Chemistry, Ingredients, and Product Selection
The basics that answer most questions
pH measures hydrogen ion concentration on a 0–14 scale; skin sits near 4.5–5.5. Solutions, suspensions, and emulsions describe how ingredients are combined, and surfactants let oil and water meet so cleansers can lift sebum. Humectants pull water in, emollients soften and fill gaps, occlusives seal, and preservatives keep water-containing products from growing microbes. Antioxidants protect formulas and skin from oxidation.
Actives worth knowing
AHAs (glycolic, lactic, mandelic) exfoliate the surface and depend on concentration and pH for strength; BHA (salicylic) is oil-soluble and suits congested pores. Retinoids accelerate turnover and increase photosensitivity. Vitamin C (L-ascorbic acid) is an antioxidant best stabilized at low pH. Niacinamide supports the barrier; hydroquinone and benzoyl peroxide are medical-leaning actives to treat with caution. Sunscreens divide into physical (mineral filters) and chemical, and SPF describes UVB protection while broad-spectrum and PA ratings address UVA.
The traps
Assuming natural, organic, or hypoallergenic means safe - none of those words guarantee anything, and essential oils are common sensitizers. Do not layer strong acids without a plan, and remember that dilution changes pH, so a weaker percentage can still be aggressive if pH drops.
Rule of thumb: for an acid, strength is a combination of percentage, pH, and time - never percentage alone.