In brief, hair porosity describes your hair's ability to absorb and retain moisture. Determine by the structure and condition of the hair cuticle, the outermost layer of each hair shaft, cite index equal sign, 16 to 1 inches. When the cuticle is tightly closed, moisture has a harder time getting in but stays locked in longer. When the cuticle is lifted or damaged, moisture enters easily but escapes just as quickly. The three porosity levels, low, medium, and high, require different product approaches and critically different protein versus moisture balances. Getting this wrong causes more breakage than the underlying damage it is meant to address. What determines hair porosity? Porosity is partly genetic, the natural structure of your hair cuticle is inherited and partly acquired through chemical and mechanical damage. Site index equal sign, 14 to 1 inches. High porosity can be genetic or a result of chemical processing or rough treatment. This means porosity can change over time. Virgin unprocessed hair tends toward lower porosity. Repeatedly colored, bleached, or heat-styled hair shifts toward higher porosity as the cuticle becomes progressively more damaged and raised. The three categories, low porosity, tightly bound cuticles, site index equal sign, 15 to 1 inches. Low porosity hair has tightly bound cuticles, making it harder for water to penetrate the hair shaft. You may have low porosity hair if hair products sit on your hair and are slow to absorb. Your hair takes a long time to air dry. Water doesn't penetrate your hair easily when you're showering. Your hair may appear dry and dull, it can be resistant to chemical treatment. The deceptive nature of low porosity hair, it often looks and feels dry, not because it lacks moisture, but because it cannot absorb moisture effectively. This is why adding more moisturizing products often makes low porosity hair worse. The products cannot penetrate and simply accumulate on the cuticle surface as buildup. Medium porosity, balanced cuticle, site index equal sign, 14 to 1 inches. With balanced medium porosity, your hair has a fairly loose cuticle that allows the right amount of moisture to absorb and escape. Your hair generally requires little maintenance. The occasional deep conditioning protein treatment will keep your hair in great shape, but it doesn't have to be a part of your daily routine. Medium porosity is the most forgiving and product tolerant porosity. Most product formulations are designed with medium porosity hair in mind. If your hair responds well to a wide range of products without becoming limp, dry, brittle, or overly greasy, medium porosity is the likely explanation. High porosity. Raised or damaged cuticles, cite index equals sign, 14 to 1 inches. High porosity hair has large holes in the cuticle. This can allow too much moisture to penetrate it. Your hair may also lose moisture quickly, which leads to frizz and tangling. High porosity hair is also delicate and can be prone to further damage, even through everyday styling. High porosity hair absorbs moisture, a rapidly in the shower but loses it equally fast as the lifted cuticle cannot seal moisture in. The result, hair that feels dry and frizzy within hours of washing despite apparently good moisture absorption. The lifted cuticle gaps that allow rapid moisture escape also make high porosity hair structurally weaker. Protein fills these gaps and restores barrier function. How to accurately test your hair porosity. Two testing methods, one behavior-based, most accurate, one physical, useful but limited. The behavior observation test, most accurate. The most accurate way to identify your porosity is through observation. Pay attention to how your hair behaves during washing, conditioning, and drying. If water beads up on your hair in the shower, products feel heavy, or hair takes a long time to fully dry, you're likely working with low porosity hair. If your hair absorbs water immediately, dries quickly, and feels dry again soon after styling, it has high porosity. This test requires no equipment and reflects your hair's actual real-world behavior rather than a single strand in artificial conditions. The waterglass float test, useful but imperfect. Take a clean, product-free strand of loose hair and put it in a glass of water. If the hair floats at the top, then it is low porosity. If it sinks slowly or settles in the middle, it is medium normal porosity, and if it sinks straight to the bottom, then it is high porosity. The limitation? The result is affected by product residue on the strand, natural scalp oils, hair thickness, and the specific location of the strand. A strand from the hairline may have different porosity than one from the crown. Use this as one data point alongside the behavior observation, the finger slide test, site index equal sign, 17 to 1 inches. Run your fingers down a hair strand to feel its texture. If your fingers move easily and your hair feels dense, your hair is low porosity. If it feels smooth, your porosity is pretty balanced. If your hair feels rough, then you have high porosity hair. A quick tactile test that takes seconds, the raised cuticles of high porosity hair produce a detectably rough, snag-prone texture when fingers move toward the root. The protein versus moisture balance. The critical practical question. This is where most hair care advice goes wrong. And where getting it right matters most. The protein moisture balance requirement is opposite by porosity type, but porosity. Hair tends to be resistant to moisture and is often protein sensitive. Too much protein can make it stiff and brittle, leading to breakage. Use protein rarely, once a month at most. Lightweight water based moisturizers. Use heat, warm towel, steamer, hooded dryer, when deep conditioning to open cuticles and allow penetration. Avoid heavy butters and oils that sit on the surface. Medium porosity, moderate protein, okay.