Short trips. Masks for everyone. Far fewer passengers than before.

Those are my top recommendations for how America’s school buses should take kids to and from school during the pandemic.

I am a professor of mechanical and aerospace engineering who studies the flow physics of particles and droplets. Since March, I have worked with my University of Michigan colleagues to measure COVID-19 risks on the buses college students use to get around our campus.

Based on our guidance, those buses now follow routes that take 15 minutes at most to complete, down from as long as 45 minutes before the pandemic. All passengers must wear masks, and the maximum occupancy is half of what it used to be. My recommendations for K-12 buses are similar.

Airborne droplets

It’s not yet clear what the minimum dose of the coronavirus is to become infected. But it is now established, based on an analysis from recent outbreaks, that the virus is predominantly spreading from one person to another through airborne droplets.

Droplets of various size are expelled when you cough, sneeze, sing or even just speak. Large droplets – to be exact, those greater than 50 microns across, about as wide as a human hair – tend to fall within a couple of feet and deposit in the confines of the seat. However, smaller droplets can remain suspended for hours and carry the coronavirus throughout the bus.

Air currents generated by heating, ventilation and air-conditioning – often referred to as HVAC systems – are capable of spreading these droplets, which scientists call “aerosols,” from an infected person to other passengers. Open windows bring in fresh air and dilute the overall concentration, greatly reducing the risk of an outbreak.

8 recommendations

The number of children regularly boarding school buses has plummeted from around 25 million before the pandemic due to the large number of students who are doing remote learning either full-time or several days a week. But some school districts have welcomed students back into classrooms.

The Centers for Disease Control and Prevention recently updated its school bus guidance, and several educational transportation trade groups have produced a task force report with detailed guidance of their own.

These eight recommendations are best practices for families and school leaders to take into consideration. To be sure, some could prove impractical due to the nature of children and the budget realities in school districts everywhere.

1. Require masks

The best way to prevent the spread of COVID-19 on school buses is to ensure that everyone wears masks, which are safe for children 2 and up to use. Not only do masks minimize the number of droplets that escape, they slow down the droplets that aren’t stopped and reduce the distance they can travel – all of which are crucial in close quarters. However, not all masks are equally effective. Loosely folded face masks and bandana-style coverings don’t capture the smallest droplets. Well-fitted masks with multiple layers are best. And no mask works if the wearer doesn’t cover their mouth and nose at all times. For that reason, it would be ideal to ban the consumption of food and beverages aboard buses.

2. Make trips brief

Bus trips ideally should take no more than 15 minutes to minimize the risk an infected person will spread the virus. Unfortunately, U.S. school bus rides average about 30 minutes, typically taking even longer in rural areas. Where it’s feasible, school districts should adjust routes to shorten ride times.

3. Leave most seats empty

Reducing the number of passengers on buses makes social distancing more viable and decreases the likelihood an infectious person is riding the bus at a given time. The CDC recommends limiting occupancy to one child per row in nonadjacent seats to reduce the risk of a bus ride turning into a superspreader event. Since most standard school buses accommodate around 72 kids, this may mean at most filling buses to one-quarter of their capacity by allowing only about 18 students on board. Keeping most of the kids off the bus will be easier if schools encourage students to walk or bike to school or catch rides with their parents. Following a hybrid approach, with at most half of students attending in person, can also help achieve this goal.

4. Keep windows open

Leaving the windows and roof hatches open helps bring in fresh air, reducing the concentration of infectious droplets on the bus and increasing the time children and the driver can be on board. It might not be feasible to keep the windows open when the temperature drops below the freezing point or when it’s raining hard. Students would need to bundle up more than usual in the wintertime and dress for the heat in warmer weather.

5. Use specialized air purifiers when windows are closed

These devices, especially high-efficiency particulate air (HEPA) filters, could potentially help reduce the transmission of the coronavirus. Some manufacturers are already retrofitting buses with medical-grade air filtration systems.

6. Don’t use AC or heat when bus windows are closed

It’s best to take this precaution because the airflow HVAC systems generate can increase the spread of airborne droplets throughout the bus. If the windows are closed, using air conditioning or automotive heating can make everyone on board more at risk of infection. However, when windows are open, HVAC systems can help bring in fresh air and remove any contagion-carrying droplets.

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7. Protect bus drivers

Drivers require specialized personal protective equipment because of their exposure to each child who enters the bus. In my view, drivers should wear N95 masks, which provide maximum filtration and would extend the time they can spend on the bus before potentially inhaling enough infectious droplets to become ill. Drivers should be frequently tested for COVID-19. Plexiglass barriers between the bus driver and the door, also known as sneeze guards, would help maintain social distancing and minimize exposure. Having additional adults on the bus would increase the risks of infection, but it might otherwise be hard to enforce new bus rules, and monitors are required for the transportation of many students with special needs. Any bus monitors would also require high-grade personal protective equipment.

8. Maintain good hygiene

It’s not clear that checking every child’s temperature would be practical or viable, especially on buses without any staff assisting the driver. But all buses should have on board a supply of spare masks for students who forget or lose their own, hand sanitizer dispensers and small touchless trash cans. The education transportation task force recommended cleaning buses between routes, disinfecting vehicles daily and not using a bus for at least 24 hours if it turns out that someone on board had an active COVID-19 case.

None of these recommendations will be easy to follow across the board. But it’s clear that there are ways that school districts can reduce the risks tied to school buses for the duration of the pandemic.

Jesse Capecelatro is affiliated with the University of Michigan COVID-19 Rapid Response Steering Committee, a centralized, University-wide team focused on supporting and enabling rapid responses to COVID-19 needs.