Tar is a brown sticky substance that accumulates in the lungs during smoking. It contains many cancer causing chemicals. Normally cell division occurs continuously in the lungs to replace the epithelium or membrane of the air sacs. Tar induces these cells to divide at an abnormal rate. Such uncontrolled multiplication of cells results in outgrowth or lumps of tissue (cancer).These blocks off the air sacs, reducing the efficiency for gaseous exchange
Tar is an irritant. It makes the sides of the respiratory passages inflamed, causing chronic bronchitis. It also damages the cilia lining these passages and causes extra mucus to be made by the goblet cells. This mucus trickles down into the lungs as the cilium is longer able to sweep it upwards.