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AirQ Profile picture
@AirQ007
Feb 12, 2021 • 15 tweets • 6 min read • Read on X
(1/n) Given recent funding directed to the use of plexiglass barriers for fighting COVID-19, I thought a thread on what experts have said on the topic would be helpful...
(2/n)According to @kprather88 they block big droplets but aerosols float right around them
(3/n) @ShellyMBoulder suggests large plexi barriers may not reduce risk
(4/n) But are effective against spray borne droplets.
(5/n) @CathNoakes says they protect some but raise risk for other:
(6/n) @VVuorinenAalto suggests it can create recirculation zones that interfere with air flow
(7/n) @jljcolorado says that face shields and plexi will not stop the coronavirus
(8/n) @JimRosenthal4 notes that plexi barriers will always create negative flow behind them that draws particles in.
(9/n) @linseymarr comments that partitions increase risk in some locations and decrease risk in others.
(10/n) And a couple articles... this one asks what plexiglass barriers do to stop Covid: qz.com/1913843/will-p…
(11/n) This Princeton study investigates whether plexiglass can assist in reducing COVID-19 risk: nj.com/coronavirus/20…
(12/n) ...last, my $.02 (based on our own testing at AirQ): low barriers can increase the time & distance it takes for aerosols to spread (giving ventilation more time to remove them). Well designed barriers may help. Poorly designed, they will interfere with ventilation.
(13/n) I’ll add this recent comment by @HuffmanLabDU which suggests we shouldn’t be using plexiglass to block aerosols that flow like smoke. BUT they can be useful to reduce droplet spray (eg checkout counters).
Another thread on plexiglass:
Another useful related thread:

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More from @AirQ007

AirQ Profile picture
Apr 1, 2021
With teachers afraid for their own safety (despite assurances otherwise) it should come as no surprise that 20-24% are choosing not to work in what they believe are high risk conditions. hwdsb.on.ca/wp-content/upl… (1/n)
In a recent TDSB survey 80% of teachers reported not feeling safe at work. 70% could not access to tools/resources for their job. And only 30% cited trust between employees and management. tdsb.on.ca/Portals/0/docs… Let’s be clear: This is a crisis. This is unsustainable... (2/n)
It’s also very predictable: exactly what’s expected when people are in risky situations they can’t control. Many are familiar with the experiments of Pavlov (you know the bell ringing and anticipation of reward). Yet a much darker experiment by Martin Seligman followed. (3/n)
Read 12 tweets
AirQ Profile picture
Mar 31, 2021
Indoor measurements have convinced me that our model of how this virus spreads is incorrect. We understand superspreading events, and we also see the impact of close contacts of long duration. But there’s a third type of transmission: let’s call it the “stray bullet” (1/n)
Big box stores, factories etc, have a large volume of mixed air w/ many occupants. Measurement in large open spaces show breath is diluted rapidly. We released a large amount of CO2 in one such space: it dispersed at equivalent 16 ACH. Affecting all sensors in 1min45sec. (2/n)
So in a large open space, a single persons breath a few feet away may only elevate CO2 levels by 30-50ppm because dispersion is rapid (much like outside). But 20+ feet away it may still stick around and be 3-5ppm higher. (3/n)
Read 11 tweets
AirQ Profile picture
Mar 21, 2021
Many experts recommend NDIR sensors for sensing CO2. But what exactly is NDIR sensing anyway? And what are the other technologies that we might use to sense CO2 in future? Let’s explore this in a short thread... (1/n)
Fortunately air is transparent. Otherwise we wouldn’t be able to see a thing: it’d all be a smoky haze. But this isn’t true at wavelengths invisible to us. At other frequencies, gases are opaque, & look like smoke in the air. Here’s a chart of these “smoky” wavelengths. (2/n)
For CO2, that main wavelength is 4.26 μm. So if we want to detect CO2 in the air, we just need a light at exactly that frequency plus a light detector. The more CO2 in the air the more smoky it will be. So we use the amount of light detected to measure CO2 concentrations. (3/n)
Read 7 tweets
AirQ Profile picture
Mar 6, 2021
Saturday morning is a great time for a thread on how to calibrate your CO2 meter. Why Saturday morning? Because we calibrate to ambient CO2 levels in outdoor air. And on Saturday mornings it’s lower than other days/times since there’s less traffic around. Let’s get started! (1/n)
So, how do you know if your CO2 meter needs calibration in the first place? The easiest way to check is to take a reading outside. If less than 400ppm that’s below global ambient. For that check Mauna Loa Observatory in Hawaii esrl.noaa.gov/gmd/ccgg/trend… (today it’s 416.75 ppm)(2/n) Image
So if you’re getting a lower reading than the clean air in Mauna Loa, it’s probably not accurate. If it’s too high, that’s trickier. Check this map for (approx) CO2 at your location. earth.nullschool.net/#current/chem/… But ensure there’s not a CO2 emission source in your area. (3/n)
Read 12 tweets
AirQ Profile picture
Feb 21, 2021
Exactly one year ago I was quite concerned about a new emerging virus. So I created a Facebook group to share practical fact-based info about it. This was one of the first posts on Feb 21, 2020: “This image below is the most important part of the daily WHO updates...” (1/n)
“...It’s the chart of number of international cases outside of China, and we’d like to see it start trending down. At first (on left), we see that almost all cases were from China (orange), then over time, the growth was from cruise ship cases (blue), and now it’s...” (2/n)
“starting to spread within international countries (green). So that green number is the one to watch, and as you can see it’s starting to grow. Hopefully that’s due to some recently-found cases they can all track down and stamp out, but if it continues to spike then...” (3/n)
Read 4 tweets
AirQ Profile picture
Feb 18, 2021
Today one of my kids took a CO2 meter to class. They said it was hard to see if windows were opened a crack or not, but here’s the chart: Image
The dip in the middle is when they left for a short break.
Also here’s the humidity chart, a gradual decline throughout the morning: Image
Read 4 tweets

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