Skinpu — Skin Barrier and SPF Basics | skinpu.com
Skin surface sits at about pH 4.5-5.5. A soap bar at pH 9-10 is a different chemical environment, and the difference is measurable.
The acid mantle is the slightly acidic film on the skin surface, measured at about pH 4.5-5.5. It is part of how the outer layer maintains itself between renewal cycles, which run roughly every 28 days in young adults and more slowly with age.
Cleansers interact with that surface directly. A mild syndet cleanser is formulated near pH 5.5, close to the skin's own baseline. A traditional soap bar sits around pH 9-10, alkaline enough to shift the surface pH upward during washing.
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The shift is not permanent. The surface recovers over time, but repeated alkaline cleansing keeps pushing it away from its baseline, which is why the pH of a cleanser is a useful number to read alongside the ingredient list.
Measured alongside pH, transepidermal water loss gives a second reading on barrier condition. Normal values stay under about 10 g/m²/h, and a disrupted barrier tends to push that number upward while lowering the water content of the outer layer.
The practical check is simple: is the cleanser near pH 5.5, or closer to 9-10? That single comparison describes most of the difference between a routine that works with the barrier and one that works against it.
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