5.3.1. pH, Active Ingredients, and Product Interactions
💡 First Principle: The skin has a naturally acidic pH (approximately 4.5–5.5) called the acid mantle. This acidity is protective — it inhibits bacterial growth and supports the skin barrier. Products that disrupt the acid mantle, especially those with a pH that is too alkaline, compromise the skin's natural protection.
pH scale basics:
- Scale runs from 0 (most acidic) to 14 (most alkaline), with 7 being neutral
- Healthy skin pH: approximately 4.5–5.5 (slightly acidic)
- Harsh soaps: pH 9–11 (alkaline) — disrupt acid mantle
- AHA chemical exfoliants: pH 3–4 (acidic) — work by dissolving cell bonds at low pH
- Toners: formulated near skin pH to restore balance after cleansing
Key ingredient categories:
| Category | Function | Examples |
|---|---|---|
| Humectants | Draw water into the skin from environment/deeper layers | Hyaluronic acid, glycerin |
| Emollients | Fill in gaps between skin cells, soften | Squalane, plant oils, ceramides |
| Occlusives | Create a barrier to prevent moisture loss | Petrolatum, beeswax, shea butter |
| AHAs | Chemical exfoliation, surface cell turnover | Glycolic acid, lactic acid, mandelic acid |
| BHAs | Oil-soluble exfoliation, penetrates follicle | Salicylic acid |
| Retinoids | Increase cell turnover, stimulate collagen | Retinol, tretinoin (Rx) |
| Antioxidants | Neutralize free radical damage | Vitamin C, Vitamin E, niacinamide |
| SPF actives | UV protection | Zinc oxide (physical), avobenzone (chemical) |
Product interaction warnings:
- AHAs/BHAs and retinoids — both increase cell turnover and can cause significant irritation if used together
- Vitamin C (ascorbic acid) is pH-dependent and may be destabilized by other acids
- Products with opposite pH values can neutralize each other's effectiveness when layered
⚠️ Exam Trap: Products labeled "natural" or "organic" are not automatically safe for sensitive skin or free from adverse reactions. Plants contain potent biochemical compounds — essential oils, botanical extracts — that can cause contact dermatitis in some clients.
Reflection Question: A client says they want to use a glycolic acid toner immediately followed by a retinol serum at home. Based on what you know about these ingredients, what guidance would you give them?
