Study: You Fart More Above 11,000 Feet

It's not their fault. As they climb higher, the outside air pressure falls so there is less resistance to the release of body gases – and uncontrollable, pneumatic flatulence occurs. The condition is High Altitude Flatus Expulsion. According to the journal, High Altitude Medicine and Biology, it kicks in above 11,000ft
http://news.independent.co.uk/world/science_medical/story.jsp?story=120500