Keratin and the "Three Bonds": Why Hair Frizzes in Humidity and Why Its Shape Changes

More than 80% of hair is a protein called keratin. Keratin forms long strands, and these strands are linked by several kinds of "bonds" that keep the hair's shape and strength.

Hydrogen bonds: broken by water, re-formed as hair dries

The most numerous and the weakest bonds. They break when hair gets wet and re-form in whatever shape the hair is in as it dries. Bed hair, styling with a hair dryer and frizz in humidity are all caused by these bonds.

Salt bonds: strength changes with pH

Bonds formed by the attraction of positive and negative charges. They are most stable when hair is weakly acidic (pH 4.5–5.5) and loosen when it becomes alkaline. Color and perm products are alkaline in order to loosen these bonds and let the product into the hair.

Cystine bonds (S-S bonds): broken only by chemicals

Strong bonds between sulfur atoms. Perms and straightening treatments break these bonds with a reducing agent, change the hair's shape, then re-form the bonds with an oxidizing agent. Repeated bleaching or coloring leaves some of these bonds broken, which weakens the hair.

Points to remember for hair care

• Frizz from humidity comes from hydrogen bonds. Drying hair thoroughly and smoothing the surface with oils that limit moisture movement helps it settle
• After coloring or perming, hair is on the alkaline side. That is why care that brings it back to weak acidity (such as acidic treatments) is recommended
• Broken cystine bonds cannot be restored with home care. Not breaking them in the first place (spacing out salon treatments, how you use heat) comes first

Related: Frizz & waves / Hair color care