Most toilets have some form of ventilation, allowing for the bad air caused by the occupants to disperse and go away. In the meantime however, the user of the toilet and other users after, have to endure the sometimes foul stenches of the human digestive waste products. If you suck air out of the toilet bowl, the possible stench will not escape into the rest of the bathroom. You could suc the air out through a T-joint in the tube that supplies the water to the toilet (yeah you need to make sure the water does not go the wrong way. gravity can help doing that)
-- nietsch, Apr 09 2004

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