Hi Gericom
A very unpleasant problem and one which others have had to deal with. I do, however, believe you are wrong about the source. The air vents do indeed run vertically to the roof but even if someone was quite unwell in a neighbouring apartment it is unlikely that you would experiance the strong smells you describe.
The sewage system is the more likely source. The water in the toilets and in the traps below the metal discs on the bathroom floors should provide an effective barrier to odours. In some apartments a bad seal or missing seal below the toilets where they connect to the pipe has been the cause of the problem. In others the trap which protects the bath and sink and is located below the metal disc on the bathroom floor has not been properly fitted and does not provide a complete seal. Either of these problems can be fixed from within your apartment by a competent plumber.
In view of the number of people who have this problem (and you are right the problem is made worse when the wind blows in 1 dirrection) Ana and I considered fitting a U trap at the end of the sewer pipe in each block before it joined the main sewer. However, the plumber we consulted did not think this would solve the problem - pointing out that the horizontal pipes in the garage area did not have a sufficient fall to empty properly and that the contents of these pipes would continue to smell even if we fitted a U bend before they joined the main sewer and as the gas would be trapped in the pipe it might even make the problem worse!
I hope no one has been eating while they read this - Gericom if you need any further info I will be happy to talk to you.
This message was last edited by davmunster on 03/06/2013.