Green Building Forum - COVID 19 and mechanical ventilation

COVID 19 and mechanical ventilation

tony — Mon, 03 Aug 2020 21:38:12 +0100

In some countries distancing is three feet!

eg HK, 24 deaths so how does that influence our thinking

COVID 19 and mechanical ventilation

Jonti — Mon, 03 Aug 2020 22:52:03 +0100

Tom,

I am not sure what you are getting at as you seem to be backing both horses. You accept the science of social distancing but also argue it is unproven ðŸ¤”

Tony,

HK did so well because it had a phenomenal track and trace system as well as support network for those that were vulnerable. It has been pointed out by some that HK never went into lockdown during the first wave but they actually did just it was not one brought in by the ineffectual government but rather by the public themselves.

Jonti

COVID 19 and mechanical ventilation

tony — Tue, 04 Aug 2020 05:34:37 +0100

Not really, very effective strategy, all positives go to hospital, symptoms or not. Trace contacts and isolate them. Quarantine incomers for 14 days, on the ball With knowing what to do before it struck.

Mask wearing was already there.

COVID 19 and mechanical ventilation

Doubting_Thomas — Tue, 04 Aug 2020 08:57:37 +0100

I'm a little late to this particular maelstrom but to answer the OP's question, I found this to be a useful resource:

https://www.bere.co.uk/press/covid-19-and-the-risk-of-air-recirculation-in-conventional-buildings/

Essentially, MVHR is potentially the only type of mech vent that can protect safely and not cost the earth, provided it is the right type of heat exchanger.

COVID 19 and mechanical ventilation

fostertom — Tue, 04 Aug 2020 09:17:23 +0100

Good article - at least mentions modifying the 2m concept to take into account 'slipstream' (someone walking) and downwind (air movement) effects.

Has anyone any idea how quickly, in still air, the droplets are supposed to sink to say waist level. Because that's the length of time we should stay out of the space where someone has just walked.

COVID 19 and mechanical ventilation

barney — Tue, 04 Aug 2020 09:51:04 +0100

It will depend entirely on the droplet size Tom (accounting for all the factors like temperature, humidity, air velocity, direction etc)

Big, gobby droplets will fall quickly - your garden hose should show that to be true

Small droplets will fall more slowly - a nice misty autumn morning shows you that

Once you get below sub micron level, the droplets are almost in a gas phase - so they almost never drop out as the are fully mixed with the air

As I mentioned, it'll be low indoor RH typical of most spaces in the UK in winter that will evaporate the "moisture" and leave viral load as a minute suspended particulate - at that point, good ventilation providing adequate dilution factors (DF's) and ideally supply at top and extract at bottom will be a dominant criteria for populated indoor spaces. If you want the example for how rapidly things get out of control then look at the meat packing industry - fully enclosed cold indoor spaces, lots of people close together, poor ventilation, minimal fresh air and almost no natural light (ie little or no UV components) all make for an almost perfect transmission environment

Essentially though, all of that stuff dropping out of the air will end up on surfaces - so handwashing is important. You've only got to look at the news channels for a few minutes to see that most people have no clue when it comes to wearing masks - it's not difficult, wash your hands thoroughly, put the mask on - don't keep fiddling with it as all you are doing is transferring stuff straight to your face (eyes, nose, mouth)

The current Government advice is actually good advice - wash your hands, weak a face covering, keep away from people (vary the last two in proportion to time and distance)

Regards

Barney

COVID 19 and mechanical ventilation

Ed Davies — Tue, 04 Aug 2020 10:04:24 +0100

Yes, good article Doubting_Thomas, thanks. Though I'm not entirely convinced by the arguments with respect to filter sizes for recirculation as it seems to muddle up virus sizes (~0.1â€¯Âµm) and the sizes of the aerosol particles carrying them (less than 5â€¯Î¼m but maybe mostly larger than 1â€¯Î¼m - it's not clear).

Posted By: fostertomHas anyone any idea how quickly, in still air, the droplets are supposed to sink to say waist level.

According to that article and other sources I've seen, for aerosol particles (< 5â€¯Î¼m) this can be hours. For larger particles (droplets) it's a lot quicker.

COVID 19 and mechanical ventilation

djh — Tue, 04 Aug 2020 10:58:21 +0100

As a rule of thumb, I estimate how close is safe by comparing distances to those where I've previously smelled people's perfume or aftershave as I walked by. I expect fragrances propagate somewhat similarly to viruses.

COVID 19 and mechanical ventilation

fostertom — Tue, 04 Aug 2020 11:21:59 +0100

Really Dave? I'd a thought that the way perfumes 'flash' quickly through a room indicates v high volatility, which AFAIK means atomised, gas state dispersing through still air by partial vapour pressure, rather than droplets carried by bulk air movement?

COVID 19 and mechanical ventilation

barney — Tue, 04 Aug 2020 12:00:39 +0100

Perfumes will move by diffusion as the particle size is very small - unless someone sprays it on you when transmission is by droplets

So, back to our virus - it can be transmitted by being enclosed in large droplets - very efficient, but limited to about 2m maximum and a short number of seconds (this is what we describe as "droplet transmission" and why you see ICU staff in double masks, face visors and gowns and gloves)

Alternatively, in the right conditions of temperature and humidity, evaporation of droplets will leave a still viable mist of minute particulate size that starts to act as a gas - what we describe as "airborne transmission" - not very effective, can spread indoors for many metres and can be airborne for many hours. Lab testing on monkeys shows a transmission is possible by spraying a diffuse mist at them - the infection rate is low, but transmission is possible. Whilst still particulate, the particle size is very small (sub micron) so it starts to acts a gas (hence filtration is difficult as even H14 grade HEPA filters struggle with particulate less than say 0.3 micron). Any mask you wear is useless at this point, except that it will stop you transmitting droplets that then become fine aerosol particulate in the right conditions

At the moment, people talk about airborne transmission as a single process, when clearly it isn't - so the analogy to perfume is quite correct as it isn't too far away from a model of airborne transmission as opposed to a model of droplet transmission

Regards

Barney

COVID 19 and mechanical ventilation

fostertom — Tue, 04 Aug 2020 12:07:03 +0100

Posted By: barneylimited to about 2m maximum and a short number of seconds

Great - getting somewhere, the Advice is making more sense.

But say more about 'short no of secs' for 'typical' or 'efficient' sneeze-droplets i.e. neither as big as hosepipe spray (falls instantly) nor as small the 'not very effective' smaller aerosol-tending (persistent in the air) particles?
Is that 3 secs, or 30 secs, scale-of ? To do what? Drop to floor, or to below face level? I'd have thought far more than 30secs, for a 'a nice misty autumn morning'.

COVID 19 and mechanical ventilation

philedge — Tue, 04 Aug 2020 12:12:00 +0100

Treat everyone like plague ridden zombies and you wont go far wrong! As has been said, if you can smell someone youll likely be able to inhale some of what theyve exhaled

A safe distance is as far as a piece of string is long. Theres some many factors to consider-2 meters is fine if the other person is downwind and breathing light and slowly. If that same person has just finished vigorous excercise or is coughing and is up wind, then youll likely be sharing a fair proportion of their breath.

COVID 19 and mechanical ventilation

philedge — Tue, 04 Aug 2020 12:31:35 +0100

Posted By: barney

So, back to our virus - it can be transmitted by being enclosed in large droplets - very efficient, but limited to about 2m maximum and a short number of seconds

I seem to recall that during lockdown it had been shown that someone sneezing/coughing in a supermarket aisle could transmit the droplets over the shelving and into the next aisle?? There were also statements made by experts in aerodynamics that the turbulence created by a cyclist would likely suspend droplets in their exhaled breath for up to 20 metres behind them. Similarly up to 10 metres behind for a runner.

I would imagine that, unless there was a strong side wind, cyclists and runners could cycle/run shoulder to shoulder with near zero risk of transmission between them whilst there was airflow between them.

COVID 19 and mechanical ventilation

barney — Tue, 04 Aug 2020 12:35:44 +0100

OK - droplets would be airborne for say between 1 and 3 seconds at most before falling onto a surface (floor, desk etc) - bigger droplets falling out faster than smaller ones. Transmission is by wet contact within 2m and all over and done with an a matter of seconds. Very efficient, big viral loads delivered

Particulate can be in the air for hours, and for very small particles possibly for days - so the mist analogy is true (ish) - quite ineffective, small viral loads delivered - exposure time increases infection risk - easily brought down to a manageable DF by basic ventilation (even natural ventilation)

Regards

Barney

COVID 19 and mechanical ventilation

djh — Tue, 04 Aug 2020 14:21:38 +0100

Posted By: barneyPerfumes will move by diffusion as the particle size is very small

That'll be why you can smell it downwind of people but not upwind then? And the same reason hunters try to stay downwind of their prey? Smells move largely by bulk movement of the air. Diffusion is quite slow by comparison.

COVID 19 and mechanical ventilation

djh — Tue, 04 Aug 2020 14:25:41 +0100

Posted By: barneywhy you see ICU staff in double masks, face visors and gowns and gloves

Medical staff in ICUs and operating theatres mostly wear PPE to protect the patients, not themselves. The gowns and gloves are sterile on the outside, not the inside. Surgical masks are designed to stop droplet transmission from the surgeon et al to the patient. In cases of infectious disease, which are usually thankfully rare, they wear respirators rather than masks, and take other precautions.

