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In another scenario, if the first filter has an efficacy of just 15% and allowed a flow of 1200 cubic feet per minute, it would give us 180 cubic feet of clean air, as opposed to the 199 from the second one. 20/

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The big difference is the bigger flow allowed by the first guy more than makes up for its lower effective filtration. That is why we want to check CADRs and not just effective filtration. 19/

So, every minute, the first guy delivers 1000 cubic feet air, 70% likely to be clean. So that is about 700 cubic feet clean air every minute. The second guy delivers 199.8 cubic feet clean air every minute. 18/

Hypothetical time. Say, there is a filter which picks out stuff you don't want with 70% probability. Another, filters the stuff out with 99.9% probability. Every minute, the first lets 1000 cubic meters of air pass through, the second lets 200 cubic meters of air pass through 17/

The CADR tells you the overall impact, taking into account how good the filter's efficiency is in the size range of concern and the flow rate through the filter. 16/

That sounds complicated. Which is why we talk about something called clean air delivery rate. Depending on the size of the pollutant of concern, a CADR can be calculated, following some standards (ahamverifide.org/ahams-air-fi...) 15/

ahamverifide.org

Air Filtration Standards

So, based on the pollutant size of concern, we decide on a filter type. There will be a range of filters in this group. They may have different flow restrictions. The compromise we need to strike is the bet value of efficiency times flow rate. 14/

Let me take it to an extreme. You could have an impermeable plastic film - that will give you 100% efficiency but 0 flow rate. We certainly do not want that. 13/

We need to strike a balance of an optimal flow rate and filtration efficiency. 12/

There is air in the room, there are pollutant sources, the air cleaner is continuously passing the air through a filter, getting stuff out of air and doing it again while the sources continue put stuff into the air. 11/

And this is not like filtering water - its not like you are going to let the air pass through the filter and then collect in a large tank and breathe some of it in whenever you feel out of breathe 10/

At the same time, higher rated filters (filters with better efficiency) need to be more restrictive to do their job. Means, they let less air flow through them than a comparatively lower rated filter if the driving force is the same 9/

To be fair, even your handkerchief will filter some 0.3 micron particles. That does not mean you will use it to protect against asbestos dust. Things have to be "fit for purpose" 8/

At the same time, if you are interested in particles around 1 micron size, you would not even think of getting a MERV 8 filter. Not that they would not filter anything at 1 micron. Just that the values are not of practical interest and MERV 8s don't get rated at that size 7/

Similarly, for MERV 13, particles between 0.3 and 1 microns, have about a 75% chance of getting trapped. Again, not a definitive NO or YES. More than an even chance, so not too shabby. 6/

For example, have a look at the performance of different MERV rated filters, MERV 8 - 16 (hpac.com/industry-per...) For MERV 16, you can't say 1 micron particles will be trapped - they almost certainly will be, but not a definitive YES. 5/

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When it comes to taking tiny particles out of air, it is no longer a YES or NO thing. It gets all probabilsticky around there. 4/

However, when it comes to filtering stuff out of air, the action is very little like a sieve. There are a range of mechanisms that lead to particles in air being filtered and straining (the sieve like action) has a very minor role 3/

If you have a sieve which allows anything smaller than 4 mm to pass through, it is like a YES-NO question, either you go through or you do not. 2/

Orla Hegarty @orla-hegarty.bsky.social · 13/10/2023

When we think of things being filtered out of something else, it is natural to think of a sieve like action. Sieving is the macro-scale example we are most familiar with and can relate to. So its natural, even though highly inaccurate for thinking about air filtration. 1/

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Is the air quality in your office fit for purpose? A new code of practice for indoor air quality is now in operation so what does it mean for employers and employees? Excellent new explainer from @akm76.bsky.social RTE Brainstorm (Pls share to employers in Ireland) t.co/Y0o47gzXYP

t.co

Is the air quality in your office fit for purpose?

A new code of practice for indoor air quality is now in operation so what does it mean for employers and employees?

