I flew a Malaysian 747-400 from Kuala Lumpur back in the late 1990s - upper deck Business Class. The air was so thick with cigarette smoke that it was hard to breathe. Malaysian didn't have smoking/non-smoking sections, so the flight attendants help move us around to segregate the smokers to the back - it helped but not much. At the time, smoking had been banned on all the North American airlines, but not foreign airlines operating into North America.
It was a mistake I never made again - 12 hours in that sort of smoke was barely tolerable.