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September 3, 2020
So…COVID, school openings, ventilation, cold climates, what can we do about air quality in closed buildings?
Three key things you need to know:
Install/use/measure/verify mechanical ventilation with filtration that can provide 3-6 air changes per hour
Control relative humidity to reduce lifespan of virus in aerosols.
Source control (masking) and distancing (room occupancy) cannot be separated from ventilation.
Fresh Air Flows, Relative Humidity, MEASURE THAT STUFF
There’s a lot of noise out there: 'faster' air movement = more air changes per hour (ACH) = better dilution/shorter exposure.
Well, yes, assuming you’re starting with a well-designed delivery system that can provide the amount of fresh air required (3 to 6 ACH/room). Duct size, system layout and ventilation equipment capacity dictate this.
All systems need MERV-13 filters (but these impact air flow). Forced air systems recirculate interior air, so that's why filtration is so very important. Note: ventilation - bringing fresh air inside - is not the same as recirculation of interior air. Air flow measurements need to be taken AFTER recommended filtration is in place.
Not all buildings have a forced air system. Canadian energy engineer @DavidElfstrom shared this flow chart from Harvard Healthy Buildings on ventilation.
Cap Occupancy Based on Floor Area and Ventilation Rates
@DavidElfstrom also suggests capping occupancy on floor area and ventilation. This makes sense. Arbitrary classroom caps (15? 12? Half of what a jurisdiction considers a ‘full’ classroom?) are not useful, because classrooms are different sizes and shapes, with a massive variation in ventilation rates. Use SCIENCE to tailor the solution to the space, not an arbitrary % reduction of the current teacher to student ratio.
Relative humidity (RH) levels mitigate the success of ventilation because humid air can extend the life of aerosol droplets. Relative humidity levels need to be controlled, both in classrooms and in hallways (think rainy days, wet gear for a few 100 kids, and a closed building with steamed up windows).
Here’s a nice infographic that breaks down the process of improving air quality/reducing risk for COVID.
What About Ventilation Systems That Don’t Measure Up?
Measuring air flows is great, but where systems are found deficient - who's going to do that work, when are they going to do it, with what budget, to what standard, and what about hazard mitigation (whoops! Who could foresee finding asbestos wrapped ducts/pipes/plaster in old schools, lead paint, etc.)?
Adequately sized stand-alone HEPA filters should be in every classroom, regardless. And they need to be maintained.
When we talk about indoor air quality in building science, there are three primary ways to create and maintain a healthy indoor environment: elimination (source control of air and moisture-borne pollutants), ventilation (dilution of pollutants), and filtration. Our best chance at creating safe interior spaces is to make the actual spaces as safe as possible, restrict the occupancy based on room area and actual, measured ventilation rates, control for humidity, and then reinforce the importance of occupant behaviour: distance/PPE, hand washing and room sanitizing.
***NOTE: BHE specializes in building science in the residential arena, but the physics of what is going on are the same, regardless of the building.
For deeper dives into #IAQ, #ventilation, #buildingscience, follow these folks: