COVID-19 Outside Air vs. Filtration

"This work was funded in part by the National Air Filtration Association (NAFA) Foundation."  I wonder what work funded by the National Fresh Air (NFA) Foundation would have shown in comparison? JK!  Thanks, Luke.  This dovetails nicely with some discussions we have been having with clients on the subject of filtration versus ventilation.

Thanks for sharing Luke. The study is basically an application of the Wells-Riley model for probability of infection. That's the same model that underpins our Flu Infection Risk Estimator tool (again, it's free and anybody can use it), so anybody that wants to do a similar study but change the parameters should be able to do so.  That said, this study is actually a good example of precisely *why* we built the tool. The parameters and assumptions change the outcomes greatly. For their example, they chose an office environment and stated: Per ASHRAE Standard 62.1, the minimum outdoor air ventilation rate should be 8.5 m3/hr per person + 1.1 m3/hr per m2 of floor area, which is equivalent to ∼760 m3/hr in the assumed space (yielding an air exchange rate (AER) = 0.51/hr). Assuming the outdoor air supply fraction of total airflow is 25%, the total supply airflow rate is ∼3000 m3/hr, with ∼2300 m3/hr provided as recirculated air. It would be a refreshing change for me to go into an existing building and find that the OA meets ASHRAE 62.1. I saw Joseph Allen referenced a couple studies on schools that estimated around 90% don't meet 62.1 requirements. I'm not at all surprised.  So, what about all those buildings that are underventilated, often significantly? In those cases, the same model shows far more benefit in upgrading the ventilation - i.e. bringing it up to ASHRAE 62.1 levels and perhaps a bit beyond. I was curious, so I ran it through our estimator.  Keeping all things equal to the study, here's the absolute probability of infection I get:

• MERV 7 Filters: 18.9% 
• MERV 13 Filters: 10.2
• MERV 7 + 30% more OA: 12.3%

So, in those parameters, I would validate that MERV-13 is a better choice. But let's say that instead of a ASHRAE 62.1 compliant VAV system, these 25 people are in a space served by a RTU that only responds to temperature, so it's cycling on and off about half the time, but we go ahead and throw MERV-13 filters in it. I can run that model by cutting the OA in half, and cutting the recirculated air changes per hour in half and bumping up the filter. Result: 

• MERV 13 Filter plus 50% less OA: 24% probability of infection

Similarly, I could have also kept it as a VAV system, but assumed that instead of full OA, it was only delivering 30% of the 62.1 amount because the OA was set based on peak cooling and is compromised in heating season when the box is at 30%, or a damper failed/stuck, reducing the OA. Again, super common everyday issues and why we get high CO2 levels in so many buildings. Basically, I get similar conclusions.  During the SMEPL show and tell, I know some folks were asking what worked better, filters, OA, humidity, etc. and I was squirrelly in my answer. This is exactly why - it's situation dependent.  tl;dr: Consider whether your building actually has ASHRAE 62.1 levels of ventilation before looking at filter upgrades. 

This is a LITTLE off topic, but one of the Infection Control docs that we are consulting with sent us this:  Researchers Create Air Filter that Can Kill the Coronavirus - University of Houston.  https://uh.edu/news-events/stories/july-2020/07072020ren-coronavirus-filter.php I look forward to your comments.  Full disclosure, being a graduate of a "competing" university from the old Southwest Conference, I have a disinclination toward the academic merits of what we referred to as "Cougar High".

"The researchers reported that virus tests at the Galveston National Laboratory found 99.8% of the novel SARS-CoV-2, the virus that causes COVID-19, was killed in a single pass through a filter made from commercially available nickel foam heated to 200 degrees Centigrade, or about 392 degrees Fahrenheit. It also killed 99.9% of the anthrax spores in testing at the national lab, which is run by the University of Texas Medical Branch." Most every living thing will die when heated to 200 C / 392 F. . .  not sure that makes for practical IAQ solution in an existing commerical building. . . Erik