Redesigning Ventilation Systems to Reduce COVID Transmission

NYC Subway. Source: Mtattrain / CC BY-SA

In a recent post, Rethinking Society, I adovcated for adopting enhanced ventilation standards utilized in biosafety laboratories. New York City Subways have up to 18 air changes per hour according to reporting from New York Times reporter, Christina Goldbaum. Check out the full report: “Is the Subway Risky? It May Be Safer Than You Think.” This means that the entire volume of air in a train car exits approximately every 3 minutes. MSNBC’s Morning Joe covered the matter on a recent show.

MSNBC MorningJoe Segment on Subway Safety. Subway safety comments start at 2:20s.

The air flow rate (air changes per hour or ACH) in a room is the number of times the entire volume of air exits the room (See Figure 1).

Figure 1. ACH Formula and Components

In the United States, ventilation or HVAC air flow rates are measure in cubic feet per minute or CFMs. I created a basic example of a room that we will assume is pressure-tight (leaks make it harder to maintain the air change rate), has supply (200 CFM) and exhaust air (300 CFM), as well as a 100 square foot floor area and the room height is 10 feet (See Figure 2).

Figure 2. Room ventilation

The resulting air change rate for this example is 6 — meaning that the air turns over in the room once every 10 minutes (See Figure 3).

Figure 3. ACH Calculation.

Ventilation Hacks

As stated in Rethinking Society, studies have shown that COVID-19 can be viable in aerosol form (in the air) for up to 3 hours. Most ventilation or HVAC systems recirculate building air; or , in the case of rooms without exhuast vents, harbor stale air with potentially infectious aerosols. The New York Metro Transit Authority (MTA) is doing a lot of things correctly for safer subway travel in a pandemic:

1. Require face masks
2. High ventilation air change rate
3. Daily disinfection of all car surfaces

Recently, Dr. Anthony Fauci, head of the U.S. National Institute of Infectious Diseases (NIAID), recommended that teachers forced to go back school have class outside or open the windows for indoor classes. There is a good deal of wisdom in Dr. Fauci’s insights.

“I’ve spoken to superintendents and principals, and recommend if possible, outdoors, better than indoors. If possible, keep the classrooms well ventilated with the windows open if possible, wearing a mask, physical separation, desks that are put further apart, if you could possibly, physically do that”

Dr. Anthony Fauci in a Washington Post interview

The wisdom in Dr. Fauci’s suggestion for indoor classrooms is that it calls for increasing the air change rate so that poentially contaminated aerosols are expelled from the room. This could be improved by placing fans to blow outward at the open windows — effectively a DIY hack to increase the flow of air out of the room.

Improving Building Ventilation

A few critical steps could be used to improve the ventilation systems in buildings to minimize COVID (or other pathogens) transmission:

1. Treat and filter supply air (HEPA filters)
2. Pressurize hallways & corridors
3. Make rooms pressure-tight/minimize leaks
4. Test room pressure
5. Apply a 100 CFM minimum airflow offset between exhaust and supply air
6. Monitor inter-room differential pressure and supply/exhaust airflow

Each one of these steps deserves some discussion so I will address them in future posts. Stay tuned and be safe.