1/ Previous analysis by my team of occupied-day average & peak CO2 concentrations in K-8 classrooms in Central Texas and Rio Grande Valley. Results shown as cumulative distribution plots. So, 35% of classrooms w/ CO2 less than 1,000 ppm & 65% greater (note ref on absenteeism).
2/ 20% of classrooms with average CO2 concentrations greater than 2,000 ppm and peak concentrations greater than 3,000 ppm! Sadly, conditions in the Rio Grande Valley were far worse than in Central Texas (an indoor socio-environmental justice issue). next ....
3/ Mean occupied day CO2 concentration in Central Texas = 1,300 ppm and in RGV = 1,800 ppm with similar disparity for peak values. Also compared portables versus traditional classrooms. more ...
4/ Across the two school districts the mean CO2 concentration was about 150 ppm greater in portable classrooms than in traditional classrooms. This was skewed by tampering w/ AC units in portables in the RGV (often switched off or cardboard & duct tape over inlets).
5/ Two major reasons given. Noise = #1. Energy savings = #2.

Results from early 2000s, but more recent sampling by others suggest little change. Can only hope for better during and post-pandemic.

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More from @CorsIAQ

1 Nov
1/ I have left my blog up on Inhaled Deposited Dose at corsiaq.com. IMO we should discuss this concept more & use it as a guide for how to effectively reduce dose, risk of infection, and infection outcomes.
2/ Without knowing what the actual dose-response relationship is we should assume that any level of inhaled deposited dose leads to some risk (precautionary principle). The higher the dose the higher the risk of infection.
3/ It is also likely that the higher the dose the worse the outcome (on average) from an infection. There is certainly individual variability (elderly, diabetics, etc.) wrt dose-response.
Read 5 tweets
25 Oct
1/ Why universal mask wearing in buildings is important

First, the persons infected are unknown. So, if everyone wears a mask the infectors are also wearing masks. Second, everyone else (receptors) get an extra layer of protection for also wearing a mask.
2/ The % reduction in volume (or mass) of aerosol particles inhaled by a receptor is given by the following equation: Z = A + 0.01 x B x (100 – A)
3/ Where Z = combined % reduction in inhaled particle volume by receptors, A = % reduction in emissions by virtue of infector(s) wearing masks, B = % reduction in inhaled volume by virtue of receptors wearing masks.
Read 9 tweets
24 Oct
1/ Not surprising & consistent w/ comments I made in a recent article by @RobertRoyBritt. Exposure time is NOT the only factor that determines risk, whether in close contact or indoor far field. We should focus on inhaled deposited dose (IDD) & not just on distance or time.
2/ IDD = Concentration (C) (#/L) x Respiratory minute volume (B) (L/min) x Time of exposure (t) (min) x Fraction deposited in respiratory system (f) (-). C & f are taken for specific particle sizes and the resultant size-specific IDD values summed (integrated) for total IDD.
3/ Concentration (C) depends on a number of factors, including distance, emission mode and strength, air flow conditions (e.g., cross-flow, mixing/TKE) & whether the receptor is wearing a mask (as concentration is in breathing zone inside mask), quality of mask, etc.
Read 11 tweets
22 Oct
There has been good work on dilution near source using thermal, breathing manikins. I will try to look this up & post a couple of good papers. more
Of importance is also if someone in close contact is breathing heavily, e.g., in an aerobic workout class, etc. In that case the respiratory minute volume can be 10 to 15 x what it is at rest AND the emitter might be emitting much more. Does that mean 1 minute or less contact?
I spoke about the new CDC guideline today on a radio show with listener questions. We need to be clear that there is nothing magical about 6 ft or 15 minutes. These are gross generalization that provide something "simple" for the public. more...
Read 4 tweets
22 Oct
1/ On new CDC guidelines that move from single-event exposure time for close contact to 24-hour integrated exposure time over 24 hours. My take ....
2/ First, while exposure time is an important factor, it is incomplete. At corsiaq.com, I describe inhalation dose. Time is an important factor, but so is concentration of aerosol particles in the breathing zone & respiratory minute volume.
3/ Concentration (C) decreases with distance from the source, and so 10 ft is better than 6 ft, etc. Take advantage of this, but also know that some dose occurs in the far field as well. Further, C is taken in breathing zone, e.g., air inside mask. Thus masks reduce C!
Read 9 tweets
13 Oct
1/ Important tweet by @linseymarr.

Aerobic activities in gyms are a bad idea during this pandemic, and it is difficult to get risk down to what I believe are acceptable levels. Results of a gym simulation I did several months ago are shown here w/ specs next .... Image
2/ In this simulation I used Infector X in a restaurant in China with a significant outbreak of COVID-19. By analyzing metadata for the restaurant it is possible to ask the following question ... more ...
3/ If infector X is placed in a gym as a staff member, how much volume of virus laden aerosols would someone in the gym inhale relative to those who were infected in Restaurant X? The value "omega" on the vertical axis shows this ratio for different conditions. more ..
Read 6 tweets

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