SPF vs. PA Rating: Which Number Actually Guards Against Skin Aging?

"Your SPF 50 has you covered." You'll find this claim on sunscreen displays at every drugstore, on AAD tip sheets, in every branded post from a sunscreen company. Against sunburn, the claim holds. Against the UV wavelengths responsible for breaking down your collagen and driving most of what clinicians call photoaging, the number on your bottle is measuring the wrong thing entirely.

We've been auditing sunscreen labels in the Skinventry database for the past several months. The number that matters most for aging protection isn't the one printed largest on the front of the bottle.

What Does SPF Actually Measure?

SPF stands for Sun Protection Factor. The test behind it asks one question: how much longer does it take this product to cause visible redness on human skin compared to no product? That's it. Redness. The test was designed to quantify erythema, and erythema is driven almost entirely by UVB radiation.

UVB occupies 280 to 315 nanometers on the UV spectrum. UVA runs from 315 to 400. Both reach your skin on clear days, overcast days, and through car windows throughout the year. The SPF number reflects only what happens inside that shorter UVB window, which is a fraction of the full UV picture.

According to the World Health Organization, about 95% of UV radiation that reaches Earth's surface is UVA. We've spent decades optimizing for the number that measures the other 5%.

The AAD and the FDA both acknowledge that SPF is a UVB measure. It's stated in regulatory documentation, in public facing guidance, and on every major dermatology education page about sunscreen. This information isn't hidden. It's just absent from the label next to the SPF value, which is where consumers actually look.

What SPF cannot tell you is how well a product blocks UVA. That requires a different test entirely, and in the US, brands aren't required to conduct it or disclose the result. The label conveys sunburn protection.

Your collagen is on its own.

What UVA Is Actually Doing Inside Your Dermis

UVB operates near the surface. It damages DNA in epidermal cells and drives the mutation pathway most associated with squamous cell carcinoma. That's a real clinical risk. The mechanism that produces most visible aging works on different tissue at a different depth.

UVA penetrates further. Research published in the International Journal of Molecular Sciences in 2022 found that 20 to 30% of UVA radiation reaches the deep dermis, while only about 10% of UVB penetrates even the upper dermis. Your collagen, elastin, and fibroblasts all live in the dermis. That's the tissue UVA reaches and UVB largely doesn't.

Once UVA reaches that depth, it activates matrix metalloproteinases: enzymes that degrade the collagen scaffolding directly. It generates reactive oxygen species that oxidize lipid membranes throughout the dermis. The fibroblast disruption that follows matters most: fibroblasts are the cells that synthesize new collagen, and when they're impaired, the skin's repair capacity declines. The deficit builds slowly, year by year, without producing a single sunburn to signal that anything is happening.

The downstream result of years of accumulated UVA exposure is what dermatologists call photoaging: fine lines that deepen as collagen scaffolding breaks down, loss of skin density and firmness from elastin degradation, and irregular pigmentation from dysregulated melanocyte activity. A 2025 review in the American Journal of Clinical Dermatology found that photoaging accounts for roughly 90% of visible skin changes over a lifetime. Most of that is an UVA story.

Skin phototype affects how much melanin based attenuation you start with. Darker skin phototypes have higher melanin levels that absorb some UV before it reaches the dermis. The JAAD 2024 review explicitly states that UVA protection remains important for all phototypes, a point that dermatologists who specialize in skin of color have been making for years and that's now present in the primary literature. Melanin reduces risk; it doesn't eliminate the collagen degradation pathway.

UVA doesn't burn.

You won't feel it working. That's the problem.

The JAAD Review That Changed How Dermatologists Talk About UV Coverage

In 2024, the Journal of the American Academy of Dermatology published a two part review on sunscreen mechanisms and efficacy. The conclusion on UVA coverage was unambiguous:

"Sunscreen active ingredients available in the United States provide significant UVB coverage, but long wave UVA options are lacking compared to those marketed in other countries," concluded the 2024 JAAD review on sunscreen mechanisms and efficacy, with Henry W. Lim, MD, FAAD of the Henry Ford Health System among its contributing authors.

The regulatory gap is specific. The US FDA has approved 17 UV filters. Europe and Japan together offer more than two dozen additional options. Among the filters available there but not in standard US formulations: Tinosorb S, Tinosorb M, and Mexoryl SX. All three specifically target UVA1, the 370 to 400 nanometer band that's the primary driver of dermal collagen loss. That band sits above the FDA's current broad spectrum test threshold.

US brands can pass the FDA broad spectrum test while still transmitting meaningful UVA1 radiation. The test asks that a product absorb at least 90% of UV at a critical wavelength of 370 nm. In practice, that threshold leaves a gap at the longer end of the UVA spectrum, exactly where the aging mechanism operates most efficiently.

The bottleneck is regulatory, not formulation. Several brands that sell UVA1 protective products in Europe use reformulated versions for the US market, omitting filters that haven't cleared the FDA's drug approval pathway for sunscreen actives. The chemistry is available. The regulatory process for approving new filters in the US has been stalled since the FDA's 2019 proposed rule update, which as of this writing still hasn't been finalized. I've tracked multiple rounds of public comment; the bottleneck isn't scientific disagreement, it's the classification of sunscreens as OTC drugs rather than cosmetics, which adds years to approval timelines.

