This is an important article about the dangers of engine chemicals mixing with the cabin air. There's also the airplane air quality issue not referenced in this article about the recirculation filters against spreading airborne microbes. Who hasn't heard someone say "you always get a cold 3-5 days after taking an airflight". There must be articles about the epidemiology of airflight. (rate of appearance of novel virus serotypes around Atlanta, and Hillingdon, London, for example)