Sunscreen SPF vs PA: what they mean, how much to use, when to reapply
SPF measures UVB protection, PA measures UVA. Work out a face's dose from 2 mg/cm², reapply every 2 hours, compare chemical and mineral, and read Thai FDA labels.
- SPF tells you about UVB, which burns. PA tells you about UVA, which goes deeper and is linked to wrinkles and dark spots. Check both.
- The SPF on the label is tested at 2 mg per cm². A face needs about 1-1.6 ml, and using less cuts real protection fast.
- Reapply at least every 2 hours outdoors, and right after swimming, heavy sweating or towelling off.
- Sunscreens sold in Thailand must show protection values that match their test results, and anything from 50 up is shown as SPF50+.
SPF tells you how well a sunscreen protects against UVB, the rays that burn. PA grades protection against UVA, which reaches deeper and is linked to wrinkles and dark spots. The numbers on the label come from a test that applies 2 milligrams per square centimetre, which for one face works out to roughly 1-1.6 millilitres. Most people use far less, so the protection they actually get is well below the label.
This is part 2 of the “Beauty basics” series, after Know your skin type and keep it to 3 steps.
UVA and UVB: the difference
| UVB | UVA | |
|---|---|---|
| Wavelength | 280-320 nm | 320-400 nm |
| Main effect | Redness, sunburn, linked to skin cancer risk | Reaches the dermis; linked to wrinkles, sagging, dark spots |
| Through the day | Peaks around midday | Fairly constant all day and all year, and passes through window glass |
| Measured by | SPF | PA (or UVA-PF) |
Broad spectrum means it protects against both. Under US FDA rules, the claim needs a critical wavelength of at least 370 nm in testing.
Reading SPF
SPF is not a time. SPF 30 does not mean you can stay in the sun 30 times longer. It means roughly 1 part in SPF of the UVB gets through.
| SPF | UVB getting through (approx.) | Filtered |
|---|---|---|
| 15 | 1/15 ≈ 7% | about 93% |
| 30 | 1/30 ≈ 3% | about 97% |
| 50 | 1/50 = 2% | about 98% |
Going from 30 to 50 filters only about one extra percentage point, and no sunscreen filters 100%. The AAD recommends broad-spectrum SPF 30 or higher. The Skin Cancer Foundation warns that very high SPFs often create a false sense of security: people stay out longer and forget to reapply.
Reading PA
The PA system comes from Japan. It is based on a UVA protection factor (UVA-PF, measured with the PPD method), sorted into grades:
| Grade | UVA-PF |
|---|---|
| PA+ | 2 to under 4 |
| PA++ | 4 to under 8 |
| PA+++ | 8 to under 16 |
| PA++++ | 16 and above |
More plus signs, more UVA protection. If you are prone to dark spots or sit near windows a lot, PA matters as much as SPF.
How much is enough: the maths
SPF testing uses 2 mg of sunscreen per cm² (the test amount the US FDA sets). A 2006 letter in the Indian Journal of Dermatology, Venereology and Leprology estimated face area at about 531 cm² for women and 794 cm² for men.
- Women: 531 × 2 = 1,062 mg ≈ 1.1 g
- Men: 794 × 2 = 1,588 mg ≈ 1.6 g
- Most sunscreen lotions are close to 1 g per ml (a rough assumption), so that is about 1.1-1.6 ml.
- A measuring teaspoon is 5 ml, so this is roughly 1/5 to 1/3 of a teaspoon for the face alone, not counting neck and ears.
The AAD is more generous: at least 1 teaspoon for the face. Add more if you cover your neck and ears. In short, the calculated figure is a minimum; err on the side of more.
What if you use less? A 2007 study in the British Journal of Dermatology notes that sunbathers typically apply only about 0.5 mg/cm², a quarter of the test amount, and found that protection falls off exponentially. At half the amount (1 mg), SPF drops to about its square root; at a quarter (0.5 mg), to about its fourth root.
- SPF 50 at half the amount: √50 ≈ 7
- SPF 50 at a quarter: ⁴√50 ≈ 2.7
That study used an SPF 4 product, so these figures are estimates from the relationship it found, not measurements of every product. The direction is clear, though.
The SPF on the tube only applies when you put it on as thickly as the test did. Thin application means much less protection.
When to reapply
- Apply about 15 minutes before going out (AAD).
