UVA vs. UVB: What SPF and PA+++ Ratings Actually Tell You

What SPF and PA Ratings Actually Measure

Most people pick a sunscreen by the SPF number on the front and call it a day. That's a problem, because that number only tells you about half the story. SPF measures protection against UVB, the rays that burn you. It says nothing about UVA, the rays that age you, trigger pigmentation, and quietly damage your skin even on cloudy days through window glass.

Honestly, the labeling system is a mess. SPF goes up to 50+ in the US, while in Asia you'll see PA+ through PA++++ stamps. The EU prints "UVA" inside a circle. None of these systems talk to each other directly, which is why so many shoppers walk away confused. Here's the thing: once you know what each rating actually measures, picking sunscreen becomes much easier.

How UV Protection Works on Skin

Ultraviolet light reaches Earth in two main types that matter for your skin. UVB sits at 280 to 320 nanometers and gets absorbed by the outer layer (the epidermis), where it damages DNA in keratinocytes and triggers the inflammatory response we call sunburn. UVA runs longer at 320 to 400 nanometers and slips deeper into the dermis, where it generates free radicals that break down collagen and elastin, then activates melanocytes to produce uneven pigment.

About 95% of the UV radiation hitting the ground is UVA. That ratio doesn't change much from morning to evening or summer to winter, which is why dermatologists keep insisting on year-round protection. UVB intensity, by contrast, peaks around solar noon and drops dramatically in winter or at higher latitudes.

Sunscreens work in two ways depending on the active. Mineral filters like zinc oxide and titanium dioxide sit on the skin and absorb most UV (a smaller fraction reflects, despite the popular "physical blocker" myth). Chemical filters absorb specific wavelengths and convert that energy into harmless heat. The newer generation of filters available in Europe, Australia, and parts of Asia covers the long-UVA range up to 400nm far better than older formulas built around avobenzone alone.

What SPF Actually Means

SPF stands for Sun Protection Factor, and it's measured in a lab using burning as the endpoint. SPF 30 means it takes 30 times more UVB to burn protected skin than unprotected skin under standardized test conditions. The math gets misleading though.

Here's the breakdown most marketing skips:

• SPF 15 blocks about 93% of UVB
• SPF 30 blocks about 97%
• SPF 50 blocks about 98%
• SPF 100 blocks about 99%

The jump from 30 to 50 looks small on paper, but it cuts the UVB getting through by roughly a third. We think SPF 50 is worth choosing for daily wear because real-world application is the catch. Studies consistently show people apply 25 to 50% of the lab-test amount (which is 2 mg per square centimeter, roughly a quarter teaspoon for the face alone). At half the recommended dose, your SPF 50 is performing more like SPF 7. That's not a typo. Sun protection scales roughly with the square root of how much you actually applied versus how much was tested.

What PA Ratings and the UVA Circle Tell You

The PA system was developed in Japan and uses the PPD test (Persistent Pigment Darkening), which measures how much UVA exposure causes lasting tanning on protected vs unprotected skin.

• PA+ = PPD 2 to 4 (some UVA protection)
• PA++ = PPD 4 to 8 (moderate)
• PA+++ = PPD 8 to 16 (high)
• PA++++ = PPD 16+ (very high)

The European "UVA" inside a circle uses a different rule: the UVA protection factor must be at least one-third of the SPF. So a sunscreen labeled SPF 50 with the UVA circle has a UVA protection factor of at least 16.7, roughly equivalent to PA+++.

In the US, the FDA uses the term "broad spectrum," but its bar is lower. To earn broad-spectrum status, a sunscreen must pass the critical wavelength test, meaning 90% of its absorption happens at or below 370nm. This tells you the formula covers a reasonable spread of UVA wavelengths, but it doesn't quantify how much UVA gets blocked. That's why a US-labeled "broad spectrum SPF 50" can have wildly different UVA protection from a Korean PA++++ SPF 50, even though both look equivalent on the front of the bottle.

Why Broad-Spectrum UVA UVB Protection Matters More Than a Big Number

UVA does the slow, invisible damage. It's responsible for most photoaging (the famous 2012 NEJM image of a truck driver with one prematurely aged side of his face is the classic example). It penetrates window glass, drives melasma flares, generates oxidative stress that depletes vitamin E and CoQ10 in the skin, and contributes to skin cancer risk that never shows up as a visible burn.

We honestly think the obsession with chasing SPF 100 misses the point. A well-formulated SPF 30 with strong long-UVA coverage outperforms an SPF 100 with weak UVA filtering for the kind of damage you'll actually see in the mirror at 50. The PA rating sunscreen labels in your routine matter more than the headline SPF number.

