The idea that there's some confounding factor isn't in itself implausible, but a factor of 16 increase in risk is huge. Of course, to really be sure you should

The idea that there's some confounding factor isn't in itself implausible, but a factor of 16 increase in risk is huge. Of course, to really be sure you should also look at how lung cancer risk and smoking correlates between countries, and within a country over time. And you can segment the population by sex and ethnicity. Oh and also look for a dose-dependent effect, very important. Heavier smokers should be at a higher risk. Of course, the link between cancer is smoking is even stronger if some of the "non-smokers" were exposed to an increased level of cigarette smoke.