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Study Identifies Safest Ways to Share a Car During COVID-19

woman in the back of a car with mask on

 

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Key Takeaways

  • Riding in a car with someone outside of your home carries a risk of COVID-19 infection for yourself and others.
  • A new study finds that rolling your windows down is more effective than car ventilation for improving air circulation and reducing airborne transmission of the coronavirus.
  • Sitting in the back of the car and opening the window farthest away from you may also improve air circulation and reduce exposure to aerosol droplets.

On the fence about calling an Uber to get to an appointment? If you must share a car with people from outside of your household at this point in the pandemic, a new study highlights several methods to make it as safe as possible.

While any type of traveling can increase your chances of COVID-19 exposure, riding in a car is especially risky because passengers are in a confined space. Cars don’t have the same air filtration system as airplanes, which may be slightly safer because of their HVAC ventilation.

“Within airplanes, there’s a lot of areas for air to circulate, so there’s not as much risk of repeat exposure to the same virus. But when you’re in an enclosed space like a car, there’s not much opportunity to social distance,” Sri Banerjee, PhD, an epidemiologist at Walden University who previously studied infectious diseases at the Centers for Disease Control and Prevention (CDC), tells Verywell.

Currently, the CDC recommends opening car windows or setting the air ventilation/air conditioner system to non-recirculation mode. However, Varghese Mathai, PhD, assistant professor at the University of Massachusetts Amherst and lead author of a December study published in Science Advances, says this might not be ideal for reducing transmission.

“When you are in a confined environment, there is a risk of airborne infection, especially in ride-sharing trips that take just 15 to 20 minutes," Mathai tells Verywell. "If you have all your windows closed, you are germinating in a closed space without a lot of circulation. We wanted to understand how the air flows in the car and how to improve this."

Investigating Airborne Transmission

Researchers from the University of Massachusetts at Amherst and Brown University performed computer models on the likelihood of airborne transmission from one occupant in a car to another. They looked at the differences in air flow when specific windows were open versus when all of the windows were open versus being closed.

In a simulation of two people in a car, the results suggest that better air circulation and less exposure to airborne contaminants were most effective when the passenger sat in the back of the vehicle.

While having all the windows down was more beneficial than turning on the ventilation, opening specific car windows also made a difference.

“The most surprising finding was that if one occupant could potentially infect the other, opening the window next to you might not necessarily be the best option,” Mathai says. “Instead, open windows that are farthest from you. This may allow for cross-ventilation in the cabin.”

Mathai explains car air ventilation flows from the back to the front window. 

“This could help Uber, Lyft, and taxis in knowing which windows to open for the safety of the passenger,” Mathai says.

What This Means For You

While experts recommend limiting yourself to essential travel, if you have to travel in a car with a person outside of your household, it’s necessary to take precautions. By wearing a mask, rolling down car windows, and sitting in the back of the car, you can help to reduce your risk of COVID-19 infection.

A Research Stepping Stone

While the study focused on passenger cars, it opens doors to other areas of study.

Mathai and his team are looking at applying similar simulations to improving airflow in other confined spaces, from helicopters to restaurants. They are also investigating the differences in airflow of air conditioning and heating units, and how it affects the airborne transmission of the coronavirus.

Mathai says that currently, this study does not apply to other modes of public transportation. Because each vehicle is designed differently, a study design would need to be customized to apply to the specific dimensions of the vehicle.

Is Riding in a Car Safe?

Understanding how to reduce COVID-19 transmission in the air is important in preventing future infections. Mathai says aerosol droplets are very tiny, and they linger in the air for long durations. This is especially relevant for long car rides where aerosol droplets can build up in concentration over time, like in an hour-long ride.

Mathai says that while his study looks at how to improve air flow in a car if you must, it is by no means a recommendation to ride in a car with another passengers from outside your household.

“It’s important to note that this work was looking at airborne infection—looking at how air flows—not how you cough and the respiratory droplets that can be released,” Mathai says. “Since we did not look at this or the risk of getting infected, we are not in a position to make health recommendations.”

Because the risk of respiratory droplets spreading between passengers remains, Banerjee advises people to continue wearing masks in cars. “The mask is there to protect others from other respiratory droplets or anything that is coming out. About 40-50% of the spread is from asymptomatic people,” he says. “So even if someone isn’t showing symptoms, they can still spread the virus. This makes respiratory droplets dangerous. There’s a risk of the coronavirus on surfaces, but a majority of transmissions are not from it.”

For this reason, Banerjee warns that a passenger not taking proper precautions can potentially lead to community spread, which is dangerous because of the difficulty to contact trace.

“A lot of the spread is from places where we don’t necessarily know where the transmission took place,” he says. “With ride-sharing, there’s a lot of unknowns, so you always need to take your precautions: wearing a mask that covers your nose and mouth, keeping a hand sanitizer with you, maintaining distance, and opening the window.”

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  1. Centers for Disease Control and Prevention. Protect yourself when using transportation. Updated November 27, 2020.

  2. Mathai V, Das A, Bailey J, Breuer K. Airflows inside passenger cars and implications for airborne disease transmission. Science Advances. 2020. p.eabe0166. doi:10.1126/sciadv.abe0166