- 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.