How to Minimize Public Transit COVID Risk
Photo by JC Gellidon on Unsplash

How to Minimize Public Transit COVID Risk

TLDR; To minimize COVID-19 exposure risk:
⮕ Stand in the center of subway cars. 
⮕ Sit at a window seat in the very front (windows open) or back (windows closed) of a bus.
⮕ Sit at a window seat in the back of a plane.

The holidays are a time for family. We’ve spent the year in isolation, seeing only a select few friends or loved ones, always worried about which behaviors might put ourselves or others at risk. Now, as the holiday season is upon us, we’d love to take advantage of our vacation days and spend some time at home.

Unfortunately, it’s not that simple.

You can quarantine, get tested, and wear a mask, but unless you can drive home in your own car, you’ll be taking on some level of risk during your journey.

Short of wearing a gas mask or hazmat suit, there’s no way to entirely eliminate the risk of public transit. There are, however, some simple ways to mitigate it.

The Golden Rule: Always Stay Upwind — He who smelled it is at risk.

COVID-19 is an airborne disease. It spreads both through large droplets that quickly settle to the floor and through aerosols that can remain airborne for hours.

For areas with poor ventilation, like most homes, these aerosols will spread throughout the room and remain there for long periods, potentially infecting anyone present.

For areas with excellent ventilation, like trains or airplanes, the air is refreshed fast enough that virus particles remain airborne for only a few minutes. In these cases, positioning is critical. Those positioned in between the infectious source and the exhaust vent are more likely to be hit by a concentrated stream of virus particles.

This was clearly demonstrated by the recent case of a high school student infected after spending only five minutes in a restaurant, 20 feet away from a saleswoman later found to have been sick with COVID-19.

Dr. Lee Ju-hyung, a professor at the Jeonbuk National University Medical School, analyzed the restaurant’s airflow along with a team of researchers.

“Incredibly, despite sitting a far distance away, the airflow came down the wall and created a valley of wind. People who were along that line were infected,” Lee said, according to the Los Angeles Times.

Schematic diagram of the outbreak restaurant equipped with ceiling-type air conditioners. The arrowed solid streamlines represent the airflow directions in the restaurant. Curved air streamlines represent that air streams from the ceiling air conditioners are reflected from the wall or barrier, and move downward toward the floor. SOURCE: https://doi.org/10.3346/jkms.2020.35.e415

We can learn a valuable lesson from this study: 

When spending time in any indoor space, sit close to a source of fresh air. Avoid return vents like the literal plague.

The main exception to this rule is when the HVAC system is recirculating air without filtering it properly–like a window AC unit. In this case, sitting by the AC will actually increase your exposure.

We’ll rarely have advance knowledge of the detailed airflow patterns in any given building, making the lessons learned from this study often tricky to apply.

If only buildings were all designed according to a minimal number of potential layouts, with well-defined airflow characteristics… like buses, trains, and planes.

For most forms of public transit, we can use what we know about their design to determine which seats are the safest and which are the riskiest. 

We can then all fight over those seats (while holding our breath, of course) or choose to offer them to the most at-risk riders.

Riding the Subway? Stand in the center or at the edge.

The air in a typical subway car moves very quickly. Fresh air comes down from the center and gets sucked back up at either end. The air is then passed through a filter and mixed with 1/3rd fresh air from outside before being recirculated back into the car.

Unfortunately, most subway cars use MERV-7 rated filters, which are mostly ineffective at filtering out virus-sized particles. This means that recirculated air could still contain infectious particles. Therefore, the primary source of contaminant removal is the fresh air coming in–18 subway cars worth per hour, or 18 ACH (Air Changes per Hour). This may sound like a lot, and it is, but high ACH numbers can be misleading.

That being said, it is far more dangerous to sit directly downwind of an infectious person than it is to be subjected to their diluted, recirculated particles.

The safest position on the subway is therefore either directly beneath the fresh air vent in the center of the car, where the air has been maximally diluted, or at the very edge of the car where there is limited direct airflow and fewer people encountered in close quarters.

