# Humectant

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Humectants work by being thirstier than your hair. They attract water molecules from the surrounding air and pull them into the cortex through any gaps in the [cuticle](/glossary/cuticle/). When the balance is right, hair feels supple and plump.

The caveats matter. In extreme dry air (desert winters, airplane cabins), there's no ambient water to pull in — the humectant pulls water *out* of the hair instead, and strands get drier. In extreme humidity (Miami in August), the humectant over-pulls, flooding the shaft with more water than it can hold — the cuticle lifts, and you get frizz.

Glycerin is the workhorse humectant, in both cheap shampoos and expensive leave-ins. Panthenol (provitamin B5) is gentler and also doubles as a mild repair ingredient. Hyaluronic acid is the newer entrant — molecularly large, so it sits more on the surface than deep inside the shaft.