How much does placing an air freshener/deodorizer in a toilet change the unpleasantness of fecal odor? In a study that generated simulated fecal odor in apartment toilets, odor unpleasantness was rated lower with either of two air freshener/deodorizers than without a product.
Another aim was to develop a way of measuring effects under conditions closer to everyday use. Alongside tests mixing fragrance and malodor in sealed bags, the researchers examined how odor released from a toilet bowl was perceived over time in ventilated toilet rooms.
Releasing odor from the bowl and evaluating it through a small window
The study used three apartment toilet rooms with identical layouts and specifications and no initial differences in odor. Each had a volume of approximately 1.9 m³. Extractor fans ran throughout the experiment, providing about 12 air changes per hour. Reported room temperature was 19±4°C and humidity was 51±10%.
One room received floral sample A, another herbal sample B, and the third served as a no-product control. A and B were liquid air freshener/deodorizers containing the same deodorizing ingredient in the same amount. B’s fragrance had shown relatively high efficacy against fecal odor in the established bag test. The products were left in place for at least 24 hours to allow room fragrance levels to stabilize.
The simulated fecal odor combined hydrogen sulfide, methyl mercaptan, and allyl methyl sulfide. It was placed in a box inside the toilet bowl, with odor release synchronized by pulling a string from outside to open the lid.
To avoid disturbing airflow as participants entered and left, the doors were replaced with acrylic panels fitted with small sniffing windows. Participants evaluated the odor through a window 135 cm above the floor without entering the room. The product location was covered by a container in every room, including the no-product room, to match visual conditions.
Twenty-three people rated unpleasantness and acceptability
The target participants were men and women aged 20–49 who routinely used air freshener/deodorizers in their toilets. Twenty-three people who met the olfactory screening and other criteria evaluated the odor before release and at 1, 3, 5, 7, and 9 minutes afterward.
Two outcomes were measured: a nine-point rating running from pleasant to unpleasant, and whether the toilet odor was acceptable. The study examined people’s perception of the mixed air, not how many odor molecules had been removed.
The original paper’s text reports that both A and B significantly reduced unpleasantness relative to no product at every post-release assessment. Mean rating differences were 0.9–1.3 points for A and 1.4–2.0 for B. However, at three minutes, Figure 10 marks a significant difference only between B and the control; a corresponding marker for A cannot be confirmed. The text and the figure’s markings need to be distinguished at this point.
At five minutes, for example, the no-product mean was −2.0, corresponding to unpleasant; A was −1.0, slightly unpleasant; and B was −0.3, closer to neutral. The presence of the products changed the overall impression of the malodor-containing air. The product rooms were also rated more pleasant than the no-product room before malodor release. These are ratings of the pleasantness or unpleasantness of the room’s air as a whole, not the strength of the malodor alone or the quantity of odor molecules removed.
The herbal sample had lower mean unpleasantness
Comparing A and B, B had a less unpleasant mean rating at all five post-release time points, with differences of 0.4–0.8 points. Because the type and amount of deodorizing ingredient were the same, the authors interpreted this as a difference in sensory effects arising from the fragrances.
However, the figure does not show time-point-specific significance tests directly comparing the two fragrances. Consistently directed means for these two formulations do not establish a general ranking in which every herbal fragrance outperforms every floral fragrance.
The proportion finding the odor unacceptable was also generally lower in the product conditions. At five minutes, for example, it was 82.6% without a product, 65.2% with A, and 39.1% with B. At one minute, however, A and the no-product condition both stood at 52.2%. The paper treats the acceptability results from 23 participants as reference values because the cited standard calls for at least 60 participants.
Why include time and ventilation?
Unpleasantness increased after odor release, generally became greatest around five to seven minutes, and then began to decline. The percentage finding the odor unacceptable peaked at five or seven minutes, depending on the condition.
These changes also occurred in the no-product room. The authors considered diffusion from the toilet bowl and exhaust from the continuously operating ventilation fan to be major influences. The later decline in unpleasantness cannot be attributed entirely to the air freshener/deodorizer.
Capturing this time course is an advantage over sealed-bag testing alone. On the other hand, malodor and fragrance concentrations will differ in unventilated toilets or spaces with different volumes, temperatures, or humidity. Although the study used real residential rooms, it was a controlled test of simulated fecal odor, not a reproduction of every household’s conditions.
The bag test showed the same direction of results
The researchers also compared the products using the established method of Japan’s association for air fresheners, deodorizers, and odor eliminators. Fragrance and simulated fecal odor were mixed in sealed air bags, and 18 people rated pleasantness and unpleasantness. Overlap with the 23 residential-test participants was not specified, so the two sample sizes cannot be added together.
In the bag test, mean ratings were −2.9 for malodor alone, −0.8 when mixed with A, and 0.2 when mixed with B. Both A and B significantly reduced unpleasantness relative to malodor alone. B’s mean being on the more pleasant side of A’s also matched the residential test.
From this agreement, the authors concluded that the residential method could serve as a way to evaluate air freshener/deodorizers under conditions resembling actual use. Only two samples were tested; how well the methods agree for other products and environments remains to be investigated.
Summary
When simulated fecal odor was released in ventilated residential toilets, conditions with an air freshener/deodorizer were rated less unpleasant than the no-product condition, and the tested herbal formulation had more pleasant mean ratings than the floral formulation. The conventional bag test showed the same direction of results.
This study demonstrated a way to evaluate sensory deodorizing effects in a space that includes the odor source, the passage of time, and ventilation. It illustrates the value of measuring how unpleasant people find a space, separately from chemically analyzing what has been removed and in what quantity.
All three authors were affiliated with Kobayashi Pharmaceutical at the time of the study. No separate funding or conflict-of-interest statement was found in the original paper examined.
Reference
Matsumoto K, Suzuki T, Matsumune N. Verification of the deodorizing effects of air fresheners/deodorizers for toilets in a space assuming the actual use environment. Journal of Japan Association on Odor Environment. 2019;50(5):358–364. doi:10.2171/jao.50.358.
