection claim, it wants a critical wavelength of 370 nm and a UVA protection factor at least one-third of the labelled SPF.

That is arithmetic. One-third of a labelled 30 is a UVA-PF of at least 10. One-third of a labelled 50 is 50/3. Broad Spectrum SPF 50 on a US label has cleared 370 nm. It has not cleared a UVA-PF of 50/3.

Cancer Council Australia says sunscreens tested to AS/NZS 2604:2021 must clear that same bar. The Therapeutic Goods Administration requires listed sunscreens to meet the standard. TGA’s shopper page defines broad spectrum as UVA and UVB. It does not print the fraction. The one-third line here is Cancer Council’s account of a paywalled standard.

FDA has described the gap and left the US words alone. A proposed order from 24 September 2021 says a Broad Spectrum SPF 30 can block less UVA than a Broad Spectrum SPF 15. It would add a second pass: at least 0.70 of the absorbance area from 290 to 400 nm in UVA I (340–400 nm). That ratio is still a proposal. The 10 September 2026 final order removed aminobenzoic acid (PABA) and trolamine salicylate and left the label change for later.

The same order calls SPF primarily, though not exclusively, a UVB indicator. The digit is not a UVA score.

Twenty bottles in 2017

Wang, Xu, Stanfield, Osterwalder, and Herzog measured 20 US sunscreens that year, labelled SPF 15 to 100+, with the US plate method and the ISO in vitro UVA method. Critical wavelength ran 367–382 nm. UVA protection factor ran 6.1–32. Nineteen of 20 met the US bar. Eleven of 20 met the European ratio as they scored it: UVA-PF greater than one-third of SPF.

They called the set small. A letter that year said the in vitro method often reads lower than an in vivo UVA test when the formula uses zinc oxide or titanium dioxide. One bench. Not this year’s shelf.

Before you buy the higher number

1. Read the US stamp as a pass at 370 nm. It is not a UVA factor scaled to the SPF.
2. Buy the higher digit for sunburn protection. Under the test in force, Broad Spectrum SPF 30 can block less UVA than Broad Spectrum SPF 15.
3. Use one-third only where that rule is the claim. European UVA-protection claims, and Australian sunscreens tested to the 2021 standard as Cancer Council describes it.
4. Thickness and cloth are separate tests. The bottle SPF assumes a heavy film. A cotton T-shirt is a weak filter of both bands.
5. At a peak of 3 or above, cover anyway. That line is the homepage number. The index barely counts UVA, and the stamp does not repair it.

Limits

Do not skip a coat because two stamps can hide different UVA blocks. This is not a brand ranking or advice about a mark on your skin. The European fraction is a recommendation, Australia’s is Cancer Council on a paywalled standard, 0.70 is still a proposal, and the twenty bottles are from 2017.

Sources

Critical wavelength (90% of absorbance from 290–400 nm; mean ≥ 370 nm), 4 MED / 800 J/m² pre-irradiation, and the Broad Spectrum versus Skin Cancer/Skin Aging Alert rules: 21 CFR 201.327. UVA-PF at least one-third of labelled SPF, plus 370 nm: Commission Recommendation 2006/647/EC. Australia’s one-third reading of AS/NZS 2604:2021: Cancer Council Australia. TGA shopper page, fraction not stated: Reading sunscreen labels. SPF 30 versus SPF 15, proposed 0.70 UVA I/UV ratio, SPF “primarily, though not exclusively” UVB: FDA proposed order OTC000008 (24 September 2021). Ratio still proposed; 10 September 2026 order limited to PABA and trolamine salicylate: FDA sunscreen Q&A. CW 367–382 nm, UVA-PF 6.1–32, 19/20 and 11/20: Wang et al., J Am Acad Dermatol 2017. In vitro method can read low for zinc oxide and titanium dioxide: Poh Agin, letter, 2017. Protect from index 3: WHO UV index.