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Orla Hegarty @orla-hegarty.bsky.social · 08/10/2023

‘Why good ventilation is key to stopping the spread of Covid-19. Clean air far more important than clean hands’ In the 3 YEARS since I wrote this in The Irish Times 7,473 more people died (80% of pandemic deaths) We have the knowledge & tools to end this www.irishtimes.com/opinion/why-...

irishtimes.com

Why good ventilation is key to stopping the spread of Covid-19

Clean air far more important than clean hands

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Also, important to bear in mind that it applies to all workspaces. Offices, retails shops, warehouses, nursing homes, even schools and universities, all of these are workspaces for certain populations. Here is a link to the Code of Practice www.hsa.ie/eng/publicat...

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It is an important advancement. It encourages employers to be proactive in ensuring the workplaces have a healthy indoor air quality and ensure the best environment for performance.

Is the air quality in your office fit for purpose?

I wrote a piece in RTE Brainstorm, going over the important aspects in the document, from the perspective of employers and employees. I have tried my best to not include my own opinion and just summarize the document in an easy-to-understand form.

rte.ie

A new code of practice for indoor air quality is now in operation so what does it mean for employers and employees?

In May 2023, Health and Safety Authority Ireland published a code of practice for Indoor Air Quality in workspaces. A health and safety code of practice has (legal) consequences. The HSA just put Ireland in the forefront of the move towards healthy indoor spaces.

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For my first post, I would like to clarify that I hate the idea of Chicken butter masala/Chicken tikka butter masala. It is a very basic dish and it does not deserve to be one of the most popular Indian food in the world 😈

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tern @1goodtern.bsky.social · 09/10/2023

There would still be concerns at such high levels even if the filtration is on because the aircraft filtration system would not be able to deal with VOC and related gaseous stuff.

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There is covid everywhere in my networks at the moment, and the people with it aren't brushing it off. People who are feeling very ill.

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Faye McNeill @vfmcneill.bsky.social · 09/10/2023

So it's 49 F and 9 days past the legal start of heating season... will today be the day my landlordemployer turns the heat on?

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Orla Hegarty @orla-hegarty.bsky.social · 08/10/2023

🇮🇪 Claim poor ‘road infrastructure’ could cause 77 deaths in next 5 years makes headlines Silence about poor ‘indoor air infrastructure’ that caused 9,262 deaths in last 3.5 years In 2023 Covid claimed 6.5x more lives than Irish roads- 923 v 139 people

image of classroom with viral particles/aerosols floating in the air

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VC @vgc1720.bsky.social · 09/10/2023

Photo with three doctors presented bare faced and masked, reminding patients that 59% of all Covid transmission is asymptomatic. 35% pre-symptomatic and 24% from those who never develop symptoms. Which doctor do you want treating (and breathing on) you?

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Big Dracula Energy @mayawest.online · 06/10/2023

Listen, I know things are tight all over but- The Humane Society here is stretched to the breaking point. ATP the pet foodbank line stretches around the block. If you can kick 5-10 their way, it'll feed 3-4 animals for a week. help foster at your local if you can. (please RS if you can't help 💕)

cp24.com

Toronto Humane Society urgently appeals for help as families struggle to care for their pets

The Toronto Humane Society says that the economic downturn is forcing some families to choose between putting food on the table or providing for their pets.

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Orla Hegarty @orla-hegarty.bsky.social · 04/10/2023

910 Covid-19 deaths in Ireland in 2023 « Live your life, nothing to fear » 136 road deaths in Ireland in 2023 « slow down, buckle up, get an NCT, check your tyres, use your lights, watch for cyclists, wear a hi-viz, don’t drink & drive, don’t risk a conviction »

Derek Franksenstein: 1507 Americans died of COVID last week. That’s over 4,500 deaths in the last 3 weeks. All of them were told the pandemic is over & they should “live your lives” just like you were. Now they’re gone just like you could be.

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Asit Kumar Mishra @akm76.bsky.social · 04/10/2023

Formaldehyde sensors respond to ethanol (false positive) as do tVOC sensors. An NDIR CO2 sensor would not. should be okay with typical cleaners, disinfectants.