If you have long hair, you probably don’t need to look up a weather report to get an idea of how much humidity’s in the air: You can simply grab a fistful of hair. Straight hair goes wavy and if you have curly hair, humidity turns it curlier or even frizzy. Why? Because human hair is extremely sensitive to humidity, specifically the chemical element Hydrogen.
The majority of your hair is made up of long keratin proteins. These keratin proteins are chemically bonded together in two different ways.
The first, a disulfide bond, is permanent—it’s responsible for the hair’s strength—and isn’t affected by the level of humidity in the air.
The second is a hydrogen bond, which is much weaker and temporary, with hydrogen bonds breaking and new ones forming each time your hair gets wet and dries again. (This is the reason why, if your hair dries in one shape, it tends to hold that shape until it’s wet again).
On a humid day the air has a much higher number of water molecules (and therefore Hydrogen – the H in H2O) than dry air. When hair is exposed to this humid air it forms many more hydrogen bonds. When these bonds are formed between the keratin proteins in a strand of hair, it causes the hair to fold back on itself at the molecular level. When more hydrogen bonds are formed than usual, it causes naturally wavy or curly hair to become curlier or frizzier – Now you know!
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