The term SOLID TOBACCO SMOKE RESIDUE is composed of three words, the first of which is TAR, a three-letter word. TAR is a dark, viscous material that is produced when tobacco smoke is condensed and cooled. It is composed of various chemicals, including polycyclic aromatic hydrocarbons, which are known to be carcinogenic. TAR is one of the main components of cigarette smoke and is responsible for many of the health risks associated with smoking.