David Elfstrom Profile picture
Jun 13, 2021 23 tweets 9 min read Read on X
1/ Oh hai Clinical Excellence Commission @NSWCEC. Looks like you wrote an IPAC Manual for Quarantine Hotels recommending air pressure differential limits without considering the implications. Would you like some assistance with that? THREAD... Clippy from Microsoft Office 97Image of web page showing IPAC Manual for Quarantine Hotels
2/ Allow me to introduce you to Bernoulli's Equation as it pertains to flow through an orifice. In this case, under and around a door. Air flow through a sharp-edged circular orifice. Q = Cd * Af
3/ Bernoulli's equation allows us to predict airflow from a pressure difference, and vice-versa. The Florida Energy Center tested various orifices for flow & pressure difference. ba-pirc.org/casestud/retur… Using these results I calculate a Cd of 0.71 for various slots. Table from Florida Energy Center study on return air paths s
4/ Not wanting to be wrong, I confirmed this at home. I measured the gap dimensions under a bedroom door, the air pressure difference, the flow entering the room, and the average velocity across the bottom of the door into the hallway with a mini vane anemometer traverse. Measuring airflow entering the room at a floor register of 3Photo of differential pressure reading of 2.2 PaPhoto of mini vane anemometer traverse of the velocity of th
5/ For a 2.2 Pa positive pressure in the room relative to the hallway, the flow was 33.4 CFM (16 L/s) and average velocity 235 fpm (1.2 m/s) into the hall. I noticed at one edge the flow was reversed, entering the room. It was also much higher in the center.
6/ With the measured crack area and 2.2 Pa the calculated values are 16 L/s and 1.4 m/s, so the measurements match well with the model. The velocity measurement is difficult to make, especially with the reversed flow, so it makes sense it would be a bit lower. Calculations in a spreadsheet for air flow and velocity give
7/ Turning to @NSWCEC's IPAC manual, it has a cutoff of up+5 Pa pressure difference (room higher than corridor), where "the room has very low risk of particles leaving the room when the door is opened".

Except there is always a crack, usually by design, and air flows freely. Excerpt from NSW IPAC Hotel Quarantine Manual  on page 22 of
8/ The crack under the door is there for typical corridor positive-pressure makeup air. If an exhaust fan is running, the makeup air has to come from somewhere. Positive pressure in the hallway keeps odours in the suites and supplies fresh air.
9/ It turns out that average velocity is independent of the area. In the limit the crack may become sharper, affecting the discharge coefficient, but otherwise this chart shows the average air velocity through a crack for a Cd of 0.71. Why allow up to 5 Pa? Chart showing plot of average velocity under a door vs press
10/ Shouldn't it be negative? A 3 Pa positive pressure still shoots air out into the hallway through a crack at 1.6 m/s. Why is that "low risk"? Here's an example from @drajm in a Perth hotel:
11/ I get it, this stems in part from the quarantine leak report which cited opening and closing of doors. Were differential pressures measured? Pressure is fundamental to airflow between zones.
12/ If the door is very tightly sealed with a gasket you can achieve very little air flow (volume) and the pressure difference will likely *increase*. But sealing gets only a very short mention once. How likely will this be done? Excerpt from NSW IPAC Hotel Quarantine Manual  "Overarc
13/ Also, the door should only be completely sealed if it is compatible with the design of the building system. This is likely only for compartmentalization, where every suite has a dedicated supply and exhaust system and is air sealed from the rest of the building.
14/ Without compartmentalization the gap is needed for makeup air. With a gap, you need to ensure the suite is in negative pressure with continual exhaust.
15/ In typical hotels and apartments, opening windows messes with pressures and can cause all sorts of unpredictable airflow patterns.
16/ Here are recommended quarantine accommodation types, in order of preference, based on building science:
a) air-gapped
b) compartmentalized
c) negative pressure (with sealed windows). Figure of the three main ideal quarantine/isolation accommod
17/ Looking at the contributors to this document I think we have an idea why this guidance is missing out on the fundamentals of fluid mechanics and building science that we've known for decades. Is @NSWCEC unintentionally practicing professional engineering here? Image of the Acknowledgements page of the contributors to th
18/ There's many other problems with the NSW IPAC Manual for Quarantine Hotels that have been pointed out by others (cleaning is NOT an engineering control! gappy surgical masks instead of P2/N95 respirators for essential workers!) and I recommend heeding their advice.
19/ For further reading, I continue to add to a longer thread on the fundamentals of uncontrolled in air flow within buildings that can explain why there are outbreaks happening in apartment buildings as well as hotels, which have a similar design.
20/ @CathNoakes summarizes this thread well: "It’s the continuous flow under the doors that probably matters more than the brief flow when a door is opened."
21/ Overall the TooLongDidntRead summary is that because IPAC doesn't understand fluid dynamics they permit for airflow in the wrong direction that can lead to a repeat of transmission events within quarantine, which their guidance is trying to avoid in the first place.
22/ December update on this topic of pressure-driven under-door air flow from @CathNoakes

