What the Microbiome Is
A community of microorganisms lives on skin: bacteria, fungi and others. This is a normal state rather than contamination, and their presence is part of how skin defends itself.
On the scalp the makeup of that community differs from the face and hands, because conditions differ. The oilier environment favours certain species over others.
What Is Known and What Is Not
It is known that microbiome composition differs between a healthy scalp and one with dandruff, and that certain Malassezia fungi are involved. That much is well described.
Less clear is whether a cosmetic product can change that composition lastingly and in a useful direction. Research here is at an early stage, and much of the data comes from raw-material suppliers.
So the sensible expectation from a pre- and probiotic product is a calmer scalp and less tightness, not a rebuilt microbiome.
Prebiotics, Probiotics, Postbiotics
Prebiotics are substances that feed beneficial microorganisms. Probiotics are the living microorganisms themselves. Postbiotics are the products of their metabolism.
Cosmetics most often use prebiotics and postbiotics, because they are stable. Living microorganisms struggle to survive in a preserved shampoo, so a probiotic claim on a shampoo bottle deserves scrutiny.
What Actually Upsets the Balance
Washing too often with a harsh shampoo, very hot water, products that build up and are not rinsed out, and keeping the head covered while sweating.
The opposite extreme causes problems too. Washing too rarely with an oily scalp builds up sebum and cells, which favours exactly the organisms you would rather have fewer of.
How often to wash
There is no universal answer. Judge by your scalp rather than your lengths: if by the evening of the second day it is oily and itchy, you are washing too rarely. If it feels tight and flaky after washing, the shampoo is too strong.
Dandruff: When It Is a Cosmetic Question
Light flaking without redness or itching often responds to changing shampoo and gentler care. That is within the scope of cosmetics.
Persistent redness, greasy yellowish scales, intense itching, or involvement of other areas of the face and body are signs that call for an appointment. Several conditions look like dandruff and are managed differently.
What Helps in Practice
A mild shampoo with suitable surfactants, lukewarm water, thorough rinsing, and conditioner on the lengths rather than the roots. How to choose a shampoo is covered in detail in a separate guide.
And patience. The scalp responds slowly, and switching products often makes it impossible to tell what helped. Give a new shampoo at least a month before judging it.
Scalp and Hair Have Different Needs
This is the distinction missing from most routines. The scalp is skin with oil glands and needs cleansing. The lengths are a dead structure that needs softening and protection.
From which follows the practical rule: shampoo is for the scalp, conditioner for the lengths. Shampoo worked through the ends dries them out needlessly, and conditioner at the roots weighs hair down and speeds up greasiness.
Itching Without Flaking
An itchy scalp with no visible flaking is most often a sign of irritation rather than a microbial problem. Common causes are fragrance, strong surfactants and insufficient rinsing.
The first step is a fragrance-free shampoo and more thorough rinsing, not an anti-dandruff product. Anti-dandruff products act against specific organisms and can do nothing about irritation from an ingredient.
Scalp Exfoliation
Scalp scrubs and acid products have become popular, and with styling-product build-up they genuinely help. Mechanical particles irritate easily, though, particularly on an already sensitive scalp.
Once every week or two is enough, and if there is redness or itching, stop altogether. Exfoliation does not resolve dandruff and should not be used as a treatment.
