Ingredient Beauty BenefitsAntioxidant · Ascorbic Acid
Vitamin C: above pH 3.5 it does not get into skin
In pig skin, L-ascorbic acid entered only when formulated below pH 3.5, and 20% was the concentration for optimal absorption. Pure ascorbic acid is not among Korea's listed whitening actives; four of its derivatives are.
Last checked Sep 16, 2026
- In pregnancy
- Not established
- Irritation
- Moderate
- Daytime use
- ✓ Fine
Not a measurement — reported irritation grouped into five steps. It varies between people.
The most important number on a vitamin C product is not the percentage. It is the pH.
Ascorbic acid exists in its un-ionised form only in acid conditions, and only that form crosses the stratum corneum. Above pH 3.5 it does not get in, however high the concentration. Few brands print the pH.
This page is about pure ascorbic acid. Many products on the shelf use derivatives instead: more stable, but they have to convert to ascorbic acid in the skin, and how much converts differs by compound.
Contents
What is established
The same ingredient at a different strength, in a different formula, is a different product. These are the conditions the studies used.
It entered skin only below pH 3.5, and 20% was optimal
L-ascorbic acid and its derivatives were applied to pig skin and the levels reached inside the skin were measured. The result reduces to two sentences: 'L-ascorbic acid must be formulated at pH levels less than 3.5 to enter the skin', and 'maximal concentration for optimal percutaneous absorption was 20%'.
Tissue levels saturated after three once-daily applications. Applying more did not accumulate more.
This is not a result from human skin. Pig skin is structurally similar and is used for permeation work, but these numbers cannot be carried over as efficacy established in people. The 1992 study showing protection against UV damage was also porcine.
Hence early research. The pH condition itself, though, is genuinely usable when choosing a product.
- Excised-skin study — not a result from applying it to skin · Funding not declared PMID 11207686
- Animal study — not a result from applying it to skin · Funding not declared PMID 1390169
One derivative improved periorbital wrinkles in an 8-week trial
The trial used tetra-isopalmitoyl ascorbic acid (VC-IP), an oil-soluble derivative. Three groups of 23 women, 69 in all, applied 1%, 2% or 3% cream to the periorbital area on one side twice daily for 8 weeks, with vehicle on the other side. Double-blind, split-face.
Periorbital wrinkles improved on visual grading and photographic assessment, confirmed by topographic measurement. The interesting part is that higher was not simply better — measures associated with dermal structure improved sufficiently at the lower concentrations.
It is a single trial, and the paper does not declare its funding. And the result belongs to VC-IP, not to pure ascorbic acid.
- Randomised controlled trial in people · 69 participants · 8 weeks · Compared with vehicle (same formula minus the active) · Funding not declared · Endpoint scores from blinded assessors PMID 34910367
Pure ascorbic acid is not on Korea's listed whitening actives
Of the nine whitening actives in Annex 4, item 2 of the Regulation on the Review of Functional Cosmetics, four are vitamin C compounds: ethyl ascorbyl ether at 1–2%, ascorbyl glucoside at 2%, magnesium ascorbyl phosphate at 3% and ascorbyl tetraisopalmitate at 2%.
Pure ascorbic acid is not among them. Stability is the likely reason, but the consequence is plain: in Korea you cannot file a whitening claim on pure ascorbic acid without submitting your own data.
If a label says 'whitening functional cosmetic', what is inside is usually one of those derivatives.
How to use it
- Choose a product that publishes its pH. For pure ascorbic acid, above 3.5 the absorption evidence does not apply.
- Opaque, air-tight packaging. Ascorbic acid oxidises with light and air and turns brown. A product that has changed colour has already oxidised.
- Absorption saturated after three days. There is no reason to reapply several times a day.
- It is a daytime-friendly ingredient. Using it under sunscreen is the ordinary pattern.
Cautions
- The low pH is itself irritating. On sensitive skin, the derivatives are gentler.
- Do not read derivative evidence and pure ascorbic acid evidence as one. The 8-week result above belongs to VC-IP.
- On the claim that vitamin C must not be combined with niacinamide, see the niacinamide page: we could not find the basis for it.
- We found no established data on use during pregnancy.
With a new ingredient, apply a small amount somewhere inconspicuous, such as the inner arm, for two days first. Stop if it reddens or stings.
Related skin concerns
What this article relies on
- 기능성화장품 심사에 관한 규정 [별표 4] 자료제출이 생략되는 기능성화장품의 종류 — 식품의약품안전처고시 제2025-88호식품의약품안전처 · 시행 2025-12-16 · Checked on 2026-09-16
- PubMed — 미국 국립의학도서관 문헌 데이터베이스National Library of Medicine · Checked on 2026-09-16