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More from @DavidElfstrom

May 31
Vancouver Coastal Health has released an updated Ventilation and Indoor Air Quality resource for Schools and Childcare Facilities () but their CO2 page needs some edits. vch.ca/en/document-li…
States high levels of CO2 and other contaminants ... can contribute to [list of five health effects, such as headaches, fatigue, difficultly concentrating], says CO2 should be below 1000 ppm, then says (incorrectly) that health effects from CO2 occur at levels above 5000 ppm. No, those effects can occur below 5000 ppm.  It also states the CO2 concentrations do not indicate a risk of infectious disease transmission in a space. No, ASHRAE's position statement on CO2 states that elevated CO2 concentrations CAN reflect higher risk. Obviously, because it indicates poor ventilation, which IS an i...
Vancouver Coastal Health "CO2 concentrations do not indicate a risk of infectious disease transmission in a space". No.
ASHRAE's position document on indoor CO2 says "higher CO2 conc correspond to lower ventilation and potentially increased risk of airborne transmission"
Vancouver Coastal Health "Note that health effects from CO2 occur at levels above 5000 ppm". Did WorkSafeBC interfere? Because that's contradicted by your Health Canada reference in the sentence immediately before it.
Read 10 tweets
Mar 28
Three years ago on this day ASHRAE stated that Airborne transmission of SARS-CoV-2 is significant.

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field. /1 iris.who.int/handle/10665/3…

Landing page for the WHO document titled: Indoor airborne risk assessment in the context of SARS-CoV-2: description of airborne transmission mechanism and method to develop a new standardized model for risk assessment
Four years ago on this day WHO said : "FACT: #COVID is NOT airborne."

Today, WHO finally released a model () of #CovidIsAirborne risk by experts in the field that warned of airborne even earlier, but were shut out. /2
iris.who.int/handle/10665/3…
WHO Tweet March 28, 2020 FACT: #COVID19 is NOT airborne.   The #coronavirus is mainly transmitted through droplets generated when an infected person coughs, sneezes or speaks.  To protect yourself: -keep 1m distance from others -disinfect surfaces frequently -wash/rub your hands -avoid touching your eyes / nose / mouth  Cover page of Indoor airborne risk assessment in the context of SARS-CoV-2
This document has been a long time coming. As described by @jljcolorado, Lidia Morawska, co-chair of the group that published the new WHO airborne model, was previously cut off by John Conly when making the case that #COVIDisAirborne to WHO. /3
Acknowledgements: WHO ARIA Technical Advisory Group:  First name: Lidia Morawska (Co-Chair) (Queensland University of Technology, Australia) (many other names) John Conly (University of Calgary, Canada)
Read 4 tweets
Feb 23
Air purifier manufacturers say HEPA should always be the filter of choice, and their product's proprietary filter delivers. Which HEPA? ISO 35H at 99.95% or ISO 40H at 99.99%? Why not ISO 50U? That's 10x better at 99.999%. Why stop there? Go for ISO 70U at 99.99999%! /1
The answer is, single-pass filtration efficiency DOESN'T MATTER except in specific cases like Powered Air Purifying Respirators (PAPR), clean rooms, operating theaters, or nuclear laboratory exhaust—HEPA's original purpose. /2
For portable/in-room air cleaners, all that matters is the Clean Air Delivery Rate (CADR) for a target particle size and type, within acceptable for sound power and frequency characteristics for the people in the room. /3
Read 13 tweets
Jan 3
Four years into this and we can't keep duct-taping in-room filter solutions for clean air. It's just filter(s) and a fan. We need open-source, optimized design, certifiable product, efficient, repairable using commodity filters and commodity components. /1
We need air cleaners assembled and distributed by not-for-profit community-based social enterprise. No more lock-in to proprietary filters. Verified replacement commodity filtration performance for safety. /2
Low income with donated CR boxes will pay over time in electrical costs for the duct-taped solution for clean air.
Power utilization for Smoke CADR, same filters:
Conventional CR Box: 4 CADR/W. (77 W)
PC fan array air cleaner: 24 CADR/W. (8 W)
/3
Read 4 tweets
Aug 28, 2023
1/ Levoit Core 400S versus Austin Air HM400 in a challenge to see which portable air cleaner removes submicron salt particle aerosols the fastest! Which do you think will win, and by how much? Poll in next tweet below... A large indoor tent with taped seams on a concrete floor is shown. On the left is a bench with instruments. A webcam is mounted on the top of a chair facing the instruments. In the middle are two air purifiers: The cylindrical Levoit Core 400S and the larger boxy Austin Air HM400. In the back right corner is a circular floor fan pointed diagonally upward into the middle. At the front right is a chair holding what looks like a tiny pitcher with a spout. A long thin clear plastic tube is connected to the side of this device, and exits the frame. Power cords are on the floor.
2/ Which has a higher CADR (Clean Air Delivery Rate):
Levoit Core 400S, or Austin Air HM400?
See if you can find the manufacturer's claims for both, and then come back and vote:
[sarcasm] Not only is the Austin Air bigger and far heavier, it also draws way more power, is much louder, and more expensive. It couldn't possibly be *worse* than the Levoit, right? Right?
Read 13 tweets
Jun 12, 2023
When to change the filter on DIY #CorsiRosenthalBox? Holder et al (onlinelibrary.wiley.com/doi/10.1111/in…) showed that performance degradation depends on contaminant: Dust reduces airflow. Smoke kills electrostatic properties. @JimRosenthal4 @joeyfox85 @CorsIAQ Table for MERV 13 single-fi...
@JimRosenthal4 @joeyfox85 @CorsIAQ Note the test substance for the CADR PM2.5 performance measurement was simulated wildfire smoke.
The same would likely apply to a commercial HEPA air cleaner. Monitor for clearing time for a fixed release of contaminant with a simple PM meter.
Read 6 tweets

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