Tar is a mixture of sticky toxic chemicals. It sticks to the cilia and damages them. This means that they cannot get rid of mucus. The mucus sticks in the airways, causing more coughing. If this damage continues, it can lead to various pulmonary (lung) diseases. Bronchitis causes the bronchi to become inflamed, restricting breathing. Emphysema destroys the alveoli (air sacs), reducing the amount of gas exchange that can take place.