The Science of Hair Health — Porosity, Density, and What Your Strands Need

Walk into any drugstore and you will find a wall of shampoo bottles promising to repair, restore, hydrate, smooth, volumize, define, protect, and revive. Almost none of them ask the only question that matters: what is your hair actually like? In twenty years at the chair we have come to believe that almost all wasted money on hair products is the result of not knowing three numbers about your own hair. Once you know them, the wall of bottles narrows to maybe three options, and they are the right three. This essay is a working introduction to the three numbers — porosity, density, and texture — and to what each one means for how you should be caring for your hair.

Number one: porosity.

Porosity is how readily your hair absorbs and releases moisture. The cuticle — the outer layer of each strand — is made of overlapping scales. When the scales lie flat, the strand resists water and product. When the scales are lifted or damaged, the strand absorbs water and product easily but also loses them quickly. Porosity is the most important single property of your hair for daily care decisions, and almost no one has ever measured it.

The home test: take a clean, dry strand of your hair — pulled from a brush is fine — and drop it into a glass of room-temperature water. Wait three to five minutes. If the strand floats on top, you have low porosity. If it floats halfway, you have medium porosity. If it sinks to the bottom, you have high porosity. The test is not perfect but it is directional and useful.

Low-porosity hair has a tightly closed cuticle. Water and product struggle to get inside, which means low-porosity hair often feels coated, takes a long time to wet through in the shower, and resists color. Low-porosity hair benefits from heat — a warm water rinse, a heated deep conditioner cap, a lukewarm rather than cool finishing rinse — to gently lift the cuticle so product can penetrate. It also benefits from lighter, more liquid products. Heavy creams sit on the surface and never absorb.

Medium porosity is the easiest to care for. The cuticle opens enough to absorb product, closes enough to retain moisture, and responds predictably to most treatments. Most products on the market are formulated for medium porosity. If you have it, the world is on your side.

High-porosity hair has a damaged or naturally lifted cuticle. Water and product enter quickly and leave just as fast, which is why high-porosity hair feels dry by lunch even after a heavy morning leave-in. High-porosity hair benefits from cool rinses to seal the cuticle, from heavier creams that physically coat the surface, and from regular bond-building treatments that repair the cuticle structure itself. It is the most demanding porosity to maintain but it is also the most responsive to good care — the difference between treated and untreated high-porosity hair is dramatic.

Number two: density.

Density is how many individual strands of hair you have per square inch of scalp. It is not the same as thickness — density is the count, thickness is the diameter of each strand. You can have low density of thick strands or high density of fine strands or any combination.

The home test: pull your dry hair into a low ponytail and measure the circumference at the base with a piece of string. Less than two inches is low density. Two to four inches is medium density. More than four inches is high density. Less precise than porosity but useful as a starting frame.

Density determines how much product you actually need. Low-density hair gets weighed down by even small amounts of leave-in and benefits from very light formulas. High-density hair drinks product and needs roughly twice the quantity most labels suggest. Density also determines styling time. The blowout we describe in our blowout guide takes seven minutes on a low-density head and twenty-five on a high-density head. The same technique. Different time budget.

Density is a useful number to give a new stylist on your first appointment. “I have high-density medium-thickness hair” tells them more in five seconds than ten minutes of lifestyle conversation.

Number three: texture.

Texture is the diameter of each individual strand and the natural curl pattern. The diameter scale runs from fine to medium to coarse. The pattern scale runs from straight (Type 1) through wavy (Type 2), curly (Type 3), and coily (Type 4), with sub-types for the tightness within each.

Texture is the property most people already roughly know about themselves. The piece they tend to underestimate is the diameter. Run a single strand of your hair between your thumb and forefinger. If you can barely feel it, the strand is fine. If it feels like sewing thread, it is medium. If it feels like fishing line, it is coarse. People with coarse hair often think they have “a lot” of hair when they actually have moderate density of thick strands. The styling implications are different.

Coarse hair holds shape well, takes color well, but resists the bend a blowout is trying to create. Fine hair takes shape easily, holds it briefly, and needs help staying lifted. Medium hair is a forgiving baseline.

The combination is what matters.

The reason you need all three numbers is that they interact in non-obvious ways. Low-porosity, high-density, coarse hair behaves nothing like low-porosity, low-density, fine hair, even though the porosity is the same. The product that saves the first head will drown the second.

Three of the most common combinations and what each one needs:

Low porosity, high density, medium texture. Common in clients of East Asian descent. Loves warm water, light creams, and patient processing time. Resists color and may need extra time at the bowl. Avoid heavy oils — they sit on the surface and weigh the hair down.

High porosity, medium density, fine texture. Common in clients with chemically lifted or heat-styled hair over many years. Needs cool rinses, leave-in conditioners with proteins, and weekly bond-building treatments. Avoid sulfate shampoos and over-washing.

Medium porosity, low density, fine texture. Common in clients who feel like nothing they put in their hair stays. The trick is small amounts of light product applied carefully. A pea of cream, not a palm-full. Volume mousse at the roots only. Patience with the cool shot during styling.

If your combination is none of these, do not worry. The point is not to memorize a chart. The point is to know your three numbers and to use them as filters when you stand in front of the bottle wall. A stylist who knows how to think about your hair will ask about all three within the first appointment. If they do not, you can offer the information and watch their face — a good stylist will light up at being given useful inputs.

What changes with age, weather, and color.

Your three numbers are not fixed. They drift over time and with treatment.

Porosity rises with chemical service, sun exposure, daily heat styling, and chlorine. It can be partially reversed with bond-builders and consistent low-heat care. The reversal is real but slow.

Density falls with age for most people, beginning gradually in the late thirties and accelerating after menopause for women and after the late forties for men. The fall is gradual. Style changes that work with the lower density — softer perimeters, more layering for movement, lighter products — are usually more flattering than denial.

Texture stays largely the same throughout adult life, but the relative diameter of each strand falls slightly with age. Hair that was “medium” at 25 may feel “fine” at 55. The strand has not changed direction; it has gotten quietly slimmer.

Knowing this lets you adapt your routine instead of fighting it. The morning routine we describe in our twenty-minute morning routine is written for medium porosity and density and is a useful baseline that you can dial in either direction.

What to ask at your next appointment.

Bring this essay to your next consultation. Ask your stylist to test your porosity at the bowl during the wash. Ask them to estimate your density and texture out loud. Take notes if you have to. Once you have your numbers, write them on a card and keep it in your wallet. The next time you stand in front of a wall of shampoo, the wall will feel smaller.

If you would like us to do this with you in person, the consultation is free and takes about twenty minutes. Come in. Bring your three questions. We will tell you the truth. To prepare further, the color essay is the natural companion to this one — color decisions are dramatically easier once you know your hair’s actual properties.

About the author

Sean

A senior member of the Trio Salon studio in Burlingame, California.

More from this author →