What is frizz?
We all know what frizz looks and feels like, but what is going on when it happens? Basically, frizz happens when the structure of a hair strand is altered in a way that prevents it from having its intended form. A frizzy strand is usually neither straight nor curly, and seems to fit in nowhere on the head. Frizz can create a halo type effect on the head, as the unruly strands float around patternless on the surface, or it can yield an overall shapeless, bushy effect. The usual cause of this behavior is that the structure of either the cuticle or the hair shaft (or both) is disrupted by either damage or interaction with the environment, which then causes it to lose its suppleness, smoothness, and curl pattern.
What causes it?
The most common culprit for frizzy hair is a substance that seems innocuous: water. The porous nature of hair enables it to absorb moisture from the environment. When this happens in excess, it causes expansion of the cushiony endocuticle layer, which lifts cuticle scales away from the axis of the hair, making the surface rough and unruly. This swelling and roughening of the cuticle layer is compounded by diffusion of moisture into the cortex, where it can cause the protein-rich gel matrix to swell substantially. Swelling due to water absorption also increases the diameter of individual hair strands, which can encourage them to lift and separate from one another instead of nesting together properly. Excess water molecules also disrupt hydrogen bonding between keratin strands, which can alter the curl pattern and cause straw-like, frizzy hair.
What hair types are most affected?
Hair that has undergone chemical and/or heat treatments is definitely prone to frizz, due to its damaged cuticle and cortical protein structures. However, normal, healthy hair can also have a tendency to frizz. Curly hair is more susceptible to this frizz than straight hair, due to its less cylindrical shape and the fact that individual scales in its cuticle layer are less tightly overlapping. This makes curly hair more delicate and damage-prone than straight hair, but also more porous and likely to be affected by water in the environment even in its virgin, healthy state. The curlier a person's hair is, the greater the likelihood she will experience with frizz. Also, the finer someone's hair is (fine hair meaning hair strands with smaller diameter), the higher the overall surface area, which results in higher relative amount of moisture absorption from the air and more problems with frizz. So, the finest and curliest hair will often be frizzy even if it is not damaged or dry.
What can you do about it?
If your hair is already very healthy, yet you still struggle with frizz, it may be easy to become disheartened. Since frizz is often a result of hair taking on too much moisture from its environment, the most effective tactic is to minimize this as much as possible. Making sure your hair is well-hydrated will reduce the amount of water it will absorb. Also, using conditioners that have good emollients in them will help make your cuticle layer smoother and will make it more hydrophobic, so that it will be less susceptible to water from the environment. Oils will also add suppleness to your hair and act as sealers or anti-humectants. While many botanical oils are wonderful for this purpose, some silicones and mineral oil also perform really well for this application, if you do not mind using a mild shampoo to remove them. If your hair is fine though, it may have a tendency to easily become weighed down by too many oils, so you will have to work carefully to find the right oil type and amount for your hair.