A blackhead is a tiny plug made mostly of dead skin cells and oil, packed tightly inside a hair follicle that has stretched open at the surface. The dark color isn’t dirt. It’s the result of a chemical reaction that happens when the material at the top of the plug is exposed to air. Understanding what’s actually in that plug helps explain why blackheads form, why they look the way they do, and why some are so stubborn to get rid of.
The Main Ingredients: Keratin and Sebum
The bulk of a blackhead is made up of two things your skin produces naturally. The first is keratin, the structural protein found in your outer layer of skin, hair, and nails. Skin cells lining the inside of your pores are constantly being replaced, and the old ones are supposed to shed and get carried out by oil flow. In a blackhead, those dead cells stick together instead of shedding normally, forming a dense, compacted mass. Lab analysis of comedonal material shows it contains the same keratin proteins found in your skin’s outermost layer, along with degraded, broken-down fragments of keratin that have been sitting in the pore for a while.
The second major component is sebum, the waxy, oily substance your sebaceous glands produce to keep skin moisturized. Sebum normally travels up through the pore and spreads across the skin surface. In a blackhead, it gets trapped behind and within the keratin plug, mixing with the dead cell debris. The result is a dense, waxy mass that fills the pore from the inside.
There’s also melanin in the mix. Melanin is the pigment that gives your skin its color, and it’s present in the dead skin cells that make up the plug. This melanin plays a key role in why blackheads look the way they do.
Why the Surface Turns Black
The name “blackhead” comes from the dark dot visible at the pore’s opening, but nothing dirty or unusual is causing that color. Unlike a whitehead, which is sealed beneath a thin layer of skin, a blackhead has an open surface. When oxygen reaches the top of the plug, it triggers oxidation of both the lipids (fats from sebum) and the melanin trapped in the material. This is the same type of chemical reaction that turns a sliced apple brown. The deeper material inside the plug is typically white or yellowish. Only the exposed surface darkens.
Bacteria Living in the Plug
Blackheads also harbor bacteria, most notably a species called Cutibacterium acnes. This bacterium naturally lives on everyone’s skin and thrives in oily, low-oxygen environments like the inside of a clogged pore. It feeds on sebum. Each individual pore tends to be dominated by a single strain of this bacterium, even though multiple strains coexist across your skin. In a blackhead, the bacterial population is generally contained and doesn’t cause the redness or swelling you see with inflamed pimples. But if the plug traps enough bacteria and the follicle wall becomes irritated, the blackhead can progress into an inflammatory breakout.
How the Plug Forms in the First Place
Blackheads start with a process where the skin cells lining the inside of a hair follicle begin behaving abnormally. Normally, these cells divide, mature, and shed into the pore canal, where oil flow pushes them out. In people prone to blackheads, the cells become stickier. They develop stronger connections to each other and don’t separate the way they should. Instead of shedding individually, they clump together and start building up inside the follicle.
At the same time, tiny fat droplets accumulate on these cells, and the structures responsible for helping cells detach from one another decrease in number. The combination creates a keratinous plug that blocks the follicular canal. Meanwhile, sebum production often ramps up, adding more oil behind the growing blockage. One theory for why this happens involves a deficiency of a specific fatty acid (linoleic acid) within the follicle lining. People with higher sebum production may have lower concentrations of this fatty acid in their pores, which could trigger the abnormal cell buildup.
This entire process begins microscopically, as what dermatologists call a microcomedone, long before anything is visible on the skin surface. Over time, the accumulating material stretches the pore open, and the characteristic dark dot appears.
Blackheads vs. Sebaceous Filaments
Many people mistake sebaceous filaments for blackheads, but the two are structurally different. Sebaceous filaments are thin, threadlike structures that line your oil glands and help move sebum to the skin’s surface. They’re a normal part of your skin’s anatomy, not a form of acne. They tend to appear as small, flat, grayish or light brown dots, most often on the nose and chin.
The key difference is that sebaceous filaments don’t contain a true plug. Oil flows freely through them. A blackhead, by contrast, has a solid mass of keratin and sebum blocking the pore. If you squeeze a sebaceous filament, a thin, waxy thread comes out and refills within about 30 days. If you squeeze a blackhead, a darker, denser plug pops out. Treating sebaceous filaments as blackheads, through aggressive extraction or pore strips, can irritate the skin without solving anything, since filaments always come back.
What Breaks Down the Plug
Because the plug is made of compacted dead skin cells bound together by oily material, effective treatments target both components. Salicylic acid, a common over-the-counter ingredient, is oil-soluble, which means it can penetrate into the pore rather than just working on the skin’s surface. Once inside, it breaks the bonds holding dead skin cells together and dissolves the oily sebum that glues the plug in place. This loosens the compacted material so it can clear out of the follicle naturally.
Retinoids work differently, by normalizing the way skin cells inside the follicle mature and shed. They essentially address the root cause: the sticky, abnormal cell turnover that created the plug. This is why retinoids are often recommended for persistent blackheads that keep returning in the same areas.
Can Blackheads Permanently Stretch Pores?
A blackhead that sits in a pore for a long time can physically stretch the opening. In extreme cases, this results in what’s known as a dilated pore of Winer, a single, visibly enlarged pore filled with a large keratin-and-sebum plug. This happens through the same mechanism as a regular blackhead, just on a bigger scale: dead skin cells and sebum collect and expand the follicle over months or years. Once a pore has been significantly stretched, it may not return to its original size even after the plug is removed, because the surrounding skin has lost some of its elasticity. Keeping blackheads from forming in the first place is the most reliable way to prevent this kind of permanent enlargement.