Does Leaving Home Quickly Reverse Adaptation to Its Scent?

When you spend time in a room with the same scent, you gradually notice it less. That change does not necessarily reverse as soon as you leave. In a 1996 study, all eight women exposed to the same odor at home for two weeks became less sensitive to it.

Even two weeks after the odor was removed, four had not fully regained their pre-exposure sensitivity. The researchers compared adaptation during experiments lasting a few minutes with adaptation following prolonged exposure in everyday surroundings, and examined how perceptions at home related to sensitivity measured in the laboratory.

Does short-term adaptation extend to another odor?

First, eight men and women took part in a short-term experiment. The researchers used two single compounds with different odors: citrus-like citralva and isobornyl acetate, which evokes conifers. After matching their perceived intensities for each participant, they had participants repeatedly sniff one for five minutes while rating the intensity of both odors at intervals.

The repeatedly sniffed odor began to seem weaker early on and returned to its original level within the two-and-a-half-minute recovery period after exposure stopped. Ratings of the other odor also fell slightly, but much less than those of the repeatedly sniffed odor.

This check supported a design for the long-term experiment that compared the odor released at home with the other odor, encountered only in the laboratory. The aim was to distinguish changes in overall olfactory sensitivity from adaptation to a particular odor.

A scent dispenser at home for two weeks

Eight different women took part in the long-term experiment. They were aged 18–42, with four assigned to each odor. A fan-driven fragrance dispenser was placed in their bedroom or dormitory room and operated continuously for two weeks.

Laboratory testing took place twice before installation, twice at weekly intervals during use, and twice at weekly intervals after removal. Participants were also tested at home 24 hours after removal. The measures were the detection threshold, the lowest concentration at which a faint odor could be distinguished from an odorless sample, and perceived intensity at five clearly detectable concentrations.

Participants also recorded how long they spent in the room, how strong the room smelled, and how long they continued to notice the odor after entering. Time spent at home varied between participants. Some were tested in the laboratory one to two hours after leaving the room, and others four to seven hours afterward.

After two weeks, a higher concentration was needed to detect the familiar odor

After two weeks, every participant found the odor encountered at home harder to detect than immediately before exposure began. In other words, a higher concentration was needed to notice its presence. The reduction after two weeks was also statistically significant for the group as a whole.

By contrast, the control odor, encountered only in the laboratory, became easier to detect over successive tests. The researchers considered practice or perceptual learning from repeated testing as possible explanations. Sensitivity to the odor encountered over the longer period changed in the opposite direction to that improvement.

Intensity ratings also differed by concentration. During exposure, only the two weakest of the five tested concentrations were rated as weaker. Higher concentrations did not uniformly seem weaker, and no participant became completely unable to detect the odor during testing.

Sensitivity recovered after removal, but at different rates

Sensitivity began to recover after the dispenser was removed. After one week, however, only two participants had returned to their pre-exposure level, two had partially recovered, and four had not yet shown recovery. After two weeks, four had recovered to their original level or better, while the other four had not fully recovered.

Individual differences need to be distinguished from the group average. Two weeks after removal, the average was no longer statistically different from immediately before exposure, but a difference remained relative to the control odor, sensitivity to which had improved over testing. The results do not mean that everyone recovers at the same speed or inevitably recovers within a fixed number of days.

An important conclusion is that recovery occurred within minutes in the short-term experiment, whereas the long-term experiment showed changes lasting days or longer.

A weaker smell at home was associated with lower sensitivity in testing

Participants who perceived the room odor as weaker also tended to be less sensitive to that odor in the laboratory. Those who spent more time in the room also tended to show reduced sensitivity earlier. Time spent at home was not, however, randomly assigned.

The time for which participants continued to notice the scent after entering the room also shortened; some reported only one to two minutes by the second week. Meanwhile, according to participants’ reports, visitors still noticed the odor. This finding connects with the everyday experience that the same room can smell different depending on a person’s history of exposure.

The researchers also tested whether simply returning home made the odor harder to detect. Testing at home 24 hours after removal did not reveal an additional loss of sensitivity compared with laboratory testing the previous day. However, because testing at home did not take place while the dispenser was operating, this alone cannot rule out contributions from context or attention.

Summary

Two weeks of exposure to the same odor at home reduced sensitivity to that odor and made weak concentrations seem less intense. These changes were measurable in the laboratory away from the room, and recovery after removal differed between individuals.

The authors suggest that long-term adaptation may involve several processes, including changes in sensory responses, attention, and memory. Although the study involved only eight women and two single odorants, it is important for capturing the experience of a familiar smell becoming weaker not just through everyday self-reports but through changes in detection sensitivity.

The research received US National Institutes of Health research grants and a training grant to Monell. The acknowledgments credit a Waterbury representative for odor-delivery equipment and International Flavors and Fragrances for odorants and advice. No separate conflict-of-interest statement was identified.

Reference

Dalton, P., & Wysocki, C. J. (1996). The nature and duration of adaptation following long-term odor exposure. Perception & Psychophysics, 58(5), 781–792. https://doi.org/10.3758/BF03213109 (Original paper PDF)