The International Agency of Research into Cancer (IARC), an arm of the World Health Organization, is notable for two things. First, they’re meant to carefully assess whether things cause cancer, from pesticides to sunlight, and to provide the definitive word on those possible risks.
Second, they are terrible at communicating their findings.
Their failings are in full view today. As my colleague Olga Khazan reported, IARC’s working group of 22 scientists reviewed a large buffet of existing studies and “classified the consumption of red meat as probably carcinogenic to humans (Group 2A),” and processed meat “as carcinogenic to humans (Group 1).”
To explain, the organization slots everything into five possible categories. The highest tier, Group 1, is reserved for established carcinogens, including smoking, asbestos, alcohol, and now processed meat. The next two tiers, 2A (“probably carcinogenic”) and 2B (“possibly carcinogenic”), are for things whose relationship to cancer is less certain. Group 3 is for substances that can’t be classified, due to lack of data.
Here’s the thing: These classifications are based on strength of evidence not degree of risk.
Two risk factors could be slotted in the same category if one tripled the risk of cancer and the other increased it by a small fraction. They could also be classified similarly even if one causes many more types of cancers than the other, if it affects a greater swath of the population, and if it actually causes more cancers.
So these classifications are not meant to convey how dangerous something is, just how certain we are that something is dangerous.
But they’re presented with language that completely obfuscates that distinction.
Group 1 is billed as “carcinogenic to humans,” which means that we can be fairly sure that the things here have the potential to cause cancer. But the stark language, with no mention of risks or odds or any remotely conditional, invites people to assume that if they specifically partake of, say, smoking or processed meat, they will definitely get cancer.
Similarly, when Group 2A is described as “probably carcinogenic to humans,” it roughly translates to “there’s some evidence that these things could cause cancer, but we can’t be sure.” Again, the word “probably” conjures up the specter of individual risk, but the classification isn’t about individuals at all.
Group 2B, “possibly carcinogenic to humans,” may be the most confusing one of all. What does “possibly” even mean? Proving a negative is incredibly difficult, which is why Group 4—“probably not carcinogenic to humans”—contains just one substance of the hundreds that IARC has assessed.
So, in practice, 2B becomes a giant dumping ground for all the risk factors that IARC has considered, and could neither confirm nor fully discount as carcinogens. Which is to say: most things. It’s a bloated category, essentially one big epidemiological shruggie. But try telling someone unfamiliar with this that, say, power lines are “possibly carcinogenic” and see what they take away from that.
Worse still, the practice of lumping risk factors into categories without accompanying description—or, preferably, visualization—of their respective risks practically invites people to view them as like-for-like. And that inevitably led to misleading headlines like this one in the Guardian: “Processed meats rank alongside smoking as cancer causes – WHO.”
This confusion isn’t new. It happens again and again. It happened when IARC ruled on mobile phones, and Round-Up, and diesel fumes. And yet, nothing changes. Which wouldn’t be a problem, except IARC always issues press releases about its new classifications.
That latest press release offers only this by way of numbers: “The experts concluded that each 50 gram portion of processed meat eaten daily increases the risk of colorectal cancer by 18 percent.” But without context, that information is useless—increases by 18 percent over what?—and says nothing about how processed meat compares to other Group 1 carcinogens like smoking or asbestos.
It’s not actually hard to convey this information well: Check out Cancer Research UK’s excellent explainer about the new ruling, complete with absolute risks, context, and simple, clear visuals. (Disclosure: I used to work there until 2011.) And although this new ruling on meat comes with a (separate) Q&A, if your classification system is so arcane that you need a five-page document to clarify it, it might be time to re-tool your system and the way you discuss it in public.
Until then, what we have is a classic ivory-tower mentality: a group of academics who hole up in a room, make proclamations to the world, and ignore the chaos that consistently ensues. Perhaps we need a separate classification scheme for scientific organizations that are “confusogenic to humans.”
New plant-based meats are an exciting development for nutrition and the environment.
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