Odour, how to measure?

As Aerox we offer our clients sustainable solutions for industrial odour emissions. Together with our customers we solve the nuisance resulting from these emissions. We therefore also regularly get questions about odour, what exactly this is and how you can measure it.

Measuring of smell

The objective measuring of odour is not easy. Odour in itself is a very complex matter to quantify and qualify. In addition, the degree of odour that we experience is depending on the individual. The values we assign to the vast majority of odours are taught. However not every appreciation of odours is culturally determined: the primary purpose of odour is shedding or attracting. Our limbic system responds to questions like "Is this dangerous?" or "Is this edible?". Certain odors directly trigger a defensive response and that's a good thing. After all it can be an odour that is associated with something that is a direct danger. Other odours do just the opposite and evoke positive responses. The experience of the vast majority of all odors however depends on what one has learned and how something is labeled.

From biological, chemical and psychological point of view, odour and our perception of it is a very interesting field. Only fairly recently much research is done to the subject. Because of being a subjective topic, the hedonic value is the criterion most commonly used for odor measurements. This scale runs from-4 to + 4, in which -4 stands for unbearable and + 4 for very pleasant. A hedonic value of zero stands for neutral. This scale is used by governments and industries to determine the odour problem. It is also used to determine if the extent of the odour (the smell concentration) is still acceptable, where for an odour with a low hedonic value (more stench) a lower odor concentration is permissible.
 
In addition to the hedonic value of an odour, it is important to take the sensitivity to a odourous substance and its concentration into account. The sensitivity for an odour is expressed in a threshold value: 50% of that concentration at which a representative group of people perceive the odour. This concentration is usually expressed in the number of odour units per volume unit. The value of the odour concentration, expressed in odour units per m3 air (ouE/m3), is the number of times the odour in the air is to be diluted with odourless air to reach the threshold. 1 ouE/m3 is, by definition, the odour threshold as determined by a panel of 4-6 selected persons who statistically smell the same odour. The methodology for measuring odour concentrations is laid down in European standard EN 13725.

Odour

Odour nuisance is the cumulative result of repeated disruptions by odour concentrations. The peak concentrations usually determine the extent to which people experience discomfort, whether or not affected by odour habituation. This discomfort is often reflected in changed behavior. This modified behavior can manifest itself active (e.g. complain, close windows, less sitting in the garden, no more inviting acquaintances) or passively (e.g. through different negative signals in survey responses). Bad odors are usually only annoying, but can also affect: one can feel less pleasant, the other gets headaches or feels nauseated. Odour nuisance can hence lead to both physical and mental deterioration of the welfare (health effects and negative experience). The extent to which depends, among other things, on a person's sensitivity to smell, which for every human is unique.

Many factors contribute to the perception of odour nuisance: a low hedonic value, a high frequency of exposure, the local situation, a problem-oriented communication style, a negative attitude towards the source or the government or the fear of increase of the odor concentration and its effects for welfare and health. In fact the local nuisance can be actually greater than that could be calculated on the basis of the general relationship between odor concentration and odor nuisance. With this we often have very complex situations in which objectification and cooperation are often the only tools available.

As Aerox we always focus on this objectification and cooperation. The extra investment in time always delivers great benefits later in the process. The final result of the joint efforts is that the surrounding areas of our customers worldwide are ensured of fresh air and thus quality of life.

Erik C. Versteeg, Managing Director, Aerox B.V., 12 sept 2017