I have been discussing smoking with a fellow who states that for all the decades of study of the tobacco leaf, still nobody has managed to find what part of the process of smoking the tobacco leaf causes lung cancer. He pointed out that so far most of the evidence is from epidemiological studies, which can only show correlations, but not causation, and he pointed out flaws in some of these. For example, he said in one of them - the original epidemiological study, I believe - that showed that 90% of people with lung cancer are smokers, it was the case that 90% of the population were smokers, so the study could with as much certainty have said that 90% of the people with broken legs were smokers therefore smoking causes broken legs.
He pointed out that already many cancers that were previously attributed to smoking, such as throat and cervical cancer, have actually been found to be caused by a virus (HPV). So given there has been no causal link discovered, it may very well be that something other than tobacco, such as a virus, causes lung cancer, and hence humanity is missing a huge opportunity to find the actual cause for lung cancer and thus avert untold suffering.
I found one New York Times that states:
The findings, published today in the journal Science, report the first evidence from the cell biology level linking smoking to lung cancer. The scientists say a chemical found in cigarette smoke has been found to cause genetic damage in lung cells that is identical to the damage observed in many malignant tumors of the lung.
The findings reported today establish the long-missing link, in the opinion of experts in the field of cancer genetics, and may also play a role in pending litigation about smoking illnesses and passive smoking.
If that part is true, this means that at the very least, no causal link had been discovered before the article was published, in October 1996.
The article states that:
[...] the scientific team exposed human lung cells to the chemical BPDE, a powerful carcinogen that is produced in the body from the breakdown of benzo$(a$)pyrene, a chemical in tobacco smoke.
Two researchers [...] examined the p53 gene and found that molecules of BPDE had bound chemically to the p53 DNA at precisely the three points that had already been identified as mutational hot spots in lung cancer.
If correct this means that
benzo(a)pyrene is the chemical found in tobacco which, when smoked, causes lung cancer. Link to the paper itself: Preferential formation of benzo[a]pyrene adducts at lung cancer mutational hotspots in P53..
However the fellow said that they thought they had discovered the causal link but later had to retract it. The NY Times article is written that it is pretty cut and dried, but I have found indicators that it isn't. This rebuttal by Lauren A. Colby, published 9 days after the NY Times article, points out several flaws in the study (severely edited for length):
The authors of the article start out by pointing out that, in about 60% of lung cancer cases, there is mutational damage to the p53 gene [...] This is another way of saying, of course, that in 40% of lung cancer cases, there is no damage to the gene, meaning that people can get lung cancer even if their p53 genes are in perfect condition.
The authors did not study any actual human lung cancers. [...]
The authors of the study apparently concede that [Benzo(a)pyrene (BAP)] [...] is not terribly carcinogenic [...]; it must be converted to BAPDE. There is no evidence that the lungs, themselves, can metabolize BAP into BAPDE. Even if they could, the amount of BAP reaching the lungs from cigarette smoke is dwarfed by the amount reaching the lungs in the blood supply [...] from consumption of burnt food. [...] The study could just as well be said to prove an indirect link between the consumption of burnt food and lung cancer.
Further evidence: [This FAQ] about benzo(a)pyrene, last updated in 2009, states that "Benzo[a]pyrene is a probable cancer-causing agent in humans." Why 'probable' if it has been proven, as the 1996 study claims?
So, is he right? Is there no as-yet proven causal link between smoking and lung cancer?