Good morning, the list of known carcinogens is quite short. It is quite short because the evidence required to get on that list is quite extensive. This is an article explaining the basics of how that list works: https://my.clevelandclinic.org/health/articles/25081-carcinogens It is not a comprehensive explanation of how it is made, but it explains the basic reasoning behind it.

Knowing a compound is carcinogenic is not enough to link it to something being carcinogenic with high statistical reliability. The reason why that list exists is that statistical evidence is more reliable than scientific evidence. You can build a scientific model to explain why something happens, but that always has some assumptions. A large enough statistical sample has a much higher confidence. We know that pure benzene is carcinogenic, that is why it's mostly prohibited to use in research and production. Do we know if other products containing benzene are carcinogenic because of benzene? Not really.

We can make an assumption: products who contain benzene and to whom people are regularly exposed are carcinogenic. It does indeed make sense as an hypothesis, however we don't know if it is true with statistical reliability. It may be true for some mixtures and not for others. This is in my opinion sufficient evidence to link some in general to cancer. Do we have any proof that any kind of smoke other than tobacco causes cancer? No, we do not.

However, the fact that we do not have such evidence does not mean that smoke does not cause cancer. We have strong evidence that smoke causes cancer, however we can not prove it with enough statistical reliability. We can not prove it statistically because this kind of studies is complicated to perform as it requires data that is difficult to obtain. You have to study a large sample of people, tens or hundreds of thousands, throughout their life while they perform one such action. As such you'd need to follow a large group of users who smoke cannabis and are open to receiving regular checkups and interviews. You also need a negative control: a group of people who does not smoke marijuana. However, tobacco and red meat are also known carcinogens; to exclude whether they got cancer from these causes you'd need separate groups to compare to: people who smoke marijuana but not tobacco, people who also smoke tobacco, people who smoke both and eat red meat, people who smoke marijuana, no tobacco but eat meat, people who smoke marijuana not tobacco and no meat. Now add to this that being an hairdresser is a known carcinogens and you'll start to understand why obtaining a definitive answer is difficult despite the fact that many studies exist.