Airflow on the New York Subway and the Safest Places to Stand. SOURCE: The New York Times

Taking the Bus? Choose a window seat up front. 

Buses can be the riskiest of the modes of public transit. This is especially true for coach buses, where the transit times are long, windows are kept closed, and the air is generally recirculated. Multiple outbreaks have been traced to bus travel, including one case where 23 out of 67 passengers on a trip were diagnosed with COVID-19 via an asymptomatic passenger, and disease particles were shown to have been recirculated through the bus’s centralized ventilation system.

Bus designs tend to vary more than subway cars, especially when it comes to city buses vs. coach buses. Coach buses often get ~6 ACH, whereas city buses can get 10–13 ACH with the windows closed and anywhere from 20–40 with them open. Nonetheless, we can still determine some effective guidelines for choosing the safest seat:

  1. Always choose a window seat. This will maximize the distance between you and the other passengers.
  2. Sitting all the way at the front or back will similarly minimize the number of other passengers you are close to.
  3. If the windows are open, sit as close as possible to the front of the bus, so that you are upwind of most passengers.
  4. If the windows are closed, sit at the back, because most buses have the return vent towards the front.

If possible, when entering a bus, see if you can spot the air return grill and avoid it at all costs. They’ll often look something like this:

Return air grill location on a coach bus. SOURCE: louair.com

One study showed that, as expected, droplets’ dispersion is strongly influenced by the positioning of the exhaust grill and its fan. Here is an example simulation for when the air is moving towards the front.

Transmission of 10 μm droplets on a bus with airflow moving towards the front. SOURCE:10.1016/j.jhazmat.2020.122609

Taking a Plane? Pick a window seat at the back.

Airplane ventilation systems were originally designed to handle on-flight smoking. Thankfully, that is no longer an issue on most flights. Still, the result is a ventilation system that works exceptionally well.

Air exchange rates for the Boeing 767 and 777 planes were 32 and 35 air changes per hour (ACH). This is achieved through 50% outside air and 50% air recirculated through a HEP filter. Unlike the filters on buses or subways, HEPA filters eliminate more than 99.97% of virus-sized particles, making them excellent for the prevention of airborne disease transmission.

The riskiest portion of air travel is the boarding and deplaning process. This is both because the primary air circulation system is often turned off at those times and because there is more close-proximity contact with a wide range of people. Back-to-front boarding was shown to be the best for minimizing the risk of COVID transmission.

During the flight, a modeling study found that there is a downward airflow at the sidewalls of the plane that suppresses aerosol transmission, whereas there is upward mixing in the center. In support of this model, a case study of a norovirus outbreak on a plane found that aisle seats were most likely to become infected.

Cross-sectional measurement of airflow in an airplane. SOURCE: https://doi.org/10.1080/02786820902736658

Putting all of this (and more) together, we get the following rules for air-travel safety:

  1. Avoid the aisle seats–they have the most contact with other passengers.
  2. The best seats are first-class or business-class window seats. These are spaced farther apart from each other and limit exposure to other passengers.
  3. The best economy-class seats are window seats at the back of the plane.
  4. Keep the gaspers (the air valves above each seat) on at all times. This will help provide a pressurized bubble of clean air around you, lowering the risk of infectious particles reaching you.
  5. Avoid using the lavatory if possible. This will both minimize exposure to other passengers, and limit the odds of infection via aerosolized fecal matter.

Conclusion

This is going to be a tough holiday season for the world. COVID-19 cases are on the rise, and even with vaccines beginning distribution, it will be many months before things start returning to a pre-pandemic normal. 

For those of us who plan on seeing our families, especially our older parents and grandparents, it is essential that we do everything in our power to minimize the risk that we expose them to. 

Following the practices outlined here will by no means eliminate the risk. They should be combined with proper masking, distancing, and, ideally, timing your travel to minimize contact with others. But, as we have learned over the course of the pandemic, every added layer of protection counts. Even one as simple as the choice of a seat.


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