1/ Source Reduction.

Ventilation & filtration are important, but so is source reduction.

"If there is a pile of manure in a space, do not try to remove the odor by ventilation. Remove the pile of manure." (Max von Pettenkofer, 1858)
2/ Humans who are infected are not manure, but we should do everything possible to reduce the emissions of infectors indoors (in addition to grater ventilation and improved filtration). So, how do we do that?
3/ First, anyone who has had contact w/ an infector should quarantine and not go to work, school, and other indoor environments other than their homes. They need to be removed from the playing field. Testing is also valuable for keeping positive cases in isolation & off field.
4/ Second, EVERYONE should be required to wear masks in buildings, as infectors at work, school, etc. are largely not identifiable. Even if the infector's mask reduces aerosol particle volume by 40% & receptor's masks are 40% efficient, that's a 64% dose reduction.
5/ De-densify building occupants to the extent possible w/ staggered work schedules, those being remote who can work or learn remotely. If total occupants are reduced by 50%, there is a 50% reduction in the probability (or total number) of infectors in the space.
6/ None of this is rocket science. But we need to focus on source reduction as an integral part of layered dose (and therefore risk) reduction. Listen to Max von Pettenkofer. Different time, but he understood over 160 years ago the importance of source reduction.
That is "greater" ventilation. "Grater ventilation" sounds painful!

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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
31 Oct
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 ...
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

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