The foul pipe system needs a vent to atmosphere.
Normally within roof space to external soil stack, opr you can have a AAV "Dergo valve" which normally sits next to the cistern above the bowl rim which allows air into the system but not out.
As the use of the foul drain causes pressure it may blow out water left in the traps which would normally seal away bad odours found in the pipes.
Paul
August 2009