Systematic review with meta-analysis of the epidemiological evidence in the 1900s relating smoking to lung cancer.

The difference in RRs by continent was very marked and highly significant (p < 0.001) for all lung cancer and adeno, but less marked, though still significant (p < 0.01) for squamous.

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There was no significant effect of sex, and the weakly significant (p < 0.05) effect for number of adjustment factors was associated with an erratic pattern, with lower RRs where the number of factors was 1, and higher RRs where it was 0 or 2+.

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A clear trend of risk with increasing duration is also seen for other definitions of smoking status and product smoked (see Additional file 5 : Detailed Analysis Tables).

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