For the average person, washing their hair every day or every few days is a matter of hygienic preference rather than necessity. Unless their work or pastime brings them in contact with dirt or involves physical exertion, it is unlikely that their hair would go from being clean to truly dirty in the span of twenty-four hours. In this scenario, what probably occurred was a buildup of sebum.
The Science Behind Shampoo and Your “Dirty” Hair
When your hair feels greasy, sebum is the likeliest culprit. Sebum, however, is not guilty of making your hair dirty, at least not directly. Sebum is a natural oil that the sebaceous gland inside each hair follicle secretes to coat hairs with a protective layer (which can attract dust and other particulate matter). Without sebum or modern hair care products hair would become dry, brittle, and break apart due to the elements. Sebum locks in moisture, though at the same time keeps hair from getting or remaining overly wet, and it accomplishes this due to its hydrophobic properties.
Stemming from Greek root words, hydrophobic breaks down into water (hydro) and fear (phobic), meaning that hydrophobic substances are afraid of or repel water. Technically, there is no repelling force involved, merely an absence of an attracting force (somewhat analogous to how cold is really the absence of heat), but for our purposes thinking of hydrophobic substances as rejecting water is fine. Among their other characteristics, oils and fats are hydrophobic. This is why they do not readily mix with water, why some raincoats are oiled, and why washing a greasy dish requires more than just water. It might sound strange at first, but we actually use the same substance to clean our dishes and our hair.
Whether you are washing your hair because of dirt, sweat, or a buildup of sebum, the combination of water and detergent is what gets it clean. Detergent is both a cleansing agent and shampoo’s principle active ingredient.
Sodium laureth sulfate is a common detergent found in shampoos and soaps for both bathroom and kitchen uses. SLES and other detergents clean hair via a two-step process. First, the surfactant portion lessens the surface tension of the water it comes into contact with, which allows it to bind more freely with the oil and other debris in your hair that might otherwise not be molecularly attracted. As this occurs, the hydrophilic portion of the detergent kicks in, and this attracts the newly bonded dirt-water compounds to the water around it so that when you are rinsing the suds from your hair the dirt and excess sebum gets washed away as well.
Conditioner is both its own product and a common component of many shampoos, these usually being marketed as two-in-one shampoos. Different detergents have different strengths, and if one does not have an excessive amount of sebum in their hair before they wash it, it is possible that the detergent in their shampoo could strip away too much of this protective coating. Standalone conditioners tend to have additives that give them multiple properties, but the conditioning agents that they share with two-in-one shampoos are meant to moisturize and lock in the moisture of hair. Conditioning agents replicate the effects of sebum via silicones like dimethicone, and fatty alcohols like panthenol.
Is Conditioner Necessary?
If plenty of shampoos can accomplish what conditioners do, are they really necessary then? The short answer is: It depends.
Whether or not conditioner is an essential part of your hair care routine comes down to a combination of factors and preferences. Some of these stem from consumer choice. If a person buys a shampoo with a strong detergent or a high pH they are likelier to want a conditioner to counteract the shampoo’s harsher effects. A preference for the additives in a certain brand of conditioner can also guide one’s purchasing decisions. Many conditioners help fight static cling or increase the shininess of hair, which for some are hard benefits to pass up. As with other fields based around chemistry intended to interact with our own biochemistry, dosage is key. A shampoo that checks all of the boxes for a certain person’s hygiene does not need an accompanying conditioner. But then, there are also people with over or underactive sebaceous glands who do benefit from using a separate conditioner.
Another consideration is hair type, or texture. Various textures of hair require unique care routines. What works for straight hair does not necessarily work for kinky hair. There are also those who forego shampoo and conditioner altogether for cultural or aesthetic reasons, which might seem strange to those on the outside looking in, until they recall that hair has evolved to care for itself.
While not the most dynamic ingredient in traditional shampoo, water is always the first one listed. This is because product labels are ordered by the amount of each ingredient contained in them, and up to eighty percent of many shampoos are water by volume. By definition that sounds like a watered-down product, but shampoo requires a delivery mechanism and this allows concentrated chemicals to be doled out at the user’s discretion.
Moldy shampoo is something no one wants to think about, and preservatives are what keep us from having to consider this disgusting possibility. When it comes to shampoo, the preservatives of choice are parabens, like methylparaben, ethylparaben, and butylparaben. If parabens sounds familiar, then it is probably because some shampoos and conditioners tout their lack of them (opting instead for other preservative agents). But why is it that parabens have a bad reputation?
At the outset the concern over parabens seems justified. Parabens contain estrogen, and elevated levels of estrogen have been causally linked to several types of cancer. However, it should be noted that parabens contain relatively miniscule amounts of estrogen, less than one would find in, say, a glass of soy milk. The United States Food & Drug Administration is monitoring studies on the effects of parabens on people’s health and as of yet has found no reports suggesting that the paraben-content of shampoos presents a risk to people’s health.
Most shampoos are designed to target hairs and their follicles, yet those containing zinc pyrithione are also intended to treat the scalp. While dandruff can have multiple causes, such as an overly dry scalp, one common cause of its more severe cases can be traced back to bacteria on the sufferer’s scalp. As an antibacterial, zinc pyrithione gets rid of dandruff in these cases at its source.
Last and maybe least are foaming agents. Cocamide and cocamidopropyl are standard chemicals that lend shampoo the ability to be worked into a lather. While satisfying, this process does not increase the effectiveness of shampoo. It is a purely aesthetic addition that marketers found consumers enjoyed, and that consumers in turn came to expect. As with scents and dyes, foaming agents add to the experience of using shampoo but serve no practical purpose beyond that.
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Pai, Deanna. “Your Ultimate Guide to What All Those Ingredients on Your Shampoo Bottle Mean”. Glamour, glamour.com, April 29 2016, http://www.glamour.com/story/shampoo-ingredients-what-are-parabens-sulfates, Accessed Aug 29 2020
“Parabens in Cosmetics”. U.S. Food & Drug Administration, fda.gov, http://www.fda.gov/cosmetics/cosmetic-ingredients/parabens-cosmetics#:~:text=Preservatives%20may%20be%20used%20in,propylparaben%2C%20butylparaben%2C%20and%20ethylparaben., Accessed Aug 30 2020
“The Science of Shampoo: What the Ingredients Mean”. Newsweek, newsweek.com, Oct 8 2009, http://www.newsweek.com/science-shampoo-what-ingredients-mean-222524, Accessed Aug 29 2020