Does Getting Used to an Odor Make It Less Unpleasant?

When we remain in the presence of the same odor, it can begin to smell less intense. But if its intensity fades, does the unpleasantness of a bad smell fade in the same way?

In a 2023 study, all four stimuli smelled weaker after a brief continuous presentation. In contrast, a clear change in pleasantness ratings appeared only at the lower concentrations of the floral mixture rated most pleasant. Getting used to an odor’s intensity and getting used to its emotional quality did not necessarily happen together.

Measuring intensity and pleasantness separately

Li and colleagues aimed to investigate how adaptation changes odor intensity and pleasantness by combining different stimuli and concentrations. Here, adaptation refers to a decrease in perceived intensity after continued exposure to the same stimulus.

The team predicted that unpleasant odors might be harder to adapt to than pleasant ones because they can signal danger. They tested this prediction using ratings of both intensity and pleasantness, rather than simply asking whether an odor was detected.

Presenting four odors to 30 people using an olfactometer

Participants were 30 people aged 18–34 whose normal olfactory function had been confirmed by testing. The four stimuli were phenylethyl alcohol (PEA), a pleasant floral mixture, hydrogen sulfide, and a mixture reminiscent of sweat.

The researchers selected the first two as pleasant stimuli and the latter two as unpleasant. In participants’ actual ratings, however, the floral mixture was pleasant and hydrogen sulfide unpleasant, while PEA was only slightly pleasant and the sweat-like mixture was close to neutral. This mismatch matters when interpreting the results.

An air-dilution olfactometer delivered the stimuli. Temperature and humidity were held constant, and stimulation was directed through a tube into one nostril. Participants listened to noise designed to mask switching sounds from the apparatus and performed a task tracking a shape on a screen, while being asked to breathe through their mouths. The design prioritized consistent stimulation over the natural way people smell in everyday life.

How does a brief puff smell after 15 seconds of exposure?

First, each stimulus was presented for 0.2 seconds at four concentration settings, and participants rated its intensity and pleasantness. Stimuli were separated by 40 seconds, with stimulus and concentration order randomized.

During the adaptation stage, the same odor was presented continuously for 15 seconds at a low setting. After a four-second odor-free interval, another 0.2-second stimulus was presented for evaluation. This post-adaptation measurement was repeated three times, and the mean was analyzed.

The paper gives the concentrations of the target stimuli as 12%, 24%, 36%, and 48%, and that of the preceding adaptation stimulus as 5%. These should be read as conditions set on the air-dilution apparatus, not as percentages of pure substances inhaled or as ingredient concentrations in a product.

All four smelled weaker, but concentration did not play the same role for each

After adaptation, intensity ratings fell for all four stimuli relative to baseline. Even unpleasant hydrogen sulfide showed a decrease, contrary to a simple prediction that unpleasant odors would show little adaptation in intensity.

However, the size of the decrease varied with the combination of stimulus and concentration. For PEA, the decrease was greater at 12% than at 36%; for the sweat-like mixture, it was greater at 24% than at 48%.

A comparable pattern of larger decreases at lower concentrations was not clear for the floral mixture or hydrogen sulfide. It is therefore more accurate to understand the concentration effect in this study as a result found at particular settings for slightly pleasant PEA and the approximately neutral sweat-like mixture.

Unpleasantness may remain, while pleasantness can decline a little

Pleasantness ratings for the floral mixture decreased after adaptation at the 12% and 24% settings. The p-value was below .001 at 12% and .022 at 24%.

For the other combinations of stimulus and concentration, changes in pleasantness ratings were not statistically clear. Although hydrogen sulfide smelled weaker, the study did not establish that it became noticeably less unpleasant.

Conversely, even a pleasant fragrance can lose not only perceived intensity but also some of its rated pleasantness. Separating the two scales revealed different changes within the single idea of getting used to an odor.

What influences adaptation?

The authors discuss whether the warning significance of unpleasant stimuli, or how much someone initially liked a fragrance, might influence changes in pleasantness. However, the study did not directly measure perceived danger or warning functions in the brain; these are hypotheses offered to explain the findings.

Importantly, the mixture selected to resemble sweat was actually rated neutral. Even if researchers prepare a stimulus as an unpleasant odor, participants do not necessarily experience it that way. Classification needs to take account of actual ratings, rather than the stimulus name or intention alone.

At baseline, changing the apparatus’s concentration setting also did not produce large differences in perceived intensity. The authors acknowledge as a limitation whether the concentration conditions generated sufficiently different intensities. This was also a small experiment involving many before-and-after comparisons across stimuli and concentrations, so further confirmation would help establish the reproducibility of these particular contrasts.

The study examined 15-second exposures and immediately subsequent ratings in healthy young adults. Extending its conclusions directly to prolonged indoor exposure, everyday perfume use, or treatment of smell disorders would require separate investigations under relevant conditions.

Summary

After brief continuous exposure, all four odors smelled weaker, but pleasantness clearly decreased only at the two lower concentration settings of the floral mixture. For unpleasant stimuli, lower intensity was not accompanied by a clear change in unpleasantness.

Larger intensity decreases at lower concentrations were also limited to particular conditions involving PEA and the near-neutral mixture. Understanding adaptation to fragrance requires considering intensity, pleasantness, concentration, and initial liking separately.

The authors declared no external research funding and no conflicts of interest. The odor mixtures came from Givaudan, and open-access publication was supported by Projekt DEAL.

References

Li Z, Mignot C, Sinding C, Hummel T. Effects of desensitization on odors varying in concentration and pleasantness. Journal of Sensory Studies. 2023;38(6):e12877. doi:10.1111/joss.12877. Open-access full text.