Nicotine, chemicals used to flavor, but especially tar.
While it is known that many of the chemicals can be addictive, tar is said to be the reason of most negative effects of smoking. That also because it isn’t always easy to determine what exactly is contained in tar(1).
What is Tar?
Tar is that dark “gooey” mass, grease which leaves most marks in cigarette filters. Tar often includes known carcinogens benzene, acrylamide, and acrylonitrile. These are highly damaging to the human body’s lung and may lead to lung cancer.
Tar also causes inflammation inside the lungs, activating the immunit system. Continued and sustained exposition to tar will eventually break down lung tissue due to the constant activity of immune cells in the lung and may lead to emphysema. Emphysema which lead to a highly inefficient pulmonary system with less oxygen absorption.
Tar, especially when combined with high cholesterol, can also form plaques on the walls of arteries, which can increase the risk of coronary artery disease (CAD). When a human has high cholesterol and also smokes, tar and cholesterol combine to a chemical reaction which increases the likelihood of those plaques and thus possible CAD.
(1) Tobacco leaves themselves, like any plant, are highly complex “brews”, consisting of multiple chemical components.