Sunday Science Lesson: “carcinogen” is not a useful category

by Carl V Phillips

I was recently asked by a correspondent if we could perhaps make the case that being in the same room as aerosolized cooking oil is more carcinogenic than being exposed to what might be called second-hand vapor.  The context was dealing with the dumb (really really dumb) law in California that requires every venue to post a sign that says, in effect, that being in this facility results in exposure to known carcinogens, and how that whole  California-style “known carcinogens” concept means that e-cigarettes are getting identified as such.

Of course, technically the California law does not require that the sign be posted in every venue, but merely those that have chemical carcinogens floating around in them.  But since this condition is met everywhere that contains, say, furniture or walls, effectively the rule requires posting the sign everywhere.  There is far more that is stupid about this law than I will cover here (e.g., a warning that appears everywhere has zero information content).  I will focus on its very premise: the concept of something dichotomously being either a carcinogen or not.

The only real dichotomy is between exposures that we have evidence that pretty clearly demonstrates that they cause cancer (under particular circumstances, for particular people, in particular doses, and with some possibly very small probability — i.e., the evidence pretty much never suggests that they always create a risk, let alone one that is large enough to matter) and exposures that probably cause cancer (with all those caveats) but the evidence does not exist yet. It is a safe assumption that under the right circumstances, any exposure experienced by a body can cause cancer.

Some of you might notice that this construction sounds disturbingly like the attitude of “public health” people (and sometimes even real public health people) that is epitomized in such statements as “this study failed to demonstrate that the exposure creates any important risks, so more research will have to be done to show that is true.”  But there is a fundamental difference here:  The “public health” people are looking for an excuse to take some action that they have a political/religious desire to take.  The implications of my point are the opposite, that the differences between the many exposures where it is surmised that their is some tiny risk are about the same as all those exposures where no one has chosen to surmise this yet, and in all such cases, almost any proposed actions or restrictions are inappropriate.

Notice that the latter statement applies to only those “known carcinogen” exposures where the guess is that there is some tiny risk of cancer, not exposures that cause important risks.  Just because there is no bright-line dichotomy does not mean that the world does not divide, and some exposures matter, of course.

Most carcinogen exposures — either the “known” ones or the billions of others — cause such a trivial risk that (a) they do not matter and (b) it is only possible to make wild guesses (often dressed up to look like science, but still just wild guesses) about the magnitude of the risk.  If an exposure does not cause a large enough risk, there is no way to detect it, let alone measure it.  It is difficult to imagine ever being able to detect a risk that causes 1  in 100,000 exposed people to get a cancer, even if that cancer were extremely rare and a lot of people were exposed.

You might argue that I should not have even go down to that order of magnitude to make the point, and that the same thing can be said about 1 in 10,000.  In reality, measuring risks in the 1 in 1000 range is beyond practical capabilities.  E.g., while there is no clear evidence that smokeless tobacco causes any risk for oral cancer, it is plausible that there is a relative risk in the order of 1.1 or perhaps even 1.3; these figures would put the risk in that 1 in 1000 range, but despite the exposure and outcome being easy to detect, the outcome being rare, and the great interest by ANTZ in the topic, it is not possible to conclude whether the risk is 1 in 1000 or zero or somewhere in between.  Similarly, any risks from second-hand smoke are down in this barely detectable range.

To the extent that quantification even exists, most of the wild guesses (that get called calculations or extrapolations) about the vast majority of “known carcinogen” exposures put the risks in the order of 1 in a million or 1 in a billion.  To put that in perspective, the risk of dying from driving 1 km is about 1 in a billion.  Yes, you read that right — the lifetime of exposure to some of these “known carcinogenic exposures” that the California-types want you to worry about cause about the same risk of getting a cancer (not even necessarily a bad cancer) as the risk of dying from driving a few blocks.

So with that in mind, there are a couple of key lessons here.

First, the one in the title: Since basically anything is a carcinogen under the right circumstances, merely calling something a carcinogen is uninformative, and quite often is fear-mongering disinformation.  Of course, most of us use the word, but usually we mean something like “carcinogenic, under the relevant circumstances, to a degree that matters or is at least measurable”, and that is how most people interpret it.  Thus, when it is used to describe something that poses a risk that is trivial or speculative, it is terribly misleading.

Second, my last disjunction is a bit misleading because the trivial and the speculative are mostly the same category.  That is, when something is guessed to produce a very small risk, it is never known for sure that this risk exists at all.

Third, given that, it should be clear that it is impossible to compare risks down in that trivial range.  Is the risk from an exposure 1 in a billion, or 1 in a million, or 1 in 10,000 (or zero)?  We will probably never know.  Thus, there is simply no basis for concluding that one exposure that is down in this range is a worse than another.  Of course, all differences among miniscule risks are themselves miniscule, so it does not matter anyway.  Anyone making such comparative claims — unless accompanied by a lot of caveats — is lying.  And, unfortunately, that would include us if we started trying to make such comparisons.

Irrational fear of and despite toward “carcinogens” is the biochemistry equivalent of racism or homophobia.  They are all driven by tendencies toward tribalism (an unconscious desire to find someone to hate) and fear (at the unconscious animal level) that are part of what we inherited from our ancestors.  There is a bit of value in responding to any of these with facts, of course — but the simplest facts (those people are mostly exactly like you; trivial carcinogenic risks are everywhere) probably matter just as much as any details.  There may be a role for the scientific details in, say, court proceedings.  But for changing the general prejudices, even we scientists have to concede that a real solution comes most from in the subconscious release of irrational hatreds and fears that comes from familiarity.