Can Heat and Humidity Change a Smell Even at the Same Concentration?

同じ濃さの香りでも、暑さや湿気で感じ方は変わる？

A familiar smell can seem different on a hot day. Such changes involve both the amount of odor entering the air and how people perceive it. But if airborne concentration is held constant, can temperature or humidity still change the experience?

In 2018, a team at fragrance company Firmenich investigated this question while developing a method for evaluating perfumes designed to reduce toilet malodor. Higher temperature or humidity lowered odor-intensity ratings. In contrast, climate produced no significant differences in pleasantness or willingness to enter the toilet. The same apparatus was also used to examine what changed as more perfume was added.

How can fragrances intended to make toilets more welcoming be evaluated?

The starting point was encouraging people to use sanitation facilities. Strong malodor can make someone reluctant even to enter a toilet. The researchers aimed to develop perfumes for affordable cleaning and air-freshening products that would also be pleasant under climatic conditions found in Africa and India.

Smelling perfume alone in a small bottle is not enough for this purpose. Researchers need to examine how it works in a space with background toilet malodor, airflow, heat and humidity. In real toilets, however, odor production and ventilation continually vary, making fair comparisons between perfumes difficult.

The team therefore built three model latrines in which malodor, perfume, airflow, temperature and humidity could be controlled. Recreating the same conditions repeatedly and combining chemical measurements with human assessments were central to the study.

Supplying the same background malodor to three model latrines

Each model latrine had a volume of approximately 1.7 cubic meters. They were placed in a room with adjustable temperature and humidity, and each received 17 cubic meters of air per hour. This corresponded to approximately 10 air changes per hour.

The malodor reproduced findings from analyses of urine-diverting toilets in Mukuru, Nairobi, Kenya. It contained 5 compounds: hydrogen sulfide, methanethiol, butyric acid, p-cresol and indole. An in-house floral perfume was then added.

Rather than placing a liquid in the room and waiting for natural evaporation, the researchers used a heated evaporation device and gas-delivery equipment to supply fixed amounts continuously. This minimized the influence of room temperature on evaporation and focused the experiment on how people perceived odors presented at controlled concentrations.

To validate the apparatus, air was sampled at 3 locations in each of the 3 latrines. Measured concentrations ranged from 66% to 138% of the targets, depending on the compound. They did not match the targets perfectly, but variation within and between latrines was relatively small, indicating a reasonably uniform odor environment. An analytical method that collected airborne compounds directly onto cartridges also supported this evaluation system.

Comparing odors at 22°C and 35°C, and 30% and 80% humidity

The four climates combined temperatures of 22°C and 35°C with relative humidities of 30% and 80%. Six odors were compared:

  • The reconstructed malodor alone
  • The floral perfume alone
  • The same malodor with perfume added at four concentrations

Perfume concentrations in the mixtures were 0.18, 0.54, 1.62 and 4.9 micrograms per liter of air. The researchers examined responses to increasing perfume while keeping the malodor supply unchanged.

Assessors were employees at Firmenich’s research facility, with 23–30 participating in each of 10 sessions. This does not mean that hundreds of different people were studied. In each session, participants rated pleasantness, familiarity, intensity, fecal/toilet character and willingness to enter on scales from 0 to 10.

They evaluated the odors immediately after entering the room and again after 3 minutes of adaptation to the climate. This short adaptation period produced no significant difference, so the two ratings were averaged for analysis. The first and final validation sessions also showed no significant differences, supporting the reproducibility of the assessments.

Heat and humidity mainly changed intensity

In the analysis pooling odor types, intensity ratings were significantly lower at 35°C than at 22°C. The temperature effect had p<0.0001 in the analysis of variance and was significant with another statistical method as well. Raising relative humidity from 30% to 80% also produced a smaller but significant decrease in intensity ratings.

The effect was not uniform across odors. The temperature effect mainly involved malodor alone, perfume alone and the mixture with the highest perfume concentration. The humidity effect was most apparent for perfume alone and the highest-concentration mixture. Increasing both temperature and humidity did not demonstrably produce a further significant decrease in intensity.

By contrast, climate had no significant effect on pleasantness, familiarity, fecal character or willingness to enter. In this experiment, perceived strength and pleasantness did not change in the same way.

The study alone cannot establish why intensity decreased. The authors discuss molecular behavior inside the nose and relationships between temperature and olfactory processing, but they did not directly measure neural activity or otherwise establish the mechanism experimentally.

How did increasing perfume change the perception of malodor?

Another important result concerned the relationship between perfume dose and ratings. As perfume concentration increased, fecal-character ratings tended to fall, while pleasantness and willingness-to-enter ratings rose, with statistically significant changes. Pleasantness closely tracked willingness to enter: a model of their relationship explained approximately 80% of the variation in willingness-to-enter ratings.

Interestingly, overall odor intensity did not differ significantly between malodor alone, perfume alone and their mixtures. Fecal character and pleasantness could change without a reduction in the overall strength of the smell. This demonstrated perceptual malodor reduction, not chemical destruction of malodorous compounds or antibacterial activity.

However, the concentration-specific ratings should not be read as one smooth curve without qualification. There were only 3 model latrines but 6 odors, so the assessments were split into two sets. One included malodor alone as a reference, while the other included perfume alone.

Consequently, the odors presented alongside a sample also influenced how the perfume’s performance was rated. The authors state that the two sets cannot simply be compared directly; instead, the trend with concentration should be considered. As well as offering a method for measuring perfume performance, the experiment shows why comparison sets need to be consistent.

Conditions to consider when applying the findings to everyday fragrances

The crucial feature of this study is its control of airborne concentration. With real fragranced products, heat can also change how quickly perfume enters the air. These intensity results do not establish that perfume necessarily smells weaker in summer or lasts longer on skin in humid weather.

The experiment also tested one particular model malodor and a floral perfume, using company employees as assessors. Real toilets vary more widely in odor distribution and users’ experiences. A rating of willingness to enter must also be distinguished from actual toilet-use behavior in the field.

Nevertheless, a controlled model space has advantages for product comparisons. The authors suggest that the system could also evaluate fragrance-delivery methods, including liquids, powders and fragranced materials. It provides a basis for examining not only which perfume to choose but also whether it can reach the required concentration in a space.

All four authors worked in Firmenich’s research and development department at the time. Firmenich and the Bill & Melinda Gates Foundation funded the study equally, and the work was associated with the foundation’s sanitation-improvement project.

Conclusion

With airborne odor concentrations controlled in model latrines, increasing temperature or humidity lowered intensity ratings. Climate did not significantly change pleasantness, fecal character or willingness to enter. Comparisons using more perfume also captured a trend toward lower fecal-character ratings and greater pleasantness and willingness to enter.

To make fragrance useful, it is important to consider not only its identity and dose but also background malodor, airflow, climate and the comparison method. This study presents a way to control these conditions and evaluate toilet perfumes through both chemical analysis and human perception.

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