The science behind cosmetic ingredients and skin benefits

Trying products and seeing nothing happen usually comes down to mechanism: the wrong active for the concern, the right active at too low a concentration, or a formulation that cannot deliver it. Reading a label by mechanism is what makes the difference.

Understanding your skin's needs

Skin needs hydration, barrier support and protection. Its outermost layer, the stratum corneum, is a lipid matrix of ceramides, cholesterol and free fatty acids holding corneocytes in place. This is the structure that keeps water in and irritants out. Most of what skincare does is support that structure, or act on the living epidermis beneath it.

Hydration: humectants, emollients and occlusives

Three mechanisms, one word.

Humectants bind water and draw it into the stratum corneum: sodium hyaluronate, glycerin, panthenol, urea. Emollients are lipids filling between corneocytes, smoothing rough and flaky texture. Occlusives form a surface film slowing water loss.

The interaction matters: humectants draw water from the environment and from deeper skin, so in dry air a humectant alone can leave skin drier than before. Apply to damp skin and seal with an emollient or occlusive.

Antioxidants

UV and pollution generate free radicals: molecules with an unpaired electron that strip electrons from lipids, proteins and DNA, degrading collagen and elastin. Antioxidants break the chain by donating an electron without becoming destructively reactive themselves.

Green tea polyphenols, vitamin C and vitamin E are the common ones. Vitamin C and E work in different phases and vitamin C regenerates oxidised vitamin E, so the pair outperforms either alone. They reduce oxidative damage; they do not absorb UV, so they complement sunscreen rather than replacing it.

Acids: which ones exfoliate

"Acid" on a label is not a warning, but it is also not one category.

Salicylic acid (the beta hydroxy acid) is oil-soluble, so it penetrates sebum into the pore lining. That makes it the right choice for congestion and acne, and it is anti-inflammatory. Alpha hydroxy acids (glycolic, lactic, mandelic) are water-soluble and work at the surface, improving tone and texture.

Hyaluronic acid is not an exfoliant. It is a humectant that shares a word with the others and nothing else. Grouping it with salicylic acid is a common source of confusion.

Alpha hydroxy acids increase the skin's sensitivity to UV, so sun protection is part of using them. Salicylic acid has not been shown to do the same, but daily sunscreen is still good practice.

Retinoids

The best-evidenced anti-ageing actives available. They bind nuclear retinoic acid receptors and alter gene expression, increasing cell turnover, stimulating collagen synthesis and normalising keratinisation, so they address fine lines, texture, tone and acne through one mechanism.

Potency runs from retinyl esters through retinol and retinaldehyde to prescription tretinoin, with irritation rising alongside. Introduce gradually, expect an adjustment period, and use sun protection: retinoids increase photosensitivity. They reduce the appearance of lines; "wrinkle-free" is not what the evidence supports.

Sun protection

The omission most ingredient guides make, and the one that matters most. Most visible ageing on habitually exposed skin is photo-ageing, and daily broad-spectrum sunscreen has randomised evidence behind it for slowing that accumulation, an effect size no other cosmetic ingredient matches.

Broad-spectrum SPF 30 or higher, applied generously and reapplied. Every other active in this article works better with it, and several are undermined without it. See UV-A vs UV-B filters.

Understanding pH

Skin surface pH sits around 4.5 to 5.5, the acid mantle. Strongly alkaline cleansers disrupt it and the barrier takes time to recover, which is why well-formulated cleansers are mildly acidic.

Product pH also determines whether actives function. Hydroxy acids need low pH to be active. Vitamin C as L-ascorbic acid needs a pH of about 3.5 or below to penetrate skin, and it oxidises faster as pH rises. Organic-acid preservatives fail above roughly pH 5.5. A formula at the wrong pH can contain everything it claims and do nothing.

Choosing wisely

Identify the concern, find the mechanism, then check the product contains that active at a level where it works. Introduce one new active at a time so a reaction can be traced, patch test on a discreet area for 48 hours, and give most actives eight to twelve weeks. Consistency does more than any single product choice, except sunscreen, which does more than all of them.