When you enter a scented space, the smell may initially attract your attention but seem weaker after a while. Does this “getting used to it” happen at the same speed for every odor? A study presenting 32 fragrance materials for two minutes each found that the decline in perceived intensity differed between materials and needed to be considered in terms of perceived intensity, irritation, and molecular properties together.
Measuring a change in perception, not a fading scent
The 2017 study had two aims: to establish whether habituation differed between odorants and to explain those differences through sensory and physicochemical characteristics. Here, “habituation” refers to a decline in the intensity a person perceives while an odor continues to be presented at a constant concentration.
The researchers tested 32 odorants over four sessions in healthy young adults with normal results on an olfactory test. The original paper gives both 58 and 51 as the number of participants, an inconsistency compounded by the sex breakdown in the methods section totaling 58. For the analysis, the authors report excluding 229 rating curves because of absences, recording problems, substantially delayed responses, and other reasons, leaving 1531 curves for analysis.
An apparatus delivered each odor to the nose at a constant concentration for 120 seconds. To reduce the recovery of sensation that could occur with renewed sniffing, participants practiced breathing through their mouths without actively inhaling through the nose. Presentation order was counterbalanced across participants, and there was a 90-second interval before the next odor.
Tracking intensity by changing the pressure of a hand
Participants pressed an air-filled syringe, easing the pressure when the odor seemed weaker and increasing it when the odor seemed stronger. The pressure was converted into numerical values, with a light scale providing feedback. Everyone began at the same point on the scale, an approach designed to capture subsequent changes rather than absolute intensity.
Analysis of the curves revealed three representative shapes: high, moderate, and low habituation. Because some curves did not fall neatly into one category, the researchers compared how close each recording was to the three representative shapes instead of assigning each record permanently to a single group.
The results showed significant differences between odorants. For example, cis-3-hexenol, classified as green, and the fruity material Manzanate were on the low-habituation side. By contrast, woody materials such as Iso E Super and Sandalore, and the musk Galaxolide, were on the side showing a greater decline in intensity. These comparisons apply to the concentrations and presentation conditions used in this study.
Intensity, irritation, and molecular properties overlapped
The researchers examined 15 characteristics, including odor intensity, pleasantness, familiarity, and irritation, as well as molecular weight, vapor pressure, number of double bonds, and hydrophobicity. The irritating sensations participants rated were prickling and warmth in the nose. Coldness and freshness were not included in this measurement.
Because these characteristics are interrelated, the researchers used a statistical model that could handle them together. Odorants with lower habituation tended to have greater perceived intensity and irritation, higher vapor pressure, lower molecular weight, and lower pleasantness. Odor activity value, or OAV, which expresses how concentrated an odorant is relative to its detection threshold, was also associated with habituation.
Importantly, the study did not isolate pleasantness, irritation, or any other single characteristic as the cause. The odorants tested did not include extremely unpleasant malodors. The authors also noted that other studies had found unpleasant substances to which people habituated readily, and discussed the results as combinations of several characteristics.
The predictive model offers clues, but some points still need checking
The model explained approximately 82.8% of the variation in a measure of distance from the representative low-habituation profile. This is the proportion of variance explained in the odorant dataset used here, not a finding that new odorants could be predicted with 82.8% accuracy. The authors identified comparing predictions with measurements in a new set of molecules as a next step.
The original paper also contains differences in how it describes the molecular-structure results. The table and results section associate fewer double bonds and greater hydrophobicity with lower habituation, while the conclusion of the discussion summarizes these relationships in the opposite direction. For these two characteristics, the direction of increase or decrease cannot be used as an established rule. The underlying OAV data are also described as confidential and are not publicly available.
Nevertheless, the approach of considering sensory and molecular characteristics together, and the continuous-rating method, are important contributions of this study. The researchers state that a separate measurement using the same method with six odorants and 30 people produced similar trends, but this was not a formal test of reliability across different days.
Extending the findings to everyday scents
The experiment measured changes while people smelled individual fragrance materials in a constant airflow for two minutes. Conditions differ when concentrations change with ventilation or movement around a room, when products combine many fragrance materials, or when exposure lasts much longer. Nor did the study directly measure how long perfume lasts or how strongly other people can smell it.
The experiment alone also cannot distinguish whether habituation occurred at receptors in the nose or in the brain. The relationship between molecular structure and habituation provides a starting point for further investigation of these mechanisms.
Summary
Even with continuous presentation at a constant concentration, the decline in perceived intensity differed between odorants. In this comparison, cis-3-hexenol and Manzanate tended to retain their perceived intensity, while materials such as Iso E Super and Galaxolide were on the side showing a greater decline.
Lower habituation was associated with a combination of factors, including intensity, nasal irritation, vapor pressure, and molecular weight. Although some inconsistencies remain in the reported directions of structural effects, the central finding is that habituation is difficult to explain through a single criterion, such as “pleasant scents are easier to get used to.”
The research was funded by Givaudan UK, which also supplied the fragrance materials. At the time of the study, one coauthor was affiliated with that company and another with Open Value. The Givaudan-affiliated author participated in study design and the selection of odorants and physicochemical characteristics. No separate conflict-of-interest statement was found in the article.
References
Sinding C, Valadier F, Al-Hassani V, et al. New determinants of olfactory habituation. Scientific Reports. 2017;7:41047. doi:10.1038/srep41047
