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Johnny Monsarrat, Cambridge, Mass asked:
When I hear a musical note, my perception of its loudness is directly related to how loud it actually is. No matter whether it's a high note or a low note or a trumpet or a piano, something that's loud sounds loud and something that's quiet sounds quiet.
Does it work the same way with smell? When I sense a very strong rotten eggs or burning rubber, does that mean that the concentration of this odour is very strong? Or is it just that we're super-sensitive to some smells?
Greg Jefferies, Medical Research Council Laboratory of Molecular Biology:
The simple answer to that is yes and I'll try and explain a little bit about why that is.
So if you're thinking about some signal in the outside world, there are two steps probably that you could divide the process up into. The first is actually taking the signal and turning it into an electrical signal inside your body and the second is the brain then processing it and telling you about it. So we'll call the second step perception and we'll assume that you're really interested in this signal.
What's going to determine how sensitive to the signal you are is really this detection step, this sensory detection step. What you have are specialised detectors at the top of the nose. These are protein detectors and the way they actually signal a smell is that a smell consists of odour molecules diffusing about in the air. They come up to the top of the nose and bind to these protein receptors. Now they don't just bind to any old
Now over time, evolutionary time that is, we've acquired receptors that fit very well with odour molecules that we might be interested in. One example of a smell that we're very sensitive to it is mercaptan that's put in natural gas, so that we can detect a gas leak. It takes only one part per billion, even less than that, for us to actually detect that smell. So, to put that in another context, that's more or less the same as three drops of smell in the volume the size of an Olympic swimming pool, and we could detect that. Of course there are plenty of other odours to which we're much less sensitive. So the questioner is quite right that there are some smells which we are very sensitive to.
The sense of smell is essentially a differential sense and you tend to note new and increasing smells in your environment and ignore ones that are static. some smells are particularly dangerous because of this Hydrogen sulphide (rotten eggs)if it is strong rapidly disables your sense of smell and when it reaches dangerous concentrations you do not smell it. Soul Surfer, Tue, 19th Feb 2008
You can't measure smell. There's not a set amount of smell in something. It's the same with taste. There's a reason computers (as far as the public has) cannot sense differences in these things. Something can be varying degrees of PUNGENT, but that does not tell you how much smell is in it. It's roughly translatable to the difference between mass and density. They depend upon each other, but that does not mean they are equal. Professor Gaarder, Sat, 23rd Feb 2008
Smell is generally measured by finding the lowest concentration of it in (generally moist) air. Lots of measurements of this have been made- they get called "odour thresholds".
but there's no actual way for a computer to sense it, right?
"Professor", I see you seldom seem to be in any doubt, but quite often wrong. Are you a teenager?
On the contrary, I often phrase things in a question. If I am right, I help. If I am wrong, I learn.
Don't you need to get elected to a professorship before you are entitled to call yourself professor? Anyway, I'm quite good at ignoring teanagers; how did you expect to be dealt with? Bored chemist, Tue, 26th Feb 2008
Woah, that's a huge step down from the other topic.
Also, I highly doubt there will ever be another Professor Gaarder due to the rarity of the last name. Within the 100 mile vicinity of my house, disregarding relatives of mine, I am the only Gaarder. Professor Gaarder, Wed, 27th Feb 2008