When cigarette smoke is passed through a filter, it leaves a black, sticky substance known as tar. The exact composition of the tar that smokers inhale is uncertain. Any sort of plant material is a veritable witch's brew of complex chemical compounds. Burning transforms these compounds in the tobacco leaves in an unpredictable manner, and the final composition of the inhaled smoke depends on exactly how a cigarette is smoked. While it's hard to determine exactly what's in tar, it's clear that it causes most of the deleterious health effects of tobacco.
Lung cancer is one of the best-known diseases caused by smoking. Some known carcinogens that are usually found in tar include benzene, acrylamide and acrylonitrile. Smoking exposes the delicate cells inside the lungs directly to these compounds, causing mutations in the DNA of the cells and leading to cancer. According to the World Health Organization's report on "Tobacco Smoke and Involuntary Smoking," 90 percent of all cases of lung cancer are attributable to smoking.
Emphysema is a disease in which the complex honeycomb of air passages and pockets inside the lungs breaks down, leading to large open air spaces. These large spaces do not absorb oxygen from the air nearly as well as the original lung tissue, which leads to shortness of breath. It is also harder to push air out of the damaged lung tissue, which leads to increased effort in exhaling. Compounds in tar cause inflammation inside the lungs, which activates the cells of the immune system. Over time, the constant activity of these immune cells causes the breakdown of lung tissue and emphysema.
Smokers have a much higher rate of coronary artery disease, CAD, than do non-smokers. Part of this increased risk is attributable to the nicotine in cigarettes, but tar also plays a significant role. Some of the compounds in tar are adsorbed through the lungs and enter the bloodstream. Once there, they promote a chemical transformation of the cholesterol in the blood that makes it much more likely to form plaques on the walls of arteries, including the coronary arteries. Smoking and high cholesterol are therefore multiplicative risk factors for CAD. Having both together is much worse than the sum of having them separately.