Turns out, we can’t tell dry hair from moisturized hair

Studies show that people identify hair with low water content as looking and feeling more moisturized.  Additionally, a study by Mendes et. al. looked at water content levels after various hair product applications and found this out [1]:

Sensory evaluations are inadequate for the assessment of the hydration levels provided by different cosmetic hair products.

In normal-people speak:  People are lousy at knowing how much moisture is actually in their hair after using various conditioning products.  So when you hear someone say, “this conditioner is so much more moisturizing than the other one,” they’re scientifically likely to be full of baloney.

Scientists have sensitive lab equipment which allows them to make exact statements like, “this swatch of hair has 10% water content at 60% Relative Humidity (RH).”  But at home, without the special and expensive equipment, the average person doesn’t know exactly how much water is in their hair.  Since we can’t know the exact water content, we have to rely on our physical senses to tell us how much water is in our hair. And that’s where the problem begins…

Our physical senses lead us to the wrong conclusions about hair hydration levels

We rely on sight and skin sensations to make the following assumptions about our hair: 

1. When hair looks frizzy, it also feels rough, so rough and frizzy hair must be dry hair.
2. The hair feels dry to the touch (aka it feels rough), therefore it must mean that the hair is actually dry.

Unfortunately, our senses are misleading us and our assumptions end up being incorrect.

Fact#1 Hair gets frizzy in high humidity because it has absorbed a lot of moisture from the air.

Contrary to the statements that fly around on the internet, when humidity is high, the water content in your hair rises, and that is what causes curly hair to frizz up [2,3].  Frizz is not “dry hair reaching out for moisture,” frizz is hair that already has too much moisture.  Also, for some people, a lack of definition in their curl pattern is genetic [4]. In either case, we shouldn’t assume that frizzy hair is a sign of dryness.

Fact #2:  Our skin does not have a sense receptor for moisture.

Our skin has nerve receptors that can respond to temperature, touch, pressure, pain, and nothing else. A “moisture” receptor does not exist [5].  This means that we combine input from the other receptors to give us the perception of moisture [6].

Fact #3: We perceive moisture from combined stimulation of pressure and temperature receptors on our skin.

The sensation of moisture is a blend of pressure and cold temperature [6-8]. Bentley et. al. did a cool (no pun intended) experiment where they put different dry substances on blindfolded people’s fingers, and when they combined coldness with the right amount of pressure on the finger, the subject would say that they’re feeling something wet.

Fact #4  Highly absorptive fibers are perceived as drier than minimally absorptive fibers

Getting closer to the hair topic, highly hygroscopic (aka absorbant) fibers such as wool are perceived to be significantly drier than weakly hygroscopic fibers such as polyester because the temperature drop at the skin surface that occurs at the moment of contact with a highly absorbant fieber is smaller than that with a weakly absorbant fiber [9-11].  In normal-people speak, we perceive colder things as being wetter, and warmer things as being drier.

The reason the higher-absorbing wool feels drier is that its absorptive nature keeps the water molecules from evaporating off its surface, thereby keeping its temperature more constant.  On the flip side, the polyester fiber, being minimally absorptive, doesn’t hold on to its water molecules.  Thus, the polyester fiber’s surface temperature is lowered thanks to the higher rate of evaporative cooling.

Fact #5 Frizzy hair feels drier than straight hair because it doesn’t feel as cool to the touch

The main concept here is thermal conductivity—the heat transfer from one object to the next. For hair that is frizzy, its meandering shape creates many air pockets, which means that less of the hair comes in contact with the skin…

Less contact = less heat transfer away from your skin = hair doesn’t feel as cold = hair doesn’t feel as moisturized.

Going the opposite way:

Less frizz (aka better hair alignment) = more contact area with skin = more heat goes away from the skin = hair feels colder = hair feels more moisturized.

Fact #6 Damaged hair feels drier because it is prone to becoming misaligned

Between 10%-90% relative humidity, bleached hair contains more water than virgin hair [12,13];  yet it is described by test subjects as feeling drier [12]. Why? Because bleaching (as well as other damages to the cuticle) strips the 18-MEA layer.  Once that happens, hair strands have higher friction and catch on each other more (aka they become misaligned and frizz-prone easier).  And it is this misalignment that we spoke of in Fact #5 that leads to a dried out feeling.

Scientists proved this by taking damaged 18-MEA-stripped hair swatches (that people previously described as dry-feeling), straightening them with a blow dryer, and giving them back to the people for evaluation.  Those same swatches were now described as moisturized-feeling, even though there was no change in water content or repair of the 18-MEA layer [12].

So there you have it folks; our senses deceive us.  The last thing I want to add is that it pays to remember that our skin and hair are different.  When our skin is dry, it looks and feels rough, and many people mistakenly assume that if their hair looks and feels rough, then it must be dry as well.  But this is clearly wrong, so let’s continue on our quest for knowledge together.

‘Till next time!

References:

1. Mendes, T. R., & PIACESKI, A. D. (2003). Evaluation of hair fiber hydration by differential scanning calorimetry, gas chromatography, and sensory analysis. J. Cosmet. Sci, 54, 527-535.
2. Gao, T. (2007). Evaluation of hair humidity resistance/moisturization from hair elasticity. Journal of cosmetic science, 58(4), 393-404.
3. Bengtsson, M. A. T. I. L. D. E. (2016). All-day hair manageability for textured hair types. Chalmers University of Technology, 1-47.
4. Nagase, S., Tsuchiya, M., Matsui, T., Shibuichi, S., Tsujimura, H., Masukawa, Y., … & Tsujii, K. (2008). Characterization of curved hair of Japanese women with reference to internal structures and amino acid composition. Journal of cosmetic science, 59(4), 317-332.
5. Turton, J., & Hooson, J. (Eds.). (2003). Target organ pathology: a basic text. CRC Press.
6. Lauterbach, C. E., & Crouser, R. E. (1933). Sensation cues to moisture. Journal of Experimental Psychology, 16(2), 328.
7. Titchener, E. B. (1909). A text-book of psychology (Vol. 1). Macmillan.
8. Bentley, I. M. (1900). The synthetic experiment. The American Journal of Psychology, 11(3), 405-425.
9. Li, Y., Plante, A. M., & Holcombe, B. V. (1993). The physical mechanisms of the perception of dampness in fabrics. Journal of Thermal Biology, 18(5-6), 417-419.
10. Plante, A. M., Holcombe, B. V., & Stephens, L. G. (1995). Fiber hygroscopicity and perceptions of dampness: Part I: subjective trials. Textile Research Journal, 65(5), 293-298.
11. Li, Y., Plante, A. M., & Holcombe, B. V. (1995). Fiber hygroscopicity and perceptions of dampness: part II: physical mechanisms. Textile Research Journal, 65(6), 316-324.
12. Tanamachi, H. (2011). Temperature as a moisture cue in haptics on hair. International journal of cosmetic science, 33(1), 25-36.
13. Wolfram, L. J., Hall, K., & Hui, I. (1970). The mechanism of hair bleaching. J. Soc. Cosmet. Chem, 21, 875-900.