Does Masking an Odor Make It Pleasant? Different Responses in 33 People

Adding a fresh scent can make an unwanted odor less noticeable. But does that necessarily make the overall experience more pleasant?

A study combining eucalyptol and ammonia found that although the ammonia-like odor seemed weaker, some people found the mixture pleasant and others found it unpleasant. The team examined perceptions and brain responses to explore the relationship between masking an odor and experiencing comfort.

What happens in the nose and brain when odors mix?

The nose senses more than odor quality. Cooling sensations and sharp irritation also involve another sensory pathway, the trigeminal system. Everyday scents often engage both smell and these sensations.

Franziska S. Müschenich and colleagues investigated cooling eucalyptol and urine-like ammonia. Both were used as substances that stimulate the olfactory and trigeminal systems.

The team had previously reported that eucalyptol makes ammonia’s odor less noticeable. This study aimed to confirm that perceptual finding while using fMRI to examine changes in pleasantness and brain processing between individual substances and their mixture.

They initially expected the mixture to be about as pleasant as eucalyptol alone and to evoke similar brain responses. The results were more complicated.

Thirty-three people evaluated the components and mixture

Thirty-nine people participated. After exclusions for imaging-analysis issues and other reasons, data from 33 healthy adults aged 20–36 were analyzed. All were nonsmokers and underwent olfactory testing.

The stimuli were eucalyptol, ammonia, their mixture, and propylene glycol as an odorless reference. Two air streams were combined within the apparatus to match nasal airflow between single-component and mixture presentations.

Inside the MRI scanner, participants received four-second stimuli through one nostril at a time. The nostrils alternated, and presentations were synchronized with breathing. Two approximately 30-minute sessions, separated by a break, comprised 80 trials in total.

After each presentation, participants rated eucalyptol intensity, ammonia intensity, overall intensity, and pleasantness. Intensity used a 0–100 scale, and pleasantness ranged from very unpleasant to very pleasant. They rated the overall experience rather than separately evaluating nasal irritation and olfactory sensation.

Ammonia became less noticeable, but the whole mixture became stronger

The mean ammonia-component intensity rating fell significantly from 52.2 alone to 33.1 in the mixture. Eucalyptol-component intensity fell from 70.0 to 65.4, a change that was not statistically significant.

The two components were therefore not weakened equally: ammonia became much less prominent. This is consistent with perceptual masking, in which another scent makes an odor less noticeable.

Overall intensity, however, was rated 65.2 for eucalyptol alone, 50.3 for ammonia alone, and 73.5 for the mixture. The unwanted component could seem weaker while the entire nasal experience became stronger. Asking separately about component and overall intensity revealed this distinction.

A lower rating does not mean that ammonia molecules were removed from the air. What this study established concerned people’s perception.

Individual differences in pleasantness were hidden by the average

Across all participants, eucalyptol was rated more pleasant than the odorless reference, while ammonia was unpleasant. The mixture’s mean rating was 47.9, not significantly different from the odorless reference at 49.9.

Individual responses, however, diverged. Fifteen people rated the mixture as more pleasant than the odorless reference, and 18 rated it as less pleasant. The researchers divided participants into these two groups during the analysis, based on those ratings.

In the pleasant-mixture group, stronger perceived eucalyptol within the mixture was associated with greater overall pleasantness. In the unpleasant-mixture group, stronger perceived ammonia was associated with lower overall pleasantness.

Even with the same mixture, the component that stood out was related to the overall impression. Describing the odor as neutral on the basis of its average would conceal these individual experiences.

Mixing also changed brain responses

Across odor stimuli, fMRI showed responses in regions including the piriform cortex, involved in olfaction, and the insula. Comparing the mixture with eucalyptol alone revealed stronger responses in areas including those involved in bodily sensation.

An analysis comparing the mixture with the two single-odor conditions also found increased responses in somatosensory regions and the insula, among others. The authors interpret these findings as difficult to explain through simple addition of responses to the components.

In analyses separated by pleasantness group, the unpleasant-mixture group showed significant mixture responses in secondary somatosensory cortex and the insula. The pleasant-mixture group showed no responses exceeding the statistical threshold in the same comparison. Significance in only one group, however, does not itself directly establish a difference between the groups.

The researchers suggest that nasal irritation or attention toward an unwanted component may contribute to this pattern. Brain images alone cannot establish that a participant felt pain or focused on ammonia. The absence of separate irritation or pain ratings also limits that interpretation.

Masking an odor and making it pleasant require separate checks

A key finding was that eucalyptol masked ammonia’s odor, but the expectation that the mixture would be pleasant for everyone was not supported. Nor were the mixture and eucalyptol alone processed identically in every aspect of the brain response.

The authors suggest odor familiarity, attention, and sensitivity to trigeminal stimulation as possible contributors to individual differences. These are hypotheses for future testing, not established causes in these participants.

The experiment briefly presented two substances at a limited set of concentrations. Household fragrance and deodorizing products vary in ingredients, concentrations, and settings, so the findings cannot be directly generalized to all products. They nevertheless highlight the value of separately asking whether the unwanted odor is less noticeable, the overall mixture is too strong, and the user finds it pleasant.

Summary

Mixing eucalyptol with ammonia reduced the perceived ammonia-like odor. Yet the overall mixture was stronger than either odor alone, and participants differed in whether they found it pleasant or unpleasant. Mixing also increased fMRI responses in regions including somatosensory areas, rather than simply producing the same processing as eucalyptol alone.

Making an odor less noticeable, reducing the overall sensory impact, and creating a pleasant scent are distinct goals. Evaluating each helps reveal comfort from the user’s perspective. Assessing a mixture requires attention not only to its components, but also to the whole experience and individual differences.

The research received funding from Symrise, and a company employee participated as a co-author in manuscript revision. The authors state that the funder did not influence study design, experiments, analysis, or interpretation.

References

• Müschenich, F. S., et al. (2021; published online in 2020). Some like it, some do not: behavioral responses and central processing of olfactory–trigeminal mixture perception. Brain Structure and Function, 226. DOI and original article. Open full text.