Here is my original question: How do we know lung disease is NOT caused by the nicotine in tobacco products but by the tar and/or other chemicals? If the curl pit turns out to be the nicotine, what does that say about vaping? As to naming my sources, try the surgeon general's report as a start.
Because scientists have isolated the Tobacco-specific nitrosamines and other chemicals in smoke that they are reasonably certain cause cancer and have not been able to induce cancer in cells using nicotine alone.
The closest they have to linking nicotine to cancer is one or two small studies that MAY have linked nicotine to aggravating existing cancer cells. Meaning (put very simplistically) that there could be some risk that if you already have cancer tumors, adding nicotine may increase the risk it could get worse. But that research has not even been completed.
Otherwise, they have not been able to tie nicotine to causing any kind of cancer. Which is why the FDA has deemed nicotine gums, patches and inhalers to be safe and why they don't tell us to avoid eggplants and tomatoes.
Additionally, at the population level, 30 years of studies of snus users (a type of smokeless tobacco with a lot of nicotine) show no increased risk of any disease, including cancer.