In February 2011, Nature ran a journalistic piece on the development of technologies designed to increase the accuracy of measuring exposures, spurred by various dissatisfactions with questionnaires. (Thanks to Thad Metz for pointing me to this.) The “exposome” is presented in that piece as the logical conclusion of improved measurement techniques. It is supposed to be a device (I am imagining an enormous plastic bubble) capable of measuring every exposure of study subjects. A quick hunt around the internet reveals that the idea is capturing at least a few imaginations, including some at the US Centers for Disease Control.

The CDC’s Overview of the exposome defines the exposome like this:

The exposome can be defined as the measure of all the exposures of an individual in a lifetime and how those exposures relate to health.

The idea of the exposome suggested two questions to me.

First, the idea of the exposome puts pressure on the concept of an exposure. In most epidemiological practice, the question “What is an exposure?” is of no practical importance. But if the aim is to measure every exposure, then we must answer the question in order to know whether we have succeeded.

The CDC article contrasts the target of the exposome with genetic risk factors, suggesting that exposures exclude genetic make-up. But the CDC article also suggests that exposures measured by the exposome may begin before birth. (I am imagining babies born in little plastic bags.) So it is not clear exactly what the rationale for excluding genetic make-up from “exposures” would be. If the goal is simply to measure anything that might affect a given health outcome then we should include genetics. We should also include our entire solar system, indeed the galaxy, so as to account for the effects of solar flares, meteorites, and so forth. (The plastic bubble in my imagination is getting very big.)

My first worry, then, about “exposomics” is that it will not get very far without circumscribing the notion of exposure, so as to be something less than what the authors of the CDC overview probably think they mean – that is, something less than all factors potentially affecting health outcomes.

My second question is whether striving for an exposome is a good idea, judged by the goals of epidemiology, which I take to be providing information which can be used to improve public health.

One central point of epidemiology is that it studies people, not in labs, but as they actually live their lives. The exposome is a sort of lab, and striving for it is nothing other than striving for the controlled experiment. Aside from the complete fantasy of ever achieving an exposome (my imaginary bubble just burst), it does not seem helpful even to “study” the exposome, or whatever else it is “exposomists” are supposed to do. (And, incidentally, it does not seem that the exposome is a logical extension of increasing accuracy of measurements of exposure.) Epidemiologists want to know what happens in reality, not in the exposome.

Epidemiology and laboratory sciences complement each other in this way. Tar may be shown to produce cancer in the skin of laboratory rats, but epidemiology tells us what happens when humans smoke cigarettes. The two sources of knowledge complement each other. Each has flaws. Causal inference is harder in epidemiology because of the lack of control over potentially relevant variables: exposures, for short. But lab sciences suffer a different inferential limitation: not in making a causal inference, but in inferring that the results obtained in the lab will apply outside. So it is hard to see how doing away with either source of knowledge could be a good idea, and hard to see what “exposomics” could add to epidemiology, except another buzz word.