Is a Stronger Green Scent More Pleasant? A Study of Leaf Alcohol

A fragrance reminiscent of green leaves or grass does not necessarily become more pleasant as it grows stronger. In a 2002 study comparing a single compound known as leaf alcohol, more participants preferred a 0.1% sample than a 10% sample.

A brain-wave measure recorded at the same time also differed by concentration. What it measured, however, was the brain’s response during a task that required distinguishing sounds while smelling the odor. The study highlights why research on odor pleasantness needs to consider concentration and individual preference, not just the name of the ingredient.

Investigating green odors through compounds, mixtures, and concentration

The “green odor” examined in this study consisted of six-carbon alcohols and aldehydes associated with the fresh green smell of leaves. Representative compounds include (Z)-3-hexen-1-ol, also called leaf alcohol, and (E)-2-hexenal, also called leaf aldehyde.

The aim was to investigate how eight such compounds relate to a human brain-wave measure, testing individual compounds, two-component mixtures, and different concentrations. Because the smell of natural leaves is a mixture, the researchers also looked for synergistic effects in which combining compounds strengthens the response.

The first experiment broadly compared eight compounds and 28 two-component mixtures. The second focused on leaf alcohol alone, comparing concentration, pleasantness, and brain responses in greater detail.

P300 was a response during an auditory-attention task

P300 is a change in the brain’s electrical potential associated with an event. In this experiment, 1,000-Hz and 2,000-Hz tones were presented randomly with probabilities of 80% and 20%, respectively. Participants pressed a button when they heard the less frequent 2,000-Hz tone.

This is known as an oddball task. The researchers averaged 20 artifact-free responses to the less frequent tone, comparing the amplitude of the wave during odor presentation with the amplitude in a condition using only an odorless solvent.

Participants sat with their eyes closed while filter paper impregnated with the sample was held approximately 2 cm in front of the nose. P300 therefore needs to be understood as a response to an auditory task in the presence of odor, not as a direct olfactory response to the arrival of the odor itself.

No clear synergy across eight compounds and their mixtures

Experiment 1 involved 128 healthy women aged 18–22. Of these, 39 were assigned to the single-compound experiments and 89 to the two-component-mixture experiments. Individual compounds were prepared as 0.03% solutions; mixtures combined equal amounts of these solutions. Triethyl citrate was the solvent.

P300 sometimes decreased with leaf alcohol or (Z)-3-hexenal and sometimes increased with leaf aldehyde. However, each compound or mixture was tested in only about two to seven people, and analyses of the data as a whole found no significant association for either the individual compounds or the mixtures.

The authors described tendencies toward increases or decreases for particular compounds, but these findings alone make it difficult to classify one compound definitively as calming and another as activating. Mixing two compounds also failed to demonstrate the clear synergistic effect the researchers had expected.

Leaf alcohol was preferred at the lower concentration

Experiment 2 involved 34 healthy women, also aged 18–22. Three whose P300 could not be identified were excluded, leaving 31 for analysis. The researchers compared 0.1% leaf alcohol, 10% leaf alcohol, and the odorless solvent. After the EEG measurements, participants were asked about intensity, pleasantness, and their impressions of the odor.

The result was that 0.1% was significantly more pleasant than 10% (p < 0.01). Among the 31 participants, 26 preferred the lower concentration, two rated the concentrations equally, and three preferred the higher concentration.

In other words, the same molecule’s pleasantness varied with concentration, and people also differed in which concentration they preferred. Here, 0.1% and 10% refer to weight concentrations in the solution applied to filter paper. They do not describe concentrations in the air reaching the nose or suitable amounts of a product to use in a room.

The lower concentration also produced the larger brain-response change

P300 amplitude was smaller with both 0.1% and 10% leaf alcohol than with solvent alone (p < 0.01 for each). In addition, the amplitude was smaller, and its reduction greater, at the lower concentration of 0.1% than at 10% (p < 0.05).

The relationship was therefore not simply that a more concentrated sample produced a greater brain-wave change. Considered alongside pleasantness ratings, the more pleasant lower concentration was associated with a smaller P300 response to the auditory task.

The researchers also explored preference and amplitude within individuals. Of the 26 people who preferred the lower concentration, 20 had a smaller P300 at that concentration. All three who preferred the higher concentration had a smaller P300 at the higher concentration. However, preference and amplitude did not align for everyone, and these observations involved small subgroups.

These findings led the authors to consider whether pleasantness, as well as concentration, might relate to the brain-response change. However, concentration and pleasantness were not manipulated independently, so the experiment could not disentangle which caused the change.

How should a smaller brain-wave response be interpreted?

In their discussion, the authors linked the P300 reduction with sedative effects and relaxation. What the study directly measured, however, was an electrical-potential change during an auditory task and ratings such as odor pleasantness. It was not a trial measuring improvement in stress symptoms or therapeutic effects.

P300 amplitude is used as an indicator related to information processing during a task. It cannot be reduced to a simple rule that smaller is healthier or larger is worse. The firm observation here is that two concentrations of leaf alcohol differed in pleasantness and in the response to sounds.

The restriction to young women, and the small numbers in each condition of the first compound-and-mixture experiment, also limit generalization. Nevertheless, the design using the same molecule at different concentrations provides a concrete example of why knowing what someone smelled is not enough in odor research.

Summary

In the experiment comparing 0.1% and 10% leaf alcohol, 26 of 31 participants preferred the lower concentration, which also had a higher overall pleasantness rating. During a sound-discrimination task performed while smelling the samples, P300 was smaller at both concentrations than with solvent alone, and the reduction was greater at the lower concentration.

By contrast, comparisons of eight compounds and two-component mixtures did not show a significant overall association or clear synergy. Understanding odors and human responses requires examining the compound, its concentration, and the individual’s experience together, rather than treating all green-leaf smells as one category.

The paper lists author K. Sano’s affiliation as the fragrance laboratory of Soda Aromatic Co. Ltd. No separate conflict-of-interest statement was identified.

Reference

Sano K, Tsuda Y, Sugano H, Aou S, Hatanaka A. (2002). Concentration Effects of Green Odor on Event-related Potential (P300) and Pleasantness. Chemical Senses, 27(3), 225–230. Original paper.