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"Finally, looking at that figure you might wonder why the relative risk of smoking has increased so much. Based on my first pass through the literature, it seems like no one knows. There are at least three possibilities:

- Over this period, cigarettes have been reformulated in ways that might make them more dangerous.

- As the prevalence of smoking has decreased, it’s possible that the number of casual smokers has decreased more quickly, leaving a higher percentage of heavy smokers.

- Or maybe the denominator of the ratio — the risk for non-smokers — has decreased."

My first thought was "second hand smoke". My logic is, when smoking was more normalized, non-smokers were exposed to some of the same risk as smokers due to smoking indoors, etc, resulting in higher rates of lung cancer among non-smokers.