Yes stickyb, its inevitable that some of the air molecules remaining in the cabin were in the cabin at departure, but how many times might they have been through the filters? There is even a possibility that at least one molecule may have remained aboard since the aircraft was built, though there's little chance that it has managed to evade the recirculation system filters. It might be fun to calculate the probability if you're into that sort of thing....
(
I believe one of the air molecules in my left lung was inhaled in August 1947 in a delivery room of the Cape Maternity Hospital, Capetown when I was smacked on the bum by a brutal midwife. But I could be wrong 
)
As to those people who imagine that Aircraft Maintenance Engineers don't religiously follow the Approved Maintenance Programme and change filters of any and every sort when called up in the schedule, think again. We are just as professional as the people who fly the aircraft so we really do try to follow the regulations. The problem with this line of thought is that unlike the oil filters, cabin air recirculation filters have no bypass valve. Blocked filters therefore have the same effect as turning the recirculation system off.
For those who cite "well documented cases" of in-flight infection, kindly provide a link to at least one of these documents.
For those who are interested in the truth rather than the half truths and downright lies that have devastated Asian airlines and several Asian economies (and BTW, there have been NO cases of SARS whatsoever in this particular SE Asian country) follow this link to find out all about
cabin air recirculation systems - they really do filter out particles down to 0.03 microns in size and this includes all bacteria and most viruses. Follow the links provided on the page for the full story including the 'legend' of the plane load of people who were infected by a single person.
Face facts, you're no more likely to be infected with anything in an aircraft cabin than in any other crowded environment where people might sneeze in your face or touch the door knobs with their infected hands. Like the office for example - there's no fresh air at all in ours, the wall mounted air conditioners endlessly recirculate the same stale air, while there are no HEPA filters either. (Cross infections are all our mothers' fault actually - "Don't be rude! Put your hand over your mouth when you cough!" )
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Through difficulties to the cinema
Cough-cough!! Gasp!! Aaattchooooo!!!
