Most hyaluronic acid products list the ingredient once on the label and call it a day. The molecule actually comes in a wide weight range, and where it sits on that spectrum changes everything — from how deep it penetrates to whether it draws water out of your skin instead of into it.
How molecular weight changes the behaviour
Hyaluronic acid (HA) is a long-chain polymer. The 'large' version, around 1,000-2,000 kilodaltons (kDa), sits on the surface, holds water against the skin and forms a film. The 'medium' version, around 100-1,000 kDa, penetrates the upper layers and plumps fine lines temporarily. The 'small' version, under 50 kDa, penetrates deeper but is also more pro-inflammatory in some studies.
Reputable formulas blend several weights. The Inkey List Hyaluronic Acid Serum and The Ordinary Hyaluronic Acid 2% + B5 both use three or more weights for layered effect. Single-weight HA products are usually budget options for a reason.
The desert-air problem
HA pulls water from wherever it can find it. In humid environments that means pulling humidity from the air into the skin. In dry conditions — central heating, cold UK winters, planes — there's no humidity to pull from, so it pulls from the deeper skin layers instead, leaving you drier than you started.
How to use it without the side effect
Apply to damp (not dry) skin straight after cleansing, then seal with a moisturiser within thirty seconds. The water trapped underneath the moisturiser is what the HA holds. Without the seal, the water evaporates.
Where 'hyaluronic acid' on the label can mislead
Sodium hyaluronate is the salt form, more stable and slightly smaller than the parent molecule. Most products list one or the other; functionally they're equivalent for skincare. 'Hydrolysed hyaluronic acid' is enzymatically chopped into smaller fragments — useful for penetration, but check the rest of the formula for soothing ingredients to offset the inflammation risk.
Picking by climate and skin type
Humid summer or oily skin
Single low-weight HA is enough; high-weight feels tacky.
UK winter or dry skin
Multi-weight HA layered under a ceramide cream is the standard. Avene Hydrance, La Roche-Posay Hyalu B5, or CeraVe Hydrating Cleanser as the base.
Sensitive skin
Stay on medium-weight only. Skip 'oligomeric HA' marketing claims (very low weight) — patch test first.
Hyaluronic acid is reliable when used correctly and frustrating when used alone in dry air. Always damp skin, always sealed, always blended weights.