There are different ways to measure odor scientifically. The first involves filling a bag with the air to be analyzed and taking it to one of the few laboratories that perform dynamic olfactometry, using human noses. In these centers, a group of panelists or “odor testers” are exposed to the contents of the bag. They start off with a minimal dilution, which is gradually increased. The moment when at least half of the noses perceive something is when the result is determined. This, subsequently, provides an estimated measurement of the amount of odor in the sample.
Rosa Arias is a chemical engineer. She previously worked in these types of laboratories before creating her own methodology with the firm Science for Change. In an interview with EL PAÍS, she explains that the aforementioned practice is standard procedure in Europe (the UNE 13725 standard) and is mainly used by companies to determine, for example, whether the air coming from a chimney or a slurry lagoon exceeds the odor levels that they are authorized to have.
The procedure can even be complemented with a mathematical model, which simulates how that odor will spread in the surrounding area. However, this system isn’t useful when it comes to truly assessing the discomfort that the same stench is causing to individuals at a distance. “This method measures how strong a substance smells at the point of emission, but not how it affects people later on; it tells you nothing about the type of smell,” the expert explains. Arias also trains people’s noses, so that they can learn to identify what they smell.
Citizen mobilization
In Spain, citizen mobilization against foul odors is growing. But the reality is that measuring the impact of bad smells isn’t so simple, and there’s practically no regulation for this type of odor pollution on the books. There are only some municipal ordinances approved by city councils, not all of which are very effective.
To measure the incidence of a foul odor in inhabited areas, a second system is used (also standard practice in Europe), which consists of field studies. People are paid to go and smell the air in certain places, in 10-minute increments several times a day, over a period of six months or a year.
According to Arias, the people selected to perform these types of measurements, whether in the laboratory or through field studies, don’t require any special olfactory skills. “In reality, almost everyone – around 90% of people – have a standard sense of smell. Only 5% have anosmia, which means that they cannot detect some or all odors, while another 5% have hyperosmia, which means they’re ultrasensitive to smells. This can also occur, for example, when women are pregnant,” explains the founder and CEO of Science for Change. However, even when ordinary noses are used, they can be calibrated by measuring a person’s sensitivity to a reference substance: 1-butanol. This allows researchers to determine the olfactory range of any individual and standardize the measurements.
These types of field studies, which can be expensive, have some useful applications. However, they still raise many questions about how to assess the level of annoyance that citizens experience from foul odors. “These people who are hired to take measurements — sometimes students — stand in a place for 10 minutes, without considering the weather conditions. [They] simply note whether or not there’s a smell, then calculate average values. But this doesn’t give a true picture of the impact,” José Cid details. He’s the technical director of Socioenginyeri, a company that provides solutions to bad odor problems.
This specialist, who introduces himself as a certified environmental odor inspector, advocates for a different methodology. Instead of standard sampling, he takes wind conditions into account, in order to measure the worst-case scenario. And, to do this, he places a Nasal Ranger portable olfactometer on his nose. The device, based on American technology, allows him to smell different dilutions of ambient air, so as to determine at what proportion the odor is perceived. This provides something similar to the dynamic olfactometry measurements taken in a laboratory. The device can determine not only if there is a smell, but also how strong the smell is.
The scientific use of this device, imported from the U.S., isn’t recognized in Europe. Experts like Arias reject it. However, it’s being used in some lawsuits concerning foul odors in Spain, and there’s even an ordinance from the Barcelona Provincial Authority regulating odor pollution that validates its use. “In Europe, especially in the Netherlands and Germany, they’re very reluctant to use field odor meters, because they have their own established systems in place,” Cid argues. He asserts that his measurements have already been used in 45 trials in Spain, including one that ended last July with a landmark ruling by the High Court of Justice of Galicia regarding a reservoir that was contaminated by manure slurry from nearby factory farms. In this case, the affected residents of the town of Lobeira, in addition to being exposed to dangerous bacteria, spend periods of time unable to even open their windows due to the nauseating smell.
Lower tolerance
“I have more and more work; there was a boom in 2025,” Cid, an environmental chemist by training, explains. “Society is less tolerant of these nuisances than before… that’s a fact,” he emphasizes. “It happens both outdoors and inside homes. Nowadays, there are neighbors complaining about the cooking smell from the apartment downstairs.”
To all these methods of measuring foul odors, we must add the one created by Rosa Arias, which is based on citizen science. For this chemical engineer, neither laboratory tests nor field sampling with external participants are effective when it comes to helping those affected. Therefore, she launched another system that only requires the noses of the residents themselves, as well as an app to create collaborative odor maps. In 2023, she even managed to get her methodology standardized in Spain.
“The human nose is the best sensor that exists for measuring environmental odors. What we do is train citizens to recognize the smells in their surroundings, so that they can map them,” Arias emphasizes. In this case, it matters little that the noses aren’t calibrated; the goal is that the sum of all the odors reported by many different residents, according to each person’s own perception, allows for the identification of the situations that are having the greatest adverse impact. One of the locations repeatedly identified at the moment is in the municipality of Villanueva del Pardillo, about 15 miles from Madrid, supposedly due to a brewery’s yeast factory.
The app used is called OdourCollect and allows users to pinpoint an odor on a map and classify it using different parameters, such as its type, which can be related to a wide range of activities. “There are many different odors. Within wastewater, there’s hydrogen sulfide, which is the typical rotten egg smell. In the industrial sector, there are many more chemical odors like those from paper mills, or the cabbage soup smell, which is caused by mercaptans (sulfur-containing organic compounds),” the chemical engineer explains. In her work with citizens across Spain, she points out that waste and wastewater treatment facilities are the most frequent sources of neighborhood complaints.
Paradoxically, despite the increase in protests across Spain against biomethane or biogas plant projects, both Cid and Arias agree that these facilities can reduce bad odors if they’re properly constructed. “I support biomethane plants if they’re done right,” the environmental chemist points out. “But I have the experience of a magnificent plant in La Galera, [a municipality in the Catalan province of] Tarragona, where the engineer didn’t take into account that this gas has to be very pure and that it was coming out through a tiny pipe. I showed that you could smell it from miles away,” he recalls.
This opinion is shared by the founder of Science for Change: “Before, what we had was a lot of diffuse biogas emissions that stank all the time, so these plants shouldn’t be a cause for alarm,” Arias points out. “If the emissions aren’t properly treated, what might happen now is that some neighbors may experience a more concentrated smell… but it would be easier to eliminate that smell than in the past.”
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