It is possible for mixtures of gases to be less odorous than any of their constituents. Because machines cannot replicate this masking phenomenon, odour quantification requires a reliable human nose. An olfactometer measures the amount of dilution with de-odorised air needed to render an odorous sample barely detectable. Because odour sensitivity varies enormously between individuals, each observer must be calibrated with reference to a standard, for which the population's mean sensitivity is now proposed.
Hydrogen sulphide is a common constituent of sewage air, but until now its correlation with sewage odour has proved somewhat elusive. Research on real sewage odours at two sites has now demonstrated that such cor1'elations are quantifiable. These correlations reveal that the H2S in sewage air is naturally odour-masked. Such masking of odours from toxic gases is considered dangerous in certain circumstances.