Head Odor Is Not Made by Just One Compound

The odor of the head and hair differs somewhat from underarm or foot odor. A 1994 study investigating its characteristics found a variety of substances emitted from the head, including aldehydes, fatty acids, and alcohols. The authors described the odor as a mixture of compounds rather than the result of a single culprit.

In addition to chemical analysis, the study examined changes in scalp bacteria after hair washing and attempted to recreate head odor by combining analytical results with perfumers’ sensory judgments.

Examining the air around the head and compounds remaining on the hair and scalp

The participants were five healthy men aged 24–38. After they washed their hair with an unfragranced shampoo, the researchers sampled the air around their heads at several time points from immediately after washing to 72 hours later. They covered the head with a specially designed bag, trapped emitted volatile compounds on an adsorbent, and analyzed them by gas chromatography.

Separately, they examined compounds extracted with a solvent three days after washing. The design used different methods to investigate airborne compounds and lipid components remaining on the hair and surrounding areas.

The researchers also sampled bacteria from the crown of the head and cultured them to measure bacterial counts. The bacterial sampling times listed in the Methods section extend from immediately after washing to 56 hours afterward, distinct from the volatile sampling schedule extending to 72 hours.

A mixture of mildly pungent aldehydes, fatty acids, and other compounds

Many compounds were present in the air around the head after 72 hours. The main text reports identifying 25 compounds, including hydrocarbons, acids, and aldehydes. Examples include the aldehydes hexanal, heptanal, and decanal; fatty acids such as isovaleric acid, butyric acid, pentanoic acid, and hexanoic acid; and indole.

The authors proposed that mildly pungent aldehydes, short-chain fatty acids, indole, and other compounds combine to produce head odor. The compound list also includes ethyl acetate and various alcohols: the single impression we call head odor encompasses different chemical classes.

Meanwhile, solvent-extracted fatty acids ranged from 4 to 22 carbon atoms, with particularly high proportions of tetradecanoic acid, with 14 carbons, and hexadecanoic acid, with 16. It is important not to equate the abundant lipids present on the head with the compounds released into the air and smelled.

Bacteria increased after washing, but their role in odor remained a hypothesis

Cultures detected Staphylococcus epidermidis, Staphylococcus capitis, and bacteria classified at the time as belonging to the genus Propionibacterium. Bacterial numbers increased over time, with a more pronounced increase reported for aerobic than anaerobic bacteria.

The authors suggested that bacterial breakdown of long-chain scalp fatty acids into more strongly smelling short-chain fatty acids might help explain the intensification of odor. They also raised the possibility that differences in the scalp’s bacterial community contribute to its odor being distinct from that of the underarms or feet.

However, the study directly measured changes in compounds and bacterial counts. It did not feed fatty acids to particular bacteria and trace their breakdown pathways. Which bacteria produced which odor compounds therefore remained a causal hypothesis.

Recreating the odor with perfumers’ noses as well as chemical analysis

The researchers used the chemical analysis as a starting point for a mixture, which perfumers adjusted by comparing it with actual head odor. The paper includes the resulting reconstruction formula.

This was a model incorporating sensory comparison, not a direct copy of the measured compound ratios. The formula also includes fragrance materials absent from the analytical list. Nor did the study remove compounds one at a time to determine which omissions would make the mixture cease to smell like head odor.

Even so, an important part of the study was its attempt to move beyond separating compounds analytically and return to the overall impression perceived by people. It approached head odor through both the list of detected substances and the perception of their mixture.

Summary

This study of five men identified aldehydes, fatty acids, and other compounds emitted from the head and observed an increase in bacteria after washing. The authors explained head odor as a mixture of multiple compounds and developed a reconstruction formula incorporating comparisons by perfumers.

Although small, with limited detailed statistical information about bacterial growth and evaluation of the reconstruction, it was an early study combining chemical analysis, observation of bacteria, and odor reconstruction. It offers a way of considering head odor as an overall combination rather than reducing it to a single compound.

All five authors were affiliated with Kanebo’s Cosmetics Laboratory at publication. No separate funding or conflict-of-interest statement was identified.

Reference

Kubota, M., Komaki, R., Ito, Y., Arai, M., & Niwase, H. (1994). Investigation of the Odor Evolved from Human Hair. Journal of the Society of Cosmetic Chemists of Japan, 28(3), 295–298. https://doi.org/10.5107/sccj.28.295 (Original paper PDF)