Hyaluronic Acid Explained

Hyaluronic acid appears in nearly every skincare product sold today. Serums, moisturizers, eye creams, sheet masks, even foundations claim it as a hero ingredient. Most of the marketing is noise. HA is a useful molecule, but understanding molecular weight, application technique, and climate factors is the difference between visible hydration and wasted product. This guide covers the actual science.

What Hyaluronic Acid Actually Is

Hyaluronic acid is a glycosaminoglycan — a long sugar chain naturally found in skin, joints, and eyes. Your body produces it constantly. A single HA molecule can hold up to 1,000 times its weight in water, which is why it keeps skin plump, joints lubricated, and eyes moist.

In your skin, HA lives primarily in the dermis (the deeper layer), where it maintains hydration and volume. Dermal HA decreases with age — roughly 50% reduction by age 50. Topical HA cannot fully replace dermal HA (the molecules are too large to reach the dermis efficiently), but it can significantly improve hydration in the upper skin layers where dryness and fine dehydration lines form.

Molecular Weight: The Detail Most People Miss

HA molecules come in different sizes, measured by molecular weight in Daltons (Da). Size determines where the molecule sits in your skin and what it does there. This is the most important concept in HA skincare, and most products ignore it entirely.

• High molecular weight (1,000-1,800 kDa): Large molecules that sit on the skin surface. They form a moisture-retaining film that prevents transepidermal water loss (TEWL). Immediate plumping effect. Cannot penetrate the stratum corneum. Washes off.
• Medium molecular weight (100-1,000 kDa): Partially penetrates the outer skin layers. Provides both surface hydration and some deeper moisturizing. The most practical size for daily skincare.
• Low molecular weight (20-100 kDa): Small enough to penetrate deeper into the epidermis. Longer-lasting hydration. Some published research suggests low-MW HA may trigger a mild wound-healing response that stimulates collagen — though this evidence is preliminary.
• Ultra-low molecular weight (under 20 kDa): Nano-sized HA that reaches the deeper epidermis. Found in some premium formulations. Too small to hold as much water per molecule, so it functions more as a penetration-enhancing delivery vehicle than a standalone hydrator.

The Humidity Factor

HA is a humectant — it attracts water from its surroundings. In humid climates (above 60% relative humidity), it pulls moisture from the air into your skin. In dry climates (below 40%), there is less atmospheric moisture to pull. So where does the HA get its water?

From your deeper skin layers. High-MW HA sitting on the surface of your face in a desert climate or a heated winter room will draw moisture upward from the dermis, then let it evaporate into the dry air. The result is paradoxically drier skin after applying a "hydrating" product.

Application Technique That Actually Matters

How you apply HA determines whether it works or wastes your time. Three rules.

1. Apply to damp skin. After cleansing, pat your face until damp — not dry, not dripping. The residual water gives HA something to bind immediately. If your skin is dry when you apply HA, the molecule searches for water and pulls it from wherever it can find it.
2. Seal within 60 seconds. After applying HA serum, follow with your moisturizer before the HA dries. The moisturizer creates an occlusive barrier that traps the water HA attracted. Without it, the hydration evaporates.
3. Use 2-3 drops, not a full dropper. HA is effective at very low concentrations. A thick layer does not hydrate better — it just takes longer to absorb and may pill under subsequent products. Thin, even application is all you need.

HA in the Context of a Full Routine

Hyaluronic acid belongs in the hydrating step of your routine — after cleansing and toning, before treatment serums and moisturizer. Apply it as a water-based serum layer. It works with every other active ingredient: retinol, Vitamin C, niacinamide, peptides. No conflicts, no interactions to worry about.

One exception: if you use a direct acid (AHA/BHA) in the same routine, apply the acid first, wait until it absorbs, then apply HA. The acid needs direct skin contact to exfoliate effectively. HA applied first would create a buffer layer that dilutes the acid.

HA Myths Worth Correcting

• "HA replaces moisture your skin has lost." Not exactly. HA holds water in the upper skin layers, reducing dehydration lines and improving texture. It does not restore the HA in your dermis that depletes with age. Injectable HA fillers address dermal volume — topical HA addresses surface hydration. Both are useful; they target different layers.
• "More HA is always better." Formulas above 2% HA concentration become sticky, pill under makeup, and provide no additional hydration benefit. The optimal range is 0.1-2%. If a product advertises "5% HA," that concentration is likely causing texture issues without added benefit.
• "HA is anti-aging." HA is hydrating, not anti-aging in the way retinol or Vitamin C are. Plumped, hydrated skin looks younger because dehydration lines soften. But HA does not stimulate collagen, regulate melanin, or accelerate cell turnover. It is a support ingredient, not a treatment ingredient. Use it alongside actives, not instead of them.

HA Products Worth Knowing

Vichy Minéral 89: Combines hyaluronic acid with 89% volcanic mineral water rich in 15 minerals. The gel texture absorbs instantly and works under every moisturizer. No fragrance, no parabens — one of the most universally tolerated HA serums available. The mineral water adds electrolyte hydration that pure HA serums lack.

The Ordinary Hyaluronic Acid 2% + B5: Multiple molecular weights of HA at the maximum effective concentration, paired with pro-vitamin B5 for barrier support. At under $10, it is the most cost-effective multi-weight HA on the market. The texture is slightly thicker than Vichy's — better for dry skin, but can feel tacky on oily types.

Dime Beauty Hyaluronic Acid Serum: Clean beauty approach to HA with added watermelon extract for additional antioxidant benefit. Lightweight texture suitable for all skin types. A mid-range option for those who want HA without synthetic additives.

How to choose between them: Budget-conscious buyers start with The Ordinary. Sensitive skin gravitates toward Vichy's mineral-rich formula. Clean beauty preference points to Dime. All three deliver effective multi-weight hydration — the difference is texture, secondary ingredients, and personal preference. There is no wrong choice among well-formulated HA serums.

HA Beyond Serums: Where Else It Appears

Hyaluronic acid appears in moisturizers, eye creams, sheet masks, foundations, and even lip products. In most of these formats, HA functions as a supporting hydrator rather than the primary active. A moisturizer with HA in the ingredient list provides additional water-binding capacity alongside its emollient base. An eye cream with HA helps the thin under-eye skin retain moisture. The concentration in these products is typically lower than in a dedicated HA serum — which is fine for support, but insufficient if deep hydration is your primary goal. For targeted hydration, use a standalone HA serum. For maintenance hydration, HA in your moisturizer is enough.

How HA Interacts With Other Actives

Hyaluronic acid is one of the few ingredients with zero negative interactions. It plays well with retinol — applying HA first creates a hydration buffer that reduces retinol irritation. It pairs with Vitamin C without diminishing either ingredient's effectiveness. It layers under niacinamide, peptides, and AHAs without conflict. This universality is why HA appears in so many multi-active formulations.

The one consideration is layering order. HA goes on first after cleansing, always on damp skin, before any other serum. It is a water-phase product and needs to sit closest to the skin surface where it can bind moisture directly. Applying an oil-based serum before HA creates a barrier that prevents the HA from reaching water on your skin. Thin to thick, water to oil — HA is always the first serum in any multi-step routine.

Frequently Asked Questions