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.
…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.
…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.