Comment by nostrademons

So the processed meat -> colorectal cancer link is actually a textbook example for explaining what statistical & scientific concepts mean.

Processed meat is a known Group 1 carcinogen, in the same category as asbestos, tobacco smoke, and plutonium. Carcinogens are grouped by the strength of scientific evidence on them, not by how hazardous they are. In this case, there are abundant studies with solid methodology that show a link between consuming nitrites in processed meat and a higher risk of cancer, and we also understand how the process works at a molecular level.

This shows how well we understand the link, not how strong the link is. And it turns out that processed meat is a very minor carcinogen. When you average the studies together, the hazard ratio for someone eating 50g of processed meat per day, over an 85 year lifetime, over somebody who eats no processed meat is about 1.18. That means that folks in the former group are about 18% more likely to get colon cancer than folks in the latter group. By contrast, the hazard ratio for cigarette smoking is about 20x (smokers are 2000% more likely to get lung cancer than non-smokers), for asbestos exposure it's about 35-45x (industrial asbestos workers without PPA develop mesothelioma at a rate 3500-4500% greater than the general public), and plutonium exposure is about 7-50x for a single unprotected exposure.

There's a further piece to the statistical puzzle, and that's base rates. Colorectal cancer is relatively rare, with a lifetime incidence (again assuming an 85y lifespan) of about 7%. Combined with the 18% hazard ratio, that means a diet full of processed meats increases your lifetime risk of colon cancer by about 1%. There are far worse activities you could be doing.

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A related concept is the Prop65 warning labels in California. If you've ever been to the restrooms in the SFO airport, you might notice that there's a warning label of "This restroom contains chemicals known to the State of California to cause cancer." These are pretty ubiquitous, you'll see them everywhere else in California too, like on new cars. Are you going to get cancer just because you transferred through SFO or LAX? No. But the list of chemicals that have scientifically demonstrated carcinogenicity is long, and the good people of California don't really make a distinction between minor risks and major risks.

Whether the known carcinogenicity of processed meats is behind the uptake in colon cancer rates is a separate scientific question. You would need data on processed meat consumption at a population level, over time, to see if that epidemiologically explains the increase we've been seeing. I don't know if this data exists and kinda suspect it would go the other way, but that's what the scientific method is for.