Re: Why does metal smell?

From: Mark Fergerson (nunya_at_biz.ness)
Date: 02/18/05 

Date: Fri, 18 Feb 2005 08:56:07 -0700

Dave P. wrote:
> On Tue, 15 Feb 2005 16:42:43 -0700, Mark Fergerson <nunya@biz.ness>
> wrote:

>>Ian Stirling wrote:
>>
>>>The vapour pressure of the metal is obviously bugger-all, at room
>>>temp.
>>>Why do things smell metallic?
>>
>> Lots of interesting responses to this question.
>>
>> No, I don't have any better answers, but I do have another question.
>>
>> _Why_ can we smell metals?
>>
>> Our sense of smell is said to be our most ancient, so you'd think
>>that the ability to smell metals must have some evolutionary value. But
>>pure metals are rare absent technology (except for frinst Cu and Au).
>>
>> So, is our ability to smell metals really an evolutionary "feature",
>>or a "bug" resulting from a fortuitous sensitivity in our receptors that
>>the "odor of metals" just happens to match?
>>
>> Off the top of my head I can think of only two "natural" things with
>>a smell commonly characterized as metallic; blood, and water so
>>mineralized it isn't safe to drink.

> Humans don't selectively smell... we smell everything that has an
> odour.

   Everything "that has an odour" may evoke a response in our olfactory
bulbs, but that doesn't necessarily mean that we are aware that we
smelled it. Humans are pathetically smell-blind compared to most other
animals.

   I specifically mentioned things with characterizable odors, which
excludes the odors that influence us beneath our awareness.

> Evolution only affects the sensitivity of our non selective
> ability to smell (unless there are some evolutionary connections to
> taste as well, which there might be... I know you can't taste well
> without your sense of smell, but I don't know if the reverse is true).

   Last things first; me either. But I'm not sure what the first part
was intended to mean; do you think we still have the full complement of
odor-sensitivity all our ancestors had? We indeed may, but the
weightings sure have changed.

> Your query would only be a brainteaser (in my opinion) if metal was
> the only thing humans could smell.

   Some Googling found this:

http://www.news.uiuc.edu/scitips/03/0221olfactory.html

   which sheds some light on the matter. Apparently the actual proteins
that do the detecting of a given odor are metalloproteins that are not
only particularly sensitive to molecules that like to bind to metal
ions, but also to zinc and copper (doesn't say if other metals have
similar effect). This seems to support my "bug, not a feature" guess.

   Note that we can apparently detect _single atoms_ of copper and zinc.
So much for the "bugger all vapor pressure" debate.

   Note also that it ascribes our ability to smell nearly 10K distinct
smells to our possession of 1K specific genes that apparently code for
specific metalloprotein molecules in our smell receptors.

   OTOH this:

http://www.hhmi.org/news/buck.html

   suggests a composite nature of specific smells, rather like the
four-component (sweet/salt/sour/bitter) sense of taste, except with
vastly more components. So, we may have inherited the ability to detect
metals because our (single-celled) ancestors needed trace metals just to
stay alive.

   Maybe it isn't an either/or situation; the ability to smell metals is
a "bug" that had solid survival value. I kinda like the cognitive
dissonance aspect.

   Mark L. Fergerson

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