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Re: [APD] Atrazine

On 28/05/2006, at 5:27 AM, Vaughn Hopkins wrote:

> Bill, I don't like to argue either!  (No snickering, now!)  A few  
> years
> ago I read that the number of chemicals that cause cancer is  
> remarkably
> small compared to the total number of chemicals in use.  So, when  
> it is
> learned that a cupful a day of Chemical X causes cancer in mice, that
> is not really a joking matter.  It means that that chemical is highly
> likely to cause cancer in any animal in even very small doses.  Or, to
> put it another way, a chemical can't be just a little bit  
> carcinogenic.
>   I'm not a chemist or a biologist so I can't say this is true, but it
> makes sense to me.


Many chemicals that cause problems in humans do so on a dose related  
basis: increase the dose and the effects get worse. Doses are  
expressed in dose/weight terms, ie so much per kg of body weight.

This kind of relationship can break down with some of the chemicals  
known to cause cancer. For example, there seems to be no dose  
relationship between asbestos fibre exposure and development of  
mesothelioma or a related lung cancer. Development of cancer in that  
case seems to depend on what happens when a fibre embeds in the lung,  
and many fibres don't embed or at least don't embed in the way required.

Now, "a cupful a day of Chemical X" for a mouse is a high dose,  not  
a low dose as you stated. In fact it's a very high dose. Consider a  
mouse's weight, divide that into an average human weight of, say, 80  
kg, then give a human that many times a cupful a day and you're  
talking equivalent doses. A mouse weighs less than a kg so we're  
talking about giving a human more than 80 cups a day to get an  
equivalent dose. That's a damn lot of cupfuls and no human is likely  
to ingest that much of any chemical a day. Even water is going to  
have a lot of toxic effects at that dosage level—I don't know what  
your weight is but try drinking 80 cups a day and see how you go.  
They run tests on animals at high doses simply because high doses  
often accelerate processes so results can be seen sooner. The results  
of such tests do not automatically mean that " that chemical is  
highly likely to cause cancer in any animal in even very small doses".

You're right that a chemical can't be "just a little bit  
carcinogenic"—it either is or it isn't. That does not mean, however,  
that all carcinogenic chemicals are equally effective in triggering  
cancer. It takes much greater exposure to some in order to trigger  
the necessary cell damage than it does to others. It is simply not  
the case that a chemical which can cause cancer in a mouse exposed to  
extremely high doses is also going to be "highly likely to cause  
cancer in any animal in even very small doses". That may be true for  
some carcinogenic substances, but it most definitely isn't true for all.

We need to be cautious about any chemical which is shown to induce  
cancer in animals, especially since the action may well be enhanced  
in the presence of exposure to other substances or conditions and  
those may well have not been present in the lab test which may well  
have been intended simply to determine whether the specific chemical  
is or is not carcinogenic, and not to examine how it causes problems  
and what exposures and conditions are necessary for it to do so under  
normal circumstances. "A cupful a day" is  not normal circumstances.

Be careful with substances known to cause problems of any kind, but  
also don't read any more into the results of any test than you can  
legitimately infer from the test.

David Aiken

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