A new study of airflow patterns inside a car’s passenger cabin offers some suggestions for potentially reducing the risk of COVID-19 transmission while sharing rides with others.
The study, by a team of Brown University researchers, used computer models to simulate the airflow inside a compact car with various combinations of windows open or closed.
The simulations showed that opening windows – the more windows the better – created airflow patterns that dramatically reduced the concentration of airborne particles exchanged between a driver and a single passenger. Blasting the car’s ventilation system didn’t circulate air nearly as well as a few open windows, the researchers found.
“Driving around with the windows up and the air conditioning or heat on is definitely the worst scenario, according to our computer simulations,” said Asimanshu Das, a graduate student in Brown’s School of Engineering and co-lead author of the research. “The best scenario we found was having all four windows open, but even having one or two open was far better than having them all closed.”
Das co-led the research with Varghese Mathai, a former postdoctoral researcher at Brown who is now an assistant professor of physics at the University of Massachusetts, Amherst. The study is published in the journal Science Advances.
The researchers stress that there’s no way to eliminate risk completely—and, of course, current guidance from the U.S. Centers for Disease Control (CDC) notes that postponing travel and staying home is the best way to protect personal and community health. The goal of the study was simply to study how changes in airflow inside a car may worsen or reduce risk of pathogen transmission.
The computer models used in the study simulated a car, loosely based on a Toyota Prius, with two people inside—a driver and a passenger sitting in the back seat on the opposite side from the driver.
The researchers chose that seating arrangement because it maximizes the physical distance between the two people (though still less than the 6 feet recommended by the CDC). The models simulated airflow around and inside a car moving at 50 miles per hour, as well as the movement and concentration of aerosols coming from both driver and passenger. Aerosols are tiny particles that can linger in the air for extended periods of time.
Part of the reason that opening windows is better in terms of aerosol transmission is because it increases the number of air changes per hour (ACH) inside the car, which helps to reduce the overall concentration of aerosols.
But ACH was only part of the story, the researchers say. The study showed that different combinations of open windows created different air currents inside the car that could either increase or decrease exposure to remaining aerosols.
Because of the way air flows across the outside of the car, air pressure near the rear windows tends to be higher than pressure at the front windows. As a result, air tends to enter the car through the back windows and exit through the front windows. With all the windows open, this tendency creates two more-or-less independent flows on either side of the cabin.
Since the occupants in the simulations were sitting on opposite sides of the cabin, very few particles end up being transferred between the two.
The driver in this scenario is at slightly higher risk than the passenger because the average airflow in the car goes from back to front, but both occupants experience a dramatically lower transfer of particles compared to any other scenario.
The simulations for scenarios in which some but not all windows are down yielded some possibly counterintuitive results. For example, one might expect that opening windows directly beside each occupant might be the simplest way to reduce exposure. The simulations found that while this configuration is better than no windows down at all, it carries a higher exposure risk compared to putting down the window opposite each occupant.
“When the windows opposite the occupants are open, you get a flow that enters the car behind the driver, sweeps across the cabin behind the passenger and then goes out the passenger-side front window,” said Kenny Breuer, a professor of engineering at Brown and a senior author of the research. “That pattern helps to reduce cross-contamination between the driver and passenger.”
It’s important to note, the researchers say, that airflow adjustments are no substitute for mask-wearing by both occupants when inside a car. And the findings are limited to potential exposure to lingering aerosols that may contain pathogens.
The study did not model larger respiratory droplets or the risk of actually becoming infected by the virus.
Still, the researchers say the study provides valuable new insights into air circulation patterns inside a car’s passenger compartment—something that had received little attention before now.
“This is the first study we’re aware of that really looked at the microclimate inside a car,” Breuer said.
“There had been some studies that looked at how much external pollution gets into a car, or how long cigarette smoke lingers in a car. But this is the first time anyone has looked at airflow patterns in detail.”
