Surely the amount of exhaled Entonox per patient (expectant mother) is going to be minuscule. This sounds to me like a (yet another) case of trying to solve a "problem" that doesn't actually exist. In my opinion normal (or even, natural) ventilation of the room shall suffice.
Further clues:- when used as originally intended (that is, with anaesthesia machines in operating theatres) AGSS removes exhaled gases from a continuous (and often prolonged) system. On the other hand, when used it Delivery Suites et al, Entonox in only given in response to patient demand (demand valve) and (normally, at least) should not be either prolonged or excessive; otherwise we end up with a new mother too dozy to respond to encouragement.
If unconvinced, it would be interesting to take an analysis of the room air when all this drama is taking place; how many parts per million etc. of this and that gas.
