Someone said:

Nevertheless, non-smokers do contract lung cancer. So does cigarette smoking cause lung cancer, or merely aggravate an existing pre-disposition toward forming lung cancer?

Two comments:

The first sentence of course brings up the dangers of secondhand smoke. If you don't smoke, but your work (or life-pattern otherwise) makes you spend a lot of time in heavy tobacco-smoke areas, it's quite possible to be exposed to as much tobacco smoke as you might if you smoked.

The last quoted sentence (merely aggravate; what's "mere" about it?)really doesn't matter. If a substantial portion of the population have such predisposition and the cancer-trigger is tobacco smoke, it's still the tobacco smoke that's to blame. Blame the bullet, not the target.

And from the original post, a portion of the remarks went as follows:

By the early 20th Century almost one in every two people smoked, but the incidence of lung cancer remained so low that it was almost immeasurable.

It is misleading or disingenuous (if not outright fraudulent) to cite the experience in the early years of the 20th Century in comparison to today, because over the years the tobacco industry has added chemicals to the tobacco which are known to be harmful, and thus comparative statistics between the period are at least suspect. Cigarettes sold in the 20s are not the same as cigarettes sold in say the 90s.

Dave Oesterreich