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Since its introduction to western world in 16th century, smoking has been one of the most popular parts of human life. Its health hazards, however, has rarely been evaluated before mid 20th century. After early suggestion of association with lip cancer and pipe smoking, which was falsely associated with the heat of the pipe smoking, association between rapidly increasing incidence of lung cancer and increasing popularity of smoking habit in the western world has been suggested in late 1940s. Initial case-control studies, in spite of its proneness to various biases, aroused the relevance of the relationship. It was supported by following well-designed case-control studies and new method, cohort studies in both coast of the Atlantic. Consistency of the results of epidemiologic studies and additional support from animal experiments made the causal relationship to be accepted from scientific community, and finally from public and governments. Establishment of criteria of causal relationship was also
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