Point of Order: tobacco smoke was suspected of being harmful a couple centuries before the scientific method started to catch up to the suspicion. King James I famously penned a treatise against tobacco use in 1604 and slapped some eyewatering taxes on it's importation.
That's true—lung cancer specifically was a concern that was growing at the time, which is almost certainly the impetus for this ad campaign—but you're right that it was strongly questioned for a long time before that.
https://en.wikipedia.org/wiki/A_Counterblaste_to_Tobacco