The science of ingrown hairs

An ingrown hair is not a skin problem that happens to involve a hair. It is a geometry problem — a hair too curved, too sharply cut or too obstructed to leave the follicle cleanly, met by an immune system that treats the result as a foreign object.

This page works through that process structure by structure, then through every ingredient in the ungro Ingrown Hair Oil and the specific pathway each one acts on. The plain-language version lives at what an ingrown hair actually is. This is the long version.

What is a hair follicle, properly?

A follicle is not a simple hole in the skin. It is one part of a structure called the pilosebaceous unit: the follicle itself, a sebaceous gland that drains into it, and the arrector pili muscle that makes hair stand on end.

Anatomically, the follicle is divided into three segments.

• The infundibulum. The funnel at the top. It runs from the skin surface down to the point where the sebaceous duct opens into it. Almost everything that goes wrong with an ingrown hair goes wrong here, or just below it.
• The isthmus. The middle section, between the sebaceous duct opening and the bulge — the region where the arrector pili muscle inserts and where the follicle's stem cells sit.
• The inferior segment. The lower follicle, ending in the bulb, wrapped around the dermal papilla. This is where the work happens: the matrix cells there divide faster than almost any other cells in the body, and they build the hair shaft.

Two consequences follow from that layout, and they run through the rest of this page.

First, the infundibulum is the choke point. It is the narrowest and most superficial part of the channel, it is lined with the same keratinising cells as the skin surface, and it is where the hair has to make its exit. A follicle can be perfectly healthy for its whole length and still fail at the last two millimetres.

Second, the infundibulum is full of sebum. The sebaceous duct empties directly into it. Chemically, the inside of a follicular opening is a lipid environment, not a watery one. That single fact decides what kind of product can reach it at all, and it is returned to below.

Anatomy of the follicle and its segments is set out in StatPearls, Anatomy, Hair Follicle.

What are transfollicular and extrafollicular penetration?

A hair can re-enter skin in two distinct ways. The distinction is worth knowing because the two have different causes, and one of them is largely about how you remove the hair.

Transfollicular penetration is re-entry from the inside. The cut hair never clears the surface. It curves while still inside the channel and pushes through the follicular wall into the surrounding dermis. Pulling the skin taut for a closer shave, plucking, and multi-blade razors that cut the shaft below the skin surface all make this more likely, because each leaves a sharpened tip sitting inside the follicle rather than above it.

Extrafollicular penetration is re-entry from the outside. The hair does emerge. It is cut at or very near the surface, which leaves it with a bevelled point rather than a blunt one, and as it grows and curls it pierces the epidermis a short distance away from the opening it came out of, then continues growing downward.

One honest note on terminology. DermNet describes these as transfollicular — piercing the skin surface — and intrafollicular, where the hair retracts beneath the surface and pierces the follicular epithelium. Much of the pseudofolliculitis literature uses transfollicular for the internal route and extrafollicular for the external one. The two mechanisms are not in dispute. The labels are not entirely settled, and anyone telling you otherwise has not read widely.

Why does hair curvature decide so much of this?

A curved follicle produces a curved hair. The follicle acts as a mould, and a hair grown in a curved channel emerges with a curl already built into it and an elliptical rather than round cross-section.

That curl is not cosmetic. It is a stored trajectory. A straight hair emerging from a straight follicle points away from the skin. A tightly curled hair emerging from a strongly curved follicle points along the skin, and then into it.

Two further points of physics compound it.

Stiffness rises steeply with thickness. The bending stiffness of a fibre scales with the fourth power of its diameter. A coarse hair is not slightly harder to deflect than a fine one; it is dramatically harder. Coarse and curly together is the worst combination, because the hair has both a re-entry trajectory and enough rigidity to push through skin once it gets there.

A cut hair is sharper than an uncut one. An uncut hair tapers to a fine, flexible tip. A shaved hair ends in an angled, blunt-but-firm point. That is why this problem is created by hair removal rather than by hair.

