If you own half a duplex, a rowhouse, or a condo with a shared wall, your air does not stop at your drywall. Every exhaust fan on the other side is a pump pulling from somewhere, and that somewhere is often your house. ACHR News has a piece on multifamily air that makes the core point well: "Exhaust, make-up air, ventilation, filtration, and building leakage all interact". That sentence applies to a two-unit house just as well as to a large building, with fewer fans and a shorter path to your living room.
Map Every Opening Before You Measure
Start with a notepad, not a gauge. Walk both sides of the shared wall and list every path air can use: outlet and switch boxes, recessed lights, plumbing penetrations under the sink, the top plate where the wall meets the attic, the band joist if the building has one, and any soffit or chase that runs between units. Note which exhaust sources sit on your side, including your bath fan, range hood, and clothes dryer. Then note what is on the neighbor's side, if you can see it or ask about it. Each of those fans is a pull, and any furnace or water heater in either unit is a combustion appliance that needs air to draft safely.
Read the Pressure Across the Wall
A cheap digital manometer with a tube to each side of the wall will show whether one unit runs negative against the other. Close the doors, switch off every fan, and record a baseline. Then run one exhaust at a time, starting with your bath fan, then the kitchen hood, then the dryer, and log what the gauge does after each one. You are looking for two things: which direction air moves across the wall, and whether the change is large enough to matter at the doors and at any opening where a combustion appliance draws its air. Repeat the test with the neighbor's fans if they will cooperate. Keep the readings in a notebook with the time and which fans were running, because a single reading tells you very little.
Smoke Shows Where the Pressure Goes
A smoke pencil or an incense stick is the cheapest way to see the leak paths. Hold it near outlet boxes, the gap at the top of a baseboard, and the edges of a recessed light while the exhaust runs. Smoke that bends toward the box is air being drawn into your unit, and smoke that leaves the box is air being pushed out. Check the attic too. Shared walls often open into an attic space that runs over both units, and a bath fan that terminates in that space can send humid, dusty air along the framing until it finds the next gap.
Make-Up Air Is the Missing Piece
Every exhaust fan removes air, and that air has to come back from somewhere. In a tight house it arrives through the nearest unsealed opening, which is often a shared wall, a chase, or the return side of a central system. This is why the same discussion has to cover exhaust and intake together. A fan without a planned intake is just a pump with no designated source. Sealing a shared wall is not a fix on its own. If you close every gap without giving your own exhaust a planned intake, the fan will pull from the next weakest opening, which may be a furnace flue or the gap under a door.
Filtration and Leakage Work Together
Filtration is the other half of that list. A filter on your return catches only the air that passes through it, and air that enters through a shared wall bypasses the filter entirely. If you upgrade the filter in one unit, check the actual air path before you assume the house is cleaner. Dust and cooking smoke that come through the wall never see the filter. Seal the leakage first, then size the filtration to the air that actually reaches the return. Check the filter frame for bypass gaps as well, since a filter that does not seat tightly leaks around its edges.
Know the Limits of a DIY Pressure Plan
A manometer and a smoke pencil will show you the pattern, but they will not tell you whether a fan is safe to run or whether a combustion appliance is drawing properly. If a furnace, water heater, or fireplace sits in the zone the shared wall affects, have a licensed heating contractor run combustion and draft tests before you change anything. Install a carbon monoxide alarm on each level if you do not already have one. Check your local building code before adding a dedicated intake or a new penetration, since requirements vary and the building department has the final word. If you rent any part of the property, the lease and local rules decide who can make these changes, and that question belongs with a local attorney or housing office.