Here is a defense of using those rat tests to judge what will cause cancer in humans:
The "junk science" they are referring to is the long-standing and
well-confirmed practice of identifying chemicals likely to cause cancer
in humans by testing them in animals. The animals (rodents) are a
standard model for biological processes of relevance to humans (which
is why drug companies and medical researchers have been using them for
a century). They are well understood and are the only sentinels for
detecting carcinogenicity of any use to public health. Since chemically
induced cancer has a latency period of decades (typically 20 years or
more), waiting for it to appear in human populations would meant that
once detected, even if exposure would cease instantly (which can never
happen), it would take another 20 or more years to eliminate the
cancers from exposure (all the cancers induced in the 20 years exposure
prior to detection). But even then, the chances of detecting any but
the most powerful carcinogens in human populations (via epidemiology)
is small. Epidemiology is a very insensitive tool. I say this with some
authority, as I am a cancer epidemiologist specializing in chemical
exposures and have authored numerous peer reviewed studies in that area
over many years.
The main rhetorical lever ACSH employs is the
use of high doses in the animal studies, doses that are much higher
than usually faced by humans. But as ACSH knows well (but didn’t
divulge) there is a technical requirement for using these doses. If one
were to use doses in animals predicted to cause cancer at a rate we
would consider a public health hazard, we would need tens of thousands
of animals to test a single dose, mode of exposure and rodent species
or strain. This makes using those doses infeasible. Thus a Maximum
Tolerated Dose is used, one that causes no other pathology except
possibly cancer and doesn’t result in more than a 10% weight loss. The
assumption here is that something that causes cancer at high doses in
these animals will also do so at low doses. This is biologically
reasonable. It is a (surprising) fact, that most chemicals, given in no
matter how high a dose, won’t cause the very unusual and specific
biological effect of turning an animal cell cancerous. Cancer cells are
not "damaged" cells in the individual sense but "super cells," capable
of out competing normal cells. It is only in the context of the whole
organism that there is a problem. It is not surprising, then, that very
few chemicals would have be ability to turn a normal cell into a
biological super cell of this type. Estimates are that is far less than
10%, perhaps only 1% of all chemicals that have this ability. Thus
western industrial civilization doesn’t have to come to a screeching
halt if we eliminate industrial chemical carcinogens from our
environment.We know of no false negatives with this process.
Every chemical we know that causes cancer in humans also does so in
rodents (with the possible exception of inorganic trivalent arsenic,
which is equivocal).
Here is the full post. I’m not close to having the expertise to evaluate these claims, but two points. First, the author is highly qualified; as a blogger he is anonymous but I can vouch for his credentials. Second, it should be the self-appointed task of bloggers to pass along arguments which either struck them or which might shake up their readers.
Seems like a sensible defence of science against the claque of “junk science” hacks. Since economics is ontopic for this blog, though, I’m not so sure about this bit:
["Estimates are that is far less than 10%, perhaps only 1% of all chemicals that have this ability. Thus western industrial civilization doesn't have to come to a screeching halt if we eliminate industrial chemical carcinogens from our environment."]
Not all chemicals are equal, surely? I would be much less confident in a blanket pronouncement that there was no set of chemicals equal to 1% of all chemicals which could not be withdrawn without bringing Western industrial socity to a halt.
The real question, however, is “What is the safe dose of any chemical for humans?”. These are the facts that allow us to determine risk of exposure, identify at-risk populations and apply meaningful regulations. The “high-dose in rats” experiments only identify the potential for human carcinogenic activity. It is a profound logical and biological error to assume that a compound which is harmful at high doses will exert the same effect at low doses; an assumption which is clearly made by this anonymous blogger.
“The assumption here is that something that causes cancer at high doses in these animals will also do so at low doses. This is biologically reasonable.”
There are many things which at high doses are harmful, even lethal, for humans: water, salt, even excessive food consumption. Yet all of these things are safe and even necessary at low doses.
This is another area, like nuclear power, where scientists and engineers have frustrated the public good due to poor communication. The scientists assume everyone is clear regarding the meaning of the results, their scope and potiential uses so no abuse of the data will occur. Consumer advocates have proven this idea false.
This type of study has its merits to determine possible carcinogens but has little use with regards to risk to humans. Maximizing banifit to potential risk is the true end goal but the work seems to end at the identification of the offending substance. Instead of having clear advice for public health we have litigation against teh offenders and advertisements promoting(defending) chemistry on TV.
Dr Q: I’m no expert in any of this, but I think you responded to a different argument than the anonymous blogger made. He said that using high doses of chemicals on rats gave good information about whether they were carcinogenic–not whether they were generically harmful. He’s trying to detect a specific effect of the chemical (causing mutations to the DNA in some cells that helps them along toward becoming cancerous), and because the effect is small at the low doses we usually see in the environment, he’s trying to amplify the effect by using much higher doses. Now, how well this tracks to P(cancer|exposure at dose X) in humans is an empirical question, but it’s not obviously unreasonable to use this technique to determine that probability / dose relationship.
The problem with using these tests in court is that they are well off point. Water is dangerous when consumed in large enough amounts. The question is whether or not a particular drug poses an unreasonable risk. These kinds of tests offer no insight into that question.
Regarding the “anything can be toxic” argument: these tests distinguish between general toxicity and carcinogenesis.
The reason that we need these special tests for carcinogenesis is that it is such a delayed effect, so it’s often hard to identify the cause of cancer.
The basic argument is that what happens in the lab is not the same as what happens in reality: guess what–that’s true of ANYTHING done in a lab.
It’s pretty obvious that the “American Council on Health and Science” is a corporate front-group that with no concern for “health and science”–and only views them as obstacles to making a buck.
Their problem is not with the science, per se, but with how that knowledge is applied in our social/legal system. However, for PR, it is much easier to attack “junk science” than it is to push the idea that they should be able to dump anything that they want into our environment until it has absolutely, positively, been shown to cause immense damage to humans.
“We know of no false negatives with this process. Every chemical we know that causes cancer in humans also does so in rodents” answers the wrong question. Is it the case that everything that causes cancer in rodents causes it in humans?
Had this discussion at Reason earlier today. Besides the Ames interview in Reason that Mr Schwarz linked, which is very good (Ames invented the Ames test, which is an important test for mutagens and often confused with a test for carcinogens), this article ( http://www.sciam.com/article.cfm?articleID=00019A70-0C1C-1F41-B0B980A841890000&sc=I100322 ) from Scientific American is very good. Simple summary: many chemicals and other effects that are harmful in large doses are actually beneficial in small doses; the body overcompensates for the damage and actually becomes less likely to suffer from cancer or toxicity. So the testing in rats may well give good information on whether a chemical can be a carcinogenI’m not qualified to judge, although it seems totally plausiblebut it doesn’t tell you anything about which doses are helpful and which harmful. I suspect that we consistently tend to set thresholds far lower than we ought.
Add to this Dr Ames’s point that you get exposed to more known carcinogens from a cup of coffee than you will from a years’ worth of pesticides on produce, and I’m pretty skeptical.
After following jadaqul’s link to the Sci Am article, MR readers may have made ALL the points in my long post!
Biotech Combany in India Supplying Jatrobha Curcas Feeds and Tissue culture Saplings of Orchids and Other Cut Flower Varietiess.
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