Tobacco produces nicotine. Nicotine is a pesticide. Some insects are pretty much immune to nicotine and other pesticides have to be used for those. Tobacco plants are sometimes not all harvested at the same time. A different method is to take the lower leaves first and allow the top leaves to grow longer. The lower leaves are often inferior to the top leaves.
Burning any plant materials will produce tars. Many of the additives placed in cigarette tobaccos are actually flavorings. Like many plants tobacco will pick up contaminants in the soil. Because of this some plants have even been proven to be slightly radioactive. If pesticides are used it is possible they can be picked up and distributed in the leaves of the plant.