Do More Fragrance Diffusers Make a Room Smell Stronger?

If you want a room to smell stronger, will adding another fragrance diffuser make the perceived scent stronger by the same amount? In 2025, Thomas Warburton and colleagues compared airborne compound levels with human perceptions of scent strength under the same conditions using plug-in liquid fragrance diffusers.

The aim was to examine indoor air and olfaction together. The authors also considered whether changes in odor and acceptable intensity as more devices are added might discourage overuse. However, this was not an observational study of how many devices consumers actually use.

Measuring room air and, separately, people’s perceptions

The electric diffusers used the same fragrance formulation and a fixed emission setting. One to five devices were placed in a room of about 10 cubic meters representing a small bathroom and a room of about 34 cubic meters representing a living room, with a no-device condition as well. Both rooms had mechanical ventilation bringing in outdoor air at 7.5 room volumes per hour.

After the devices had operated for 24 hours, air was sampled over 72 hours, and its constituents were separated and measured using gas chromatography and other techniques. Six fragrance-related compounds were the focus: α-pinene, β-pinene, γ-terpinene, benzaldehyde, p-cymene and eucalyptol. The paper’s “total fragrance-related VOCs” refers to these six compounds, not a measurement of every fragrance constituent.

Human evaluations took place in the smaller room of approximately 10 cubic meters. Assessors who had undergone four to six months of training smelled through a small window without entering the room and rated intensity from 0 to 100. They were not told the number of devices or the research purpose; evaluation order was controlled and conditions were repeated. The same sensory evaluation was not performed in the larger room.

Compound concentrations rose, but gains in perceived intensity diminished

Measured fragrance-related VOC concentrations generally rose as more devices were added. For the same number of devices, concentrations were higher in the smaller room. However, not every comparison between adjacent device counts showed a statistically clear difference. The overall increasing trend must be distinguished from individual comparisons.

Perceived intensity also increased, but the increments became smaller. Mean ratings differed by about 25.4 points between zero and one device, 8.7 points between one and two, and 2.2 points between two and three. Adding the first device produced the largest change; each subsequent addition produced a smaller one.

There was no significant increase in intensity in the one-device comparisons from two to three, three to four or four to five devices. However, two and five devices did differ significantly. This was not a complete plateau at which “nothing changes above two”; rather, perceived strength became less likely to rise in proportion to the number added.

The easiest compounds to measure are not necessarily the main contributors to scent

Another important finding was that concentrations of the more volatile fragrance compounds quantified in air were below published odor detection thresholds for smelling each compound on its own. A detection threshold is an estimate of the lowest concentration at which people can notice an odor. Nevertheless, assessors perceived a scent in rooms containing diffusers.

The authors suggested that less volatile compounds with very low odor thresholds, which were difficult to quantify with the analytical method used, might contribute substantially to perception. Candidates included γ-undecalactone and β-caryophyllene in the fragrance formulation.

This does not mean that those compounds were quantified in the air and their individual contributions to scent proven. Single-compound detection thresholds vary between people and measurement methods, and mixtures are not necessarily perceived like isolated compounds. Nonetheless, the findings question how well readily measured monoterpene concentrations can represent a room’s scent intensity.

Potential to discourage overuse and to change formulations

If gains in intensity are small, users may see little benefit in adding more devices and refrain from doing so. This is the possibility of partly self-limiting use discussed by the authors. However, preferences and tolerance for intensity also matter, so behavioral studies are needed to establish whether use is actually limited.

The authors also discussed implications for fragrance formulations. Reactive compounds such as monoterpenes can react with indoor ozone and contribute to the formation of other gases and fine particles. If components contributing relatively little to scent perception could be reduced, it might be possible to reduce secondary-pollutant formation without greatly compromising the perceived fragrance.

This proposal is a hypothesis derived from the lack of a simple relationship between airborne concentrations and perception. The study did not create a new, lower-monoterpene formulation and test whether both fragrance and air quality improved. The authors identified testing different ventilation rates and analytical methods able to measure lower concentrations of low-volatility compounds as next steps.

Not a test that sets device limits or establishes safety

The study involved one fragrance formulation and device under specified room conditions. It used trained assessors, and the main paper does not explicitly state their number. It does not establish “two devices as the limit” for every household and every fragrance.

Indoor concentration modeling also considered how concentrations could rise when a small space and low ventilation coincide. The authors discussed some concentrations under the study conditions as low compared with existing residential measurements and exposure indicators, but regarded health-impact assessment as outside the study’s scope. Weak perceived intensity, or individual compounds being below odor detection thresholds, cannot be treated as proof of safety.

The research was funded by Givaudan and Bath & Body Works, and employees of both companies were coauthors. The paper states that, to preserve independence, analytical measurements and data analysis were performed by the University of York and there were no restrictions on freedom to publish.

Summary

In this study, increasing the number of diffusers increased measured fragrance-related VOCs, but gains in perceived intensity diminished. In particular, adding devices one at a time beyond two did not produce clear differences in intensity.

The authors drew from this a behavioral hypothesis that use might naturally be limited, and an opportunity to consider formulations that retain scent with lower emissions. Understanding how fragrance works requires examining not only the amounts of compounds easily measured in air but also which compounds actually cause people to perceive the scent.

References

Warburton, T., Lewis, A. C., Hopkins, J. R., Andrews, S. J., Yeoman, A. M., Owen, N., Jordan, C., Adamson, G., & Xia, B. (2025). An assessment of VOC emissions and human strength perception of liquid electric fragrance diffusers. Environmental Science: Advances, 4, 739–752. DOI: 10.1039/d4va00388h · Full original article.

© 2025 The Author(s). Published by the Royal Society of Chemistry. CC BY 3.0.