Alter them before?
Andy, I thought it was baked beans that caused flatulance.
By the way Co2 is carbon dioxide not H2S hydrogen sulphide, thats the gas that produces the rotten egg smell. But both gases do the same thing to the human body, preventing the transition of enregy in the body. Thats why farting in bed and holding your girlfriends head below the duvet is dangerous...she might clout you one.