What's the risk of COVID in apartment buildings? This review covers much of the basics but I believe the prevalence is much higher ... (THREAD) /1
https://ncceh.ca/documents/evidence-review/contextualizing-risks-indirect-covid-19-transmission-multi-unit

We've long known air flows throughout a building despite walls and floors because there's strong driving forces and little attention to air sealing, both on the building envelope, and between units. /2
Figure: Ueno, Lstiburek, Bergey 2012 https://www.buildingscience.com/documents/bareports/ba-1209-multifamily-ventilation-retrofit-strategies/view

The three driving pressure forces are: Wind, Stack (chimney) effect, and Mechanical (both negative and positive). /3
Figure: Air Leakage Control for Multi-Unit Residential Buildings, CMHC 2017 https://www.agency.coop/media/747/download

The stack effect is a dominant factor in winter, and is directly related to the height of the building AND the temperature difference between indoors and outdoors. Remove the temperature difference and the stack effect is minimal. /4
Figure: Cdn building digest number 104, 1968

Wind effect can exert very strong pressures that overwhelm other pressures, from minutes to hours, causing significant air movement within a building. /5
Figure: Handegord & Hutcheon, Building Science for a Cold Climate, 1983

Dynamic pressures on a building can be very high. Note a 5 Pa pressure difference from a kitchen exhaust fan in a home is enough to backdraft a naturally aspirated hot water tank in the basement. /6
Figure: plot based on physics

Mechanical pressures: It depends. Nearly all MURBs use corridor positive pressurization with fresh air to make up air exhausted from suites from kitchen and washrooms. Typically these flows are not balanced between units at all. /7
Figure: CMHC 2017 https://www.agency.coop/media/747/download

The implications are that in cold climates in winter, there are significant amounts of uncontrolled air movement between suites, sometimes substantially more than the intentional air supplied by mechanical systems. /8
Figure: Fitzgerald & Bohac, 2012 https://www.slideshare.net/mnceeInEx/mf-sealing-and-ventilation-bb-wi-2012-v32

*IF* any centralized exhaust fan is off, there is a potential for direct connection to other units with flows driven by wind, stack, and remaining mechanical pressures. /9
Figure: Center for Energy and Environment multifamily ventilation assessment 2016 https://www.cards.commerce.state.mn.us/CARDS/security/search.do?method=showPoup&documentId=%7BDB45217D-CD15-483D-87EE-E657487CAA7F%7D&documentTitle=282726&documentType=6

In Ontario alone there have been outbreaks reported in the news of MURBs in London, Sudbury, North Bay, Collingwood. All of these had significant numbers, all were in winter. There are likely many more, but health units aren't aware of them because they are overwhelmed... /10

Dr. Alex Summers: A heavy load of case area-wide and the ability to notice trends contributed to delay. "It wasn’t until we had a few weeks of seeing this common address pop up multiple times, that we were able to see that trend and declare outbreak" /11 https://london.ctvnews.ca/why-did-it-take-18-days-before-apartment-residents-knew-of-covid-19-outbreak-1.5248949

So what can those in mid-rise and higher apartments do?
DO NOT cover up exhaust vents, or block the suite door, they exhaust stale air and provide fresh air!
Check with toilet paper for this necessary air movement out of and into the suite. /12

Look for areas of uncontrolled leakage entering the suite from other suites. Pipe penetrations are common. /13
Figure: https://www.slideshare.net/mnceeInEx/mf-sealing-and-ventilation-bb-wi-2012-v32

Pandemic or not, if kitchen & washroom exhaust fans are not functioning (toilet paper not sticking to the grill) or worse, slight air movement into the apartment, bring it to management's attention immediately.
/14

Remember, #COVIDisAirborne.
Properly functioning mechanical systems help.
Portable HEPA air cleaners will provided lasting indoor air quality benefits.
Open your windows a crack.
Wear a tight-fitting quality respirator outside your home.
Don't panic: Be aware of shared air. /15

Whoops, forgot the figure. You want central exhaust fans to be operating. If not, there's potentially direct connection to other suites.
https://twitter.com/DavidElfstrom/status/1379814623004798976?s=20