What a percentage on a bottle actually buys

A percentage on the front of a bottle is a number without a unit of usefulness. It tells you how much of an ingredient is in the formula and nothing about whether that ingredient is stable, in the right pH, in a vehicle that will carry it through the barrier, or at a dose where more has stopped meaning better. Here is what the numbers actually mean for the ingredients people buy.

Vitamin C

L-ascorbic acid, the form with the best evidence, works between 10 and 20 per cent and needs a pH below 3.5. Below 8 per cent the effect falls away sharply. Above 20 there is no further benefit and rising irritation. It also oxidises in light, air and warmth, turning yellow and then amber, and an amber serum is a serum that has become a mild pro-oxidant.

Which means the packaging is part of the formula. Opaque, airless, small enough to finish in three months. A clear dropper bottle of vitamin C is a product designed to look like chemistry and behave like juice. The gentler derivatives, magnesium ascorbyl phosphate and ethylated forms, are more stable and less potent, which is a reasonable trade for sensitive skin and a poor one if you bought it for the evidence base.

Niacinamide

The useful range is 2 to 5 per cent, where it supports the barrier, moderates sebum and reduces redness. The evidence for benefit above 5 is thin and the evidence for flushing above 10 is abundant. The 10 per cent and 20 per cent products that appeared when the ingredient became fashionable are selling a number rather than a result.

Retinoids

This is the family where time matters more than dose. Retinol converts in the skin to retinoic acid in two steps, losing potency at each one, so 0.3 per cent retinol is roughly comparable to a low strength prescription retinoid used consistently. Consistency is the active ingredient. Twelve to sixteen weeks at a tolerable strength beats six weeks at a strength that produces a fortnight off to recover.

• Start at two nights a week and add a night every fortnight if the skin is calm.
• Buffer with moisturiser underneath while building tolerance. It reduces irritation and barely reduces the effect.
• Retinaldehyde sits between retinol and prescription strength, one conversion step instead of two, and is the sensible upgrade before asking a doctor.
• Anything promising retinol results without irritation in a week is promising something the chemistry does not do.

With retinoids the dose that works is the one you can still be using in four months.

Acids

Glycolic at 5 to 10 per cent is a reasonable weekly exfoliant, lactic at similar strengths is gentler and more hydrating, and salicylic at 2 per cent is the one that gets into an oily pore because it is oil soluble. The percentages above those are professional treatments sold at retail, and the main thing they produce in home use is a compromised barrier that takes six weeks to rebuild.

Hyaluronic acid, where the number is a distraction

It is a humectant. It holds water. Two per cent does not hold meaningfully more usable water than 0.5, and high concentrations in a dry room can pull moisture out of the skin rather than into it. What matters is molecular weight, because low weight fractions sit in the upper layers and high weight fractions stay on the surface, and a formula with a spread of both does more than a formula with a large number on the front. Apply to damp skin, seal with something, or it does nothing at all.

Reading an ingredient list instead

Ingredients are listed in descending order by weight down to one per cent, after which they may appear in any order. Preservatives are almost always used under one per cent, so anything listed after the preservative is present at under one per cent too. That is the single most useful trick available to a consumer: find the phenoxyethanol, and treat everything below it as a trace.

It explains a great many products. The extract on the front of the bottle, the one in the name, sitting three places below the preservative, present at a fraction of a per cent, doing nothing except appearing on the label.

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