Thread by @DrJayVarma

There seems to be a lot of misunderstanding about how to make indoor air safer to prevent #COVID19 in @NYCSchools. Much of this argument is about terminology, science, technology, evidence, and how to turn evidence into policy (1/n)

In infectious disease epidemiology:

Ventilation = bring new air into a room, send old air out of a room

Filtration = pull old air through a machine, filter the old air for small particles, push the newly-filtered old air out (2/n)

For ventilation in school, there are 2 approaches:

“natural” = windows & doors that use air currents to pull new air in & push old air out

“mechanical” = central heating/ventilation/AC that pulls new air in & pushes old air out (3/n)

Natural ventilation can be enhanced by adding fans (to increase air going in & out) and by adding filters

Mechanical ventilation can be enhanced by upgrading the system to pull & push more air and by adding filters (4/n)

Both forms of ventilation—natural & mechanical—are widely accepted as ways of reducing transmission of infectious diseases.

(NB: I spent 8 years living SE Asia and China where I worked on TB prevention, including by improving ventilation.) (5/n)

The mainstay of @NYCSchools COVID-19 indoor air safety, therefore, is natural & mechanical ventilation, with enhancement of mechanical ventilation using MERV-13 filters. (6/n)

There is no rapid, accurate way to measure the primary outcome (⬇️viral particles in air), so the scientific consensus is to rely on proxies for how well the windows & HVAC systems move air in and out: 1/air-changes-per-hour (ACH) estimates using anenometers, 2/CO2 levels (7/n)

Neither of these methods tell you how well the MERV filter itself is working. They just give you an estimate for how much new air is coming in and how much old air is going out (8/n)

So what about portable devices (aka purifiers, filters, cleaners)? Portable devices are completely different than everything I’ve described above. These attempt to mimic the HVAC + MERV-13 by using a filter and trying to turn old air into new air. (9/n)

For simplicity, we can classify these portable devices as having 2 different types of filters: mechanical or electrical. (10/n)

Mechanical filters can be classified as "HEPA" if they meet certain standards. “Electrical” filters are now used by @NYCSchools & some other schools and try to achieve the same level of filtration as mechanical "HEPA" devices(11/n)

People are criticizing @NYCSchools, saying portable *mechanical* devices are proven to decrease infections, while *electrical* devices are not proven to do that. (12/n)

But real world evidence that ANY portable device (mech or elect) prevents respiratory infections in humans doesn't exist. This means devices MAY benefit, but we should be cautious claiming that one is best or one makes a room safe & the other unsafe dx.plos.org/10.1371/journa… (13/n)

For this reason, @NYCSchools is using windows & HVAC (enhanced w/filters) as its primary way to improve indoor air quality. Portable devices are a supplement just in case they help. (14/n)

For your home or work, I recommend trying to improve ventilation using windows & HVAC (enhanced w/filters), while not arguing about which portable device is best. @NYCSchools is appropriately measuring CO2 & ACH to monitor how well windows & HVAC are ventilating classrooms (end)