Scrub-A-Dub Dub: How Often Should You Wash Your Hair & Scalp?

Dr Paul Cornwell, Director at TRI

Dr Samuel Gourion-Arsiquaud, Director at TRI

Dr Philippa Cranwell, Technical Content Creator at TRI

How often should you wash your hair to keep both your hair and your scalp healthy?  This short article will summarize the current thinking in the area, diving into the literature, and work that is currently being undertaken at TRI Princeton, to answer this question once and for all!

Introduction

It would seem that everyone has an opinion on how often you should wash your hair, with the two camps advocating very different things. On the one hand, there are those who emphatically state that it’s a crime to wash hair too often, that it should be avoided at all costs, and hair can be ‘trained’ to produce less grease. On the other hand are those who insist that washing three or four times per week is best, as it avoids lanky, greasy hair and they feel better for it. Usually, those with curly hair (Walker type 3 or 4) tend to opt for washing less frequently, and those with straighter hair (Walker types 1 and 2) tend to wash more frequently. However, regardless of hair type, what is often cited by both groups is that hair shouldn’t be washed too often, as washing too regularly can cause scalp irritation, lipid stripping of the hair fiber and product build-up. But is this actually true? Or is there a sweet-spot that means you get the best of both worlds: smooth, luxurious locks without the risk of dryness, hair or scalp damage, and irritation?

This short article will summarize the current thinking in the area, diving into the literature, and work that is currently being undertaken at TRI Princeton, to answer this question once and for all! We’ll start by looking at the impact of washing on the scalp followed by the impact on hair, and then consider three clinical studies, one completed at TRI Princeton, that explicitly investigate the effects of hair-washing frequency.  

The basics: Scalp skin vs body skin

A common theme for the washing versus non-washing debate is that over-washing can cause irritation and damage to the scalp. However, we probably wash our face and body every day, so is there a difference between scalp skin and body skin that means scalp might behave differently? 

Beneath the surface, the skin on your body and the skin on your scalp are very similar, in that they both have three layers (the epidermis, dermis, and hypodermis) and they both have sweat glands, hair follicles and sebaceous glands. However, unlike skin on the body, your scalp is usually covered with long hair that can be thick. In addition, the scalp has more sebaceous glands per unit area in comparison to other parts of the body, therefore build-up of sebum, the skin’s natural lubricant, will naturally occur over time. 

While sebum secretion is necessary to maintain the barrier function of skin and to prevent surface dryness, a build-up of oil can be a problem. Two studies have been disclosed that consider the outcomes of infrequent hair washing in different work-place situations. In one study, a group of people working in the Antarctic was monitored for changes in scalp microbe content and, in another, the scalp health of astronauts on the International Space Station was monitored. In both cases, scalp itch and flaking increased dramatically, which was accompanied by an increase in Malassezia (formerly P. ovale) levels. The itching was likely due to the lipids in sebum being oxidized to give products that can irritate the skin, and the build-up of oil likely contributed to the elevated Malassezia levels. 

But did they get itchy scalps in the Antartic?

But did she get itchy scalp in space?

While the issues of oil build up clearly impact upon the scalp, it can also influence the development and growth of new hair fibers. The hair on your head originates approximately 4 mm underneath the skin, with a newly formed fiber around taking two weeks to reach the skin’s surface. While under the surface, the new fiber is in close contact with the surrounding scalp tissue. When young, the fiber is soft and malleable and as it matures it hardens, therefore any poor scalp health can affect the developing hair fiber, impairing hair growth and also causing premature hair loss, low shine, a weak or roughened cuticle and hair ‘pimples’, or folliculitis. 

Washing hair is a good thing

Thus far, all the evidence has suggested that removing sebum is a good thing as sebum build-up can cause scalp and hair issues, such as irritation and scales. The next logical question to address is “how often should I actually wash my hair to remove the sebum?”

A small clinical study on 7 young (19-23 years old) Japanese male volunteers has shown that not washing the hair for four days allows more sebum triglycerides to be converted into fatty acids and that the build-up of these fatty acids is correlated with skin itchiness (Kato, K. Bulletin of Hokkaido University 10, 47-58, 1997).  Interestingly, washing the hair and scalp, removed the decayed sebum and immediately reduced scalp itch.  Clearly, washing only every four days was not enough to control scalp itch.

