Pilots are the worst culprit lah. While deadheading on a certain airline in a B744, the Pilot in Command (PIC) and First Officer (FO) along with their relived counterparts were basically smoking non-stop even before we began the take-off roll. During cruise, a check captain came in and the first thing he did as he sat down on the jump seat was to light a stick of unfiltered cigarette.
Sometime later, a flight stewardess came in and informed us that whiff of odour from the tobacco can be smelled from in first class so I essentially reach over and opened the cockpit smoke evacuation port and now everyone began to wonder what that loud hissing noise coming from upfront is.
During maintenance, repair and overhaul. It still shock me in some instance how thick the layer of nicotine stain can be with even some forming into a sort of "layered nicotine cake" while removing the cockpit instruments for inspection and recertification.