Friday, December 16, 2005

Rats and cancer, Part I

Tyler Cowen, an economist whose excellent blog Marginal Revolution sometimes links to us, recently noted an old post of ours on rodent testing for carcinogenicity and it was picked up by another blog, The Right Coast, which, since it is actually on the left coast when looking at the map in its usual North-South orientation, is a signal I presume about where it is situated in the political spectrum. There law professor Michael Rappaport says that we have elided a key issue:
It is interesting that every human carcinogen is a rodent carcinogen, but the real question is whether every rodent carcinogen (at high doses!) is a human carcinogen (at low doses). Or more relevantly, how often rodent carcinogens are human carcinogens.
We believe it is not just a key issue for carcinogen testing in rodents, but the key issue. Our original post was a topical one addressing an attempt by a notorious industry front group to gut rodent bioassays as a means to identify human carcinogens. Thus it didn't lay out the full argument. So we will try to plug some of the holes to try to get Professor Rappaport further down the road about what is behind the scientific thinking.

No one knows exactly how many chemicals are "out there" in the stream of commerce, but a reasonable order of magnitude would be about 100,000 (about 4 million chemicals have ever been synthesized and identified, but most of them are laboratory curiosities of no special concern for public health policy). Again, current thinking is that only a small percentage of these chemicals are capable of causing cancer, no matter how large a dose you administer. The actual proportion of carcinogens is unknown, but estimates of less than 10% are reasonable, maybe even less than 1%.

That's the good news. If we want to get rid of chemical carcinogens in our environment we don't have to bring modern industrial society to a crashing halt since 90% - 99% of chemicals are not carcinogens (we are leaving off the vexing problem of other effects like endocrine disruption). But there's bad news, too. We don't know which of the 1,000 - 10,000 chemicals are the bad actors. We do know there are some universally agreed upon carcinogens out there, like asbestos and benzene, that are close to ubiquitous in or environment and the number of people estimated to die of environmental or workplace associated cancer each day in the US is 75 to 150. If that were an industrial accident it would make headlines in every newspaper in the country. It is an industrial accident, but one in slow motion so we don't pay attention to it (see an earlier post). The death toll in this country alone is about the same as the death toll from motor vehicle accidents and probably much worse in other countries where workplace and environmental standards are lower. So it's a big deal from the public health point of view. For the most part we don't know which of these 1000 to 10,000 chemicals are the bad ones.

Thus we need a way to identify them.

Part II here. Part III here.