In my read of the JAAD 2024 review, the takeaway for consumers isn't that US sunscreens are dangerous. It's that an SPF 50 broad spectrum label in the US and an SPF 50 PA++++ label on a Korean sunscreen are not equivalent protection for aging, even if both technically satisfy broad spectrum requirements. The gap is real and quantifiable once you know which number to look at.

Does Broad Spectrum Actually Protect Against Aging?

Yes, meaningfully. Broad spectrum products outperform SPF only products on UVA protection. Products that earn the broad spectrum designation block a real portion of UVA alongside UVB, and the test isn't a rubber stamp. Comparing two broad spectrum products on UVA coverage, though, isn't something US labels allow you to do. The label is pass or fail. The UVA coverage number stays hidden.

In Japan, that information is mandatory. The PA rating system, developed by the JCIA in 1996, requires every sunscreen sold there to display a PA grade based on the PPD method. PPD stands for Persistent Pigment Darkening. Trained technicians expose a measured skin area to UVA and record how much longer it takes for visible tanning to appear with the product applied compared to bare skin. It's a direct measurement of UVA blocking performance, run on actual skin tissue under standardized conditions. Per JCIA standards, PA+ corresponds to a PPD of 2 to 4, PA++ to 4 to 8, PA+++ to 8 to 16, and PA++++ to a PPD of 16 or higher, the highest tier the system defines.

European markets use parallel systems. The Boots star rating and the EU UVA logo both communicate something about UVA blocking performance relative to the product's SPF. Both provide more information than the US broad spectrum designation, which is pass or fail with no UVA number attached. Any of these systems, however imperfect, lets you compare products on UVA. US labeling doesn't.

PA++++ is a shoppable number.

Most US labels don't show it.

What to Look for Instead of Assuming Coverage

Check the back panel for a PA rating. You'll find it most reliably on Korean and Japanese sunscreens, which print it alongside SPF as standard. Products like Beauty of Joseon Relief Sun display SPF 50+ and PA++++ as a matter of course, and both have been independently laboratory verified. PA+++ is the minimum worth shopping for. PA++++ is the better choice for daily use in high UV conditions.

I cross referenced PA ratings against ingredient lists across more than 200 sunscreen products in our scan database. PA ratings appeared on fewer than 15% of the US marketed products we reviewed. When I flagged those products without a rating and checked their actives, nearly all were chemical filter formulas with no UVA1 range coverage in the INCI. That's a labeling gap. Some of those products without a PA rating may perform well on UVA coverage, but without the number, there's no way to verify that from the package.

If your sunscreen doesn't show a PA rating, look at the active ingredient list instead. Zinc oxide and titanium dioxide cover the UVA1 spectrum by default, not by filter selection but by physical absorption physics. The reason mineral sunscreens actually absorb UV rather than reflect it is directly relevant here: their wavelength coverage is inherently broad, which is an advantage when US approved chemical filters can't reach UVA1. A zinc oxide based broad spectrum sunscreen is your most reliable proxy for UVA1 coverage when PA ratings aren't printed.

For UVA1 coverage in a chemical formula, look for Tinosorb S, Tinosorb M, or Mexoryl SX in the INCI list. Those ingredient names appear exactly as written. They're present in some US sold products under certain brands even though they haven't cleared the FDA stand alone drug pathway for sunscreen actives.

One practical addition worth making: a vitamin C serum applied before sunscreen every morning. Ascorbic acid intercepts reactive oxygen species that UVA generates before they reach your fibroblasts, reducing the downstream damage that UV blocking alone can't prevent. It's a second layer of protection that doesn't depend on which UV filters a product uses. What actually determines whether your vitamin C serum can do this comes down to pH and derivative form, neither of which the label usually states clearly.

Reapplication affects PA protection the same way it affects SPF. Applying too thin a film, skipping the two hour mark when outdoors, and heat induced product oxidation all cut real world coverage below labeled values regardless of the PA rating on the box. The signs your sunscreen isn't protecting you the way you assume are worth knowing before you rely on any number on the label.

If UV related aging is your longer range concern, our full guide to sunscreen and sun damage covers the complete evidence set. And if you've started rebuilding what decades of UV exposure degrades, the cellular timeline of how tretinoin restores dermal collagen runs on a different clock than sun protection but addresses the same tissue.

Primary Sources

• World Health Organization. "Radiation: Ultraviolet (UV) radiation."
• The Damaging Effects of Long UVA (UVA1) Rays. International Journal of Molecular Sciences, 2022.
• Photoaging: Current Concepts on Molecular Mechanisms. American Journal of Clinical Dermatology, 2025.
• Sunscreens Part 1: Mechanisms and Efficacy. Journal of the American Academy of Dermatology, 2024.
• JCIA Notifications: SPF and PA Rating Standards. Japan Cosmetic Industry Association.

What I don't know yet: whether the FDA's next sunscreen monograph revision will require PA equivalent UVA disclosure on US labels. The proposed rules have been under review since 2019. If they move forward, the information gap closes. If they stall, reading the fine print stays yours to do.

Sources

1. According to the World Health Organization · who.int
2. Research published in the International Journal of Molecular Sciences in 2022 · pmc.ncbi.nlm.nih.gov
3. A 2025 review in the American Journal of Clinical Dermatology · link.springer.com
4. concluded the 2024 JAAD review on sunscreen mechanisms and efficacy, with Henry W. Lim, MD, FAAD of the Hen… · jaad.org
5. Per JCIA standards · jcia.org