Every apartment in a multifamily building shares its air with the rest of the building, whether the residents notice it or not. A bath fan in one unit pulls air out of that unit, and something has to replace it. Where that replacement air comes from decides whether the hallway door is hard to open, whether a stairwell drafts, and whether a gas appliance in the next unit backdrafts when the kitchen hood runs. ACHR News has a piece on how exhaust, make-up air, ventilation, filtration, and building leakage interact in multifamily properties, and its core warning fits any service call: "getting one wrong can affect the entire building."
Start With an Exhaust Inventory
Before you measure anything, list every device that moves air out of the unit or the building. That usually means bath fans, the kitchen range hood, the clothes dryer vent, and any shared or central exhaust that runs on a timer or a switch in a common area. Read the labels. Bath fans and hoods are rated in CFM, and that number tells you how much air each one wants to move. Add them up, then ask which of them can run at the same time. Two fans running together pull harder than either one alone, and that is where most of the trouble starts.
A smoke pencil or a stick of incense makes the airflow visible. Close the doors, run one fan at a time, and hold the smoke at the door undercut, the frame gap, and any wall penetration you can reach. If the smoke gets pulled toward the fan, air is moving in through that opening. If it streams out, the unit is pushing air somewhere it should not. Repeat with the front door closed and the hallway door in its normal position.
Where the Replacement Air Comes From
Make-up air is whatever enters to balance the air leaving through exhaust. In a sealed unit with closed doors, it has to come through leakage: door undercuts, gaps around outlets and recessed lights, pipe chases, or the corridor. If the corridor is the source, the building's shared system is part of the problem, and the unit's fan is only one input. If the air comes through a chase that also serves a combustion appliance, you have a backdraft risk to check.
Look at the gas water heater and furnace first. Atmospherically vented appliances rely on a natural draft up the flue, and a strong exhaust fan can reverse that draft. With the fans running and the appliance firing, hold a tissue near the draft hood. If the tissue gets pulled toward the hood, the flue is not carrying combustion products cleanly. That is a call for a licensed technician, not a homeowner fix. A carbon monoxide alarm belongs in any unit with fuel-burning equipment, and it should be tested on a schedule, not just after it chirps.
Filtration and Ventilation Are Separate Questions
A filter rating describes how the air is cleaned as it passes through the air handler. Ventilation describes how much outdoor air is intentionally brought into the building and how it gets distributed. These are different jobs. A building can have decent filters and still starve its units of fresh air. It can bring in plenty of outside air and then pass it through filters that are too low in rating for the load. It can be filtered and ventilated on paper while leaking badly through the envelope, so the filtered air never reaches the rooms where people sit.
Check the filter size, the rating, and the change schedule on the air handler that serves your unit. Those details are printed on the filter rack or the equipment label, so you do not need special access to find them.
Questions for the Building Manager
When you talk to the owner or manager, keep the questions specific so the answers are useful:
Which units share a stack, shaft, or chase with my exhaust?
Is there a dedicated make-up air path, and where does it connect?
Who maintains the central exhaust, and when was it last checked?
What is the filter size, rating, and change schedule for the equipment serving my unit?
Has the building had any pressure or leakage testing, and can I see the results?
A Note for Families
If an older relative lives in one of these units and spends long stretches indoors, air quality becomes a daily check rather than a one-time project. Someone should walk through the unit regularly, run the bath fan with the door closed, confirm the carbon monoxide alarm is working, and note any new drafts, odors, or stale spots. Anything that points to the equipment or the building goes to the manager and the licensed contractor. A local option such as non-medical in-home support for older adults in St. Louis can handle the routine walkthrough and the reporting, while repairs stay with the people qualified to make them.