Researchers at the University of Florida collected the SARS-CoV-2 virus which causes COVID-19 from the air within a car driven for 15 minutes by a patient confirmed to have the illness. The driver had minimal symptoms, without a cough, and was not wearing a face mask during the drive.
Senior author J. Glenn Morris Jr., M.D., M.P.H & T.M., who directs UF’s Emerging Pathogens Institute, says the findings underscore the importance of wearing a mask when using public transportation or sharing a vehicle with another person. Their work is currently posted on the medRxiv preprint server and is publicly available.
“In a way, we showed what you expect to find,” Dr. Morris says. “However, in contrast to prior studies, we were able to isolate the virus from the air of the car. We also used an air sampler that let us stratify particles collected from the air by size, and we only found culturable, or viable, virus in a size range that can be inhaled into the lower lungs.”
A virus must be viable in order to cause an infection in a person. Non-viable viruses will not cause infection and are harmless.
UF virology expert John Lednicky is the paper’s first author and says that the work has important implications for understanding COVID-19’s airborne risks.
“We were able to retrieve this virus from the infected person’s air space, and then we propagated the virus in cell cultures,” says Lednicky, who is a professor in UF’s College of Public Health and Health Professions Department of Global and Environmental Health. “This indicates viable virus was present and posed an inhalation risk.”
Public health authorities have debated the role of both large and small respiratory droplets in the spread of COVID-19. While large droplets expelled with a cough or sneeze fall to the ground within a few feet of the ill person, smaller respiratory droplets can become aerosolized and ride on air currents, like meandering cigarette smoke, for a considerable distance.
Prior work by this same research group collected viable SARS-CoV-2 virus in the air of a hospital room occupied by a patient ill with COVID-19.
Genetic sequences of the virus collected from the air, and a swab from the patient’s nose, were identical; this indicated that the viral particles in the air came from the patient. Samples from the air of a hallway adjacent to the patient’s room did not detect viable virus.
“For this study, we wanted to move beyond the medical setting and test air in the community, in a setting where normal people go about their day,” says Dr. Morris, who is also a professor of medicine and infectious diseases in UF’s College of Medicine.
Epidemiological studies have identified public transportation as a risk for transmission, and at least one study has estimated that someone’s risk of contracting COVID-19 is about three times higher if they share a vehicle with an infected person.
In the study, UF investigators clipped a portable air sampler to the passenger-side sunshade of a car driven by a patient who had received a confirmed laboratory test for COVID-19 a few days earlier.
The patient drove for 15 minutes with the air conditioner on, and then the car was left with the windows up for another two hours while the air sampler continued to run. This allowed the sampler time to process about 1.22 cubic meters of air.
The team then retrieved the sampler and transported it back to a secure lab at the University.
The air sampler used by the research team separates particles captured from the air into five size categories that range from less than 0.25 micrometers, to between 0.25 and 0.50 micrometers, between 0.50 and 1.0 micrometers, between 1.0 and 2.5 micrometers and greater than 2.5 micrometers.
The investigators recovered viable, culturable virus from only the filter that captured airborne particles ranging from 0.25 to 0.50 micrometers in length, which is roughly equivalent to 1/200 to 1/100 the thickness of a human hair.
“This was the most surprising part,” Morris says. “With the air sampler running for two hours, we’d have expected the smaller-sized particles to dry out and be rendered non-infectious.”
Instead, when particles from this size range were introduced to animal cells cultured in a secure lab, they successfully infected the cells. A molecular test suggested that the infective virus was SARS-CoV-2, which the researchers then confirmed by sequencing the entire virus genome. The researchers even matched mutations in the sampled virus to a previously known and identified strain isolated from the UF community.
Other studies have shown that wearing cloth face masks, especially those constructed with multiple fabric layers, effectively reduces the emission of respiratory droplets by the wearer.
“These findings show that there is a risk of airborne transmission from people ill with COVID-19 who are not wearing a mask,” Morris says. “It really underscores the importance of mask-wearing, especially in small spaces with poor ventilation, or even wearing two masks as the CDC has recently recommended.”
