Of course cigarette smoke was not the only human added thing that caused problems back in the olde days, hopefully not now.
Way back in the 1960s when I was an Ansett Apprentice and in heavy maintenance at the time we were the first Airline in the World to have to do a complete overhaul on our 727s, and we had all sorts of people out here from Boeing to assist etc.
I was on the engine crew at the time, so once we had the engines and APU out, we assisted with the airframe, mainly on section 48, couldn't believe all the tools we found at the bottom of it, riveting tools mainly that had obviously been dropped when building the aircraft and left there.
Myself and another Apprentice were given the very long and boring job of checking the bolts securing section 48 to section 46, a ring of bolts right around the circumference of the fuselage, until we found EVERY bolt we checked below floor level was VERY badly corroded and our boring job suddenly became VERY important, the Boeing people were concerned that IF our aircraft were typical of the 727 fleet in general, there was a strong possibility 727s MAY actually LOSE their whole section 48 in flight, that is the whole tail including all 3 engines.
Boeing determined that the problem was not foreseen earlier, and was caused by ''urine'' leaking from the rear toilets and reacting with the dissimilar metals of the bolts and the structure.