We have seen the benefits of olfactometry in our blog post Measurement of odor emissions – Olfactometry or chemical analysis?
In general, it is difficult to use the chemical analysis method for mixtures of odorous compounds due to the phenomena of synergy, inhibition and masking between different compounds (Gostelow et al., 2003).Complex mixtures, such as environmental air samples, contain many odorous compounds, generally at very low concentrations (Gostelow et al., 2001) (Schiffman et al., 2001) (Parker et al., 2002) (Filipy et al., 2006). To analyze all the odorous compounds that are present, the composition of the sample must be known in advance, and the detection limits of the chemical analysis devices are often too high to identify and quantify all these odorous compounds (Gostelow et al., 2003). Finally, the olfactory perception threshold values are not always available in the existing literature, the values reported vary by several orders of magnitude (AIHA, 1989) (US EPA, 1992), and the available references are not recent.
The effects of synergy and masking between different odorous compounds can be observed in samples. For example, in a sample of food odor, the volatile compounds were identified and regrouped in five key odorous families. This was done to study the effect on odor resulting from different combinations of the five groups of compounds (Hallier et al., 2004). Synergy and masking effects were thus observed.
Numerous researchers have studied odorous mixtures and have created models to predict the effect that the mixtures’ composition has on the perceived odor (composition and concentration) (Gostelow et al., 2003). In general, the use of these models is limited and applies only to the experimental conditions of the study. As well, the mixtures of compounds are mostly studied in the laboratory because of the complexity of mixed odors.
Studies have identified dominant odorous compounds in environmental samples. For example, a positive relation can be established between the odor concentration determined by olfactometry and the odor principle identified in the odor samples of liquid hog manure (Hobbs et al, 2000) and odor samples of composting mushrooms (Noble et al., 2001). However, these studies also show that a relation between the mixture composition and the odor concentration is still misunderstood and difficult to predict. For wastewater treatment processes, where H2S is the predominant odor, Gostelow and Parsons (2000, from Stuetz and Frechen 2001) show the values of r2 between the H2S and the odor concentrations to be as low as 7 to 69%.
The American Industrial Hygiene Association (AIHA, 1989) compiled numerous studies and established a critical analysis of odor threshold values. The AIHA document is a recognized reference today and is often used as a source for odor threshold values. The scale of acceptable odor threshold values was established for H2S from 0,001 ppmv to 0,130 ppmv (1 µg/m3 to 181 µg/m3). The recommended value held by the AIHA (1989) is 0,0094 ppmv (13 µg/m3). H2S is a well-studied odorous compound and yet the AIHA proposes a scale of values for the threshold of two orders of magnitude, after their critical review. The example of H2S illustrates why it is often inappropriate to work with odor threshold values because reliable values are not always readily available. New studies with dynamic dilution olfactometers shows 0.0004 pmmv as perception threshold values.
Olfactometry generates standard sensory analyses, and the principal tool to measure odor characteristics is a trained jury of “noses” or a group of selected experts chosen according to rigorous and precise criteria. An olfactometer is a device designed to dilute the odorous gas samples and to present these dilutions to the jury. After obtaining the responses of the jury, a statistical treatment of the data permits the olfactometric result to be calculated.
Olfactometric analyses are tested in the laboratory (EN 13725 and ASTM E679-04) or in the field during which the odor samples are gathered and then exposed to the target population in the study area. However, olfactometric analyses of ambient air in the field are not recommended because of frequent variations of odor concentrations in ambient air and the low resolution of these methods.
In England, the Environmental Agency published a guide on the measure of H2S and the reduced sulphur totals at the source of ambient air (Environment Agency, 2001). This guide recommends that the measuring strategy be directly related to the objective of the measurement study. Thus, if the objective establishes the required abatement to eliminate the nuisance odor, it is specified in the guide that the odor concentration measurements expressed in odor units per cubic meter (o.u./m3) are more appropriate than the kind obtained through chemical measurement.
The main advantage of olfactometry is the direct correlation between the odor and the sensitivity of the detector used, i.e. the human nose.
Despite the advantages of the classic analytical methods (accuracy, reproducibility, etc.), olfactometry remains the best available approach to measure odors directly, in order to objectively quantify the perception of odors.