In addition to the addictive drug nicotine, the other principal harmful substances in cigarettes are tar and carbon monoxide. Tar is a sticky, brown residue that collects in the lungs. Primarily made up of chemicals known as hydrocarbons, tar is a powerful cancer-causing agent that has been linked to the development of lung cancer. Carbon monoxide is a poison that partially replaces the oxygen normally carried throughout the body by red blood cells, robbing the body of sufficient oxygen. Switching to a low-tar cigarette usually does not help because the person usually compensates for the change by inhaling longer or by smoking more cigarettes. read more