Hair absorbs moisture from humid air, causing it to swell and become frizzy.

Hair is primarily composed of a fibrous protein called keratin, which is held together by various chemical bonds. One of the most significant types is the hydrogen bond, which is relatively weak and easily broken by water molecules. When humidity levels rise, water vapor from the air penetrates the hair's outer layer, known as the cuticle, and enters the inner cortex.Inside the cortex, these water molecules form new hydrogen bonds with the keratin filaments. This chemical reaction causes the hair fibers to swell and change shape on a molecular level. Because the absorption is often uneven across the hair shaft, some parts of the strand expand more than others, resulting in the characteristic bends, loops, and kinks we call frizz.Research conducted by cosmetic scientists at institutions like the TRI Princeton have shown that a single strand of hair can increase its weight by 12% to 15% in high humidity environments. This effect is significantly more pronounced in damaged hair where the protective cuticle scales are lifted or missing. This increased porosity allows moisture to flood the cortex much faster than it would in healthy, sealed hair strands.The relationship between hair and humidity is so predictable that it was historically used in scientific instruments. In 1783, Swiss physicist Horace Bénédict de Saussure invented the hair hygrometer, which used a human hair to measure atmospheric humidity. As the hair lengthened or shortened based on moisture levels, it moved a mechanical needle to provide a reading. This principle remained a standard for meteorological measurement for over a century.