Science Questions

Size of Smell Sense

Sun, 24th Feb 2008

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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.

The Human NoseSo 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
receptor. There's sort of a lock and key fit between the smelly molecule and the receptor. So if you don't have receptors which can fit with a particular molecule then you won't smell it.

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.


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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".
Here is Wiki's take on the concept."Quantifying olfaction in industry

Nasal Ranger in use.Scientists have devised methods for quantifying the intensity of odors, particularly for the purpose of analyzing unpleasant or objectionable odors released by an industrial source into a community. Since the 1800s, industrial countries have encountered incidents where proximity of an industrial source or landfill produced adverse reactions to nearby residents regarding airborne odor. The basic theory of odor analysis is to measure what extent of dilution with "pure" air is required before the sample in question is rendered indistinguishable from the "pure" or reference standard. Since each person perceives odor differently, an "odor panel" composed of several different people is assembled, each sniffing the same sample of diluted specimen air. A field olfactometer can be utilized to determine the magnitude of an odor. One example is the Nasal Ranger® field olfactometer, which is often utilized in odor studies. "

Of course, like many biological measurements they differ from one person to the next so, for example, I can barely percieve the smell of acetonitrile, but some of my coleagues think it stinks.
In much the same way the strength of chilli peppers etc is measured in Scovile units
Bored chemist, Sun, 24th Feb 2008

but there's no actual way for a computer to sense it, right?

maybe I'm on another track altogether, here. Professor Gaarder, Mon, 25th Feb 2008

"Professor", I see you seldom seem to be in any doubt, but quite often wrong. Are you a teenager? Bored chemist, Mon, 25th Feb 2008

On the contrary, I often phrase things in a question. If I am right, I help. If I am wrong, I learn.

Also, Professor is only in my username so that in 8 years when I have my degree in science, I can still have this account.

So yes, I am a teenager. But one very interested in chemistry, and one who would like to learn if people would just understand that you should probably ignore my compulsions and get to the point.

Don't mean to be demanding, but for your sake I am warning you that is prettymuch the only way to deal with me. Professor Gaarder, Tue, 26th Feb 2008

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.

I expected that you would assume that my intentions were good despite my apparent lack of bits and pieces of vital information. Forgive me if I am a bit detatched from modern technology.

Also, I do not appreciate being refered to as a "teen". In fifth grade I read at an eleventh grade reading level, and right now I am taking algebra a year early.

Also, unlike most teens, 99% of my online grammar is sufficient. Professor Gaarder, Wed, 27th Feb 2008

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

Although the question regarding the sense of smell was already answered I want to add one thing that I think to be important here: The statement made in the question about the acoustic sense is quite a misconception. Well "the loudness is related to how loud something is" is so tautological that is has to be true but what Mr Monsarrat probably actually meant was that the intensity of the perception of a stimulus (what's called the loudness of a sound) is linearly correlated to the intensity of the physical stimulus itself (called the sound pressure) and independent from other sound properties such as pitch or timbre. This couldn't be less true though. First of all both loudness and sound pressure are measured on logarithmic scales which means doubling the actual energy i.e. the sound pressure level of a given sound only increases its value measured in dB by 3 regardless of the sound pressure level you're doubling. The perceived loudness is often measured in phon. At 1 kHz 1 dBSPL equals 1 phon and 100 dBSPL equals 100 phon. This still doesn't mean that doubling the phon value means doubling the perceived loudness. For example an 80 phon sound is perceived to be 16 times louder than a 40 phon sound because the perceived loudness is more or less doubled for every increase by 10 of the phon (or dB) value. There is yet another unit called sone which takes care of the exact mapping from phon to perceived loudness because there's a nonlinear relation between these two, too, especially for more silent sounds. To make things worse the dB/loudness mapping varies hugely depending on frequency. So while a 70 dBSPL sound at 20 Hz has a barely audible 3 phon (actually the absolute threshold of hearing is at 3 phon), the same energy at 4 kHz which is the frequency we are most sensitive to has about 80 phon which corresponds to the noise of standing 10 metres from a road with heavy traffic. Finally there are still several other factors which influence the perceived loudness of a sound such as your age or whether you're tired or not.

So long story short: Yes, whether you hear a low note or a high one does have a very large influence on the perceived loudness indeed.

For a diagram showing the relation between loudness and sound pressure level for the whole range of frequencies humans are able to hear have a look at Remember the scale is logarithmic already. mudd1, Wed, 27th Feb 2008

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