Why 10% Isn't Stronger Than 5%: The Science of Bioavailability

A founder note from Kusuma. The skincare aisle has trained us to read percentages like prices: bigger must mean more. It doesn't. Here's the chemistry nobody puts on the label.

Walk down any skincare aisle and you'll see it: "20% Vitamin C!", "10% Niacinamide!", "15% Azelaic Acid!". The number gets bigger, the claim gets louder, and we assume the result gets better. But here's the truth that formulators know and marketing teams hope you don't: the percentage on the jar tells you how much active was added, not how much ever reaches the living skin where it works. That second number, the part that actually gets in, is called bioavailability, and it's the only one that matters.

A poorly delivered 10% can leave less active in your skin than a well-formulated 5%. Let me walk you through exactly why: pH, the skin barrier, molecule size, and the clever delivery systems that change the game.

Bioavailability: the number that actually matters

In pharmacology, bioavailability is the fraction of a dose that actually reaches its site of action in an active form. A 10% serum that sits on the surface, can't cross the barrier, or is at the wrong pH to be absorbed might deliver a fraction of a percent into living skin. A thoughtfully built 5%, right pH, right molecule, right delivery vehicle, can deliver several times more.

So "higher percentage" is only an advantage if everything else is equal. It almost never is. Three things decide the real outcome: pH, the skin barrier (size + solubility), and delivery system.

Why pH matters

Your skin isn't pH-neutral. Its surface, the acid mantle, sits naturally around pH 4.5 to 5.5, slightly acidic, which keeps the barrier healthy and the right microbes happy.¹

pH matters for actives for two reasons:

1. It controls absorption. Many actives are weak acids or bases that exist in two forms, charged (ionised) or uncharged (non-ionised), depending on pH. The uncharged form slips through the skin's oily barrier far more easily; the charged form mostly sits on top. So the same molecule at the same percentage can be well-absorbed at one pH and barely absorbed at another. This is exactly why a vitamin C (L-ascorbic acid) serum needs a low pH (~3.5) to work, and why a poorly buffered formula underperforms regardless of its percentage.
2. It controls irritation and stability. A formula too far from skin's natural pH can sting, disrupt the barrier, and degrade the active itself. On Indian skin (Fitzpatrick IV to VI), barrier disruption means inflammation, and inflammation means more pigmentation. Percentage means nothing if the formula is irritating your skin into producing dark marks.

The takeaway: a great formula tunes its pH to deliver the active and respect the acid mantle. A high percentage at the wrong pH is just expensive sting.

The wall: your skin's lipid barrier

Here's the structure every active has to get past. The outermost layer, the stratum corneum, is built like a brick wall: flattened dead skin cells (the "bricks") held together by a mortar of lipids: ceramides, cholesterol, and fatty acids.²

The crucial detail: that mortar is lipophilic, oil-loving. To cross it, a molecule generally has to be small enough and friendly enough with oil to dissolve into the lipid layer and diffuse through. A big, water-loving, electrically charged molecule hits this wall and mostly stays on the surface, no matter how much of it you apply.

This is why the barrier is so good at its day job (keeping the outside world out) and so frustrating for skincare (keeping your actives out too).

The 500-Dalton rule: size is the gatekeeper

How small is "small enough"? Dermatology has a well-known rule of thumb: the 500-Dalton rule. A Dalton (Da) is a unit of molecular weight. The observation, from decades of penetration research, is that molecules under ~500 Da can penetrate the stratum corneum reasonably well, while molecules much larger than that struggle to get through intact.³

This single rule explains a lot of skincare mysteries:

• Niacinamide (~122 Da) and 4-n-butyl resorcinol (~166 Da) are both small: they cross the barrier well, which is part of why they're so effective.
• High-molecular-weight hyaluronic acid (often hundreds of thousands of Daltons) is far too big to penetrate. It's a superb surface humectant, it holds water on the skin, but it isn't diving into your dermis. (This is why good HA products use a range of molecular weights, including small fragments, for layered hydration.)

A quick caveat that sets up the next section: molecular weight isn't the only gate. Size tells you whether a molecule could fit through, but it still has to be dissolved to get there in the first place. That's exactly where pure azelaic acid trips up.

So when you compare two products, molecular size tells you more about what will actually work than the headline percentage does.

Case study: 10% pure azelaic acid vs 8% PAD

This is the comparison that makes the whole principle click, and it's personal, because it shaped how I formulated Lucènci.

Azelaic acid is a brilliant active for pigmentation and acne. By molecular weight alone (~188 Da) it's small enough to penetrate, but weight isn't the whole story, because pure azelaic acid is notoriously hard to formulate. It's a stubborn, gritty powder that doesn't dissolve easily. To force a high percentage like 10 to 20% into a product, it often ends up partly undissolved: micro-crystals suspended in the base. When you rub it on, much of it is essentially sitting on your skin like fine salt or sand: it pills under makeup, it can sting, and a lot of that impressive "20%" never dissolves enough to make it past the wall.

Now compare PAD (Potassium Azeloyl Diglycinate), the form we use at 8%. PAD is azelaic acid that's been chemically bonded to glycine (an amino acid). That bond makes it water-soluble and skin-friendly, so instead of gritty crystals, you get a smooth, evenly dissolved active that glides into the skin with far less irritation.⁴

So which is "stronger", 10% pure azelaic acid or 8% PAD? On the label, the 10% wins. On your skin, the 8% PAD often delivers more usable active with less inflammation, exactly the bioavailability-over-percentage story in one comparison. (We go deep on this in our azelaic acid guide.)