This is why pseudofolliculitis barbae — the medical name for razor bumps — predominantly affects men of African ancestry, at a reported prevalence of roughly 45 to 80 per cent, and why it is common on the groin in women of all backgrounds. DermNet also notes a genetic contributor: a single nucleotide substitution in the follicle companion layer keratin K6hf is an additional risk factor. It is a structural condition, not a hygiene one.

What is the bump actually made of?

Once a hair is in the dermis, the body does not recognise it as its own. Keratin in the wrong compartment is treated as foreign material, and what follows is a foreign body inflammatory reaction.

Simplified, but accurately:

1. Recruitment. Local cells release signalling molecules. Neutrophils — the first-response white blood cells — arrive within hours.
2. Vasodilation. Local blood vessels widen and become leaky so those cells can get out of the bloodstream and into the tissue. That is the redness, the warmth and the swelling. The papule is largely fluid and cells, not hair.
3. Attempted clearance. Macrophages arrive to remove the foreign material. A hair shaft is heavily cross-linked keratin and does not break down easily, so the response persists rather than resolving. If it persists long enough, macrophages fuse into multinucleated giant cells — the classic microscopic signature of a foreign body reaction.
4. Pus, sometimes. An injured follicle is easy to infect. When bacteria establish, the papule becomes a pustule and the diagnosis shifts from pseudofolliculitis to folliculitis. That is a different problem with a different answer.

The dark mark is the last act. Inflammation stimulates melanocytes, the pigment-producing cells sitting at the base of the epidermis. They deposit extra melanin, and some of it drops into the dermis below, where it is slow to clear. This is post-inflammatory hyperpigmentation, and it is why the mark outlasts the bump by months. It is more pronounced and more persistent in richly pigmented skin.

Repeated inflammation in the same follicles can also produce scarring, including keloid scarring. The useful conclusion is that the inflammatory phase is worth shortening, not just tolerating.

What is hyperkeratinisation, and why does scrubbing make it worse?

The skin surface renews itself continuously. Cells called corneocytes — flattened, dead, keratin-filled cells — are produced at the base of the epidermis, migrate upward, and are shed. The shedding process is called desquamation, and it depends on enzymes dissolving the protein bonds that hold neighbouring cells together.

Hyperkeratinisation is that shedding failing to keep pace. Corneocytes are retained instead of released. Inside the narrow infundibulum, retained cells accumulate along the wall, and the effective diameter of the opening drops.

The result is a channel that is narrower at exactly the point where the hair needs the most room. Shaving accelerates it, because the blade abrades the surface and the skin responds by increasing turnover. This is why dermatologists reach for retinoids and benzoyl peroxide in razor bump management: both suppress follicular hyperkeratosis.

Now the part almost everyone gets wrong. The instinctive fix is to scrub. Physical scrubs, stiff brushes, exfoliating mitts, pressure.

Mechanical abrasion removes surface cells indiscriminately, including cells that were doing structural work. The skin registers the damage and increases keratinocyte turnover to replace what was lost. More cells arrive at the surface, faster, and the follicle is no clearer than it was.

Worse, scrubbing over an inflamed follicle is direct trauma to a site that is already inflamed. It can rupture a follicle that was going to settle. The way out is regular and gentle, not occasional and aggressive.

Why does the vehicle matter more than the active?

This is the single most important formulation argument ungro has, and it is a chemistry argument rather than a marketing one.

The infundibulum is filled with sebum, which is a mixture of triglycerides, wax esters, squalene and free fatty acids. It is a lipid compartment. Like dissolves like: a lipophilic vehicle mixes readily with sebum and partitions into that space. A water-based vehicle does not. It largely sits on the surface of the skin, spreads across it, and evaporates.

An active is only as useful as the distance it travels. A well-chosen ingredient in the wrong phase is an ingredient sitting on top of the problem.

This is covered at length in the lab notes: why an oil-soluble acid reaches an ingrown hair that a water-based one cannot.

What is each ingredient in the oil actually doing?