The research grew out of a COVID-19 research task force established at Brown to gather expertise from across the University to address widely varying aspects of the pandemic. Jeffrey Bailey, an associate professor of pathology and laboratory medicine and a coauthor of the airflow study, leads the group. Bailey was impressed with how quickly the research came together, with Mathai suggesting the use of computer simulations that could be done while laboratory research at Brown was paused for the pandemic.
“This is really a great example of how different disciplines can come together quickly and produce valuable findings,” Bailey said. “I talked to Kenny briefly about this idea, and within three or four days his team was already doing some preliminary testing. That’s one of the great things about being at a place like Brown, where people are eager to collaborate and work across disciplines.”
International Organization of Motor Vehicle Manufacturers (OICA) has estimated that over 1 billion passenger cars travel on roads by 2019 worldwide, indicating that one out of seven people of the world has a passenger car. When the world is open back to normalcy by lifting the present state of lockdown, people will resort to traveling by passenger cars, and consequently, there will be a propensity of spreading the COVID-19 unless precautions are taken. We, therefore, bring in a hypothesis to illustrate the best possible ways of preventing the COVID-19 from spreading while traveling in a passenger car.
A crucial attribute that supports the spread of COVID-19 is the interior ventilation rate in the passenger vehicle, usually expressed in ACH, which depends on the vehicular speed, ventilation setting and window positions (Ott et al., 2007). Engelmann et al. (1992) have estimated that with the air-conditioning (AC) system off, the ACH for a stationary vehicle was in the range of 0.42–1.09 per hour. With the AC on, ACH was between 1.96 and 3.23 per hour, and with the AC off and the fans on, it varied in the range of 8.7–10.7 per hour. Park et al. (1998), with the windows closed and no mechanical ventilation, have reported the ACH between 1.0 and 3.0 per hour, and with the ventilation set on recirculation, between 1.8 and 3.7 per hour.
With the windows closed and the fan set on fresh air, the ACH was between 13.3 and 26.1 per hour, and with windows open, but no mechanical ventilation, the ACH ranged from 36.2 to 47.5 per hour (Park et al., 1998). Offermann et al. (2002) have measured the ACH by letting the vehicle move with an average speed of 29 km/h and have found that with the window open and the ventilation system off, an ACH of 71 per hour, with the ventilation system on and the windows closed, 60 per hour, and when the ventilation system was turned off, 4.9 per hour.
Following the study done by Khatoon and Kim (2020), a typical pattern of velocity streamlines inside the vehicular cabin with a moderate level of ACH assigned to a vehicle moving at a moderate speed under conditions of “AC on and windows closed” is shown in Fig. 6 a. Fig. 6a illustrates that cooled air travels to the back seats and returns towards the front on either side at a lower level.
Under such circumstances, an infected person sitting in the back seat may cough and the resultant cough-jet in the form of droplets and a plume of aerosols (with an average speed of 10 m/s; relative humidity < 50%; temperature < 25 °C; ACH < 60 per hour) spreads towards the front seat, and the plume of aerosols may drop the advective transport phenomena with lower velocities and get carried away with existing velocity streamlines once again towards the back seats (Fig. 6b).
Such phenomena may expose all passengers in the vehicle, and the risk of contracting the disease seems to be high. Two such cases have been reported in Sri Lanka, where an infected passenger had travelled sitting at the back seat in a rented car for a period not greater than 1 h with AC on and windows closed, and the driver was subsequently reported to have got infected of the COVID-19.
The other case was reported that a person had accompanied one of his siblings (an asymptomatic person) in his car with AC on and windows closed for more than 15 min. Such situations seem to be somewhat controlled when the infected person wears a surgical mask. However, the risk factor remains the same, as loose ends of the mask shed both droplets and aerosols, although the expiration from the front of the mask is substantially reduced (Fig. 6c).
