What a water-based hair serum is, and how to tell from the bottle

A water-based hair serum is one where water makes up most of the formula, with everything else dissolved or dispersed in it. You can tell from the bottle: water sits at the top of the list on the back, and no oils or silicones appear near it. On the scalp it feels like a liquid, absorbs, and dries down without leaving a film on the hair.

"Water-based" is one of the most useful things a hair product can tell you and one of the least explained. It is a description rather than a defined term, so it appears on some bottles that are not, and is missing from many that are. Learning to check for yourself takes about ten seconds per bottle.

The short version

1. Turn the bottle over. Look at the top of the list.
2. Water at the top means water is the largest component.
3. Scan the next few lines for oils, butters and silicones. Their absence near the top is what makes a water-based formula light.
4. Do not read too much into the bottom of the list. Below one percent, the order no longer has to follow quantity.

Why the top of the list matters

Cosmetics sold in the US have to list what is in them, and the order is not up to the brand. Federal regulation requires each component to be declared in descending order of predominance,¹ and the FDA's guidance for cosmetic packaging repeats it: the components must be declared in descending order of predominance.² So whatever is listed at the top is present in the greatest amount.

There is one exception worth knowing. Components present at one percent or less may be listed in any order after those above one percent.² The top of the list is reliable. The bottom half often is not, because a lot of small amounts can be shuffled freely.

Water is the most common thing to find at the top. The Personal Care Products Council, the US cosmetics industry's trade body, notes that water is used in the formulation of virtually every type of cosmetic and personal care product, primarily as a solvent.³ In a water-based serum, it is the medium everything else travels in.

What to look for next

Water at the top is necessary but not sufficient. Creams and lotions lead with water too, and they are full of oils; they are emulsions, oil and water held together. What makes a serum light is what comes after the water.

Scan the next five or six lines for these:

• Oils. Anything ending in "oil" or "seed oil", such as jojoba, argan, castor or coconut. Also mineral oil, sometimes printed as paraffinum liquidum.
• Butters. Shea, cocoa, mango.
• Silicones. Names ending in "-cone" or "-siloxane", such as dimethicone or cyclopentasiloxane.
• Esters that behave like oils. Names such as caprylic/capric triglyceride or isopropyl myristate.

These materials have a job, and they do it well: they coat. A review of hair cosmetics describes dimethicone adsorbing onto the hair surface, and notes that thick films of oil may leave an oily and heavy look.⁴ That is useful on dry lengths. It is the opposite of useful at the roots of fine hair.

What you will often find instead in a water-based serum are humectants such as glycerin and propanediol, which attract and hold water,⁵ plant extracts, and small amounts of preservatives and pH adjusters.

Water at the top tells you what most of the bottle is. What comes straight after it tells you how it will feel at the roots.

Why it matters on a scalp

Oil does not dissolve in water. The same review explains that oily residues need a surfactant, a molecule with one end that holds onto oil and another that holds onto water, to be removed from hair.⁴ In practice, that surfactant is shampoo. An oil on the scalp stays there, and on the hair around it, until the next wash.

A water-based serum has no such film to leave. The water evaporates, the dissolved components stay on the skin, and the hair either side of the part looks the way it did before. For anyone whose roots flatten under a hair oil, or who styles in the morning and wants the style to last, that is the whole argument.

It also changes when you can use it. An oil is a wash-day product, because it needs washing out. A water-based serum can go on after styling, between washes, or straight before bed without changing anything about the next morning.

Three quick checks without reading anything

The list is the reliable test, but these back it up:

• The pour. A water-based serum runs like water, or like slightly thickened water. An oil moves slowly and clings to the dropper.
• The back of the hand. Rub a drop in. Water-based formulas absorb and dry within a minute or two. Oils leave a sheen you can see in the light.
• The rinse. Water-based products rinse off skin with water alone. Oils bead and need soap.

Three kinds of list, read side by side

It helps to see the pattern. These are the shapes the top of a list tends to take for three common kinds of hair product. They are illustrations, not any particular brand.

An oil sold as a serum. The list opens with something like Simmondsia Chinensis (Jojoba) Seed Oil, then another plant oil or two, then fragrance, then a botanical oil or extract in small amounts. No water anywhere. Whatever the front says, this is an oil, and it will behave like one: slow to spread, a visible sheen, and a shampoo to remove it.

A cream or lotion. The list opens with water, then quickly brings in oils, butters or fatty alcohols such as cetearyl alcohol, plus an emulsifier that holds the two together. It leads with water but is designed to leave a conditioning film. Excellent on dry lengths; heavy at the roots of fine hair.

A water-based serum. The list opens with water, then a humectant such as glycerin or propanediol, then extracts, peptides or other actives, then preservatives and pH adjusters. No oils, butters or silicones near the top. It absorbs, dries down, and leaves the hair either side of the part as it was.

Once you have read a few lists with this in mind, you start to recognise the shape in seconds, without needing to know what every name means.

What "water-based" does not tell you

It describes the base, not the results. A water-based serum is still a cosmetic, which in US law means a product for cleansing, beautifying, promoting attractiveness or altering the appearance.⁶ A product sold to restore hair growth is a drug, whatever its base, and the FDA names that as one of its own examples.⁶

It also does not say anything about strength. Water at the top is normal, because water is a solvent. What matters is which other components are listed, and how high.

Ours

Both of our serums lead with water, followed by glycerin, and neither contains an oil or a silicone. You can check that on the back of either bottle, which is the point.

Botanical serum

Max Density

Rosemary leads the blend: only water and glycerin come before it. Water-based, no oils, no added fragrance. Left on for at least one to two hours before washing, which makes it the morning serum. 2 fl oz.

See Max Density

Peptide serum

Max Growth

Five peptides, each one printed on the bottle, in a water-based serum with no oils and no added fragrance. Nothing to rinse out, which makes it the evening serum. 1 fl oz.

See Max Growth

If a serum has already left your roots greasy, why a scalp serum can make hair greasy covers the technique side. And serum, oil, tonic, essence explains why the word on the front of a bottle tells you much less than the list on the back. If flat roots at the part are the real concern, how to cover a wide part goes through the styling.

Eloura products are cosmetics. They are not intended to diagnose, treat, cure or prevent any condition. This article is general information about hair care and is not medical advice.