Caprylic/Capric Triglyceride, C12-15 Alkyl Benzoate, Simmondsia Chinensis (Jojoba) Seed Oil, Squalane, Mandelic Acid, Bisabolol, Tocopherol, Avena Sativa (Oat) Kernel Oil, Bacillus Ferment.

Nine ingredients. Each is below with the pathway it acts on. The plainer ingredient-by-ingredient version is on the ingredients page.

Mandelic acid at 0.3 per cent

Mandelic acid is an alpha hydroxy acid, and it is the only common one with a benzene ring attached.

That ring is the whole story. Glycolic acid has a molecular weight of about 76 and lactic acid about 90; both are small, highly polar molecules that dissolve happily in water and very poorly in oil. Mandelic acid is about 152, and the aromatic ring gives it meaningfully more affinity for a lipid phase than either of them has. That is why it can be carried in an anhydrous oil at all. Glycolic acid at the same percentage in the same base would not stay in solution in any useful way.

Now the honest part. At 0.3 per cent, this is not exfoliation.

AHAs exfoliate by loosening the bonds between surface corneocytes, and that effect depends on both concentration and free acid availability — which in turn depends on pH. Mandelic acid used to resurface skin runs at roughly 5 to 10 per cent in a water phase at a controlled low pH. There is no water in the ungro oil, so there is no pH to control. An acid without water has no pH at all.

What 0.3 per cent contributes is antibacterial activity. Mandelic acid has documented in-vitro activity against Cutibacterium acnes and against Staphylococcus aureus, including methicillin-resistant strains. In a follicle that has been traumatised by a blade and is vulnerable to tipping from irritation into infection, that is a reasonable contribution and it is stated as a contribution rather than as the mechanism.

For context on what AHAs do at exfoliating strength, DermNet's overview of alpha hydroxy acid treatments is a good reference point — and a reminder of the gap between 0.3 per cent and a peel.

Jojoba seed oil

Jojoba is not a triglyceride and, strictly, not an oil. It is a liquid wax ester: a long-chain fatty acid joined to a long-chain fatty alcohol. Almost every other plant oil in cosmetics is a triglyceride, three fatty acids on a glycerol backbone. Jojoba is structurally different.

That structure is close to the wax esters that make up a substantial fraction of human sebum, which is why jojoba integrates into the follicular environment rather than sitting on top of it.

Why that matters for a hair. Hair is keratin, and keratin's mechanical properties are strongly influenced by what surrounds it. A shaft that is lubricated and conditioned is more able to bend and less able to punch back into skin. Softening the hair does not change its curvature, but it changes what happens when the curve meets skin.

Squalane

Human sebum contains squalene. Squalane is the hydrogenated, saturated version of that molecule — the same skeleton with the double bonds removed, which makes it stable enough to sit in a bottle without oxidising. Cosmetic squalane is produced from sugarcane or olives.

It is a barrier lipid. The stratum corneum works as a brick-and-mortar arrangement, with corneocytes as the bricks and a lipid matrix as the mortar; squalane supports that lipid phase. It is very well tolerated and non-comedogenic, meaning it does not itself tend to block follicles — a non-trivial requirement in a product whose whole purpose is to keep follicles clear.

Tight, dehydrated skin is stiffer skin, and stiffer skin is harder for an emerging hair to push through.

Bisabolol

Bisabolol is the principal active constituent of chamomile, though most cosmetic-grade material now comes from candeia wood or is made synthetically.

The pathway, plainly: it dampens the inflammatory signalling that follows follicular injury, reducing the local mediators that drive redness and swelling. It is among the better-characterised anti-irritants in cosmetic chemistry, which is why it is here rather than as decoration.

Since an ingrown hair is fundamentally an inflammatory response, shortening that response is real work. It is also the pathway most directly connected to the dark mark, because pigment deposition follows inflammation.

Oat kernel oil and Bacillus ferment

These two travel together as a single lipid active, and they address the part of this that most of the category ignores: a follicle is not sterile.