Conversely, when the infected passenger is equipped with an N95 respirator, under the same conditions, a minute payload of droplets and a faint cloud of aerosols may come out (Fig. 6d). However, because of the circulation within the cabin, one cannot rule out that there is no element of risk.
Thus, a hypothesis could be built speculating that traveling in a passenger vehicle with people aboard under conditions of AC on and window closed, has a discernible risk factor of getting susceptible hosts infected, though masks are worn.
When a passenger car moves at a certain speed with windows open, the velocity streamlines are generated from front and rear windows, and finally, sweeping the passengers aboard, they exit the cabin from the rear windows (Fig. 7 a). Such transport-phenomena are simulated using computational fluid dynamics, but detailed information on the behavior of streamlines under different environmental settings is poorly investigated.
In the case of passenger cars with windows open, different behaviors could be expected depending on the environmental settings prevailing in different geographical regions. In other words, the environmental settings for temperate climates such as East Asia, Europe, and North America (relative humidity < 50%; temperature < 25 °C; ACH > 60 per hour) and tropical climates, including South East Asia, Africa, and South America (relative humidity > 50%; temperature > 25 °C; ACH > 60 per hour) could be expected.
The studies done on the sustenance of SARS-CoV-2 have manifested that there may be a better chance for the viral-laden cough-jets to sustain in temperate climates than tropical climates, as the daily mortality of COVID-19 has been positively associated with diurnal temperature range, but negatively with the absolute humidity (Ma et al., 2020).
Fig. 7b shows how the cough-jet behaves in a passenger car with windows open and AC off when the car moves at a speed of less than 30 km/h. Under such conditions, the droplets fall in the entire length of the vehicle, while the aerosol-cloud drives to the front and returns with the airflow streamlines, spreading the aerosol plume every part of the cabin in no time.
When the car moves at higher speeds (> 30 km/h) with the same environmental settings, the droplets do not travel far and confined to a limited space (even not beyond the driver’s seat), but the cloud of aerosol will drift far and finally exits from the rear windows. The explanations given in this paper restrict the analysis only for the case where the speed is less than 30 km/h, as such speeds become the worse scenario for the sustenance of the SARS-CoV-2 virus.
The cabin environment becomes much improved when the infected person wears a surgical mask while traveling (Fig. 7c). There seems that only a minimal payload of droplets being shed from the front, but considerable load may come from either side of the mask, as the surgical mask is usually loosely fitted to the face.
Conversely, the aerosol cloud may still travel to the front area of the cabin and returns with the airflow stream coming from outside the vehicle. Nevertheless, the cabin airflow streamlines drive such virus-laden plume out of the cabin in seconds. The cabin environment is further improved when the infected person wears an N95 respirator (Fig. 7d). Still, one has to admit the fact that there is an element of risk for susceptible hosts to get infected.
When two scenarios (Scenario 1: AC on and windows closed; Scenario 2: AC off and windows opened) are critically reviewed, one can speculate that the scenario 2 will be better in controlling the SARS-CoV-2 virus; hence strongly recommended at least until the COVID-19 pandemic ceases.
For example, the second patient of COVID-19 in Sri Lanka was a tour guide, and when he became symptomatic, he travelled to the hospital by his car driven by his son, with his wife sitting in the front seat. He made it a point to open all windows and sat behind until they reached the hospital.
The traveling time was more than 30 min, and no person in the car was infected with the COVID-19. This story epitomizes the rationale postulated above, and the relevant authorities of affected countries should come out with strict guidelines to get such best practices implemented for reduced morbidities and mortalities.
Conversely, two cases were reported in Sri Lanka, where drivers of rental cars got infected with scenario 1. Besides, letting the car park under direct sunlight with windows open for at least 30 min would be a better option to eradicate the potential payloads of the SARS-CoV-2 virus from the cabins of passenger cars.
More information: Varghese Mathai et al, Airflows inside passenger cars and implications for airborne disease transmission, Science Advances (2020). DOI: 10.1126/sciadv.abe0166