COVID 19 and mechanical ventilation

barney — Tue, 04 Aug 2020 15:17:53 +0100

I think you're mixing up the scenarios from my comment - as a general rule, yes, you want to protect patients susceptible to infection from the staff - gloves, basic masks and aprons do this normally - along with the pressure regime of the room (generally positive - eg an aseptic room)

In this case, the patient is highly infectious already, as are patients in the same room - so you are protecting the staff - with everything up to full respirators or even piped medical air supply, depending on what's available - again enhanced by/with the pressure regime of the room (generally negative - eg an isolation room)

If we accept that diffusion is the movement of molecules from a region of high concentration to one of low concentration, then perfumes move by diffusion but will be greatly influenced by the air movement path - which is what I said ie "so the analogy to perfume is quite correct as it isn't too far away from a model of airborne transmission as opposed to a model of droplet transmission"

Regards

Barney

COVID 19 and mechanical ventilation

Jonti — Tue, 04 Aug 2020 16:45:23 +0100

Posted By: philedge

I seem to recall that during lockdown it had been shown that someone sneezing/coughing in a supermarket aisle could transmit the droplets over the shelving and into the next aisle?? There were also statements made by experts in aerodynamics that the turbulence created by a cyclist would likely suspend droplets in their exhaled breath for up to 20 metres behind them. Similarly up to 10 metres behind for a runner.

I would imagine that, unless there was a strong side wind, cyclists and runners could cycle/run shoulder to shoulder with near zero risk of transmission between them whilst there was airflow between them.

Current recommendations for runners is 2 meters when side by side and 5 meters when running in single file. All this talk of concentrated aerosol remaining in still air for long periods of time is fascinating but I am not sure how relevant when you take into consideration the number of confirmed cases of outside transmission is less than 5.

Air does not remain still regardless of what many people think. It is the recirculation or rather poorly ventilated indoor spaces that create a problem even with social distancing. Masks don't work so great in stopping aerosol spread but good ventilation coupled with social distancing do.

COVID 19 and mechanical ventilation

djh — Tue, 04 Aug 2020 17:28:15 +0100

Another great example of how aerosol particles move by diffusion is the plume of 'steam' that rises above a tea cup. Small particles of water rather than invisible vapour of course, and mostly being moved in the convection plume rising from the cup rather than diffusing out in a sphere.

Even the slow movement of air produced by our MVHR largely prevent smells emerging from bathrooms, and keeps most cooking smells in the kitchen.

COVID 19 and mechanical ventilation

Cliff Pope — Tue, 04 Aug 2020 18:45:50 +0100

"How long does it take the droplets to fall; how long should I wait before moving into space that someone has recently occupied? "

Perhaps what's needed is a way of making the virus droplet cloud visible, or perhaps have a distinctive smell, like the way someone smoking a pipe on a cold still morning leaves a characteristic vapour trail behind even after he has moved on?

If people left a lingering, visible, slowly fading image behind them then you could simply dodge around the darker bits.
:)

COVID 19 and mechanical ventilation

tony — Tue, 04 Aug 2020 20:32:04 +0100

https://drive.google.com/file/d/1T4Rb-HFbWp4DOua--uBmMhhRqf9JNQfT/view

COVID 19 and mechanical ventilation

Mike1 — Wed, 05 Aug 2020 00:26:09 +0100

Potentially linked to ventilation, the latest figures in France show that, excluding family & retirement homes, work is the biggest source of infections - accounting for 26%.

Next come:
Public & private events - 13%
Family gatherings - 12%
Healthcare establishments - 9%
Social housing + youth reintegration centres - 9%

So best to continue to work remotely if you can, until someone works out how much ventilation does makes a difference.

https://www.leparisien.fr/societe/sante/coronavirus-des-medecins-s-inquietent-de-l-acceleration-de-l-epidemie-01-08-2020-8362245.php

COVID 19 and mechanical ventilation

Mike1 — Wed, 05 Aug 2020 00:40:02 +0100

Posted By: tonyIn some countries distancing is three feet!

Yes, it does vary:

UK - 2m
USA - 1.8m (6')
Spain, Germany & Australia - 1.5m
France & Hong Kong - 1m

More to do with the application rather than the specific distance in the rules, I suspect.

COVID 19 and mechanical ventilation

tony — Wed, 05 Aug 2020 08:32:06 +0100

For those like me who canâ€™t read very well

https://www.youtube.com/watch?v=f57LXUbrtPU&feature=youtu.be

And the hierarchy of What to do: (1) distance, (2) always masks indoors (except your own house), (3) attention to ventilation, (4) avoid crowds, (5) hygiene.

I think worries about domestic ventilation are unfounded, if someone in the house has it everyone is going to get it. The most likely way in for it apart from people is on deliveries, mail, shopping etc.

COVID 19 and mechanical ventilation

djh — Wed, 05 Aug 2020 17:44:16 +0100

There's a news article about an interesting paper on breath-related aerosol/droplet transmission. What's interesting is that both large and small droplets can be transmitted for large distances, whilst droplets intermediate in size drop to the floor more rapidly.

https://www.theengineer.co.uk/aerosol-extractor-spread-of-airborne-diseases/