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

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

We distinguish two major families of hair serums. The first family groups scalp serums, such as anti-dandruff treatments, hair-growth-stimulating serums, and rebalancing formulas for oily scalps. The second family covers serums for the lengths, including moisturizing or fortifying treatments. Although terminology varies, these products broadly share the same goal: to nourish, protect, and strengthen the hair fiber. Most hair serums also incorporate cationic agents to reduce frizz, since hair carries a positive charge.

The hair oils are also designed to maintain and enhance the hair. They are primarily distinguished by their composition. Most hair oils are formulated from plant-based oils, although some may include synthetic molecules, such as silicones, to improve the feel. Hair oils work on several levels. First, they form a protective film on the surface of hair fibers, reducing water loss and shielding against external aggressors like UV rays, pollution, or certain surfactants found in shampoos. Some hair oils are also formulated to penetrate the fiber to strengthen its intercellular cement and structure.

Did you know? The use of hair oils is far from recent and dates back to ancient Egypt. Several historians report that these civilizations already employed plant-based oils to care for their hair.

Although hair serums and oils are often used to care for hair, they are not the same products. They stand out primarily in their formulation and texture. Hair serums are generally aqueous products or light emulsions, sometimes enriched with active ingredients targeting either the scalp (anti-dandruff, growth stimulators, sebum regulators) or the hair lengths (hydration, strengthening, frizz control). This fluid formulation allows for quick, non-greasy application. In contrast, hair oils are always intended for the lengths and ends and rely predominantly on lipid-based substances, whether plant-derived oils or synthetic components. They are primarily used to nourish and protect the hair. The denser, oil-rich texture of hair oils promotes the formation of a protective film around the hair fibers.

The choice between a hair serum and a hair oil depends on the intended purpose: serums come in various formulations, offering versatility, while oils primarily focus on nourishing hair lengths.

• Thierry MORANT's dissertation. 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).