To prevent the unhealthy, constant presence of sewage odours in bathrooms, you need to look at how the fittings have been installed, because it is precisely the poor way in which they have been installed that causes these unbearable odours; so the falls of the waste pipes of the sanitary fittings must be corrected, and proper bottle traps must be installed correctly. The trap, dear friend, is a wonderful invention. Let us explain.
The problem you are suffering from is not a rare one; unfortunately many users suffer from it, since bathrooms often have bad odours attributable to defects in the waste pipes of the sanitary fittings. How are they corrected? By doing as the Romans did, who were masters of handling gravity and siphons.
With the help of a trusted, experienced plumber (perhaps, if he is descended from the Romans, it will be in his genes), you should check that the waste from toilets, slop sinks and squat toilets always runs directly into the downpipe. The steeper the fall, the more gravity, the better.
And the waste from kitchen sinks, utility sinks and pumped appliances must have an individual trap. The distance from the bottle trap to the downpipe must not exceed 1 m. The distance from the furthest fitting to the bottle trap must not exceed 2.5 m. And the distance from the furthest trap to the downpipe must not exceed 2 m.
We have sometimes seen toilet waste pipes that run for up to 2 m to reach the downpipe on the attic floor, a run which, together with the current situation of some toilets discharging independently, reveals the incorrect fall of the waste pipes.
We have also found in some homes that the bathroom bottle traps do not form a water seal to prevent odours escaping, because the water does not completely cover the waste outlets. That is the botched work done nowadays.
So now you know that good gravity and traps are the principles that must prevail in your bathroom, and it is in the waste pipes of washbasins and toilets that they perform their greatest function, preventing the foul smell of rotting matter in the sewers from coming out through the plugholes of those fittings. But do not think these inventions are only useful for that prosaic function, or that they are a recent invention. The Aqueduct of Segovia, or those of Terrassa or Mérida, the ones in France and Italy and, in short, all Roman aqueducts are based on those two principles: gravity and the U-shaped inverted siphon, which, as well as enhancing the landscape and giving a sense of grandeur, are today a source of tourism. Of the cultured people who travel to where they still stand in order to admire them, not all understand the keys to their science (the different heights, gravity and siphons) but only look at the stones, and at the place where they are going to eat roast suckling pig afterwards.
Any questions?