Outbreaks of respiratory diseases, such as influenza, severe acute respiratory syndrome (SARS), Middle East respiratory syndrome, and now the novel coronavirus [severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)], have taken a heavy toll on human populations worldwide.
They are redefining a myriad of social and physical interactions as we seek to control the predominantly airborne transmission of the causative, SARS-CoV-2 (1–3). One common and critical social interaction that must be reconsidered is how people travel in passenger automobiles, as driving in an enclosed car cabin with a copassenger can present a risk of airborne disease transmission.
Most megacities (e.g., New York City) support more than a million of these rides every day with median figures of 10 daily interactions per rider (4). For maximum social isolation, driving alone is clearly ideal, but this is not widely practical or environmentally sustainable, and there are many situations in which two or more people need to drive together.
Wearing face masks and using barrier shields to separate occupants do offer an effective first step toward reducing infection rates (5–10). However, aerosols can pass through all but the most high-performance filters (8, 11), and virus emissions via micrometer-sized aerosols associated with breathing and talking, let alone coughing and sneezing, are practically unavoidable (12–21).
Even with basic protective measures such as mask wearing, the in-cabin microclimate during these rides falls short on a variety of epidemiological guidelines (22) with regard to occupant-occupant separation and interaction duration for a confined space. Preliminary models indicate a buildup of the viral load inside a car cabin for drives as short as 15 min (23, 24), with evidence of virus viability within aerosols of up to 3 hours (25, 26).
To assess these risks, it is critical to understand the complex airflow patterns that exist inside the passenger cabin of an automobile and, furthermore, to quantify the air that might be exchanged between a driver and a passenger. Although the danger of transmission while traveling in a car has been recognized (27), published investigations of the detailed airflow inside the passenger cabin of an automobile are unexpectedly sparse.
Several works have addressed the flow patterns inside automobile cabins, but only in the all-windows-closed configuration (28–30)—most commonly used so as to reduce noise in the cabin. However, intuitively, a means to minimize infectious particles is to drive with some or all of the windows open, presumably enhancing the fresh air circulating through the cabin.
Motivated by the influence of pollutants on passengers, a few studies have evaluated the concentration of contaminants entering from outside the cabin (31) and the persistence of cigarette smoke inside the cabin subject to different ventilation scenarios (32, 33). However, none of these studies have addressed the microclimate of the cabin and the transport of a contaminant from one specific person (e.g., the driver) to another specific person (e.g., a passenger).
In addition to this being an important problem applicable to airborne pathogens, in general, the need for a rigorous assessment of these airflow patterns inside the passenger cabin of an automobile seems urgent in the current coronavirus disease 2019 worldwide public health crisis.
The current work presents a quantitative approach to this problem. Although the range of car geometries and driving conditions is vast, we restrict our attention to that of two people driving in a car (five-seater), which is close to the average occupancy and seating configuration in passenger cars in the United States (34). We then ask the question: What is the transport of air and potentially infectious aerosol droplets between the driver and the passenger, and how does that air exchange change for various combinations of fully open and closed windows?
To address this question, we conducted a series of representative computational fluid dynamics (CFD) simulations for a range of ventilation options in a model four-door passenger car. The exterior geometry was based on a Toyota Prius, and we simulated the flow patterns associated with the moving car while having a hollow passenger cabin and six combinations of fully open and closed windows, named as front left (FL), rear left (RL), front right (FR), and rear right (RR) (Fig. 1).
We consider the case of two persons traveling in the car—the driver in the front left-hand seat (assuming a left-hand drive vehicle) and the passenger sitting in the rear right-hand seat, thereby maximizing the physical distance (≈1.5 m) between the occupants. For the purposes of simulation, the occupants were modeled simply as cylinders positioned in the car interior.
