On Mon, 25 Jul 2005 17:30:15 +0000 (UTC), "vsop"
wrote:

My new neighbour has relocated a row of Raspberries to up against the wall
of her garage so they can be watered by rain dripping off from the
roof......(No gutters on the roof)

However, the roof is asbestos and when I pointed out that there might be a
danger from asbestos being taken up by the plants, either thro the roots,
leaves or fruit, she was quite certain that it was not possible for asbestos
to be absorbed in this way and there was no danger to her family eating the
fruit in future years.

She may be right, I don't know, perhaps I'm being over concerned and am
totally wrong, but I certainly wouldn't take the risk and I'm very worried
that she's placing her family in danger from ingesting cancer causing
toxins.

Whets the general opinion on this ?

Your neighbour is right. It's the physical presence of asbestos
particles that causes cancer. Asbestos minerals (and there are several
types, see below), have a fibrous structure. The individual crystals
grow in the form of long thin filaments. When these are ingested, most
commonly as dust into the lungs, the body attempts to get rid of them
by enclosing them in a scavenger cell (a macrophage IIRC) and then
dissolving them. Asbestos fibres are too long to be enclosed
completely, and persist. The repeated but unsuccessful attempts of the
body to get rid of them eventually result in the formation of a tumor.

But a lot depends on the type of asbestos. The general impression
given is that it's all the same stuff, which it isn't. There are two
types commonly used by industry and met with by the general public:
white asbestos (the mineral chrysotile) and blue asbestos (the mineral
crocidolite, aka fibrous riebeckite). They are entirely different
minerals, with different chemical properties and crystalline
structures. The only thing they really have in common is their fibrous
nature.

Most asbestos-cement products made since the 1960's contain white
asbestos only, at about the 10% level IIRC. White asbestos dissolves
slowly in acid, which means it can and may dissolve in the stomach
(but whether it remains there for long enough, I don't know). The
resulting products are harmless (and also occur in many pharmaceutical
preparations). It can also dissolve in the lungs, although much more
slowly than in the stomach as the acidity is very much less. Residence
times in the lungs are in the order of 6 to 12 months, I believe.
Consequently, white asbestos is less of a hazard than other types
because it can dissolve and doesn't persist for long enough for tumors
to form. However, prolonged exposure to high concentrations can result
in cancers. The asbestos deposits in Quebec are of this type, and
claims by the workers for compensation nearly bankrupted parts Lloyds
insurance a couple of decades ago.

OTOH the really nasty stuff is blue asbestos, extensively used in
thermal insulation and fire-proof fabric because of its longer
filaments. It is not readily dissolved in acid and will persist e.g.
in the lungs for many years, eventually causing tumors to form. But it
takes a long time. Anything ingested by mouth moves quite rapidly
through the intestines and is excreted within a few days, so I'd be
surprised if even blue asbestos caused stomach or intestinal cancers.
If it has been shown conclusively that blue asbestos does cause such
cancers, it must surely be only after exposure to high concentrations
over a long period.

Cement products made in the late 1940's and 1950's sometimes contained
a blend of white and blue asbestos, so inhaling the dust from them
could be potentially harmful. As there's no simple way of knowing
whether your old asbestos-cement shed contains just white asbestos, or
a blend of blue and white, it's sensible to treat all old asbestos-
cement as being potentially hazardous, and take appropriate
precautions when handling it.

There are other hazardous forms of 'asbestos' (e.g brown asbestos and
tremolite, both related to the blue variety IIRC), but these are
rarely encountered commercially.

I should add that I have no specific expertise in the field of
asbestos. But I spent my career working for a company involved in the
mining of finely powdered white minerals from all over the world.
Inevitably one picked up a certain amount of background information on
the hazards of mineral dusts in general, including asbestos.


--
Chris

E-mail: christopher[dot]hogg[at]virgin[dot]net