Types of Hair: 5 Factors That Determine Your Hair Type

Quick Summary:

• Genetics influence our hair patterns based on follicle shape and size
• Oil production and dispersal throughout hair can vary among individuals due to different factors such as follicle structure, hair pattern, ethnicity, hormones, climate, and styling products

Why is my hair straight, curly, or wavy?

As we know, there are a variety of different hair patterns. The most common variations are straight, wavy, or curly. These variations are mainly a result of genetics that determine follicle shape and size.^[1] Hair shaft angle and climate can also play a role in the texture of hair.

1. Straight hair is caused by having round follicles.^[1] The follicles are positioned vertically into the skin which also contributes to the straight pattern.^[2] Previous research has found specific gene variants in European individuals with straight hair.^[3] A study done in 2013 found that the 370A gene variant is strongly associated with straight hair in East Asian populations including Han, Tibetan, Mongolian, and Li.^[3]
2. Curly hair is caused by having oval-shaped follicles.^[1] The flatter the oval is, the curlier the hair will be.^[2] The follicle is slightly angled which causes the curling effect.^[2] Curly hair has also been found to have more disulfide bonds on the hair shaft since the curl shape makes it easier for these bonds to form.^[2]
3. Wavy hair is caused by having a follicle shape in between round and oval.^[1] This type of hair might exhibit a slight follicular angle but is not as extreme as the angle in curly hair.

Climate can also play a part in hair texture. Many people may experience frizzing hair in humid climates. As previously mentioned, hair contains disulfide bonds responsible for hair strength.^[4] A disulfide bond has two sulfur atoms strongly bonded together by covalent bonds.^[4] Hydrogen bonds between keratin proteins also contribute to hair structure.^[4] These bonds are a lot weaker than the disulfide bonds.^[4] In humid climates there are more water molecules in the air containing hydrogen atoms thus causing the hair to create more hydrogen bonds when exposed to humidity.^[4] The hair folds back onto itself creating a frizzy texture.^[4] The curlier the hair is originally, the more it will frizz or curl in a humid climate due to the hydrogen bonding.^[4]

Oil Production

Sebaceous glands are the type of oil producing glands everyone has on the scalp and other areas on the body.^[5] They produce sebum which is an oily substance containing squalene, cholesterol, cholesterol esters, wax esters, and triglycerides.^[5] The largest sebaceous glands are found on the scalp and face.^[5] Many things can influence the regulation of sebum production and oil distribution on hair. Some factors contributing to this include hair pattern, climate, ethnicity, hormones, and different styling products. Therefore, some people may have more oily or dry hair on a regular basis. 

Why do I have oily hair?

1. Hair Pattern

• Individuals with straight hair may be more prone to oily hair. This is due to the straight direction of the follicle.^[2] The vertical position of the follicle makes it easier to disperse the natural oils throughout the hair from top to bottom.
• However, oil production comes down to sebaceous gland regulation.

2. Ethnicity

• A study done in 2013, evaluated different ethnicities sebaceous gland production and found Northern Asians and Caucasians to have similar levels of oil production.^[6]
• Northern Asians had slightly elevated levels compared to Caucasians.^[6]

3. Hormones

• Androgen hormones and retinoids play a role in the function of sebaceous glands.^[5]
• Testosterone gets converted into DHT by sebaceous glands.^[7] Elevated testosterone levels have been linked to increased sebaceous gland activity. Those with oilier hair may have elevated levels of testosterone. Depending on the individual, this could be a normal condition or caused by underlying issues if this is an irregular symptom.
• Elevated levels of DHT have also been seen in those with alopecia or hair loss.^[7]
• Growth hormone is another hormone shown to increase sebaceous gland secretions.^[7] Sebaceous glands become more active during puberty due to these elevated hormone levels.^[8]
• Since sebaceous glands are also located on the face, over stimulation of them may result in acne as well.^[7]

4. Climate & Environment

• Environmental factors including a humid tropical climate, medications, and industrial exposure to halogenated hydrocarbons can all cause overactive sebaceous glands.^[7]
• When in a warmer climate the body tries to maintain homeostasis by sweating in efforts to balance body temperature.
• Some people may experience an oily feeling scalp after sweating from being in a hot climate or exercising. Eccrine glands are the sweat glands responsible for this which are different from sebaceous glands.^[8] Eccrine glands are mainly found on the palms, soles of feet, and scalp.^[8]
• However, too much UV exposure will dry out the hair overall.^[9]

5. Products & Styling

• Conditioners are commonly used to moisturize the Silicone, quaternized proteins, and polymers are the chemicals responsible for adding shine to hair in most leave-in conditioners.^[10]
• Hair emollients and petroleum have also been used to add moisture to hair.^[10] These work by coating the hair shaft and creating a water barrier.^[10]
• Overuse of these products over time without consistent hair washing can lead to product build up on the scalp leaving it with a greasy texture.^[10] Some of these products may also cause breakouts if overused as well.^[10]

Why do I have dry hair?

1. Pattern

• Curly hair is typically associated with a dryer hair type due to its shape.
• Since the follicle is curved, it is harder for the oil secretions to make it to the end of the hair strands.^[2] As a result, more oil might be left at the scalp leaving the rest of the hair strand dry and dull.

2. Ethnicity

• Although African Americans are found to have the most oil production from their sebaceous glands, they tend to have dryer hair.^[6] This is due to their hair having a tight curl pattern preventing the oils from fully distributing throughout the length of the hair.^[2]
• Other ethnicities may be prone to dry hair as well. However, hair types have been commonly classified as Asian, African American, or European in research.^[11] This type of classification is limiting as it is quite broad and there are other ways to classify hair pattern and texture.^[11]

3. Hormones

• Dry hair can also be caused by hormonal imbalances. Specifically, thyroid hormonal imbalances can affect overall hair moisture.
• Hypothyroidism is a condition lacking the proper amount of thyroid hormone.^[12] A common symptom associated with hypothyroidism is dry, brittle hair due to decreased sebum secretion.^[12]
• Birth control pills and pregnancy may cause hormonal changes that can affect hair texture as well.

4. Climate and Environment

• As previously mentioned, while being out in the heat can cause excess sweating and lubrication on the hair, too much UV exposure can dry out and damage the hair.^[13]
• A study done in 2014 examined the effects of ultraviolet radiation on hair proteins and found decreased sulfhydryl content in the hair shaft after UV exposure.^[9] The UV exposure is thought to rupture the cysteine in the sulfide bonds that contribute to hair strength.^[9]
• Hair protein loss is caused by UVB radiation specifically while UVA radiation causes hair to change color.^[13]
• During the cold winter months, one can expect hair to be more brittle and drier from the lack of moisture in the air too.

5. Products & Styling

• Any sort of heat styling can be associated with damaging hair causing it to become dry and brittle.
• A study done in 2011 found that using a blow dryer caused more damage than allowing the hair to dry naturally.^[14] However, the same researchers also found that using a blow dryer 15cm away from the hair with continuous motion causes less hair damage than letting hair dry naturally.^[14]
• An ingredient found in some cleansing and hygiene products called, Sodium dodecyl sulfate, was shown to rupture and partially erode the cuticle sublayers on human hair.^[15] This might be something to look out for when choosing different hair products to use.

Overall, there are numerous explanations for different hair types. Some of these factors include follicle structure, hair pattern, ethnicity, hormones, climate, and different styling products. These are just some of the many reasons why we may experience a diversity of hair types among individuals. If you notice a drastic difference in your hair type you should visit a health professional as there may be a more serious underlying issue.