The health effects of tobacco are the effects that use of tobacco has on human health, and concern about health effects of tobacco has a long history. Research has focused primarily on cigarette tobacco smoking. In 1950, Richard Doll published research in the British Medical Journal showing a close link between smoking and lung cancer. Four years later, in 1954, the British Doctors Study, a study of some 40, 000 doctors over 20 years, confirmed the suggestion, based on which the government issued advice that smoking and lung cancer rates were related. Tobacco use is the single greatest cause of preventable death globally. As many as half of people who use tobacco die from the results of this 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. In the 20th century tobacco is estimated to have caused 100 million deaths. Similarly, the United States Centers for Disease Control and Prevention describes tobacco use as "the single most important preventable risk to human health in developed countries and an important cause of premature death worldwide."