1/ One of my slides. Cost of good portable HEPA filter system in a 600 to 800 square ft classroom with 25 students (equivalent ACH increase of 2.7 to 3.5/h). Cost is broken down by classroom and student.
2/ Cost = year 1 capital cost + annual recurring costs for filters + electricity at $0.11 per kW-hr.

Total 3-year cost per classroom < $790 (< $11/student for 75 students over 3 years).
3/ If school has 50 classrooms (larger than my K-8 school) the total cost over 3 years < $40K for entire school. Less for less classrooms.

If noise is an issue can go with 2 HEPA air filters per classroom at lower setting for around $20/student.

How much do books cost?
4/ Maintenance time. I timed myself switching out three HEPA filters in a single portable HEPA filter system last Friday for the units I purchased in my building @Portland_State. It took slightly less than 4 mins.
5/ This will likely have to be done twice/yr in each classroom w/ some caution (careful, wear mask, filter into plastic bag, sealed, wash hands). Turn system off on Friday afternoon and replace filters on a Monday morning.
6/ This is a proven technology, folks. The cost per student is small and the overall cost to the US is very small compared to what schools will be receiving, the overall inhalation dose (and risk) reduction, and benefit toward re-opening schools. Just Do It!
6/ Note - This is only for classrooms and does not include other spaces in schools, some of which are too large for reasonable consideration of $250 HEPA filters (e.g., gymnasiums). But the classroom environment is critical. So let's get that done right.
7/ Along with masks and distancing and increased ventilation where possible, etc., portable HEPA filtration systems SHOULD be considered to reduce inhalation dose and risk. Just Do It!
To be absolutely clear, I do not endorse any of the products that are being added as replies to this tweet thread.

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More from @CorsIAQ

20 Feb
1/ Tragedy in Texas. When I was Chair of @ut_caee at UT Austin we implemented a new strategic vision that revolved around water, energy, growth of the built environment, and innovations in infrastructure that tie these elements together as a COMPLEX system.
2/ I was so proud of our early-career faculty who led an effort, for example, to develop a new graduate area in Sustainable Urban Systems (SuS) that rapidly grew into one of the most in-demand graduate areas in our department. Great effort. Great success.
3/ The ongoing tragedy in Texas reflects the importance of systems-oriented thinking to make cities, states, regions, and countries more resilient, and to understand the connection between built & natural environments as climate continues to change.
Read 9 tweets
19 Feb
Portable HEPA Filter Systems.
1/ The cost of retrofitting EVERY public school classroom in the entire US w/ a portable HEPA air filter w/ CADR = 300 cfm is approximately $1 billion w/ recurring cost of about $300M/yr for replacement filters. To be clear, EVERY single classroom.
2/ For a classroom that holds 20-25 students, say 600 to 700 ft2 w/ ceiling height of 9 ft, this portable HEPA unit will yield approx equivalent increase in ventilation (equivalent ACH increase) of 3/hr. For a classroom under-ventilated to 2 ACH the bump up is to 5 ACH!
3/ That leads to a 60% reduction in inhalation dose of aerosol particles. Capacity exists. It can be done now.
Read 10 tweets
13 Feb
1/ Ventilation of Schools. My experience working with public school districts for more than two decades is that classrooms are, in general, woefully underventilated (and that's before COVID-19 times!). Others have found the same.
2/ Some results from a 4-yr study involving 7 high schools of different ages & construction, & 46 classrooms (focus on occupied day - plot at right). Approx 80% of classrooms = permanent. Red bar shows approx ASHRAE 62.1-2019 ventilation rate (small variations around bar).
2/ For permanent classrooms, median ventilation rates are less than 50% of ASHRAE 62.1-2019, w/ mean values approx 40% lower. Schools all had mechanical systems and capability of much greater ventilation. When asked why, our team was told (firmly) - energy savings ($$).
Read 20 tweets
13 Feb
1/ Incredibly disappointed in this @CDC document aimed at "layered mitigation". But I probably shouldn't be surprised by now. cdc.gov/coronavirus/20…
2/ It addresses near-field (close contact) inhalation dose w/ masks and distancing. That's good. Far, far, far more on fomites and surface cleaning than far-field inhalation dose (shared indoor air). Ventilation is given lip service w/ little guidance. Incredibly disappointing.
3/ The lack of understanding of ventilation or its importance (or perhaps just disregard) is wholly obvious. Incredibly disappointing.
Read 11 tweets
6 Feb
1/ Clean Air Delivery Rate (CADR). The CADR of a portable filtration system is:

CADR = f x Q.

f is the fractional removal efficiency of particles that pass through a device (value = 0 to 1). Q is the volumetric flow rate of air through the device (e.g., m3/hr, ft3/min). ...
2/ A portable air cleaner may be highly efficient (high value of f) but also highly ineffective at removing particles from air if Q is very small. This is the case for a wide range of ion generators. Don't be fooled by claims of high efficiency. It's f x Q that matters.
3/ On the other hand, a portable filtration system might have a lower efficiency (say, 0.5) but a very high air flow rate that leads to significant reduction of particle concentrations in air. It is f x Q that matters.
Read 19 tweets
5 Feb
B117 is spreading in US but we have not seen the explosion yet. Brace yourselves. These outcomes in Italy & Israel should give pause, a motivation for tripling down on reducing inhalation dose in all activities, and a quintupling down on layered dose reduction in schools. more..
Dose and risk reduction in schools must include re-opening plans based on extent of community spread, rapid exit plans, rapid testing if possible, rigorous and universal mask requirements, distancing and lower occupant density, pod groups, increased ventilation ....
supplemented with single zone filtration, e.g., portable HEPA air cleaner, to achieve at least 6 effective air changes per hour, as much outdoor activity (including teaching) as possible (I used to teach lectures on occasion outdoors in Texas - it can be done when elements allow)
Read 6 tweets

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