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Abstract
In this study, the odour thresholds (OT) and atmospheric lifetimes (AL) were compared for a suite of volatile organic compounds. It was found that odour threshold, as determined by the triangle bag method, correlated surprisingly well with atmospheric lifetime for a given chemical family. Molecules with short atmospheric lifetimes with respect to the primary atmospheric oxidant OH tend to be more sensitively detected by the human nose. Overall the correlation of odour threshold with atmospheric lifetime was better than with mass and vapour pressure. Several outliers from the correlations for particular chemical families were examined in detail. For example, diacetyl was an outlier in the ketone dataset that fitted the trend when its more important photolysis lifetime was included; and similarly, the relatively low odour threshold of carbonyl sulfide (OCS) was interpreted in terms of uptake by vegetation. The OT/AL relationship suggests that OH rate constants can be used as a first-order estimate for odour thresholds (and vice versa). We speculate that the nose's high sensitivity to chemicals that are reactive in the air is likely an evolved rather than a learned condition. This is based on the lack of dependence on ozone in the aliphatics, that the anthropogenically emitted aromatic compounds had the worst correlation, and that OCS had a much lower than predicted OT. Finally, we use the OT/AL relationships derived to predict odour thresholds and rate constants that have not yet been determined in order to provide a test to this hypothesis.This article is part of the Theo Murphy meeting issue ‘Olfactory communication in humans’.
Highlights
Our sense of smell enables us to scan our environment for chemical signals
We present odour threshold (OT) data plotted against atmospheric lifetime (AL) for eight chemical families
By connecting the odour threshold (OT) and OH reaction rate datasets, we have revealed a strong correlation within chemical families for most of the species considered
Summary
Our sense of smell enables us to scan our environment for chemical signals. The chemical senses of smell and taste were the first to develop when life began, and even simple single-celled organisms are able to detect chemicals around them and react . A comprehensive compilation of odour thresholds for 223 compounds was published by Yoshio Nagata from work done at the Japan Environmental Sanitation Center [14] It was painstakingly acquired over 12 years using a panel of trained people, selected as their capability to detect smells represented the average for human beings (see Methods for details). If the nose is tuned or has adapted through evolution to the atmospheric removal rates of the various odorous compounds we may expect a relationship between the relative odour thresholds and the chemical removal rates—namely that short-lived molecules are detected more sensitively. This concept is shown schematically in figure 2. We combine the latest odour threshold data from Nagata [14] with atmospheric lifetimes calculated from OH rate constants available in the literature
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