The air coming into the cabin may be much more clean than the air we breath on the ground, but two other things are going on. First, a percentage of the air is recirculated, and the amount recirculated on the newer jets is substantially higher than on the older jets. Second, it is being contained in a rather limited space (the cabin) which is densely populated with people, some of whom have contageous diseases. So, one's chances of contracting the common cold or something worse is far greater in a plane's cabin then on the ground. Then there is the matter of whether harmful gases are being outgased from the seats, carpets, plastics, foams, etc. And that is a whole other story that it would be nice to get the strait scoop on.