1/ Some thoughts spurred by the guidance below

6 eACH (ventilation + filtration) is somewhat arbitrary, but a reasonable guidepost. Keywords here are "at least." The allusion to 95% removal of contaminant in 30 min is a bit deceiving. It ONLY relates to after source is removed. Image
2/ For a well-mixed indoor air space, an increase in eACH from, say, 2/hr (unfortunately not untypical for K-12 classrooms) to 6/hr yields a maximum decrease in indoor aerosol concentration of 67% and not 95%. Starting at 3/hr = max of 50% reduction w/ increase to 6 eACH.
3/ And the net benefit is particle size dependent, as particle deposition to surfaces is a third sink (removal mechanism) that adds to removal by ventilation and filtration.
4/ For particles in the size range of 0.5 to 10 microns, the particle decay rate by deposition can vary from approximately 0.1/hr (for 0.5 micron particles) to 10/hr for 10 micron particles.
5/ For a specific particle size there is some variation in decay rates depending on a number of factors, including the extent of mixing (kinetic energy) of air, surface to volume ratio, and nature of indoor materials.
6/ For example, a reasonable range of decay rates for a 3 micron particle in indoor spaces is 0.3 to 1/hr. These particle decay rates must be accounted for to determine actual reductions in aerosol concentrations due to increasing eACH (ventilation + filtration).
7/ Further, increasing eACH by increased ventilation or a portable HEPA air cleaner increases mixing conditions in air w/ some increases in particle decay to surfaces, which should also be accounted for in estimates. This is an area still in need of some research.
8/ Let's take a situation of a 1 micron particle with a decay rate of 0.2/hr and a 10 micron particle with a decay rate to surfaces of 6/hr & assume that eACH refers to ventilation + filtration. I will adjust for the additional removal by deposition to surfaces at steady-state.
9/ For the 1 micron particle an increase from 2 eACH to 6 eACH is really an increase from 2.2/hr to 6.2/hr. That yields about a 65% reduction in 1 micron particle concentrations in the air, close to the original calculation of 67%.
10/ A reduction of 65% is nothing to turn away from. It is good, but is not 95% and I would not count on it as a sole source of protection. Good masks + the increased eACH would together give a tremendous reduction in inhalation dose and, therefore, risk of infection.
11/ Now let's look at that 10 micron particle. In this case the 4 eACH increase from 2 to 6/hr is more like 8 to 12/hr when a particle decay rate of 6/hr is superimposed. This yields a 33% reduction in 10 micron particle concentration in the air.
12/ Concluding point. Increasing the equivalent ACH by increasing ventilation and filtration is important and we SHOULD do so to the extent possible. Focus on the words "at least" and do not think that somehow 6 eACH is magical. It is a good guidepost to try to exceed.
13/ Reducing aerosol concentrations by increasing ventilation and filtration is very important during this pandemic.
14/ However, unless one is starting at an extremely low eACH & increasing to a value exceeding 6 eACH, having everyone wearing high quality masks, e.g., N95, will yield a greater reduction in inhalation dose for all ranges of aerosol particle sizes.
15/ The solution should be what I and many others have tried to scream from rooftops since early in 2020 - layered inhalation dose (and therefore risk) reduction is the key.
16/ Use multiple proven and effective interventions to reduce inhalation dose (mask, ventilate, filter, avoid non-essential spaces w/ large crowds). Add vaccinate fully (including booster), and protection will be extremely significant.
17/ If you do these things + test and quarantine as needed, you will also protect others. We too often forget that to end this pandemic we have to also work on reducing the inhalation dose of everyone around us as well.
17/ NONE of this is rocket science, folks. It's a little bit of math and a lot of common sense. Get vaccinated and lower your inhalation dose. That's the pathway into a safer 2022 for yourself, family, friends, colleagues, & anyone else you encounter in daily life. Just do it!

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More from @CorsIAQ

31 Dec 21
1/ The governor's words might be turned on those making such absurd statements. Inhalation of aerosols that contain the SARS-CoV-2 virus causes COVID-19. Right-sized HEPA air cleaners can be effective at reducing aerosol concentrations in classrooms.
2/ It is one important intervention amongst others (vaccines, masks, ventilation, testing) & plays a magnified role when significant increases in ventilation are not possible, when children have masks off, and/or some are not vaccinated. But it provides benefit in all scenarios.
3/ There have been dozens and dozens of peer-reviewed papers in high quality journals by high quality researchers that reinforce the value of HEPA filters for lowering indoor aerosol levels, even during the past two years and in the context of COVID-19.
Read 7 tweets
23 Nov 21
1/ With COVID-19 on the rise (again). Remember, the SARS-CoV-2 virus is transported by respiratory aerosols. INHALATION of aerosols is the dominant transmission pathway. Many of us have not wavered on this fact since early in the pandemic. It's about inhalation dose, folks.
2/ Since the dominant pathway is INHALATION of virus-laden respiratory aerosols, then reducing inhalation dose of these particles or inactivating viruses in the particles is the answer to reducing infection (pre- or unvaccinated & even vaccinated breakthroughs).
3/ Science & engineering gave us all of the tools we need to drive this pandemic into the ground. Vaccinations are a gift handed to us on a silver platter. The rest is not rocket science. The rest should be common sense.
Read 7 tweets
29 Oct 21
1/ A Quantitative Risk Estimation Platform for Indoor Aerosol Transmission of #COVID19 (Open Access paper)

Some Cliff Notes in thread below.

2/ This paper offers a platform that is somewhat more mechanistic than traditional risk estimates for airborne infectious disease. It includes ability to include aerosol size distributions & emissions from infected individuals + particle fate mechanisms indoors.
3/ Insertion of control options (increasing ventilation, masks, filtration) in single zone systems can be employed. The model predicts particle deposition in the respiratory system of receptors & converts these to total volume of particle deposition.
Read 7 tweets
21 Oct 21
1/ Indoor Air Quality in Schools
Some slides from a presentation on indoor air quality in schools a decade ago. It contains reference to what I thought were important/insightful papers at the time. A lot more slides than shown here. Will post at corsiaq.com
Read 18 tweets
12 Oct 21
1/ Don't be fooled by some air cleaning companies that claim their device is highly efficient. When claims are made about removing 95% or 99% of particles from air, this nearly always refers to % of particles removed THAT FLOW THROUGH THE DEVICE (single pass removal efficiency).
2/ A device can be 99.5% efficient & highly INEFFECTIVE if it has little flow moving through it. The product of fractional removal efficiency (max = 1.0) & volumetric flow rate (e.g. cubic feet per minute) is the important parameter (& known as CADR or Clean Air Delivery Rate).
3/ Example - Company for device 1 claims it removes 99.5% of particles from air (fractional single-pass removal efficiency = 0.995), but the device moves only 30 cfm of air (CADR = 0.995 x 30 cfm = 30 cfm).
Read 5 tweets
19 Sep 21
1/ Inhalation Dose
It's been about inhalation dose from day 1. We have not kept our eye on the ball. Critical pre-vaccination. Critical post-vaccination. Been writing & speaking about this for nearly 20 mos. Reducing inhalation dose is a must. Come on folks, just do it!
2/ Reducing Inhalation dose using layered interventions is still important, whether unvaccinated or vaccinated (breakthrough cases), in close contact (near field) or far field in the same indoor space, in a car or classroom, in a restaurant or apartment, young or old.
3/ Inhalation dose associated with virus laden aerosol particles is defined by the same variables (whether close contact or far field ---- don't bicker about one or the other - I've previously tweeted about this):

Dose(inhal,i) = C(i) x B x t x fdep(i)
Read 25 tweets

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