The past 2.5 years of the COVID pandemic have put a big spotlight on indoor air quality and the spread of infectious disease. I’ve been writing about various aspects of indoor air quality since I started this blog in 2010, so a lot of this wasn’t new to me. Still, I’ve learned some new things, had others reinforced, and generally feel like I have a more complete view of the subject than I did before 2020. Here’s my big picture view of IAQ now along with an example of how it was put into action at a conference I attended last month.
Good IAQ advice
1. COVID is airborne. If you’re still wiping down groceries, washing your hands obsessively, or hiding behind plexiglass, you’re not doing the most effective things to prevent COVID or other infectious diseases. Some people call those things hygiene theater. The problem is mainly particles that float in the air for hours (aerosols), not droplets that fall quickly.
2. The best method to avoid COVID is to stay away from people who have it. That’s source control. The COVID protocols that relied on testing were based on this method. You test positive; you can’t get in.
3. If someone with COVID is in a room with you but they’re wearing an effective mask (KN95 or N95 worn properly), that’s source control and filtration.
4. If you filter the air with MERV-13 filters or better to remove particulate matter, your exposure is reduced if someone is in the room with COVID and not wearing a mask.
5. By bringing clean outdoor air into a building and mixing it with indoor air, you’re diluting any pollutants in the indoor air. That’s ventilation.
6. Ultraviolet germicidal irradiation (UVGI) is another proven air-cleaning method. But the key is that they must be properly engineered. If not, they may be ineffective or even destructive. To deactivate viruses in the air stream with an in-duct UV lamp, the power level has to be high enough for the air flow rate. And if the UV hits vulnerable materials, as is the case with some filters, it can damage them. This technology is best in hospitals and other buildings with a lot of people.
7. Carbon dioxide can be an effective way to monitor how much dilution is happening with ventilation air. With masks and good filtration of the indoor air, however, you can have high CO2 levels and little spread of infectious disease. Monitoring CO2 is only one part of getting clean indoor air.
8. Additive air cleaners may not make the air any cleaner. They may also do harm. There are no standards governing how they’re tested except for ozone emission. It’s best to avoid them.
9. Control the humidity. Neither too high nor too low is good. We’ve known for a long time that humidity affects indoor air quality. Now Stephanie H Taylor, MD and some others say that keeping the relative humidity between 40 and 60 percent can reduce the spread of infectious disease. There’s still some debate about that, though. (See this paper from the journal Nature.) One thing to be aware of with keeping the relative humidity higher is that you have to have a building enclosure that can take it in a cold climate. Otherwise, you can rot a building and even destroy it, as they did at Harvard.
An example from a conference
At the 2022 Westford Symposium on Building Science, also known as Building Science Summer Camp, we had a three day conference with about 400 people. Three of the four sessions on the first day were on the topic of indoor air quality and the spread of infectious disease, including my presentation on the Corsi-Rosenthal DIY box fan air cleaner. And we practiced what we preached.
Professor Bill Bahnfleth spoke on buildings and infection control (pdf) and evaluated the layered approach to IAQ at the event. It included:
- Upgrading the filtration in the room to MERV-13
- Increasing the mechanical ventilation with outdoor air
- Using three Corsi-Rosenthal boxes
- Irradiating the air with four upper-room ultraviolet germicidal irradiation lamps (one shown above)
- Some people (I was one) wearing masks
Below is the slide showing Bahnfleth’s calculation of the exposure reduction due to this layered approach.
Based on his calculations, we had an overall exposure reduction in the conference room of about 63%. If half the people in the room had been wearing masks, it would have been over 90%. The actual mask-wearing rate was less than 20%.
Of course, this is one example and incomplete anecdotal evidence, but there’s only one case of someone getting COVID at the conference. He’s pretty sure he got it there, but not in the main conference room. He was in a meeting in a small room that didn’t have those protective measures mentioned above. It’s possible more people got it and didn’t report back, but one out of about 400 people is pretty good.
The bottom line is that the workhorses of good indoor air quality are source control, filtration, ventilation, and humidity control. Ultraviolet germicidal irradiation is appropriate in some settings but must be engineered properly.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard.
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