Just talk to them. Most people are not aware of a smell that they constantly live in, so when you talk to them don't be a jerk. It's likely that they are not aware and once you make them aware, then they will do what they can to fix it. I don't know if you have any kind of a relationship with them, but even if you don't it shouldn't be too hard.
In case you need specific direction:
1. Knock on their door and ask them if they have a second to talk. If they don't, then find a time that works for them and just say that it's important.
2. Tell them that you aren't trying to be rude, but there is a strong odor that has begun to permeate the hallway and that it seems to be emanating from their flat.
3. Explain that it is extremely unpleasant and ask if they know what it is that is causing it and that they do something to fix it.
4. Bonus points if you talk to your other neighbor first and get them to help with the confrontation.
It isn't rude to confront somebody about an issue that negatively affects your residence. It would be rude for you to do nothing and then complain about it or judge them. I'm not saying that you are doing that here. You're just asking for advice.
It's "should have" and "could have." When a native English speaker uses of in place of have, he or she looks ignorant.
It is definitely NOT myself. Hope you didn't get that idea from self-experience :-P
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They don't, as i stated in my OP.
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Yes. We usually don't have that kind of drug here, cannabis is what people use as an herbal drug.
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Your advice seems solid and reasonable. However, i am dreaded by the fact i may actually have to do this. Even with the gentler and the politer way i could muster, a part of me would feel bad and guilty for bringing this up to them, especially while they are elders.
Last edited by Fabinas; 2015-11-12 at 01:03 AM. Reason: double post