Odor Strength – The Forgotten Odor Parameter

After decades of applied olfactometry it is time that a fundamentally forgotten odor parameter of Intensity be highlighted and placed into the forefront of practice. The distinction needs to be made between odor concentration and odor intensity. The terminology difference is not just a minor linguistic matter: Odor Concentration (e.g. odor units) versus Odor Intensity. Further, there needs to be clarification between the stimulus of odorant(s) concentration and the effect, which is odor sensation.Scientists and engineers in environmental protection started to quantify the properties of odor in order to avoid odor nuisance near emission sources. Methods that were developed became standards, i.e. ASTM E679 and EN 13725, and became the acknowledged basis of “odor” quantification with trained sensory assessors (a.k.a. panelists or judges).The “problem” becomes obvious with odor abatement limits set as odor concentration without regard for the odor intensity of the limits that are set. In terms of olfactometry as applied in Europe (Weber-Fechner), I is the odor intensity (strength of odor sensation) and c and c0 are the odorants concentration or the threshold concentration of odorants, respectively.I = kW . lg (c/c0) (1)The coefficient kW is a measure of the slope of the curve (function) depicting the typical and individual relationship between odor intensity and odorant concentration of a defined source or production cycle (e.g. olfactometric fingerprint of an emission). It emphasizes the relationship between stimulus (odorants) and effect (odor intensity) with the odorant index as a mathematical link to avoid misinterpretation of olfactometric results and limit values.The knowledge of this background requires not only revision of the interpretation of odorants abatement (commonly called odor abatement), but also of the necessary degree of gas cleaning efficiency to meet any emission limits and their results. The sections of this manuscript serve to become aware of the difference between stimuli (concentration) and effects (intensity) and the importance to distinguish them in terminology.