Every chemical is dangerous if you have high enough levels of concentrations. Everyone knows that when children first smoke they become sick, have headaches and can vomit. The same can be said of tobacco pickers, they absorb tobacco through the skin, when initially exposed to it they get nausea. This is low exposure and these people recover. But for the average smoker, it would certainly not be the nicotine that would do them harm.
People buy cigarettes of a specific brand.
They prefer one brand to another, and once they've found one they like they will stick with it. Certain additives maintain the consistency of the smoke. They control the pH balance, the rate of burning etc. Just about every food we have contains arsenic. Like a large number of other chemical elements in the soil, it is taken up by plants and as a result it is present in tobacco leaves.