Tobacco use has predominantly negative effects on human health and concern about health effects of tobacco has a long history. Research has focused primarily on cigarette smoking.Tobacco smoke contains more than 70 chemicals that cause cancer. Tobacco also contains nicotine, which is a highly addictive psychoactive drug. When tobacco is smoked, nicotine causes physical and psychological dependency. Cigarettes sold in underdeveloped countries tend to have higher tar content, and are less likely to be filtered, potentially increasing vulnerability to tobacco smoking–related disease in these regions.Tobacco use is the single greatest cause of preventable death globally. As many as half of people who use tobacco die from complications of tobacco use. The World Health Organization (WHO) estimates that each year tobacco causes about 6 million deaths (about 10% of all deaths) with 600,000 of these occurring in non-smokers due to second-hand smoke. Tobacco is estimated to have caused 100 million deaths in the 20th century. Similarly, the United States Centers for Disease Control and Prevention describes tobacco use as "the single most...