​Humid air causes hydrogen bonds to form between water molecules and the proteins in your hair, triggering curls and frizz
 
By Joseph Stromberg  - smithsonian.com
 
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 and see how it feels. Human hair is extremely sensitive to humidity—so much that some hygrometers (devices that indicate humidity) use a hair as the measuring mechanism, because it changes in length based on the amount of moisture in the air.
 
Straight hair goes wavy. If you have curly hair, humidity turns it frizzy or even curlier. Taming the frizz has become a mega industry, with different hair smoothing serums promising to “transform” and nourish hair “without weighing hair down.” But just why does humidity have this strange effect on human hair?
 
Hair’s chemical structure, it turns out, makes it unusually susceptible to changes in the amount of hydrogen present in the air, which is directly linked to humidity.  Most of a hair’s bulk is made up of bundles of long keratin proteins, represented as the middle layer of black dots tightly packed together in the cross-section below.

​Bundles of keratin proteins (the middle layer of black dots above) are susceptible to changing shape on a humid day. Image from Gray’s Anatomy

These keratin proteins can be chemically bonded together in two different ways. Molecules on neighboring keratin strands can form a disulfide bond, in which two sulfur atoms are covalently bonded together. This type of bond is permanent—it’s responsible for the hair’s strength—and isn’t affected by the level of humidity in the air.
 
But the other type of connection that can form between adjacent keratin proteins, a hydrogen bond, 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 remain in roughly that same shape over time.)
 
Hydrogen bonds occur when molecules on neighboring keratin strands each form a weak attraction with the same water molecule, thereby indirectly bonding the two keratin proteins together. Because humid air has much higher numbers of water molecules than dry air, a given strand of hair can form much higher numbers of hydrogen bonds on a humid day. When many such 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 at a greater rate.
 
On the macro level, this means that naturally curly hair as a whole becomes curlier or frizzier due to humidity. As an analogy, imagine the metal coil of a spring. If you straighten and dry your hair, it’ll be like the metal spring, completely straightened out into a rod. But if it’s a humid day, and your hair is prone to curling, water molecules will steadily be absorbed and incorporated into hydrogen bonds, inevitably pulling the metal rod back into a coiled shape.

By Kendra Aarhus

Hair products are a multi-billion dollar industry in the United States. Flashy advertising and beautiful imagery of perfect hair makes us all long for better hair days. Unfortunately, those commercials are also full of deceit; careful wordsmiths bend the truth to an often unrecognizable state. While the lies in the hair industry are vast, these four in particular need to be addressed and understood by woman everywhere.
 
See if you've believed these four bold faced lies.

1) You Can Not Repair Split Ends
Once a hair has split, you can put the most expensive serums and potions on your hair but nothing will repair that split end. Sure there are products that will smooth the frayed ends, and there are products that might even make your split ends appear (temporarily) repaired. Heat from a hair dryer, curling iron, or flat iron can also make split ends appear less frizzy for a while; in reality the high heat is probably making your split ends worse.
 
The truth is, the only thing that will actually repair your split ends is a haircut. Maintaining healthy hair practices, using products that don't dry your hair out, and making sure you get regular trims can prevent split ends. Products that promise to repair split ends are preying on your hopes and dreams.
 
2) You Can't Penetrate Your Hair with Protein
Protein treatments can be a nice treat for your hair and many are healthy for your scalp, but the fact of the matter is that protein products do not penetrate the cuticle of your hair. Products that claim to strengthen your hair with protein from the inside out, simply coat your hair with protein. In fact, over use of protein sprays can often be drying and cause your hair to get brittle or frizzy due to build up on the hair shaft.
 
What about the protein and keratin treatments that promise soft, frizz free hair? Well, that's a whole different chemical and heat reaction that has nothing to do with protein actually penetrating the cuticle of your hair shaft.
 
The best way to strengthen your hair from the inside out, is to take really great care of your scalp and to eat a hair healthy diet. Once your hair has grown past the surface of your scalp, it's at the mercy of the way you treat your hair on a daily basis. Keeping your hair hydrated and healthy will go a lot further than empty protein promises.
 
3) All Natural Hair Products, Usually Aren't
There's a big all natural product push going on in the beauty industry. Cosmetic companies are removing all kinds of harmful chemicals from their products and proclaiming their success on their labels. You've likely started wondering about the sulfates and parabens in your products by now.
 
The biggest problem is that the cosmetic industry by-and-large is very under regulated in the United States, and just because a label makes claims to be all natural, chemical free, or organic, doesn't mean that the formula is meeting any standards to actually live up to it's promises.
 
In all honestly, there are very few "standards" set to begin with, so labeling ends up being a big advertising scheme.

The other side of the problem is the internet; full of conflicting advice and doomsday warnings from a lot of people that have no actual education on the subject beyond what they learned by a Google search. It's hard to know what to believe and whom to trust.
 
The only way to truly understand what's in your cosmetics and your hair products is to understand how to read the label. I'll be honest; I'm not a chemist and I don't understand the majority of the labels of the products I use. But, until the cosmetic industry gets it's act together with honest labeling, there's really no way to be completely sure.
 
My biggest advice to anyone that's concerned with the chemicals in their hair products is to do your homework by researching reputable sources to determine what exactly is in your hair care products. Don't blindly trust the label to tell you the truth.
 
4) Your Hair Products are Full of Wax
Some of the products that make your hair feel soft and healthy are actually riddled with wax and silicone. There are a few waxes found in cleansing conditioners, no-poos, conditioners, and styling products that are pretty horrible. Why? Heavy waxes and silicones start off feeling soft and smooth, but over time these feel good additives can build up on your hair. Unlike other product build up problems, a clarifying shampoo isn't usually strong enough to strip silicone build up from your hair.
 
This build up can cause serious damage and breakage to your hair, and long term product buildup can actually clog your hair follicles. Hair follicles that are gunked up with wax can lead to hair thinning, weak hair, and even baldness. Yes. You can go bald from an overdose of wax and silicone build up.
 
The only way to keep the wax off your hair and out of your follicles is to avoid the worse offenders whenever possible. Silicones can be easily identified, as they usually end in -one, -conol, or -xane (such as, Dimethicone, Cetearyl methicone, Dimethiconol, or Cyclopentasiloxane). Waxes to avoid include mineral oil (paraffinum liquidum), petrolatum, bees wax, and candilla wax (to name a few).
 
Fortunately for all of us, there are water soluble silicone based products that are great at helping you get that soft feeling of silicone, but will wash away with water and your favorite shampoo. Water soluble silicones are easily recognized by the "PEG" on the label include ingredients like this: 
 

• PEG-8 Dimethicone
• Bis-PEG-8 Dimethicone
• Bis-PEG-8/PEG-8 Dimethicone
• Bis-PEG-18 methyl ether dimethyl silane
• PEG-8-PG-coco glucoside dimethicone
• Dimethicone PEG-X phosphate

 
(source)