Common myth and total rubbish
really? why's that? not trolling genuinely interested, it makes sense in my head-is the air not recycled then?
aircraft are pretty leaky things really. while there is a bit of recirculation going on, there's also a constant supply of new air from outside coming (via the engines) in to replace that which is lost – that's how the pressurisation is maintined.
there is a bit of controversy brewing among industry types (cabin crew, flightdeck crew) about illness caused by long term exposure to that compressed air – it's compressed in the first stages of the engine and is tapped off to feed the cabin. there's a chance that the air coming in is contaminated with engine lubricants – not significant as a punter, and only (as yet) allegedly of significance even after long exposure.
as crazy-legs says though, IMO it's more about all the people in the airport/gate/jetway/cabin than the quirks of air circulation.