Tracking down unpleasant odors

Research in focus August 2013

The freshly renovated apartment forms part of a condominium development built on a prime site in the south of Germany, and sold at a correspondingly high price. But the new owner complains of a strange, fishy smell permeating the apartment – and yet there is no restaurant in the building and no fishmonger around the corner. Even after numerous attempts to banish the unpleasant odor by airing the rooms thoroughly, the problem persists, rendering the apartment uninhabitable at first. This type of problem is not unfamiliar to the scientists at the Fraunhofer Institute for Building Physics IBP. “We receive frequent requests to identify the source of offensive smells in buildings and to provide advice on how to eliminate them,” reports Dr. Andrea Burdack-Freitag, a scientific project manager in the institute’s Working Group Chemistry and Sensory. Among other things, she and her team investigate sources of emissions in many kinds of building components and the methods used to process them. They analyze the composition of indoor air and the odor characteristics of construction materials, technical materials and raw materials, and evaluate this information with a view to minimizing offensive odors. The group also develops new analysis technologies and intelligent air quality sensors that can be used to control ventilation systems.

In the case of the above-mentioned apartment, an independent expert was appointed to carry out an emissions analysis. The techniques employed are the same as those used by the Fraunhofer IBP researchers in such situations. First, a sample of indoor air is collected in a small glass tube using a pump. The tube contains a polymer material, which has the advantage of possessing a large surface area that enables it to capture volatile organic compounds present in the air. Back in the laboratory, the sample is then analyzed in a gas chromatograph (GC) linked to a mass spectrometer (MS), after being heated to release the substances bound to the polymer in their original gaseous form. The analysis is performed according to the standard method defined in DIN/ISO 16000-6. The results are evaluated according to the recommendations of the Indoor Air Hygiene Commission (IRK) of the German Federal Environment Agency (Umweltbundesamt), which specify guideline values for concentrations above which certain indoor air contaminants represent a threat or even a serious risk to health and hygiene. In this particular case, the emission analysis did not reveal anything abnormal. Subsequent attempts to eliminate the smell by fitting airtight seals to all wiring and ventilation ducts made no difference. Drying out the apartment and then airing it intensively didn’t bring any improvement. In fact, this measure was counterproductive because heating the place made the fishy smell even stronger.

Human noses to the rescue
“We often encounter cases like this,” comments chemist Andrea Burdack-Freitag. “There are substances that analytical instruments are unable to detect but that nevertheless produce an unpleasant olfactory effect.” To localize the source of these unidentified emissions, the scientists start by carrying out what they call a sensory on-site inspection. “This approach is used for the simple reason that it gives us more scope for analytical testing,” explains the Fraunhofer research scientist. In the first instance, all building components are tested to identify any that emit unpleasant odors. Samples are obtained by rubbing a hand over each surface. Protective vinyl gloves must be worn to avoid contamination by hand perspiration. Scientists then spray distilled water onto the various materials and heat them using a hot air dryer. These are all suitable non-destructive methods of releasing volatile organic compounds and intensifying their olfactory effect so that they can be more easily detected. The results of this sensory inspection of the apartment in question revealed that the fishy smell emanated strongly from both the floor and walls. Certain scientists at Fraunhofer IBP have developed the capacity to differentiate aromas on the basis of their intensity and specific qualities, even after long exposure, either as a result of long experience and training or because they have an inborn talent. This sets them apart from the average person, who soon becomes accustomed to familiar smells and loses their ability to perceive them. An estimated five percent of German citizens can be classified as anosmics, which means that they have lost their sense of smell or are unable to differentiate between individual odors. But Andrea Burdack-Freitag belongs to the category of people whose olfactory perception remains unaltered, and is thus naturally able to differentiate the components of different smells. An untrained nose can usually identify one or two aromatic compounds, whereas a trained nose can often identify four or five.
Database of smells for training sensory-test volunteers
The chemists who work for Fraunhofer IBP spend a lot of time training their volunteer testers; having a “good nose for smells” is just one of the qualities required when it comes to differentiating between various flavours and odors in a product. Without specific training, testers are rarely able to identify more than a few substances on the basis of their smell, and still less provide a detailed description of the various components. “Untrained testers tend to evaluate odors on the basis of an emotional response evoked by memories of past situations in their personal lives: hence statements such as “it smells like a church” or “it reminds me of my grandmother’s kitchen”. It is very time-consuming and sometimes even impossible to fit such statements into the framework of an odor profile,” says Burdack-Freitag. To solve this problem, she limits the group of volunteers selected to take part in these tests to a core team of 30 participants who take part in regular training sessions that enable them to describe odors according to a defined vocabulary, including terms such as fruity, earthy, malty, or fishy. The trained panel of odor testers is required to meet regularly to take part in special exercises in which they learn to identify specific odors, classify them according to a standard scale of reference, and describe their characteristics in technical terms.
Identifying and removing sources of odors
The sensor experts at Fraunhofer IBP were quickly able to identify the floor covering, or more precisely the screed on which it was laid, as the source of the unpleasant odors in the above-mentioned apartment. They began by conducting a sensory analysis, using a technique that combines the use of conventional analytical instruments with human perceptual skills. Investigations are performed in the laboratory, where a researcher puts her nose to the analytical instrument. Since scrapings of wall plaster didn’t seem to be the source of the odor, and strong traces of amines were found in the air samples, the most likely suspect was thought to be the floor or the underlying screed. This hypothesis was confirmed by gas chromatography/mass spectrometry (GC-MS) with an olfactory port (sniffing port). The fishy smell was undeniably caused by the amine additives in the screed, in particular the triethanolamine used as a milling surfactant and its breakdown products. The walls were identified as a secondary contamination source. Most likely not enough time was allowed for the screed to set before the floor covering was laid. The apartment owner was advised to remove the primary source of contamination by removing and replacing all of the flooring, and then sand down all of the walls and repaint them. After that, the apartment would be perfectly habitable.

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