As a reference configuration (Fig. 1, Config. 1), we consider driving with all four windows closed and a typical air-conditioning flow—with air intake at the dashboard and outlets located at the rear of the car—that is common to many modern automobiles (35). The intake air was modeled to be fresh (i.e., no recirculation) with a relatively high inflow rate of 0.08 m3/s (36).
The numerical simulations were performed using Ansys Fluent package, solving the three-dimensional, steady Reynolds-averaged Navier-Stokes (RANS) equations using a standard k-ε turbulence model (for details, see Methods). The RANS approach for turbulence, despite its known limitations (37), represents a widely used model for scientific, industrial, and automotive applications (38). A more accurate assessment of the flow patterns and the droplet dispersion is possible using large-eddy simulations or using fully resolved direct numerical simulations, which have a significantly higher computational cost. This is beyond the scope of the present work.
We simulated a single driving speed of v = 22 m/s [50 miles per hour (mph)] and an air density of ρa = 1.2 kg/m3. This translates to a Reynolds number of 2 million (based on the car height), which is high enough that the results presented here should be insensitive to the vehicle speed. The flow patterns calculated for each configuration were used to estimate the air (and potential pathogen) transmission from the driver to the passenger and, conversely, from the passenger to the driver. These estimates were achieved by computing the concentration field of a passive tracer “released” from each of the occupants and by evaluating the amount of that tracer reaching the other occupant (see Methods).
Here, we first describe the pressure distributions established by the car motion and the flow induced inside the passenger compartment. Following that, we describe the passenger-to-driver and driver-to-passenger transmission results for each of the ventilation options and, last, conclude with insights based on the observed concentration fields, general conclusions, and implications of the results.
RESULTS AND DISCUSSION
Overall airflow patterns
The external airflow generates a pressure distribution over the car (Fig. 2), forming a high-pressure stagnation region over the radiator grille and on the front of the windshield. The peak pressure here (301 Pa) is of the order of the dynamic pressure (0.5 ρav2 = 290 Pa at 22 m/s).
Conversely, as the airflow wraps over the top of the car and around the sides, the high airspeed is associated with a low-pressure zone, with the local pressure well below atmospheric (zero gauge pressure in Fig. 2). This overall pressure map is consistent with other computations of flows over automobile bodies (39) and gives a physical preview to a key feature—that the areas near the front windows and roof of the car are associated with lower-than-atmospheric pressures, while the areas toward the rear of the passenger cabin are associated with neutral or higher-than-atmospheric pressures.
A typical streamline (or pathline) pattern in the car interior is shown in Fig. 3, where the RL and FR windows are opened (Config. 3 in Fig. 1). The streamlines were initiated at the RL window, which is the location of a strong inflow (Fig. 3, bottom right), due to the high-pressure zone established by the car’s motion (Fig. 2).
A strong air current (~10 m/s) enters the cabin from this region and travels along the back seat of the car before flowing past the passenger sitting on the RR side of the cabin. The air current turns at the closed RR window, moves forward, and most of the air exits the cabin at the open window on the FR side of the vehicle, where the exterior pressure is lower than atmospheric (Fig. 2). There is a much weaker air current (~2 m/s) that, after turning around the passenger, continues to circulate within the cabin. A small fraction of this flow is seen to exit through the RL window.
The streamline arrows indicate that the predominant direction of the recirculation zone inside the cabin is counterclockwise (viewed from above). These streamlines, of course, represent possible paths of transmission, potentially transporting virus-laden droplets or aerosols throughout the cabin and, in particular, from the passenger to the driver.
As already indicated, for the particular ventilation option shown here, the overall air pattern—entering on the RL and leaving on the FR—is consistent with the external pressure distributions (Fig. 2). The elevated pressure toward the rear of the cabin and the suction pressure near the front of the cabin drive the cabin flow.
This particular airflow pattern was confirmed in a “field test” in which the windows of a test vehicle (2011 Kia Forte hatchback) were arranged with the RL and FR windows open, with two occupants (driver in the FL seat and a passenger in the RR seat) as in Config. 3. The car was driven at 30 mph on a length of straight road, and a flow wand (a short stick with a cotton thread attached to the tip) and a smoke generator were used to visualize the direction and approximate strength of the airflow throughout the cabin.
