Hair Structure & Blow Drying

Blow drying long hair
Photo: Africa Studio/Shutterstock
Q: What are the physical effects of blow drying on the hair structure?
A: Well, the effect blow drying has on the hair comes partly in combination with the effect of water on the hair structure. To explain properly, I need to relay the following:
Water and heat both act to break the physical side bonds in the hair. It is this fact that allows the hair to be curled and straightened using wet roller sets, curling irons, hot rollers, and flat irons. When the hair dries and/or cools, the side bonds are reformed into the new configuration.
When the hair is wet, these physical side bonds are broken and the hair usually hangs straighter than when it is dried to its natural state. Using blow dryer, while drying the hair and restoring the side bonds, also uses heat which keeps the side bonds from fully reforming.
When the hair is pulled taut using a comb or brush while blow drying, the side bonds begin to reform with the hair in a straighter configuration. Then once dry, the side bonds complete their reformation in this straight pattern.
The fact of the side bond's susceptibility to heat allows for the blow-dryer to be used to create bends and waves in the hair based on the tools used in conjunction. Round brushes to create soft curves, flat paddle brushes to give smooth even straightness.
The effects of heat and moisture on the physical side bonds have to be considered when trying to maintain the curl of naturally curly hair while drying. Diffuser attachments were designed to prevent the airflow from pulling the curl from the hair while drying it. The diffuser disperses the airflow and diminishes the amount of heat transferred by the dryer to the hair. This allows the hair to be scrunched with the fingers and dried gently so that the natural curls stay intact.
It should also be noted that heat can expand the cuticle layer of the hair and speed up the loss of moisture from the hair. If the hair is already porous or is chemically processed, the heat of a hair dryer can result in excessive moisture loss.
See also:
How to blow dry hair
Heat styling basics
The best blow dryer
Should you not brush hair when wet?