SMELL--THRESHOLD CONCENTRATION 41 SMELL--THRESHOLD CONCENTRATION A. W. MIDDLETON, B.Sc., Ph.D., F.R.I.C.* IN ^N earlier paper • in which the author developed the thesis that smell is a physical sense--primarily a matter of adsorption of olfactory molecules on a surface possibly of protein matter--he showed that the vapour pressure of the substance divided by its saturated vapour pressure under the same conditions of temperature and pressure is the proper measure of the stimulus applied to the olfactory receptor. There have been published in the literature the results of various determinations of olfactory thresholds which have been expressed as concentrations of odoriferous substance. It was considered worth while to recalculate those accessible to the writer for which the saturated vapour pressure at 32 ø C. can be calculated or extrapolated from published values: the recaiculated values are listed in Table I. The substances in that table have been arranged in order of decreasing activity. It is seen that the hydrosulphide and aidehyde radicles pre- dominate among the more active substances (although this might be a consequence of the high proportion of substances containing these radicles for which data are available). These radicles are among those which have a high affinity for protein molecules and which would be expected to have high heats of adsorption on them. The values lend some support to the theory of the adsorption being one of the gas on to the protein terminal hairs on the end vesicles of the olfactory nerve fibres. It is known that such proteins have a helical structure which might well uncurl to some extent when external molecules are adsorbed on to them. In the same way that the basis of the auditory end organs move to alter the tension of the membrane of their nerve cells and, consequently, their permeability to sodium and potassium ions, so might the uncurling of the protein helix of the olfactory hairs disturb the membrane permeability of their attached cells. However, this is purely speculation, and a matter not likely to be decided for some considerable time. Holway et al. • have determined the response of four subjects to five equally separated stimuli, using the right nostril alone, the left nostril alone, and both nostrils together. Each subject stated forty times whether the odour could be smelled or not at each concentration after inhaling for five seconds, and at two minute intervals between tests. They plot their results to show that the relation between the frequency of response to the intensity of stimuli is a Gaussian one but their results fit a Poisson distri- bution equally well, more results being needed in the intensity range 0 to 30 per cent and 70 to 100 per cent. * Chesebrough-Ponds Ltd., London, N.W. 10. ,

42 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE I Substance P/P8 Source of Threshold Data Ethyl mercaptan ...... 0.9 X 10 -? 4 Propionaldehyde ...... 1-4 ,, 4 Phenyl isocyanide ...... 1.7 ,. 4 Methyl mercaptan ...... 2.5 ,, 4 Propyl mercaptan ...... 8'7 4 Acetaldehyde ...... 1.5 X 1'(• -6 4 Carbon disulphide ...... 1.6 ,, 4 Trans dichlor-ethylene .... 2-2 ,, 4 Diethyl sulphide ...... 6- 7 ,, 4 HCN ........ 7.7 4 Isoamyl acetate ...... 9.8 4 2-Chlorophenol ...... 1.0 X i• -5 4 Pyridine ........ 3.6 ,, 4 Butyric acid ........ 2-2 ,, 5 Diphenyl ether ...... 4.5 ,, 4 Allyl isothiocyanate ...... 5.1 ,, 4 Isoamyl isovalerate ...... 6.1 ,, 4 Camphor ...... 6.25 3 ß . Methyl thiocyanate ...... 1.6 X 1'(• -• 4 Allyl alcohol ...... 1.7 4 Dichloro-ethyl-sulphide .... 2.8 ,, 4 Benzaldehyde ...... 2.9 ,, 4 Pyridine ........ 3.7 ,, 6 Phenyl isothiocyanate .... 8.5 4 Camphor ...... 1.3 X 1'(• -a 3 ß . Diphenyl sulphide ...... 1.4 ,, 4 Methyl enthranilate ...... 1.7 ,, 4 Nitrobenzene ...... 4.7 ,, 4 Skatole ........ 5.0 ,, 4 Quinoline ........ 5-0 ,, 3 Coumarin ........ 8.9 ,, 4 Quinoline ........ 3.4 x I0 -2 3 Methyl salicylate ...... 3.6 ,, 4 Methyl salicylate ...... 9.3 ,, 3 All vapour pressure at saturation data has been calculated from Jordan, T. Earl, "Vapour Pressure of Organic Compounds," l•terscience, 1954. Gundlach 8 quotes a range of threshold concentrations among different subjects for quinoline, camphor and methyl salicylate, giving the percentage of his subjects perceiving the odour at the various thresholds. The two values given in Table I for each of these substances are those calculated from the lowest concentration which he gives for perception by some of his subjects and from the highest concentration at which not all of his subjects could smell the substance (the threshold for some of them was above this concentration). Unfortunately, his results are not sufficiently numerous to indicate whether the frequency distribution of thresholds is a Poisson or Gaussian one.