inter floor ventilation

[mellie_man]

I have found that my downstairs WC has a false Pboard ceiling about 2' below original c1900 lathe+plaster ceiling. There has been a vent brick added to the void.

Why is this any different to a normal 1st floor/ceiling void to require ventialtion and as I would like to keep my house warm can i block it up (may be introducing a hole in original ceiling)

 

[noseall]

I guess being a w.c. and possibly having a wash basin and all, then the void is perhaps vulnerable to condensation build-up. Floor voids can be draughty places so are less prone to condensation.