When choosing sunscreen, most people think "higher SPF is better." But the actual protection difference between SPF50+ and SPF30 is smaller than you'd expect.
To choose sunscreen well, you need to properly understand what SPF and PA mean.
What is SPF?
SPF (Sun Protection Factor) is the UVB protection rating. UVB is the short-wavelength UV that burns skin (sunburn) and causes skin cancer.
| SPF | UVB protection | Transmission |
|---|---|---|
| SPF 15 | 93.3% | 6.7% |
| SPF 30 | 96.7% | 3.3% |
| SPF 50 | 98% | 2% |
| SPF 100 | 99% | 1% |
The protection difference between SPF30 and SPF50 is only 1.3 percentage points. What matters is applying the right amount properly. Applying even half the recommended amount (about 1/4 teaspoon for the face, 1.5–2mg/cm²) drops real protection to roughly the square root of the SPF.
What is PA?
PA (Protection Grade of UVA) is the UVA protection grade. UVA penetrates deep into the skin and causes photoaging (wrinkles, pigmentation). It passes through glass and affects skin year-round regardless of weather.
| PA grade | UVA protection |
|---|---|
| PA+ | 2–4x protection |
| PA++ | 4–8x protection |
| PA+++ | 8–16x protection |
| PA++++ | 16x+ protection |
PA+++ or higher is recommended for daily UV exposure (commuting, indoor windows), and PA++++ for frequent outdoor activity.
Physical vs chemical filters
| Item | Physical filter | Chemical filter |
|---|---|---|
| Ingredients | Zinc oxide, titanium dioxide | Oxybenzone, avobenzone, octinoxate |
| Mechanism | Reflect/scatter UV | Absorb UV and release as heat |
| Application | Can leave white cast | Clear and light |
| Sensitive skin | Low irritation, suits sensitive skin | Possible irritation |
| Water resistance | Relatively higher | Can be weak to sweat/water |
| Reapplication need | Same | Same |
How much sunscreen should you apply?
Research shows real consumers apply only 20–50% of the recommended amount. In that case, real protection drops below half the labeled SPF.
Recommended facial amount: two finger-lengths (the 2-finger rule), or about the size of a coin
Reapplication: every 2–3 hours. Reapply immediately if exposed to sweat or water during outdoor activity.
FAQ
Q: Should I wear sunscreen on cloudy days?
Yes. Clouds block only 20% of UVB, while 80–90% of UVA passes through. UV exposure continues on cloudy days.
Q: Do I get better protection using sunscreen plus a foundation with UV filters?
Protection doesn't simply add up. The correct order is sunscreen first, then foundation on top.
Q: Can sunscreen cause acne?
Some chemical filters or oil-rich textures can clog pores. Choose a non-comedogenic sunscreen or a physical (zinc-oxide-based) filter.
SKINROUTE and UV management
The SKINROUTE AI coach suggests sunscreen types and reapplication routines suited to your skin type (sensitive, oily, dry) and lifestyle (indoor work, outdoor activity).