Further to Doncas concise reply, here are a couple of links that tell it from the horses mouth.
Boeing: Commercial Airplanes - Cabin Air Quality Home
http://www.scientificjournals.com/sj/espr/Pdf/aId/2518
There are many more links available on Google, including many scare-monger sites! But as a doctor, I'm sure you are well praciced at sorting wheat from chaff!
Shinobi