Product Knowledge and Cosmetic Chemistry
Product Knowledge and Cosmetic Chemistry
This domain tests ingredient function, formulation types, pH, actives, preservatives, sun protection and how to read a label so you can select and combine products safely.
Ingredient families
- Humectants - glycerin, hyaluronic acid, sorbitol and urea draw water into the stratum corneum.
- Emollients - oils, esters and fatty alcohols soften and smooth by filling gaps between corneocytes.
- Occlusives - petrolatum, dimethicone and lanolin reduce transepidermal water loss.
- Surfactants - anionic (SLS, SLES) cleanse but irritate more; amphoteric (cocamidopropyl betaine) and non-ionic are milder.
- Preservatives - phenoxyethanol, parabens and benzyl alcohol are needed in any water-containing product.
- Antioxidants - vitamins C and E, ferulic acid.
Actives and pH
Skin surface pH is about 4.7–5.5. AHAs need a low pH (roughly 3–4) to exfoliate effectively; vitamin C is most stable and effective below about pH 3.5. Retinol converts in the skin to retinoic acid, causes photosensitivity and is avoided in pregnancy. Niacinamide is typically used at 2–5%.
Labels and sun protection
INCI lists ingredients in descending order until 1%. The PAO symbol (e.g. 12M) gives months of use after opening. SPF relates to UVB; PA or PPD ratings relate to UVA. Apply sunscreen at about 2 mg/cm².
Worked example
A client wants to use vitamin C and retinol together. You advise vitamin C in the morning with SPF, retinol at night, alternating rather than layering, because both are potentially irritating and retinol increases photosensitivity.
Traps that cost marks
Assuming "natural" means hypoallergenic; ignoring pH when combining acids; believing "chemical-free" claims; using products past their PAO; recommending retinol in pregnancy.