Even for the same molecule, changing its concentration can alter not only odor intensity but also the impression it creates. In a 2025 study of skatole, ratings at low concentrations shifted from light and mild toward heavy and rich as concentration increased; at still higher concentrations, unpleasant ratings became more pronounced.
A distinctive feature of this study was that, rather than applying the same concentration to everyone, it used each person’s detection threshold as a reference. Alongside changes in odor quality, it examined the proportion of participants who found the odor unacceptable, seeking insights relevant to indoor odor management.
How much dilution makes the odor undetectable?
The participants were 25 people aged 19–22 who had passed an olfactory screening test: seven men and 18 women. Testing took place on two days, with eight participants on the first day and 17 on the second.
Participants were given a set containing one bag of skatole odor and two bags of odorless air and asked to identify the bag with an odor. Each correct answer was followed by further dilution, and an individual olfactory threshold was calculated from the point at which the participant could no longer identify it. For the sample selected as the odorous bag, participants also evaluated intensity, pleasantness or unpleasantness, acceptability, and impressions using 18 pairs of descriptive words.
Odor concentration here differs from the chemical concentration of a substance, such as a value in ppm. In this study it was referenced to each individual’s threshold: an odor concentration of 3 corresponds to roughly three times that person’s detection threshold, and 10 to roughly ten times it.
Threshold estimates also depend on the measurement method
The skatole concentration in the original sample differed between the first and second days. The authors attributed this in part to differences in room temperature during sample preparation. They analyzed the original concentration on each day before calculating individual thresholds.
The mean threshold across all 25 participants was 1.3 × 10−5 ppm. To reduce the influence of extreme values, the authors excluded the four highest and four lowest results. They also used recovery rates from previous research to correct for losses such as adsorption to bags and syringes. The resulting corrected mean was 7.3 × 10−7 ppm.
Although this is a very small number, it is an estimate that accounts for differences between the amount placed in the apparatus and the amount actually reaching the nose. It should not be read as a fixed sensitivity shared by everyone, or as a fragrance formulation level that can be applied directly.
When lightness changes, and when unpleasantness emerges
Odor concentrations normalized to individual thresholds were compared in four groups: at least 1 but below 3, at least 3 but below 10, at least 10 but below 30, and at least 30 but below 100. Overall, increasing concentration was associated with stronger odor and a shift from neutral toward unpleasant ratings.
A factor analysis of the descriptive ratings yielded three groupings: comfort, lightness, and arousal. Lightness concerned impressions such as light versus heavy and mild versus rich. Arousal was a label grouping words such as distinct and vivid; it was not a measurement of actual physiological arousal.
Compared with the lowest group, from 1 to below 3, the group from 3 to below 10 tended to be rated as less light and less mild. The individual comparisons between these groups for light–heavy, mild–rich, and similar pairs, however, were trends at p < .1, not significant differences at the conventional 5% level.
Clearer differences appeared in the groups at 10 or above. Comparing 1 to below 3 with 10 to below 30, significant differences were found in liking, cleanliness, lightness, mildness, and other ratings, with the higher concentration shifting toward heavier and more unpleasant impressions. Liking and irritation ratings, among others, also differed between 3 to below 10 and 10 to below 30.
By contrast, the patterns of impressions were relatively similar in the 10 to below 30 and 30 to below 100 groups. The authors interpreted this as a staged change, with lightness beginning to change around 3 and unpleasant qualities becoming clear at 10 or above. The results do not establish a sudden switch in everyone’s impression at exactly 3.
More participants also found the odor unacceptable
The study also asked whether the odor would be acceptable indoors and used a statistical model to estimate the relationship between concentration and the proportion finding it unacceptable. After excluding concentrations with too few respondents and certain other data, the researchers fitted a relationship in which unacceptability increased with concentration.
In relation to the Architectural Institute of Japan’s approach to indoor odors cited in the paper, the estimated odor concentration corresponding to 20% unacceptability was about 3, and that corresponding to 30% was about 5. The authors noted that 3, the value associated with 20%, was close to the concentration at which they considered lightness to begin changing.
These are values for consideration in indoor management derived from this study’s responses and model. They are not demonstrated health-based safety limits or concentrations that can be applied to everyone in actual rooms. The original paper also identifies testing beyond odor bags in real spaces as a future task.
Summary
At low concentrations near each person’s threshold, skatole was perceived as light and mild. With increasing concentration, impressions became heavier, and unpleasantness was clearer in groups at ten times the threshold or above. The interpretation of a change around three times the threshold rests on patterns of impressions and statistical trends.
The unacceptability model likewise estimated a value of about 3 for 20% unacceptability and about 5 for 30%. The study illustrates why understanding odor quality requires considering not only the molecule’s name but also its concentration and the point at which each person begins to detect it.
All three authors listed Daido University as their affiliation at publication. No separate funding or conflict-of-interest declaration section was identified.
References
Isozaki, A., Mitsuda, M., & Tanamura, T. (2025). Consideration of the Change Point in Odor Quality of Skatole: Relationship between odor concentration and odor quality. Journal of Environmental Engineering (Transactions of AIJ), 90(838), 602–608. doi:10.3130/aije.90.602 (Full original article)
