Product Knowledge and Ingredient Science
Ingredient questions reward precision. You are expected to read an INCI list, explain why a product penetrates or does not, and match an active to a skin condition without overstating what a cosmetic can do.
Reading the label
INCI names are in descending concentration down to 1%, after which order is free. The first five ingredients are usually the base: water, humectant, emollient, emulsifier, preservative. The functional actives often sit low and work at low percentages.
Functional categories
- Humectants: glycerin, hyaluronic acid, panthenol - pull water into the stratum corneum.
- Emollients and occlusives: fatty alcohols, dimethicone, petrolatum - soften and reduce water loss.
- Surfactants: anionic (effective, more irritating), non-ionic (gentler), amphoteric (mild).
- Preservatives: needed in any water-containing formula; "preservative-free" usually means self-preserving packaging, not safety.
pH, pKa and free acid
AHAs only exfoliate when a meaningful fraction is present as free acid, which is why glycolic (pKa about 3.8) works at pH 3–4; at pH 7 it is a humectant. Salicylic acid is lipophilic and comedolytic at 0.5–2%. L-ascorbic acid needs a low pH (below about 3.5) and 10–20% to be useful, and it oxidises to a yellow-orange colour, which means it has lost potency.
Penetration rules
Molecules under roughly 500 Da cross the stratum corneum more readily; lipophilic substances cross the lipid matrix, hydrophilic ones use appendages and are slower. Hydration, occlusion and mild exfoliation increase penetration, which is also why over-processing increases irritation.
Actives worth knowing
Retinol converts to retinaldehyde then to retinoic acid; expect irritation and photosensitivity, and advise caution in pregnancy. Niacinamide 2–5% supports barrier and sebum regulation. Azelaic acid, peptides, and hyaluronic acid of different molecular weights act at different depths. Hydroquinone is a medicine, not a cosmetic ingredient, in the EU.
Worked example
A client complains her vitamin C serum "stopped working" and shows a deep orange liquid. Reasoning: oxidation, not concentration, is the issue; the correct advice is to discard it, choose opaque airless packaging, store cool and dark, and reintroduce gradually.
Traps that cost marks
- Quoting marketed percentages instead of available free acid.
- Layering a strong acid and retinol for a first-time client.
- Accepting "chemical-free" or "natural means safe".
- Ignoring PAO symbols and opening dates.