Work in this area has also been undertaken by Procter and Gamble and TRI Princeton. In the Procter and Gamble study there were two aspects. The first sought to derive a link between hair-wash frequency and self-perception of hair health. In this case, the more often a participant washed their hair, the more likely they were to report satisfaction with their hair and have more ‘great hair days’ per week, with fewer instances of dandruff, itching and dryness reported, Figure 1. In addition, there was no correlation between wash-frequency and self-reported hair loss, brittleness and roughness.

Figure 1: (a) Self-perceived scalp parameters dandruff, itch, and dryness of n = 1,500 males and females as function of wash frequency in the epidemiological study. The mean is plotted with standard error bars. Results not connected by same letter are significantly different; (b) Self-perceived hair attributes hair loss, hair brittleness, and hair not smooth of n = 1,500 males and females as function of wash frequency in the epidemiological study. The mean is plotted with standard error bars. Results not connected by the same letter are significantly different. Images reproduced from Punyani et al., Skin Appendage Disord., 2021, 7, 183 – 193 (doi: 10.1159/000512786). 

The second arm of the Procter and Gamble study required participants to refrain from washing hair for a 7-day period, then wash their hair daily with a ZPT scalp-care shampoo for 4 weeks. After washing daily, the quantity of lipids and levels of the oxidized lipid HODE-9 present on scalp was reduced. In addition, the Adherent Scalp Flaking Score (ASFS) and hair odor scores were both lower. 

In terms of the hair fibers themselves, in hair washed daily there were fewer sebum lipids and less HODE-9 on the surface, as well as a lower greasy shine score, Figure 2. Interestingly, the use of DVS to probe cuticle integrity showed that daily washing resulted in lower hair water vapor absorption, indicating a more effective cuticular barrier. When hair was washed daily the levels of hair internal lipids, which are beneficial for hair, did not change significantly compared with the baseline, therefore stripping of lipids in the hair was not an issue with the shampoo formulation in the study.  

Figure 2: (a) Technical hair measurements for total sebum lipids; (b) oxidized lipids (HODE); (c) greasy shine; (d) and malodor at 7-days refrain (yellow bar) and daily wash (blue bar). For measurements in (a, b, d), lower value indicates healthier hair. For measurement in (c), lower value indicates a less greasy shine on hair. Image reproduced from Punyani et al., Skin Appendage Disord., 2021, 7, 183 – 193 (doi: 10.1159/000512786).

What impact does hair type have upon things?

TRI Princeton have taken this concept even further and considered whether hair type also plays a role. For example, those with Walker type 3 or type 4 hair are usually encouraged to wash hair less frequently, whereas those with Walker type 1 or 2 hair tend to wash hair more frequently. 

A TRI study recruited panelists in both the Republic of South Africa (RSA) and the United States (US) with Walker hair types 2, 3 and 4. In both populations, those with type 4 hair tended to wash their hair less frequently: 35.9% participants with type 4 hair in the US had more than 14 days between washes, and 58.7% of those in RSA. Interestingly, in RSA, those with type 3 hair tended to wash their hair once per week, whereas in the US type 3 hair was washed 3 or 4 times per week, Figure 3.

Figure 3: (a) Maximum number of days between hair washes for US-based participants (n = 120); (b) Maximum number of days between hair washes for RSA-based participants (n = 120).

When participants were forced to wash their hair on alternate days for a week, across all hair-types in both participant populations, improvements were noted in terms of overall hair cleanliness, itchiness, dryness and dandruff, Figure 4. It was particularly noted that those participants who washed their hair less frequently prior to the study saw the greatest improvement in hair scalp health. 

Figure 4: (a) Self-reported changes by all participants after washing hair alternate days for a week (n = 240); (b) Relationship between normal wash frequency and degree of perceived improvement in scalp condition, Day 1 to Day 7, over all participants (n = 240).

Finally, it was noted that in both the RSA and US-based participants, those with type 4 hair had biomarkers for higher inflammation including higher pH (more alkaline) and increased levels of IL-1α and IL-1RA. After washing the hair alternate days for a week, these biomarkers were significantly reduced, suggesting that washing the hair was beneficial. 

Summary

Clinical studies show that washing hair more regularly, regardless or hair type, can give positive results and reduce scalp inflammation, dandruff and itchiness.  However, when it comes to recommending a number of times to wash hair per week, it’s likely down to personal preference and your own perception of your hair. Although it has been shown scientifically that both consumer satisfaction and hair health are improved when washing more regularly, it may not be practical. 

Contact us today to discuss your hair and scalp product testing needs.