Product knowledge and cosmetic science
Domain 10 tests ingredient function, formulation basics, pH, preservation, labelling and honest claims. You need to explain why a product works, how to store it and when it is unsafe, without repeating marketing language.
Ingredients and functions
- Humectants attract water: glycerin, hyaluronic acid, propylene glycol.
- Emollients soften and smooth: plant oils, butters, fatty acids, silicones.
- Occlusives reduce water loss: petrolatum, mineral oil, dimethicone.
- Surfactants cleanse and emulsify; they may be anionic, cationic, amphoteric or non-ionic.
- Preservatives prevent microbial growth: phenoxyethanol, parabens, benzyl alcohol.
- Antioxidants limit oxidation: vitamin E, BHT.
- Actives include AHAs (glycolic, lactic), BHA (salicylic acid), retinol, vitamin C and niacinamide.
- UV filters are organic or inorganic; SPF is a measure of UVB protection.
Formulation and pH
- Oil-in-water emulsions feel lighter and absorb quickly; water-in-oil emulsions are richer and more occlusive.
- Skin surface pH is about 4.5–5.5. AHAs and vitamin C need low pH to be effective, which also increases irritation risk.
- Any product containing water needs an appropriate preservative system.
Labelling and shelf life
- INCI names are listed in descending order of concentration up to 1%; allergens, batch number and PAO (period after opening) must be clear.
- Store away from heat and light; do not decant into unlabelled containers; use a spatula.
Claims and testing
- Cosmetics cannot claim to treat or cure disease; medicinal claims require regulation.
- 'Hypoallergenic' and 'natural' are not guarantees of safety; patch test where indicated.
Worked example
A client reacts to a moisturiser containing fragrance and essential oils. The therapist selects a fragrance-free, minimal-ingredient product, patch tests it and explains that 'natural' essential oils can still sensitise. She checks the PAO and records the reaction. The reasoning is ingredient avoidance plus patch testing, not a claim that the new product is hypoallergenic.
Traps that cost marks
- Describing a product as 'chemical-free'.
- Assuming natural ingredients are always safe.
- Ignoring PAO and expiry dates.
- Mixing retinol with strong acids without warning the client.
- Over-claiming results or implying a cure.
- Failing to check preservatives and allergens.