[jws]

For the EPA, "likely carcinogen" means:

• There is evidence of carcinogenicity in animals. (Multiple, consistent studies)

• The substance is shown to directly or indirectly cause chromosomal damage or mutations in a way that is relevant to humans.

• There are no or limited human studies, they are inconclusive, or otherwise inadequate. ((Note: This is sort of a "Why isn't this classified higher?" factor.)). ((If a substance isn't in widespread use, it is kind of hard to design an ethical human study. I mean, you aren't going to have some of your test subjects drink a bunch of likely carcinogen each morning.))

So this is a a classification for "Let's maybe not go nuts with this stuff, and someone really ought to check this out. And if you plan to ship tons of this stuff you might want to talk to your lawyers and lawsuit judgement mitigation team."

I didn't manage to find an exhaustive list of things the EPA has listed with this, but I found one that included higher risks as well, and in my little warehouse/workshop I identified 8 things at a casual glance that I have in inventory or generate. Proper use of these have minimal exposure to my squishy bits for most of them, and the others a well informed user should know to take adequate precautions. (e.g. "wood dust": wear a respirator)

The US does not currently fund the EPA to commission studies to further investigate likely carcinogens, so they stay on the list for ages.