Skinpu — Skin Barrier and SPF Basics | skinpu.com
The stratum corneum normally holds about 20-30% water by weight, and transepidermal water loss stays under about 10 g/m²/h.
The stratum corneum is the outer layer of the skin. It normally holds about 20-30% water by weight, and that water content is one of the readings used to describe barrier condition.
Transepidermal water loss (TEWL) is the rate at which water escapes through that layer. Under normal conditions it stays under about 10 g/m²/h. When the barrier is disrupted, TEWL rises — and the same disruption tends to lower the water content of the outer layer.
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The two readings move together, which is why they are usually reported side by side. A surface that holds less water and loses more of it is a surface whose barrier is not doing its usual job.
Surface renewal sets the timescale. The outer layer renews roughly every 28 days in young adults, and more slowly with age. A barrier change therefore plays out over weeks, not hours, and not within a single wash.
This is the background against which cleanser pH matters. A soap bar around pH 9-10 shifts the surface away from its baseline of about 4.5-5.5, and the recovery is measured on the same weeks-long cycle as the renewal process itself.
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