1. Grown-Out Hair Is Dead Cells, But It Still Has an Inside
Hair grows about 1 cm per month. If your hair is shoulder-length (25–30 cm), it holds about 2 years of history [1]. This grown-out part consists of dead cells and tissues containing a protein called keratin. Even if it gets damaged, it is not rebuilt like living cells in your body [1].
However, hair still has an "inside." For Japanese people, the diameter is about 0.1 mm. The outermost layer is a hard cuticle, inside that is a softer cortex, and at the center is the medulla (marrow) [1]. When discussing whether treatments enter hair, the focus is on the cortex. Before reaching it, ingredients must pass through the hard, scale-like cuticle. How do they get through?
2. Why the Outer Layer of Hair (the Cuticle) Works as a Barrier
In an observation of healthy Japanese hair cross-sections, the cuticle was found to be 5–6 layers thick. The cortex is made of cells with diameters of a few micrometers (1 μm is one-thousandth of a millimeter), separated by parts called CMC (Cell Membrane Complex) [1]. Note that this was based on observing just one strand; not everyone’s hair has exactly 5–6 layers.
Furthermore, each cuticle layer is described as having an outer surface with oil-like properties and an inner layer with water-like properties [3]. Hair diagnosis experts also write that because the cuticle acts as a barrier, it is difficult for treatment ingredients to penetrate inside [4].
Yet, even this "hard-to-enter" hair seems to have conditions where entry becomes easier. What changes in damaged hair?
3. How Chemical Damage Creates Gaps Inside Hair
Lion’s document explains that in hair repeatedly treated with chemicals, lipids in the CMC decrease, and the barrier function that keeps moisture out weakens, making the hair "sparse" or full of gaps [2].
Kao’s observations show an even more extreme example. Near the tips of hair that had been colored 8 times, the cuticle was completely gone, and cavities larger than a few micrometers were found [1]. However, this was one single strand with extreme damage from 8 dyeing sessions, and individual differences are large, so it cannot be used to explain typical hair damage.
If the barrier weakens and gaps form, there is more room for ingredients to enter. But what is inside is invisible. How did researchers confirm that things had entered?
4. How Researchers Mark Ingredients to See Them in Hair Cross-Sections
You cannot see ingredients inside hair with a normal microscope. So, in Lion’s research, they attached a "marker" called a stable isotope to an oil ingredient called isostearic acid (IS) [2]. Stable isotopes are atoms that have almost the same properties as other atoms but differ slightly in weight.
They soaked damaged model hair bundles in this formula for 24 hours, cut the hair, and examined the cross-sections with a device called nano-SIMS. Because markers appear where the isotope is present, they could map exactly where the IS was located inside the hair. The results confirmed that IS had entered into internal gaps and around the CMC [2].
However, this result applies only to a formula with at least 40% concentration of an ingredient called isopentyldiol, soaking damaged model hair for 24 hours. It does not apply to all commercial treatments in general.
So why did it enter in that specific formula?
Safety Note: Isotopes have almost identical chemical properties and differ only slightly in mass, so scientists assume they behave just like the unmarked ingredient, allowing them to distinguish location.
5. What Helps Oil Ingredients Enter Hair: Carriers and Molecular Size
Lion’s document suggests that oil ingredients are easily diluted and hard to enter wet hair on their own. It states that isopentyldiol may act as a medium to carry IS into the hair interior [2]. The phrasing "possibility was suggested" is cautious; it does not declare it definitely acts as a carrier.
Another study examined differences based on ingredient size. Published in the Journal of Cosmetic Science, this research used TOF-SIMS to examine brown hair cross-sections. It reported that large molecules like polyquaternium-10 stayed mostly on the surface, while smaller molecules like cetyltrimethylammonium bromide (CETAB) entered deeper [5]. This compared only these 2 ingredients; it does not prove a general rule that "smaller molecules always go deeper." The conditions were untreated hair, 0.5% water solution, 37°C, and 6 hours, which differs from daily treatments lasting a few minutes.
So, if it enters deeply, does it work on damaged hair?
6. Why an Ingredient Entering Hair Does Not Mean It Works
A presentation at an international conference in 2018 used X-ray micro-CT to examine gaps inside the cortex of bleached hair. The formula that reduced these gaps by about 70% was a leave-on (not rinsed out) type that did not contain cationic ingredients (those with positive charges) [6]. This means cationic ingredients are not always the most effective inside.
This 70% figure is specific to that formula and condition, not a general number. Also, when looking at the nano-SIMS results together, we did not find any study in these sources that directly measured how much enters during normal use of commercial treatments (applying for a few minutes and rinsing).
The current answer is: "Experiments show it can enter. But this depends on specific conditions, and we cannot say it applies to all commercial products."
7. How to Estimate the Age of Hair from One Strand
Try measuring one of your own fallen hairs with a ruler. Using the guideline that hair grows about 1 cm per month [1], the length in cm gives an estimate of how many months ago it grew. If it is 20 cm long, it grew about 20 months ago, or more than a year and a half ago. Growth speed varies by person, so this is just an approximate value.
The tips have experienced washing and brushing for that entire length. It might be interesting to compare the feel of the roots and tips on the same hair strand.
If you want to know more, research documents published by companies or papers in the journal of the Microscopy Society of Japan are good clues. You can check cross-section photos and experiment conditions via the URLs in the source list.