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.
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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.