Odor sensation mechanism

Many people can detect hydrogen cyanide by odor or taste sensation at a concentration of 1 ppm in air while most people can detect 5 ppm. But HCN does not have an offensive odor, and a few people cannot smell it even at toxic levels. OSHA has set 4.7 ppm as the maximum, average safe exposure limit for a 15-minute period. Exposure to 20 ppm of HCN in air causes slight warning symptoms after several hours 50 ppm causes disturbances within an hour 100 ppm is dangerous for exposures of 30 to 60 minutes and 300 ppm can be rapidly fatal unless prompt, effective first aid is administered. A small concentration of cyanide (0.02 to 0.04 mg/L) always exists in a person s body, and the body has a mechanism for continuous removal of small amounts of cyanide129. 

It has to be remarked that in spite of the widely accepted term electronic nose, current devices are still far from the structure and functions of natural olfaction sense. The unique common feature between artificial and natural system is that both are largely based on arrays of nonselective sensors. The concept underlying electronic nose systems has been demonstrated to be independent on the particular sensor mechanism indeed during the last two decades almost all the available sensor technologies have been utilized as electronic noses. Clearly, all these sensors are very different from the natural receptors. These dissimilarities make the perception of electronic nose very different from that of natural olfaction, so that the instrumental perception of the composition of air cannot be called odor measurement because odor is the sensation of smell as perceived by human olfaction. Nonetheless, the term odor analysis with electronic noses is now largely adopted, but it is important to keep in mind, especially in medical applications, that the electronic nose measurement may be very distant from the human perception. 

Smell is a rather curious property in that its perception is thought to arise from a lock-and-key mechanism, whereby a certain molecular shape will trigger a particular smell receptor, and thus produce the sensation of a particular odor. Seen in this way, it would seem that two vastly different molecules which share the same molecular side-chain (which is required to trigger a certain smell receptor) would do so irrespective of the structure of the rest of the molecule. This view would suggest that the same smell might indeed result from different molecules having different microscopic components... 

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