Every follicle carries its own resident microbial population — commensal organisms that occupy the space, compete with opportunists, and interact with local immune signalling. Cutibacterium acnes is a normal follicular resident, not an invader.

The dominant strategy in this category is antibacterial, usually tea tree oil. Broad antibacterial pressure on an already-disturbed follicle removes commensals along with anything else.

The ungro approach is the other one: fatty acids of bacterial origin, carried in oat oil, intended to support barrier integrity and the balance of the skin's own microbial population rather than to sterilise it.

The honest framing: follicular microbiome science is real and active, but it is younger than the anatomy above, and the specific claims for this material rest on supplier data rather than on independent clinical trials in ingrown hairs. It is a considered choice, not a proven one, and it is described that way here on purpose.

Tocopherol

Vitamin E, and mostly it is here to protect the other ingredients.

The oil contains no preservative, because there is no water for microbes to grow in. The failure mode of an anhydrous formula is not microbial but oxidative: unsaturated lipids react with oxygen, which produces rancidity and, more importantly, reactive breakdown products that are themselves irritating. Tocopherol is a chain-breaking antioxidant that interrupts that reaction.

There is a modest secondary benefit to skin. The primary job is formula stability.

What does the evidence support, and what does it not?

This section exists because the rest of the page would be less credible without it.

Well established. Follicular anatomy. The two penetration routes. The role of hair curvature and coarseness. The foreign body inflammatory reaction and post-inflammatory hyperpigmentation that follows it. That hyperkeratinisation narrows the follicular opening. That the infundibulum is a lipid environment. None of that is contentious.

Well established, and not what this product does. The best direct trial evidence in razor bumps sits with agents at therapeutic strength: topical retinoids, benzoyl peroxide, topical antibiotics, glycolic acid at exfoliating concentrations, and laser hair reduction. If someone has severe, scarring pseudofolliculitis, those are the interventions with the strongest data, and they belong with a doctor. AHAs at exfoliating strength have better direct trial evidence than cosmetic oils do. That is simply true, and it is worth saying on a page selling an oil.

Mechanistically sound, less directly trialled. That softening and lubricating the shaft reduces re-entry. That supporting the barrier reduces the stiffness a hair has to push through. That an oil vehicle reaches the follicular opening better than a water one. Each rests on solid physical chemistry and dermatological reasoning. Randomised trials of these specific ingredient combinations in ingrown hairs do not exist.

Supplier rationale. The microbiome contribution of the oat and Bacillus ferment pairing. Plausible, mechanistically coherent, backed by supplier testing, not by independent clinical work in this indication.

ungro's position is that the honest thing is to publish that ladder rather than flatten it.

When is this no longer a cosmetic problem?

If a lesion is hot, spreading, weeping, genuinely painful rather than annoying, or if bumps are leaving raised scars, that is beyond what any cosmetic product should be asked to handle. Infection, abscess and keloid scarring are all recognised complications of pseudofolliculitis barbae. See a GP or a dermatologist.

Reducing hair removal frequency, or changing method, remains the most effective single intervention available. DermNet's summary of hair removal techniques sets out the options, including permanent reduction.

Where to read next

• What an ingrown hair actually is — the shorter explainer.
• The science of slowing hair regrowth — the hair growth cycle, FGF-5, and why the Smooth Gel is water-based.
• What's in it, and why — both formulas, ingredient by ingredient.
• Why an oil-soluble acid reaches an ingrown hair that a water-based one can't.
• How hair growth reduction actually works.

Sources

• Martel JL et al. Anatomy, Hair Follicle. StatPearls
• Hoover E et al. Physiology, Hair. StatPearls
• Pseudofolliculitis barbae. DermNet
• Alpha hydroxy acid facial treatments. DermNet
• Hair removal techniques. DermNet

Where no source is cited above, the statement reflects settled cosmetic-science or dermatological consensus rather than a single paper. Nothing on this page is a medical claim, and no ungro product treats or cures any condition.

Published by the ungro lab.