What is the difference between a hair oil and a hair serum?

A hair serum is a treatment highly concentrated in active ingredients, formulated to address specific needs of the hair or scalp. Most serums are formulated on an aqueous base, sometimes as a light emulsion, offering a fluid and non-greasy texture. The formulation may include solubilising agents, surfactants or polymers to stabilise the ingredients and facilitate application. Some hair serums also incorporate film-forming agents to protect the hair fibre by coating it.

One can distinguish two main families of hair serums. The first encompasses serums for the scalp, such as anti-dandruff serums, hair growth-stimulating serums and rebalancing serums for oily scalps. The second family pertains to serums for the lengths, such as hydrating or fortifying serums. Although terminology varies, these products broadly share the same objective: to nourish, protect and strengthen the hair fibre. Most hair serums also incorporate cationic agents to limit frizz, as hair carries a positive charge.

The hair oils are also intended to nourish and beautify the hair. They are mainly distinguished by their composition. Most hair oils are formulated from vegetable oils, although some may contain synthetic molecules, such as silicones, to improve texture. Hair oils act on several levels. They first form a protective film on the surface of the hair fibres, reducing water loss and protecting against external aggressors such as UV radiation, pollution or certain surfactants found in shampoos. Some hair oils are also formulated to penetrate the fibre, reinforcing its intercellular cement and structure.

Did you know? The use of hair oils is by no means a recent development and can be traced back to ancient Egypt. Several historians note that these civilisations were already applying plant-based oils to maintain the health of their hair.

Although hair serums and hair oils are often used to care for hair, they are not the same products. They are primarily distinguished by their formulation and texture. Hair serums are generally aqueous products or light emulsions, sometimes enriched with active agents targeting either the scalp (anti-dandruff, growth stimulants, sebum regulators) or the lengths (hydration, fortification, frizz control). This fluid formulation allows for rapid, non-greasy application. In contrast, hair oils are always intended for the lengths and ends and rely predominantly on lipid-based ingredients, whether derived from vegetable oils or synthetic components. They are chiefly used to nourish and protect the hair. The denser, more lipid-rich texture of hair oils promotes the formation of a protective film around the hair fibres.

The choice between a hair serum and a hair oil depends on the desired outcome: serums are available in several formulations, offering versatility, whereas oils primarily target the nourishment of the lengths.

• Thierry MORANT’s thesis. Parfums et cosmétiques dans l’Égypte ancienne (2019).

• RAMACHANDRAN V. & al. Development and evaluation of herbal hair serum: A traditional way to improve hair quality. The Open Dermatology Journal (2021).