Does water damage hair? The myth of “hygral fatigue”

How water interacts with hair is one of the biggest sources of myths about hair. One of the biggest myths is the idea that wetting and drying hair over and over again inherently damages hair – common referred to as “hygral fatigue”. This isn’t true!

Let’s dig deeper into this…

This article is adapted from my video on hair hydration. Also check out Part 1 on hair and water more broadly!

Hair bonds

A quick refresher on hair bonds (check out Part 1 for the full version):

• Water is very good at breaking temporary bonds between hair proteins (mostly hydrogen bonds)
• This is why wet hair breaks more easily – there’s less “glue” holding the hair proteins together, so less force is required to break it apart
• Water also causes cuticle scales to stick up, since the top of each cuticle scale absorbs less water than the underside
• When hair is dried, these effects all reverse

“Hygral fatigue”

Because of these effects that water has, it might seem like wetting and drying hair repeatedly could damage hair. 

This very persistent myth is referred to as “hygral fatigue”, and it’s even mentioned in some peer-reviewed papers. It’s often given as a reason for not washing your hair every day. But there’s actually not much convincing evidence that this actually happens!

At the molecular level

An analogy I’ve seen is that this is like stretching a rubber band repeatedly – with enough wetting and drying, the hair eventually “fatigues” and snaps.

But hair and rubber bands are very different. In rubber bands, stretching breaks permanent bonds that can’t form again, and this eventually builds up to form a crack.

In hair, water is breaking temporary hydrogen bonds which reform really easily – they form when you just dry the hair. A better analogy would be joining and unjoining Lego pieces (not the knockoffs, the brand name ones), except the atoms in hydrogen bonds are even more durable than this, since electrons and protons don’t wear down!

Related post: Hair, hydration and water

However, hair is still more fragile when it’s wet, so washing more frequently does mean there’s more opportunity for hair to get damaged if you don’t treat it gently. But water simply going in and out of hair doesn’t cause damage.

Hair drying study

There are a few peer-reviewed papers that seem to support the idea that wetting and drying can inherently damage hair, but the evidence they show is not very convincing.

The first one is a 2011 study on hair drying methods (previously discussed in this article). The researchers looked at the damage from air drying versus using a hair dryer at different distances, which resulted in different drying temperatures. 

The researchers found that blow drying at a low temperature caused the least damage. They observed bulges in the air-dried sample of hair, and concluded that this was damage from water swelling the hair for longer:

I don’t think their explanation makes much sense. Air drying is pretty standard everywhere, including in hair experiments. If air drying was causing the bulges they saw, then other hair studies would be reporting these a lot more.

Related post: How to dry your hair, according to science

It’s much more likely that the bulges are caused by something specific to this experiment. It’s unclear how many times the researchers repeated this experiment and the measurements, so it’s difficult to work out if it’s an issue with the experiment, or a fluke problem. But one potential explanation could be something that happened to that specific hair sample before it was collected for use, such as lots of sun exposure.

Coconut oil studies

A few studies have proposed that coconut oil could potentially block hair from absorbing water, and hence protect it from hygral fatigue.

A couple of these studies explicitly use the term “hygral fatigue”. But they don’t actually cite any sources for hygral fatigue happening in the first place.

It’s also questionable whether coconut oil actually blocks hair from absorbing water to any significant extent. 

In some experiments, hairs were coated in different oils (coconut, mineral and sunflower oils), then placed in a dynamic vapour sorption (DVS) apparatus. The weight of the hair was measured at different humidities – any increased weight would be water that absorbed into the hair. The absorbed water was then converted to a percentage of the hair’s weight. Since the coconut oiled hair increased by the smallest percentage by weight, they concluded that coconut oil blocked hair from absorbing as much water.

But Trefor Evans, another hair scientist, points out that this might be an experimental error. The hair weighs more after you add coconut oil, so the same amount of absorbed water would look smaller as a percentage (essentially you’re dividing by a bigger number: hair + oil, rather than just hair).

There could be a similar effect happening with studies that weighed oil-treated hair after wetting, then again after drying.

Coconut oil’s inability to seal out much water would make sense when you consider the structure of hair. Every cuticle edge is a gap – it’s really unlikely that you could seal the majority of the hair “pinecone” against teeny tiny water molecules. This logic extends to the likelihood of pretty much any hair treatment preventing water from going into or out of hair. The water content of your hair largely depends on the relative humidity.

This doesn’t mean coconut oil isn’t beneficial for hair. Oils act as lubricants on the surface of hair, smoothing and protecting the cuticle from damage, such as during combing. Some experiments suggest that coconut oil absorbs deeper into hair than other oils. This means coconut oil could potentially fill gaps in the oily parts of hair (the cell membrane complex “mortar” between hair cell “bricks”), which can help reduce internal cracking in hair.

Related post: Busting Hair Conditioner Myths: Build-Up, Silicones, Weighing Hair Down etc.

References

Robbins CR. Chemical and Physical Behavior of Human Hair. 5th ed. Springer Berlin Heidelberg 2012. 

Lee Y, Kim YD, Hyun HJ, Pi LQ, Jin X, Lee WS. Hair shaft damage from heat and drying time of hair dryer. Ann Dermatol. 2011;23(4):455. doi:10.5021/ad.2011.23.4.455

Evans T. Measuring the water content of hair. Cosmetics & Toiletries. 2014;129(2):64-69.

Keis K, Huemmer CL, Kamath YK. Effect of oil films on moisture vapor absorption on human hair. J Cosmet Sci. 2007;58(2):135-145.

Ruetsch SB, Kamath YK, Rele AS, Mohile RB. Secondary ion mass spectrometric investigation of penetration of coconut and mineral oils into human hair fibers: Relevance to hair damage. J Cosmet Sci. 2001;52:169-184.

Rele AS, Mohile RB. Effect of coconut oil on prevention of hair damage. Part I. J Cosmet Sci. 1999;50(6):327-339.

Rele AS, Mohile RB. Effect of mineral oil, sunflower oil, and coconut oil on prevention of hair damage. J Cosmet Sci. 2003;54(2):175-192.

Kaushik V, Chogale R, Mhaskar S. Single hair fiber assessment techniques to discriminate between mineral oil and coconut oil effect on hair physical properties. J Cosmet Dermatol. 2021;20(4):1306-1317. doi:10.1111/jocd.13724

Gode V, Bhalla N, Shirhatti V, Mhaskar S, Kamath Y. Quantitative measurement of the penetration of coconut oil into human hair using radiolabeled coconut oil. J Cosmet Sci. 2012;63(1):27-31.