Why does hair curl in humid air?

[InfiniteNow]

Wasn't sure if this was an Earth Science or Medical question... feel free to move if appropriate...

So, when the humidity rises, why does my hair tend to do the same and get all curly while it's at it?

[InfiniteNow]

Well, okay, I thought this would be a fun conversation, but I got bored too quickly and went and looked it up myself. I found the following response this same question from a UMass Med Center professor:

A strand of hair is made up of invisibly small molecules (groups of atoms), mainly of a certain kind of protein called (big word) "cytokeratin". These molecules bond to each other, kind of like holding hands or being held together by Velcro. There are two main kinds of bonds between molecules in a strand of hair that affect how straight or curly it is. One kind is very strong and permanent, kind of like being stapled together. These are bonds between sulfur atoms. When you get a "permanent" to straighten or to curl your hair, the smelly chemicals that are used have lots of sulfur in them that breaks these bonds and then allows them to re-form in new places. If the hair is set straight when the chemicals are removed, is stays straight. If it’s set curly it stays curly (until new hair grows out of your head again with your own normal curliness or straightness).

Now we get to second kind of bond between hair molecules. These are called "hydrogen bonds", and they’re much weaker and more temporary than he sulfur bonds. If you think of the sulfur bonds as being like staples, then the hydrogen bonds are more like paper clips. The amount of water in the air affects these bonds. When your hair dries, these hydrogen bonds get set up. If the air is dry, like on a dry day or under a hair drier, your hair will have a certain amount of curliness to it. If the humidity goes higher, then water dissolved in the air goes bouncing around into everything, including your hair. That is, your hair is absorbing a small amount of water, even though it doesn’t feel wet. The water has a very strong effect on the hydrogen bonds, and allows the "paper clips" to shift around a bit, but it doesn’t affect the permanent sulfur bonds. So the amount of curliness in your hair changes when the humidity goes up. If you have hair that has the sulfur bonds set for curliness (which it certainly sounds like you do!) then your hair will tend to get curlier, or "frizzier", when the hydrogen bonds relax. People with straight hair who curl it with curling irons or rollers or with a brush when they dry it have exactly the opposite experience to yours - their hair straightens out when it gets more humid, but for exactly the same reason. The hydrogen bonds that set up a temporary curliness get broken, and the sulfur bonds that in their case are set for straight hair take on their natural shape.

Well, how about 'dat?

[Dr.DDunn]

an easy way of saying it is, hair is just strands of protein, and in between these molecules thers space for that humidity to get trapped in.