With the COVID-19 pandemic on our hands, people are becoming increasingly concerned about aerosol transfer from the corridors and common areas to apartment suites in buildings. Residents can feel a lot of air coming into their apartments from the corridor and are curious to know if the aerosols can be transmitted to their apartments. With airborne pathogen dissemination, the concern people have is very legitimate.
Pathogen dissemination through the air occurs through droplets and aerosols typically generated by coughing, sneezing, shouting, breathing, talking, etc.
Most of the larger emitted droplets are drawn by gravity to land on surfaces within about 3–6 ft from the source. Small aerosols (<10 μm), however, can stay airborne and infectious for extended periods (several minutes or even hours) and have an increased reach ranging from 6-15 ft from the source when expelled by a cough or sneeze at standing or slow walking speeds.
Most apartment buildings provide the ventilation air to suites floor by pressurizing the corridor via supplying outdoor air. The system relies on infiltration through the entrance doors of the individual suites. This happens through an air gap under the suite doors. In some buildings, the air goes into the suites through transfer grilles over the entrance doors to facilitate air passage.
Figure 1: Typical HVAC Apartment Design
Ideally, the outdoor air in the corridor forms a natural one-way infiltration into the suites. The corridors are a positive pressure zone, while the suites will run a negative air pressure. Ideally, one washroom exhaust fan is always on with assistance from things like kitchen hood fans to facilitate the bringing of outdoor air into the suite. If the system performs as designed, no air will transfer from suite to corridor and there would be no cross-contamination from suite to suite.
There could be several reasons for how the system can be compromised: Suites with operable windows can have wind effects that can cause pressurization and prevent corridor air from entering. This can adversely affect the operation of the conventional corridor air supply. In high rise buildings, stack effects can change the natural pressures vertically in the stairwells. A stairwell door kept open for a prolonged period has the potential of depressurization of the corridor, hence reversing the airflow. The air from suites can now come into the corridor and make its way to the other suites.
HVAC engineers have long known that tiny particles of pathogen travel in the air that is circulated, in buildings. Better known as of late, Legionnaires disease or bacterial pneumonia, has been detected in the buildings’ air-conditioning systems around the world. If the building has a basic and/or poorly designed ventilation system, the potential of cross-contamination can be quite high.
So how can we mitigate or rid the chances of cross-contamination?
One way to reduce any air coming from the corridor is to seal the opening in the corridor door. Air will always follow the path of least resistance into the suites. This would work equally when each suite is closed and remains closed. Only one suite opening into the corridor could mean the door will see an extremely high pressure in this case. The best solution might be to limit the opening of the doors to a bare minimum via a newsletter or information bulletin to residents. This will mitigate exposure and help maintain air pressures.
A good information point would be to suggest the use of filtered masks and sanitization of common areas, when possible.
Figure 2: Personal Protective Equipment
A big problem in an apartment building is the elevator. When you take the elevator, you are sharing the airspace with its current occupants along with the airborne particles from previous occupants. Again, the use of masks should be suggested along with the inclusion of sanitization materials in the elevator to allow occupants to wipe down touch points such as rails and the control panel.
Stairwells are a good alternative to the elevator as they provide a much bigger area to mitigate the risk of exposure. Maintaining social distance in the stairwell and, of course, masks are a great addition to this solution as well.
If you have HVAC questions related to COVID 19, feel free to reach out to us and discuss!