Chemistry, Ingredients, and Product Selection
Know the vocabulary
pH runs 0–14; skin sits near 4.5–5.5. Acids lower pH and bases raise it, and either can disrupt the barrier if misused. Surfactants clean because one end loves water and the other loves oil; foam is not the measure of cleaning. In formulations, a humectant (glycerin, hyaluronic acid, sorbitol) draws water in, an emollient (oils, esters, fatty alcohols) softens and fills the gaps between corneocytes, and an occlusive (petrolatum, dimethicone) blocks water loss. Emulsions are oil-in-water (light, suited to oily skin) or water-in-oil (richer, suited to dry skin).
Actives worth knowing
- AHAs (glycolic, lactic, mandelic) are water-soluble surface exfoliants; BHAs (salicylic acid) are oil-soluble and reach into the follicle, which suits oily, acne-prone skin.
- Retinoids speed turnover; they cause photosensitivity and irritation, so start low and use SPF.
- Vitamin C (L-ascorbic acid) is an antioxidant that degrades with light and air; niacinamide supports barrier and tone; peptides are signaling fragments.
- Preservatives prevent microbial growth in water-containing products; fragrance is a common allergen.
Ingredient lists run in descending order of concentration, and ingredients under 1% may be listed in any order - so a botanical at the end of a list may be largely decorative.
Worked example
An oily, comedone-prone client asks for "the strongest peel." Reasoning: stronger is not better. Salicylic acid at about 2% is lipid-soluble, targets the follicle, and is usually less irritating on inflamed skin than a high-percentage glycolic.
Traps
Confusing humectant with emollient; assuming a higher percentage is always more effective; believing "chemical-free" or "organic" claims; layering vitamin C with benzoyl peroxide; trusting SPF in makeup alone.