The Surgeon General has stated that there is NO safe level of exposure to smoke, either directly via smoking or through exposure to secondhand smoke. There are more than 7,000 chemicals in smoke, most of which have been added by the tobacco companies. Hundreds of the chemicals are dangerous and over 69 are known carcinogens. Many people have thought that filtered, low-tar, and “light” cigarettes are less harmful, but they are not. They have just fooled people into thinking they can continue to smoke or smoke more.
Tar is the oily, sticky, non-water-soluble 'black gunk' that is released from the tobacco leaf by the burning process. Tar coats the inside of the lungs and immobilizes the cilia. It traps many of the toxins in the cig smoke and causes the body to produce larger amounts of mucus (phlegm) in a constant effort to cleanse the lungs.
U.S. Department of Health and Human Services. How Tobacco Smoke Causes Disease: The Biology and Behavioral Basis for Smoking-Attributable Disease: A Report of the Surgeon General. Atlanta, GA: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, Office on Smoking and Health, 2010.