Abstract
The complexity of the odours issue arises from the sensory nature of smell. From the evolutionary point of view olfaction is one of the oldest senses, allowing for seeking food, recognizing danger or communication: human olfaction is a protective sense as it allows the detection of potential illnesses or infections by taking into account the odour pleasantness/unpleasantness. Odours are mixtures of light and small molecules that, coming in contact with various human sensory systems, also at very low concentrations in the inhaled air, are able to stimulate an anatomical response: the experienced perception is the odour. Odour assessment is a key point in some industrial production processes (i.e., food, beverages, etc.) and it is acquiring steady importance in unusual technological fields (i.e., indoor air quality); this issue mainly concerns the environmental impact of various industrial activities (i.e., tanneries, refineries, slaughterhouses, distilleries, civil and industrial wastewater treatment plants, landfills and composting plants) as sources of olfactory nuisances, the top air pollution complaint. Although the human olfactory system is still regarded as the most important and effective “analytical instrument” for odour evaluation, the demand for more objective analytical methods, along with the discovery of materials with chemo-electronic properties, has boosted the development of sensor-based machine olfaction potentially imitating the biological system. This review examines the state of the art of both human and instrumental sensing currently used for the detection of odours. The olfactometric techniques employing a panel of trained experts are discussed and the strong and weak points of odour assessment through human detection are highlighted. The main features and the working principles of modern electronic noses (E-Noses) are then described, focusing on their better performances for environmental analysis. Odour emission monitoring carried out through both the techniques is finally reviewed in order to show the complementary responses of human and instrumental sensing.
Highlights
In the last decade great attention has been paid to the issue of air quality as it directly affects both the environmental and human health
Chemicals transported by the inhaled air are trapped and dissolved into the olfactory epithelium, a small region of both nasal cavities where odorants stimulate an electrical response of the olfactory nerves: the olfactory signal is transmitted to the brain, where the final perceived odour results from a series of neural computations
An electronic nose based on conducting polymer sensors, has been widely applied in the analysis of odour samples from swine manure sources coupled with a NH3 and a H2S sensor [163,164] and as an alternative to sensory analysis for assessing the effectiveness of biofilters, showing good correlation with odour concentrations [165]; together with olfactometry and gas chromatography to analyze indoor air from swine finishing facilities, instead, the correlation between Gas Chromatography coupled with Mass Spectrometry (GC/mass spectrometer (MS)) analyses and Electronic Noses (E-Noses) was found better than between E-Nose and olfactometry
Summary
In the last decade great attention has been paid to the issue of air quality as it directly affects both the environmental and human health. As occurring in the trade industry (i.e., food, beverages, perfumes, etc.) for many years, the sensory evaluation of smells by means of panels of sensory trained evaluators has been the main odour assessment and quantification tool: the so-called dynamic olfactometry is the standardized method used for determining the concentration of odours and evaluating odour complaints [36,37] This methodology is based on the use of a dilution instrument, called olfactometer, which presents the odour sample diluted with odour-free air at precise ratios, to a panel of human assessors. Papers comparing the performances of both techniques are reviewed in order to show the complementary responses of human and instrumental detection
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