This is unofficial advice given neighbor to neighbor; accuracy is not guaranteed, and any work on a unit should be done by licensed professionals with proper permissions from the building management.
There are many avenues for smoke and odors to enter your apartment. The two main ones are in and around your bathrooms and in and around your heat pumps.
There is a very large chase behind your bathroom’s wall with all the fixtures. The void is about 2 to 3 feet wide. On the second floor of your apartment, typical walls are brought flush with the main beam which runs down the center of your apartment. Smoke may enter around this beam. Additionally, smoke may enter through your bathroom vents. Good back draft dampers may be installed to prevent smoke from entering via your bathroom vents.
Heat Pump Chase:
Another sore spot is the area around the heat pump. There is a chase which runs behind your heat pumps which contains the water pipes and the condensation drain. These pipes don’t run vertically the entire six floors of the building but instead step outward as the apartments on the higher floors have narrower exterior walls than the floors beneath them.
There are two areas to seal. One area is in and around the heat pump. This may not be sealed properly. If the heat pump is not original, the installer may not have done a good job sealing the holes to this chase. When your heat pump fan kicks on, it can draw air from your neighbors below or above you.
The other area is again in and around the main beam down the center of your unit on the second floor. In this case, to seal this area, you would need to do one of two things. Either seal the air infiltration around the chase or seal the beam around the wallboard. The former is the ideal solution.
The last area of smoke infiltration is from the front of your unit around the center beam and around the front door.
We’ve been fortunate in that we haven’t been bothered by excessive smoke or odors entering our unit. But when I do notice something, it’s often coming through the fire corridor. Were I to try to block air flow from that corridor, I’d weatherstrip around the 2nd floor fire door (being careful not to restrict its use in any way), and then turn off the lights in our loft and see where around the beam I could see light from the corridor shining through and take steps to seal those areas.
In addition to the heat pump chase and the bathroom chase/vents, there is also the vent in the kitchen to consider when thinking about backdraft dampers.
Further Information from Bill (added summer 2022)
I have a sensitive nose and odors have bothered me since I moved in. I remember Cathy [the building manager for 27 years] telling me that a lot of people have complained about odor infiltration from other units. There are three main culprits. I have now fixed two of these. The new switch I had installed in the bathroom for turning on the fan separately from the light helps with the third one.
1) Each heat exchanger or air conditioner has a raceway behind it. If replacing the unit, you will need to seal off that area completely. Steve Bedard at A1 Maintenance knows how to assist in this to some extent. On my second floor there was a large hole behind the unit which connected to the vacant area right next to the unit under the beam where odors (i.e. smoke) came from. The unit owner above me asked if I knew who the culprit was for the smoke and I told him I had no idea but it must be below me. At any rate, I blocked off that hole and this took care of the problem for both of us. Since sealing off this area, my upstairs neighbor has reported that it made a huge difference. In his case, I took care of both of his heat units because I blocked it off from below. In my case, I was only able to take care of my upper unit but the lower unit was ok anyway.
2) Also on the second floor in my apartment, there was a large hole between the bathroom and the balcony/loft area which again led under the beam. I opened the wall in the hallway to take care of this and discovered a hole about 2 feet wide and 6 feet high. Since blocking this off with drywall, it has made a huge difference.
[As Bill points out, the large beam in the second floor ceiling is often the culprit when it comes to odors. This is because it is actually two beams braced together with a channel between them that unfortunately funnels odors or smoke from wall cavities into the unit. When Bill opened the wall to properly block this “channel,” he found a virtual smokestack bringing up odors from below. —admin.]
3) The last way odors enter my apartment is through the vents in the bathrooms. I intend to get to this one day by replacing the original vents and putting in something that prevents back drafts better. I wonder if anyone has done this in their apartment.
There’s so much I could say about this odor thing so I am really glad items 1 and 2 are not an issue anymore. Directly after I opened up the hallway wall to take care of item 2, I woke up at about 5 AM one morning to the smell of perfume. The only thing I can figure is that a person below me resorts to masking the smoke or whatever with perfume.