Re: I have curly hair. Why does it frizz when there is humidity in the air?

Dear Jade,

Thanks for your question! 

To answer it, let’s think about how hair is made and held together. 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 the 
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.

I hope this answers your question. Stay curious!!!

Paul Odgren, Ph.D.
University of Massachusetts Medical School
Worcester, MA