Thread by @DrEricDing

This bothers me so much—

📍There are still no official CDC guidelines for ventilation for schools—no rules on adequate air exchanges—despite so much science on >=5 ACH/hour.

📍And 6 feet is not safe for airborne virus, says ex-OSHA chief. #COVID19 🧵 nj.com/opinion/2021/0…

2) “We’ve known for nearly a year that COVID is airborne – yet aside from hospitals, few workplaces acknowledge that the virus can hang in the air. Instead, they emphasize surface-scrubbing and physical distancing”

3) We need at 5 or more air exchanges per hour says @HarvardChanSPH researchers, either via natural ventilation or air cleaning.

4) And air disinfection such as with HEPA filters do work. New study this past month demonstrates it for the coronavirus. Reduced inhaled dose by 6x on optimal 5.7 ACH setting. Good thread 🧵 summary below.

https://twitter.com/drericding/status/1363638390432948227

5) To understand air cleaning and not be tricked by snack oil disinfection machines, you have to understand the CADR value...

The key to portable air cleaners is what is called a clean air delivery rate (CADR).

6) The CADR is the product of two numbers: CADR = f x Q. 

f is the fraction removal efficiency of particles that go into the system. Q is the volumetric flow rate in cubic feet per min.

If 100 particles go in & 70 are removed, then f = 0.7. HEPA has basically 0.999+, or ~1.0.

7) warning—some shady manufacturers who claim that their devices are nearly 100% efficient (f ~ 1.0)—sounds great.  But what they do not tell you is that the flow rate of air through the device (Q) is very low.  So, f x Q is low—many devices that are quiet, e.g., ion generators.

8) CADR ➡️ ACH conversion:

Eq ACH = CADR x 60/(A x Z)

CADR = clean air delivery rate in cubic feet per minute

A = floor area in square feet
Z = ceiling height in feet

The number 60 is to convert the CADR to cubic feet per hour

Eq ACH = air changes per hour.

9) Basically if you know the air cleaning device’s CADR, and you know the room size (volume), then you can calculate ACH that the cleaner adds. If your room has 2 ACH via natural ventilation, and your HEPA filter has +3 EqACH via above formula, then your room’s total ACH = 5!

10) The organization that certifies air cleaners for manufacturers who want certification.  It is the Association of Home Appliance Manufacturers (AHAM) - aham.org

They have a standard test with smoke, dust and pollen to determine CADR for each.

11) I want declare I have no financial relationship with any HEPA filter maker.

Also really cool is this low cost MERV13 homemade device called “Corsi Box”. This fan’s air speed is 580 feet/min—so multiply by the area of the fan—to get Q. The F of these filters is 0.9 I’m told.

https://twitter.com/drericding/status/1362895328459059200

12) Now to be fair, the CDC does put out general suggestion on ventilation and air cleaning in buildings for COVID risk. And says HEPA and UV work for Covid. But they don’t set specific guidelines on classrooms meeting 5 ACH per hour. That’s the issue.
cdc.gov/coronavirus/20…

13) one of my favorite videos—

VENTILATION is key. Both ventilation and masks needed to reduce risk indoors. But most buildings & classrooms not ventilated well enough. Hence need for air cleaners like portable HEPA filters.

14) I want to add, CDC doesn’t need to bless any specific machines or filters. That’s not their job. They need to just set standards on adequate ACH for workplaces and schools, and maybe minimum CADR for devices — some bad devices have CADR of 20-30, while good HEPA have 250-300.

15) So if total ACH we should aim for is 5+ from natural ventilation + air cleaning, then what is the typical ventilation ACH in classrooms? Well, it’s rarely ever 4. @CorsIAQ says the median he’s seen is 2, while Arlington VA schools it’s 2.4, but as low as 0.5 in some classes!

16) How important is ventilation outside of COVID risk? “There is compelling evidence, from both cross‐sectional and intervention studies, of an association of increased student performance with increased ventilation rates.”...

onlinelibrary.wiley.com/doi/abs/10.111…

17) “There is evidence that reduced respiratory health effects and reduced student absence are associated with increased ventilation rates.”

18) “The net annual costs, ranging from a few dollars to about 10 dollars per person, are less than 0.1% of typical public spending on elementary and secondary education in US. Such expenditures seem like a small price to pay given the evidence of health & performance benefits.”

19) Here is the cost breakdown of one good portable HEP filter per standard classroom, done by @CorsIAQ. The total cost across 3 years is just $11/student. That is nothing compared to the cost of other school materials. And this improves learning! We need to do this.

20) The good news is that $130 billion designated for schools is coming in the Democrat proposed bills! And it will offer funding for ventilation. But will it be prioritized? I’m working on a push for this on behalf of safe reopening. I will announce tomorrow.

21) I want to add that while HEPA cleaners are well-suited for rooms under 1,000 square feet, with ceiling heights of 8-10 feet—they are not sufficient for larger rooms like big cafeterias, gymnasiums, auditoriums, indoor swimming pools. For these, you then need “Upper Room UV”

22) Upper-room UV fixtures are mounted on ceilings and are safe for humans because they do not expose UV to students and teachers. They draw air upward, then disinfect the air above people who are in the room, adding a high number of clean air exchanges to a room’s air supply.

23) Upper-Room Ultraviolet devices can sometimes achieve 15 or more air exchanges per hour if done well.

Some UV devices can also avoid specific wavelengths that cause ozone, allowing it to be further safe.

24) Bottomline: there are MANY ways to achieve 5 or more air exchanges per hour. Natural ventilation + also via air disinfection approaches. Some don’t want to chase 5 ACH per hour, but I say $11/year per child is definitely worth it—& there is funding coming soon. See this 🧵

https://twitter.com/drericding/status/1362880620343484416

25) Ventilation with new air is best. Here is how to ventilate your school classrooms if unable to open window. #COVID19

26) let’s learn some lessons from 100+ years ago in the last great global pandemic... doctors and nurses kept all windows open. Fresh air ventilation was the key to defeating the pandemic. If only we can heed these lesson now. Great piece by @sarahzhang.
theatlantic.com/health/archive…

27) CDC needs to emphasize school ventilation. This is something all airborne virus experts agree on. Masks, ventilation / air cleaning, and rapid testing. Vaccination is good if available, but air cleaning is much easier and faster to solve if we have Biden Covid bill funding

https://twitter.com/drericding/status/1360343218177646597