There's no 100% proof as such. Cancer is a multifactorial condition, i.e. it not caused by a single factor, e.g. smoking, but by a combination of factors. The medical consensus, along with that of Tobacco Control, is that smoking is the major factor for lung cancer. However, this was based on unreliable, imprecise, inaccurate, biased epidemiological studies mainly performed half a decade ago.
Today, the emphasis of 'scientific' studies is switching to other lifestyle choices. So expect more of "alcohol/obesity/sunbathing/sex/cellphones/Facebook.... causes cancer".
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