Keith Howell wrote:

> I'm curious as to why anyone would wish to grow anything
> hydroponically in a country with the climate of a greenhouse.  Most of
> the population is coastal and water is not _that_ scarce.

  Arguably so.

> Hmm... call me Mr. Cheeky if you like, but you're not trying to set up
> a cannabis growing establishment are you? I've seen ads for
> hydroponics kits, usually selling halide lamps, etc. Somebody's got to
> want a plant really badly if they want to force-feed it light and
> carbon dioxide.

  I have to confess to a bit of a giggle when I see those huge mercury-
vapour lamps and hydroponics stuff at garage sales.  My other *big*
giggle is people trying to sell Kodak "Colourburst" instant cameras for
money!

  As to CO2, well I *hope* I know what I'm talking about.  You may
exhale 5% CO2, but that's because your body is trying to get RID of it,
and the 5% parallels the amount in your blood.  That is to say CO2 in
exhaled breath is at equilibrium with the blood, breathing is
deliberately controlled in rate and tidal volume to remove only as much
as necessary.

  If you were to breathe IN 10% CO2, you'd have to breathe out 15% to
get rid of the same amount, wouldn't you, and because what you breathe
out cannot be less than the amount in your blood (whilever your lungs
are working, that is!), that'd mean you had THREE TIMES as much as you
should in your bloodstream.

  Now, in fact, the body *does* try to control the amount of CO2 in the
blood fairly carefully.  Normal is 36-44 torr, out of one atsmosphere =
760 torr, so 5% is pretty close.  A person with severe chronic
respiratory failure might just head toward half as much again, and yes,
they do adapt over time but they'd be pretty snoozy.  The feeling of
asphyxia is more closely related to oxygen lack, but if *you* breathe 5
to 7% CO2 even on high O2, you will feel pretty crook pretty fast.  More
than that is just not on!

  All this is at sea level, and equivalent partial pressures at lowered
atmospheric pressure of course apply.  Spacecraft tend to use pure
oxygen at 1/5 atmosphere and remove as much CO2 as possible (I seem to
recall a major problem on MIR some time ago - sleepy cosmonauts!).

  Cheers,
        Paul B.