I couldn't agree more. Even when we test chemicals on laboratory
mammals, such as rodents, their metabolism may be totally different from
that of humans. A case in point is that of chlorinated solvents. These
are all toxic, of course, and precautions are necessary, but none of
them (not even carbon tetrachloride) has been shown to be carcinogenic
to humans, even though they have caused tumours (with large doses) in
rats and mice. Epidemiologists have done large cohort studies and there
is no indication, over the past century, since they have been used
industrially under shocking conditions, of any form of human tumour
having been generated from any of them. A few anecdotal cases have been
reported but none of these have ever withstood scientific scrutiny. In
fact, one study showed that if every adult in the USA were exposed to 50
ppm (twice the PEL) of methylene chloride (dichloromethane) 8
hours/working day for 40 years and he lived another 25 years after that,
the statistical likelihood of death from solvent-induced cancer would be
ONE case in 200 years. Yet this substance is often cited as a probable
cause of human cancer, because it has developed tumours in mice, with a
totally different method of metabolisation of the solvent.
Actually, the most real danger to human life from methylene chloride is
rarely discussed. At levels over 500 ppm (20 x PEL), some of it
metabolises in the blood stream to carbon monoxide, forming
carboxyhaemoglobin (CHb). This prevents Hb from performing its natural
function of oxygenating the tissues. CHb has a typical residence time of
3 - 4 weeks decay to 1/e of the original concentration, before it is
eliminated and replaced by Hb. This means that regular massive overdoses
of MC over several weeks can reduce the oxygen-carrying capacity of the
blood down to dangerous levels.
The problem is that many substances are examined by toxicologists
working on animals, but rarely are they examined by industrial
epidemiologists, who gain a much broader picture of the effects on
humans in real life. One example of good epidemiological work, also on a
solvent, was with iso-PB, an isomer of nPB. This substance was used as a
solvent, manufactured in China, in Korea and workers were found to have
severe reproductive problems, even on exposure to small doses, despite
toxicologists having given it the green light. Research went back to the
manufacturing plant, where it was found that workers there were also
suffering from similar symptoms. The epidemiologists were able to
eliminate all confounding factors, over three plants and it was proved
that the probability of iso-PB being a reproductive toxin was over 0.99.
As a result, the proportion of iso-PB in nPB is now restricted to 0.1%,
although nPB is now also suspected of being both a reproductive toxin
and, worse, a neurotoxin and is under scrutiny.
Brian
Rudy Sedlak wrote:
> Some many years ago, the infamous columnist, Molly Ivins wrote a column on
> something she thought the FDA was doing was dumb/dangerous, and I responded
> with a letter that could have some insights that might help some of us get a
> better understanding of this. I copy the important parts below....
>
> Dear Ms. Ivins:
>
> Read your recent column, I think it was Sunday in the San Jose Mercury News,
> wherein you discussed the Delaney Clause. From the sound of your discussion,
> it seems that you might not understand all the ramifications of the Delaney
> Clause, and being one of your serious fans, I felt is my duty, honor, and
> privilege to keep you from inadvertently tasting shoe leather.
>
> You talked about the Delaney Clause preventing the presence of any amount of
> a carcinogen in food or drugs. This issue that most folks have with the
> Delaney Clause has nothing to do with the amount of the carcinogen allowed,
> but rather the definition of a carcinogen. Seems that the Delaney Clause (do
> you capitalize the "C"?) calls something a carcinogen if it can cause any
> cancer-like condition in any living organism.
>
> Technology, like all of life, changes. And since the Delaney Clause was put
> in place there have been some tests invented that can show virtually any
> substance that has ever been known, to cause cancer, including an awful lot
> of substances that occur naturally in the very vegetables that everybody's
> mother urges them to eat!
>
> The test that has become the weather vane for the Delaney Clause is known as
> the Ames test, invented by Bruce Ames. Bruce Ames now says publicly that he
> is sorry he invented the test, because the results are essentially
> meaningless, especially when talking about ostensibly higher animal forms,
> like you, I, and certain politicians. Perhaps I was too generous, presuming
> that politicians belongs to the same order of life that you and I do. But I
> digress.
>
> You see, as life forms evolved, and as we have seen evolution has not been a
> steady or sure thing (re: politicians), they have developed cancer fighting
> mechanisms. Thus stuff (a technical term for chemicals) that will kill
> single celled organisms, if they are even in the same room, is utterly
> harmless to human beans.
>
> The problem that the Delaney Clause causes is that some very useful
> chemicals, that will help materially increase the quality of human life, but
> that cause cancer only in single celled critters, not in human beans, are not
> allowed to be used. This is not real clear thinking.
>
> Rudy Sedlak
>
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