A very detailed review of air cleaning devices. "Air cleaning devices where the primary principle of operation is based on fibrous filtration or germicidal UV (UVC) are likely to be beneficial." 1/3
"Devices based on other technologies (ionisers, plasma, chemical oxidation, photocatalytic oxidation, electrostatic precipitation) have a limited evidence base that demonstrates effectiveness against SARS-CoV-2 and/or may generate undesirable secondary chemical products that."2/3
" could lead to health effects" 3/3

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More from @Poppendieck

19 Oct
Air change rate is the rate of air ENTERING (not leaving) a room divided by the volume of the room. Why is this important? 1/n
Because of mixing it takes up to 3/(air change rate) for 95% of the air in a room to exit the room due to mixing. 2/n
So if the air change rate is 6/hour, it will take up to 0.5 hours to remove 95% of the air originally present. 3/n
Read 4 tweets
14 Oct
Are you opening windows and using fans to move air into or out of a room? 1) Try to isolate the fan so it doesn't capture air it just blew. 2) Open a second door or window so the air has somewhere to go/come from other than cause condensation in walls (mold). 1/6
Typical $20 box fans claim to move 2,000 cfm (cubic feet per minute) on a high setting. A typical class room around 8,000 cubic feet. This equates to an air change rate of about 15 per hour, or taking 12 minutes for 95% of the original air in a classroom to leave. 2/6
The high setting is rather loud and flows are optimistic. @JohnSemmelhack showed yesterday numbers from 1200-1400 cfm with no filter. If we assume the flow (no filter) is about 1,000 cfm then the air change rate is 7 per hour, or about 25 min to change 95% of class air. 3/6
Read 6 tweets
21 Sep
I was wrong.

Back in late January, I had multiple conversations with an awesome colleague who was raised Wuhan, China about the rapid spread of COVID19 in China.

We are both indoor air researchers with PhDs. 1/6
She was adamant that we should start wearing masks in the US. I was adamant that masks wouldn't help. I went through all the arguments: 1) Their were not enough N95s, 2) people would just touch their mask more than the face, 3) people would do more risky things.... 2/6
4) they weren't 100% effective. 5) you can't mandate people to wear them.

She was right on masks. I was wrong on all counts.

We did agree that COVID19 would be worse in the US than China. I wish I was wrong on that to. 3/6
Read 6 tweets
21 Sep
Why is @CDC new guidance on droplet/aerosol transmission a big deal and not just schematics? Think about tornadoes and hurricanes. Both are swirling masses of air that can be extremely dangerous. Both have warning systems. Both have proven harm reduction strategies. 1/5
For tornadoes we can take shelter in basements, for hurricanes we can evacuate and avoid flood prone areas. However if we are only told to worry about tornadoes in a storm, we won't take any action to save ourselves from the flooding of a hurricane. 2/5
This weekend @CDC issues a warning that we should be worried about the massive hurricane and take all necessary steps to mitigate the damage (masks, portable air cleaners filters, ventilation), but keep an watch out for any tornadoes (masks, spacing, cleaning) it spawns. 3/5
Read 6 tweets
16 Aug
How fast does air leave a room? It is complicated. Think about a cup of black coffee. How much milk do we have to add to the cup before we only taste milk? If we add one cup of milk to our cup of black coffee (allowing it to overflow) the result will still be a tan cup.
In fact, due to mixing it will be just two thirds milk. We would need to add three cups of milk to get our original black coffee cup to be 95% milk.

Indoor air behaves the same way. As outdoor air enters an indoor space it mixes with the air already indoors.
So how long does it take to replace aerosol laden air from indoor spaces with outdoor air? In residences, 95% of the indoor air will likely be replaced with outdoor air in a time frame that ranges from 30 minutes (leaky house) to 10 hours (tight house).
Read 4 tweets

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