Who Needs the Highest UVA Protection

Some people should treat PA++++ or its European equivalent as non-negotiable:

• Anyone with melasma or post-inflammatory hyperpigmentation. UVA and visible light reactivate pigment cells. Iron oxide-tinted sunscreens add visible-light defense, and clinical trials show they outperform untinted SPF for melasma.
• People on tretinoin, hydroquinone, or strong chemical exfoliants. Active ingredients thin the stratum corneum and increase photosensitivity by measurable amounts.
• Anyone with a personal or family history of skin cancer. UVA contributes to both melanoma and squamous cell carcinoma development.
• People who drive a lot or sit near windows. UVA passes through standard glass; UVB doesn't, which is why one-sided sun damage on commuters is so common.
• Darker skin tones with pigmentation concerns. The idea that melanin provides full UV protection is wrong. Skin types IV through VI burn less easily but pigment unevenly more easily under UVA.

If none of those apply and you only see daylight on the walk to your car, a broad-spectrum SPF 30 with PA+++ is plenty.

How to Actually Use Sunscreen Properly

The single biggest factor in real-world sun protection isn't the rating, it's the amount and the reapplication. Here's what we'd actually recommend:

• Use a quarter teaspoon for face and a quarter teaspoon for neck. Two finger-lengths of product squeezed onto your index and middle finger gets close. If your facial sunscreen lasts more than three months, you're underdosing.
• Apply it as the last step of skincare, before makeup. Let it set for two to three minutes so the film forms properly. Mineral sunscreens need this even more than chemical ones.
• Reapply every two hours of direct sun exposure, or after swimming, sweating, or towel-drying. Indoors with no window exposure, one morning application is fine.
• Powder and stick reapplication over makeup work, but only with enough product. A two-second dust of SPF powder isn't doing anything measurable.

For body, the standard guidance is one full ounce (about 30ml, a shot glass) for full coverage in a swimsuit. That's more than most people use across an entire summer.

What to Pair It With (and What to Avoid)

Antioxidant serums in the morning multiply the protection sunscreen provides by neutralizing free radicals that slip through. Vitamin C (L-ascorbic acid at 10 to 20%) is the most studied; vitamin E and ferulic acid stabilize it and add their own UV-defense benefit. Niacinamide pairs well too and reduces UV-induced inflammation.

Iron oxides, the colorant that makes a sunscreen tinted, block visible light, which standard SPF actives mostly don't. For melasma, the data on tinted formulas is genuinely strong.

Avoid layering sunscreen over freshly applied high-percentage AHAs or BHAs in the morning unless your routine is built for it; the exfoliated skin underneath needs more protection, not less. Retinoids belong in the evening only, both because of UV instability and because of next-day photosensitivity.

One thing we'd skip: combining a mineral sunscreen with silicone-heavy primers. The interaction tends to pill, and pilled sunscreen is uneven sunscreen.

Potential Side Effects and Things to Watch For

Most modern sunscreens are well-tolerated, but a few caveats are worth knowing.

• Eye stinging. Older avobenzone-based formulas are the worst offenders. If sunscreen burns your eyes, look for water-resistant formulas or mineral-only options.
• White cast. Zinc oxide above 15% almost always casts. Newer micronized zinc reduces this, but darker skin tones often need chemical or hybrid formulas to avoid the gray look.
• Breakouts. Often caused by occlusive carriers (heavy silicones, coconut oil derivatives) rather than the actives themselves. Look for non-comedogenic labeling and gel or fluid textures.
• Contact dermatitis. Octocrylene and oxybenzone are the most common culprits. PABA, the original chemical filter, was phased out for similar reasons decades ago.
• Vitamin D concerns. Real-world sunscreen use does not cause clinically meaningful vitamin D deficiency in studies. People who avoid sun entirely or live in northern latitudes are better off supplementing than skipping sunscreen.

The 2019 FDA pilot study showing systemic absorption of certain chemical filters scared a lot of consumers. The honest read: absorption above the testing threshold doesn't equal harm, and the FDA explicitly told people not to stop using sunscreen while further research continues.

Frequently Asked Questions

Is SPF 100 worth the extra cost over SPF 50?

Marginally. SPF 50 blocks 98% of UVB, SPF 100 blocks 99%. The difference matters most for people on photosensitizing medications or with conditions like lupus. For everyone else, money is better spent on stronger UVA filters or on actually using enough product per application.

Does SPF in moisturizer or makeup count?

Only if you apply it at sunscreen-grade thickness, which almost nobody does. A foundation with SPF 30 applied at normal makeup thickness performs more like SPF 5. Treat tinted SPF and SPF makeup as a bonus, not a replacement.

Can I skip sunscreen on cloudy days or indoors?

UVA penetrates clouds at roughly 80% intensity and passes through window glass freely. If you sit near a window or commute by car, you're getting UVA exposure. Indoor-only days with no window exposure are the only time skipping is reasonable.

Why do Asian and European sunscreens feel so much better than US formulas?

Because the FDA hasn't approved a new sunscreen filter since 1999. The newer filters available abroad (Tinosorb S, Mexoryl 400, Uvinul A Plus) are more photostable, cover broader UVA wavelengths, and feel lighter on skin. Until the US updates its monograph, that gap is going to stay open.

Related Reading

• How to Choose the Right Sunscreen for Your Skin Type
• The Truth About Mineral vs. Chemical Sunscreens
• Why You Need Sunscreen Even When Indoors