TL;DR: The International Agency for Research on Cancer (IARC), the organization famous for classifying glyphosate (the active ingredient in Roundup) and red meat to be carcinogens, is using an outdated system of classification. Their classification system does not take dosage into account, or the method of exposure. Basically, if a chemical could give someone cancer, but only at a dose that's thousands of times higher than what they'd experience normally, the IARC would still label it carcinogenic, even if the normal dose could never give them cancer. That's why they label ridiculous things like microwaves from cell phones, drinking very hot beverages, and working as a hairdresser to be carcinogenic.
"Just because something is in IARC’s top level category, it doesn’t necessarily mean it’s public health number one – it’s more complex than that. IARC does ‘hazard identification’, not ‘risk assessment’. That sounds quite technical, but what it means is that IARC isn’t in the business of telling us how potent something is in causing cancer – only whether it does so or not. To take an analogy, think of banana skins. They definitely can cause accidents – but in practice this doesn’t happen very often (unless you work in a banana factory). And the sort of harm you can come to from slipping on a banana skin isn’t generally as severe as, say, being in a car accident. But under a hazard identification system like IARC’s, ‘banana skins’ and ‘cars’ would come under the same category – they both definitely do cause accidents." - Cancer Research UK
The whole article is worth reading, if you have the time. It's an easy read, because it uses language laypeople can understand.
If you can't access .onion links, you can still find the paper here. Do a Ctrl+F for "Classification", and try the Sci-Hub Ocean link.