The clever part: delivery systems

If size and solubility are the barriers, the frontier of skincare is smuggling actives past them. Enter nano-scale delivery vehicles: tiny bubbles that wrap an active and help carry it into the skin.^5,6

• Liposomes, microscopic spheres made of a phospholipid bilayer (the same kind of material your own cell membranes are built from) wrapped around a watery core. Because they're so similar to skin's own lipids, they merge with the barrier and ferry their cargo inward, improving penetration and often reducing irritation.⁵
• Niosomes, similar bubbles, but built from non-ionic surfactants instead of phospholipids. They tend to be more stable and more cost-effective than liposomes while doing a similar delivery job.⁶
• Exosomes, the newest and most sophisticated: naturally occurring, cell-derived nanovesicles that cells use to talk to each other. Beyond just carrying an active, they come loaded with their own signalling cargo (growth factors, peptides, RNA), which is why they're being explored as next-generation regenerative ingredients.⁷

The point isn't that you need every buzzword on your shelf. It's that how an active is delivered can matter as much as how much is in there. A modest percentage in a smart delivery system can outperform a big percentage that never gets past the wall.

How to read a product like a formulator

Next time you're comparing two products, look past the big number on the front:

1. Is the active small enough to penetrate? (Roughly under ~500 Da for surface actives meant to work in the skin.³)
2. Is it the right form? A gentle, soluble derivative (like PAD for azelaic acid) often beats a raw, gritty, higher-percentage version.⁴
3. Is the pH sensible, effective for the active and kind to your acid mantle?¹
4. Is there a delivery system (liposomal, etc.) doing the hard work of getting it in?^5,6
5. Is the percentage within the evidence-based range, or just a marketing flex past the point of diminishing returns (and rising irritation)?

A "10%" that fails points 1 to 4 is weaker, in practice, than a "5%" that nails them.

Founder note: how this shaped Lucènci

Every choice in our Acne Defense + Pigmentation Serum came out of exactly this thinking:

• 8% PAD instead of higher-percentage pure azelaic acid, because the water-soluble, amino-acid-bonded form actually penetrates and doesn't sit on the skin as gritty crystals.⁴
• 4% niacinamide, not 10%, because the evidence for fading pigment and supporting the barrier sits in the 2 to 5% range, and a small molecule (~122 Da) at that level penetrates beautifully without the flushing some people get at 10%.³
• A 16-form hyaluronic acid complex, a range of molecular weights rather than one, so you get surface hydration from the large fragments and deeper conditioning from the small ones, instead of one big molecule that only sits on top.

I'd rather give you a smaller number that your skin can actually use than a big number that looks impressive on the jar and pills off your face. Bioavailability over bravado.

Frequently asked questions

Is a higher percentage of an active always better? No. Above the evidence-based range, you usually get more irritation, not more benefit, and if the formula's pH, the molecule's size, or its solubility are wrong, a high percentage can deliver less into your skin than a smart lower one.

What is the 500-Dalton rule? A dermatology rule of thumb: molecules under about 500 Daltons penetrate the skin's top layer reasonably well, while much larger molecules struggle to get through.³

Why doesn't hyaluronic acid "plump from within"? High-molecular-weight HA is far too large to penetrate, it hydrates the surface. Good HA products blend multiple molecular weights for layered hydration.

Is 8% PAD really comparable to higher-percentage azelaic acid? For tolerability and real-world delivery, yes, PAD is the water-soluble form that penetrates evenly without the grit and sting of raw azelaic acid.⁴

Do I need liposomes or exosomes in my products? Not necessarily, plenty of small actives penetrate fine on their own. But delivery systems can meaningfully boost harder-to-deliver actives and reduce irritation.^5,6,7

The bottom line

The percentage on the front of the jar is the least informative number in skincare. What decides whether an active works is bioavailability, and that's governed by pH, the skin's lipophilic barrier, molecular size (the 500-Dalton rule), the form of the active, and how it's delivered. A well-built 5% can quietly out-perform a flashy 10%.

That's the philosophy behind our serum: smart forms, sensible percentages, real delivery, formulated for what actually reaches your skin.

-Kusuma Founder, Lucènci

References

1. Lambers H, Piessens S, Bloem A, Pronk H, Finkel P. Natural skin surface pH is on average below 5, which is beneficial for its resident flora. Int J Cosmet Sci. 2006 Oct;28(5):359-70. PubMed: 18489300
2. Norlén L, Lundborg M, Wennberg C, Narangifard A, Daneholt B. The Skin's Barrier: A Cryo-EM Based Overview of its Architecture and Stepwise Formation. J Invest Dermatol. 2022 Feb;142(2):285-292. PubMed: 34474746
3. Bos JD, Meinardi MM. The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Exp Dermatol. 2000 Jun;9(3):165-9. PubMed: 10839713
4. Del Rosso JQ, Levin J. The clinical relevance of maintaining the functional integrity of the stratum corneum in both healthy and disease-affected skin. J Clin Aesthet Dermatol. 2011 Sep;4(9):22-42. PubMed: 21938268
5. Natsheh H, Touitou E. Phospholipid Vesicles for Dermal/Transdermal and Nasal Administration of Active Molecules: The Effect of Surfactants and Alcohols on the Fluidity of Their Lipid Bilayers and Penetration Enhancement Properties. Molecules. 2020 Jun 27;25(13):2959. PubMed: 32605117
6. Khatoon M, Shah KU, Din FU, et al. Proniosomes derived niosomes: recent advancements in drug delivery and targeting. Drug Deliv. 2017;24(sup1):56-69. PubMed: 29130758
7. Shen X, Song S, Chen N, Liao J, Zeng L. Stem cell-derived exosomes: A supernova in cosmetic dermatology. J Cosmet Dermatol. 2021 Dec;20(12):3812-3817. PubMed: 34536054

This article is educational and not medical advice. Molecular weights given are approximate and illustrative. Patch test new actives and consult a dermatologist for persistent skin concerns.