- Outdoors, reapply at least every 2 hours, even if your skin stays dry.
- For swimming or sweating, look for “Water Resistant (40 minutes)” or “(80 minutes)”. Under FDA rules, that means the SPF holds after 40 or 80 minutes in water. After that, reapply, and always reapply straight after towelling off.
- No sunscreen is truly “waterproof”.
Example: a beach day. You leave at 9:00 and apply at 8:45. If you stay out of the water, reapply at 10:45, 12:45 and 14:45. But if you swim at 10:00 wearing a 40-minute water-resistant sunscreen, you come out at 10:40, towel off and reapply right away rather than waiting until 10:45. Then the 2 hours restart: next time is 12:40, or sooner if you swim again.
Chemical vs mineral
| Mineral (physical) | Chemical | |
|---|---|---|
| Active ingredients | Zinc oxide, titanium dioxide, or both | Neither of those two |
| Strength | Recommended by the AAD for sensitive skin | Thin, easy to spread, little white cast |
| Watch for | Possible white cast; tinted versions help | Some people find it irritating |
Hybrids combine both. Any type works if it is broad spectrum, SPF 30 or higher, and applied in the right amount. Pick the one you will actually wear every day.
Reading a sunscreen label in Thailand
Most sunscreens in Thailand are regulated as cosmetics under the Cosmetics Act B.E. 2558 (2015).
- Values must match test results. Under the Cosmetic Committee notification on displaying sun protection values, B.E. 2560 (2017), a label whose values are not backed by test results counts as a false or misleading label.
- Anything from 50 up is shown as SPF50+ under Thai rules. A number like SPF 100 on a product sold in Thailand deserves a second look.
- Required warnings. The Cosmetic Committee notification on warnings for cosmetics containing sunscreens requires wording along the lines of: sunscreen is only one way to reduce the risk from sunlight; read the directions; if anything abnormal happens, stop and consult a doctor or pharmacist.
- Notification number. You can check it in the Thai FDA’s system, but a real number does not guarantee the SPF is right. In a test of 20 sunscreens announced in March 2025 by the Thai Consumers Council and Chalard Sue magazine, 5 did not match their SPF label and 4 did not match their PA.
Common mistakes
- Applying thinly to avoid stickiness. Real SPF ends up several times lower. If the texture is too heavy, change products, not the amount.
- Counting the SPF in foundation as sunscreen. Nobody applies foundation at 2 mg/cm². Treat it as a bonus only.
- One morning coat for a whole day outdoors. Sweat, water and rubbing break sunscreen down.
- Missing ears, neck, the backs of hands and lips. Use an SPF 30+ lip balm for lips (AAD).
- Looking only at SPF, not PA. Good UVB protection with weak UVA protection still leaves you open to dark spots.
Water- and sweat-resistant sunscreen is harder to wash off than regular sunscreen; part 4 of this series covers how. The next part looks at pores and oily skin: which beliefs are true and which are not.
Sources
- How to decode a sunscreen label — American Academy of Dermatology
- How to apply sunscreen — American Academy of Dermatology
- The relation between sun protection factor and amount of sunscreen applied in vivo — Br J Dermatol 2007 (PubMed)
- Sunscreen application: Not less, not more — Indian J Dermatol Venereol Leprol 2006
- Cosmetic Committee notification on warning labels for cosmetics containing sunscreens (No. 4), B.E. 2565 — Royal Thai Government Gazette (Thai)
- SPF and PA test of 20 sunscreens, citing the B.E. 2560 notification — Naewna (Thai)
FAQ
Is SPF 100 twice as good as SPF 50?
No. SPF 30 filters about 97% of UVB, SPF 50 about 98%. Each step up adds less. Applying enough and reapplying matter far more, and dermatologists warn that very high numbers tend to make people stay out longer and skip reapplying.
Do I need sunscreen indoors or at the office?
UVA passes through window glass, so if you sit near a window or travel during the day, apply it in the morning. Reapplying every 2 hours matters most when you are outdoors.
What if a sunscreen irritates my skin?
Wash it off and stop using it. Sensitive skin often tolerates fragrance-free mineral sunscreen (zinc oxide / titanium dioxide) better. If you get redness, swelling or strong itching, ask a doctor or pharmacist.
General guidance. Products differ between brands: always follow the label. If you react to a product, see a doctor.