SPF · PA · UVA circle · broad spectrum
SPF and PA ratings explained
Compare two SPF numbers by what actually gets through to your skin, see what a thin layer does to them, and decode the UVA marks on labels from Japan, the EU, the UK and the US.
How to use the explainer
Comparing SPFs. Enter two SPF values. SPF is measured as how much more sunburning UV skin can take with sunscreen than without, so under test conditions roughly 1/SPF of that UV gets through: 6.7% for SPF 15, 3.3% for SPF 30, 2% for SPF 50. The amount slider shows what happens with a thinner layer, using the in-vivo finding of Faurschou and Wulf that protection drops exponentially with the amount: half the dose gives about the square root of the SPF.
Worked example: SPF 30 blocks 96.7% and SPF 50 blocks 98%, only 1.3 points apart. Turn it round and SPF 30 lets through 3.3% against 2%, which is 1.67 times as much. Now slide the amount to 0.5 mg/cm², a typical real-world layer: SPF 30 behaves like about SPF 2.3 and SPF 50 like about SPF 2.7. Using enough matters more than the step from 30 to 50.
Decoding PA. Enter the labelled SPF and, if you know it, the UVA protection factor. The tool gives the PA grade from the Japan Cosmetic Industry Association's bands and checks the EU rule that UVA protection should be at least a third of the SPF, the condition for the UVA circle logo. PA grades are ranges, so a grade alone sometimes cannot settle the EU check; the tool says when.
SPF numbers here describe sunburn protection under laboratory conditions. They are not a time limit and not a promise about any product.
The marks on a sunscreen label
What each one is tested against, and who sets it.
| Mark | What it tells you | Rule |
|---|---|---|
| SPF 15, 30, 50… | Protection from sunburn, mostly from UVB, measured with 2 mg/cm² on volunteers' skin. | ISO 24444; FDA 21 CFR 201.327 |
| PA+ to PA++++ | UVA protection factor (PPD) of 2 to under 4, 4 to under 8, 8 to under 16, and 16 or more. PA++++ was added in 2013. | Japan Cosmetic Industry Association; ISO 24442 |
| “UVA” in a circle | UVA protection at least a third of the labelled SPF, with a critical wavelength of at least 370 nm. | EU Recommendation 2006/647/EC |
| Low, medium, high, very high protection | EU SPF steps: 6 and 10; 15, 20 and 25; 30 and 50; 50+ (a measured SPF of 60 or more). | EU Recommendation 2006/647/EC |
| Broad spectrum | The protection reaches into UVA: critical wavelength of 370 nm or more. Only broad-spectrum SPF 15+ products may say they help lower the risk of skin cancer and early skin ageing when used with other sun protection. | FDA 21 CFR 201.327 |
| Water resistant (40 or 80 minutes) | The SPF held through 40 or 80 minutes of activity in water in the test. “Waterproof”, “sweatproof” and “sunblock” are not allowed. | FDA 21 CFR 201.327 |
| UVA stars (UK) | A 0 to 5 star rating of UVA protection relative to UVB. The NHS and the British Association of Dermatologists suggest 4 or 5 stars with SPF 30 or more. | UK retail scheme |
Questions about SPF and PA
Is SPF 50 much better than SPF 30?
Under test conditions SPF 30 lets through about 3.3% of sunburning UV and SPF 50 about 2%, so SPF 50 cuts what reaches the skin by about 40%. In everyday use both are usually applied too thinly to reach their labels, and the amount applied makes a bigger difference than the step between them.
Does SPF tell me how long I can stay in the sun?
No. The FDA explains that SPF relates to the amount of sun exposure, not the time spent in it, because the sun's strength changes through the day. WHO also warns against using sunscreen to stay out longer. Reapply at least every 2 hours whatever the SPF.
What is the difference between PA and broad spectrum?
Both are about UVA. PA grades the UVA protection factor measured on skin by persistent pigment darkening (ISO 24442). US “broad spectrum” is a pass or fail laboratory test: the critical wavelength must be at least 370 nm. The EU UVA circle combines a UVA-to-SPF ratio of at least one third with that same 370 nm test.
Is PA++++ the same as the EU UVA circle?
Not quite. PA++++ means a UVA protection factor of 16 or more. The UVA circle needs at least a third of the SPF, which for SPF 50 is about 16.7. A PA++++ product is close to or above that, but the grade alone cannot prove it.
Why do labels stop at 50+?
Under the EU recommendation, measured SPFs from 50 to under 60 are labelled 50, and 60 or more “50+”, so higher figures are not printed. Above about 50 the extra share of UV blocked is small: SPF 100 blocks 99% against 98% for SPF 50.
Sources
- US FDA, 21 CFR 201.327 (SPF test at 2 mg/cm², broad spectrum test with a 370 nm critical wavelength, water resistance, banned claims) and Sunscreen: How to Help Protect Your Skin from the Sun.
- ISO 24444:2019, in vivo determination of the sun protection factor; ISO 24442:2022, in vivo determination of sunscreen UVA protection (persistent pigment darkening).
- Japan Cosmetic Industry Association, voluntary standards for UV protection labelling (jcia.org): PA grades from the UVA protection factor, PA++++ from 2013.
- European Commission, Recommendation 2006/647/EC (UVA at least 1/3 of SPF, 370 nm, label categories, banned claims, reduced amounts).
- Faurschou A, Wulf HC. The relation between sun protection factor and amount of sunscreen applied in vivo. Br J Dermatol 2007;156:716–719. doi:10.1111/j.1365-2133.2006.07684.x.
- NHS, Sunscreen and sun safety, and British Association of Dermatologists, The sunscreen fact sheet (SPF 30+, UVA 4 or 5 stars, UVA circle).
- WHO, Global Solar UV Index: A Practical Guide (sunscreen should not be used to prolong time in the sun).