By moving the wand and the smoke generator to different locations within the cabin, the overall flow patterns obtained from the CFD simulations—a strong air stream along the back of the cabin that exits the FR window, and a very weak flow near the driver—were qualitatively confirmed (see the Supplementary Materials). Different ventilation configurations generate different streamline patterns (e.g., figs. S4 and S5) but most of these can be linked to the pressure distributions established over the car body (Fig. 2).
An important consideration when evaluating different ventilation options in the confined cabin of a car is the rate at which the cabin air gets replenished with outside fresh air. This was measured by Ott et al. (32) for a variety of cars, traveling at a range of speeds, and for a limited set of ventilation options.
In these measurements, a passive tracer (representing cigarette smoke) was released inside the cabin, and the exponential decay of the tracer concentration was measured. Assuming the cabin air to be well mixed (32), they estimated the air changes per hour (ACH)—a widely used metric in indoor ventilation designs.
From the simulations, we can precisely compute the total flow of air entering (and leaving) the cabin, and, knowing the cabin volume, we can directly compute the ACH. Such a calculation yields a very high estimate of ACH (of the order of thousands; see fig. S6), but this is misleading, since the assumption of well-mixed cabin air is an oversimplification. Instead, a more relevant quantification of the ACH was obtained using a residence time analysis for a passive scalar released at multiple locations within the passenger cabin.
The time taken for the concentration at the outlets to decay below a threshold value (1% of the initial value) was computed, and the inverse of this time yields effective values for ACH (Fig. 4), which compare favorably with those reported by Ott et al. (32), after correcting for the vehicle speed (40).
As one might expect, the-all-windows-open-configuration (Config. 6) has the highest ACH—approximately 250, while among the remaining configurations, the-all-windows-closed-configuration (Config. 1) has the lowest ACH of 62. However, what is somewhat unexpected is that the ACH for the configuration with windows adjacent to the driver and the passenger (FL and RR, respectively; Config. 2) are opened is only 89—barely higher than the all-windows-closed configuration.
The remaining three configurations (Configs. 3 to 5) with two or three open windows all show a relatively high efficacy of about 150 ACH. The reason for these differences can be traced back to the overall streamline patterns and the pressure distributions that drive the cabin flow (Fig. 2).
A well-ventilated space requires the availability of an entrance and an exit and a favorable pressure gradient between the two (41, 42). Once a cross-ventilation path is established (as in Config. 3 or Fig. 3), opening a third window has little effect on the ACH.
It is important to point out that the ACH for Config. 3 is higher than that for Config. 2, despite the apparent mirror symmetry of the open windows. This occurs because of two effects. First, the locations of the occupants relative to the open windows influences the residence time of the released scalar, which is used in estimating the ACH (32).
Second, the cylinders representing the driver and passenger also cause a reduction in the airflow in Config. 2 where the occupants are seated next to the open windows. We will later show that the ACH gives only a partial picture and that the spreading of a passive scalar can show marked variations between Configs. 3 and 5, despite their nearly constant ACH.
The flows established through the cabin provide a path for air transmission between the two occupants and hence a possible infection route. Our focus here is on transmission via aerosols, which are small enough (and noninertial) that they can be regarded as faithful tracers of the fluid flow (43, 44).
We begin by addressing the problem from the viewpoint of an infected driver releasing pathogen-laden aerosols and potentially infecting the passenger. Figure 5 shows a comparison of the spreading patterns of a passive scalar released near the driver and reaching the passenger (for details, see Methods). To obtain a volumetric quantification, the average scalar concentration in a 0.1-m-diameter spherical domain surrounding the passenger’s face is also computed, as shown in Fig. 5B.
