Appendix 6: Odour Thresholds for Individual Chemicals
The odour threshold is the concentration of an odorous compound at which it is noticeable to the human nose. Odour threshold data are useful in assessing effects for odour emissions, particularly in situations where one compound is known to predominate the odour effect (i.e. there are no synergystic effects with other compounds). Using odour threshold data has the advantage over odour unit measurements because chemical concentration measurements can usually be more readily carried out or can be calculated by mass balance. In some situations the expense of olfactometry may not be justified, and using odour threshold data for individual compounds may be the only option.
Both detection and certainty or recognition odour thresholds for compounds are reported in the literature. The detection threshold is the lowest concentration of a compound that can just be detected by a certain percentage of the population, while the certainty or recognition threshold is the lowest concentration of a compound that can be recognised with certainty as having a characteristic odour quality. In general, recognition thresholds are approximately three to five times the detection threshold. When using odour threshold data it is important to be clear about which type of threshold is being reported.
Odour threshold data should be used with caution because many different methods have been used and there is a wide variation reported in the literature, often by four orders of magnitude. As an example, when using dilution dynamic olfactometry methods the odour threshold is usually taken as the value at which 50 percent of the panel are able to detect or recognise the odour, but some historical data are based on a range of different percentages. Most odour threshold reference data available appear to have been developed before dilution dynamic olfactometry was standardised, so the data may not be directly applicable to assessments where odour guidelines have been developed based on the standard olfactometry techniques.
Some useful references for odour threshold data are discussed below.
- Nagy (1991) undertook work sponsored by the Air Resources Board of the Ontario Ministry for the Environment. Forced-choice dynamic olfactometry was used to determine the 50% detection levels for 86 pure compounds as µg/m3 using a nine-member panel.
- The American Industrial Hygiene Association (AIHA) published odour thresholds for 102 compounds in 1989. The AHIA reference does not incorporate any odour threshold data that are more recent than the 1980s, even though it was last published in 1997, and a lot of the data they rely on are much older. The review is, however, a critical one. Of 191 primary sources, 155 references were excluded as unacceptable, and this remains one of the preferred sources of odour threshold data.
- Van Gemert (1999) is a compilation reference based on literature values of odour threshold concentrations incorporating studies since 1977. The most recent study incorporated prior to this review was Devos et al (1990). This latest reference is essentially an update of the earlier compilations. No attempt is made to critically evaluate the data, but data are given chronologically for each compound with the original data source identified. More than 1100 compounds with one or more odour threshold references are reported.