The all-windows-closed configuration (Config. 1), relying only on air-conditioning, fares the worst and results in over 10% of the scalar that leaves the driver reaching the passenger. In contrast, the all-windows-open setting (Config. 6) appears to be the best case, with almost no injected scalar reaching the passenger. An overall trend of decreasing transmission is observed when the number of open windows are increased. However, there is some variability between the different configurations, the reasons for which may not be clear until one looks at the overall flow patterns (e.g., Fig. 3).
Concentration fields of the scalar (Fig. 5C) are examined in a horizontal plane A-B-C-D within the car cabin roughly at head height of the occupants (Fig. 5A). The scalar field concentration is the highest where all four windows are closed (Config. 1). We note that this driving configuration might also represent the most widely preferred one in the United States (with some seasonal variations).
A two-windows-open situation, wherein the driver and the passenger open their respective windows (Config. 2), might be assumed as the logical thing to do for avoiding infection from the other occupant. Although this configuration does improve over the all-windows-closed situation, shown in Fig. 5B, one can see from the concentration field that Config. 2 does not effectively dilute the tracer particles and that the passenger receives a fairly large contaminant load from the driver.
To explain this result, we looked more closely at the airflow patterns. In analogy with the streamlines associated with Config. 3 (Fig. 3), Config. 2 establishes a strong air current from the open RR window (RR) to the open FL window, along with a clockwise recirculating flow within the cabin as viewed from above.
Although this flow pattern is weak, it increases the transport of tracer from the driver to the passenger. Moreover, the incoming air stream in Config. 2 enters behind the passenger and is ineffective in flushing out potential contaminants emanating from the driver.
An improvement to this configuration can be achieved if two modifications are possible: (i) a change in the direction of the internal circulation and (ii) a modified incoming airflow that impinges the passenger before leaving through the open window on the front. This has been realized when the RL and FR are open (Config. 3) (Fig. 5C), same as the configuration shown in Fig. 3). Now, the incoming clean air stream from the RL window partially impinges on the passenger (seated in the RR seat) as it turns around the corner. This stream of air might also act as an “air curtain” (45), and hence, the concentration of potentially contaminated air reaching the passenger is reduced.
The remaining configurations (Configs. 4 to 6) will be treated as modifications made to Config. 3 by opening more windows. Configuration 4 has three windows open (Fig. 5C). Since this represents opening an additional (RR) window, it may be unexpected to find a detrimental effect on the concentration field and the ACH (comparing Configs. 3 and 4 in Fig. 5, B and C). The increase in the concentration can be linked to the modified airflow patterns that result from opening the third (RR) window. First, opening the RR window leads to a reduction in the flow turning at the RR end of the cabin, since a fraction of the incoming air gets bled out of this window (fig. S4). Because of this diversion of the airflow, the region surrounding the passenger is less effective as a barrier to the scalar released by the driver. Second, the modified flow also creates an entrainment current from the driver to the passenger, which further elevates the scalar transport.
When the third open window is the FL (Config. 5), this leads to an improvement, nearly halving the average concentration when compared to when the additional window is the RR (Config. 3). The reason for this is apparent from the concentration field (Fig. 5C), since with the FL window near the driver open, the relatively low pressure near the front of the car creates an outward flow that flushes out much of released species. With the substantially reduced initial concentration field near the driver, the fraction reaching the passenger is proportionately reduced. Thus, among the configurations with three windows open, Config. 5 might provide the best benefit from the viewpoint of driver-to-passenger transmission.
Last, when all four windows are opened (Config. 6), we can again use the exterior pressure distribution to predict the flow directions. The streamlines enter through the rear windows and leave via the front windows. However, unlike the configuration with only two windows open (Fig. 3), the overall flow pattern is substantially modified (fig. S5), and the streamlines obey left-right symmetry and, for the most part, do not cross the vertical midplane of the car. In this configuration, the flow is largely partitioned into two zones creating two cross-ventilation paths in which the total airflow rate is nearly doubled when compared to the two- and three-windows-open configurations (fig. S6).
In this section, we look into the particle (and potential pathogen) transmission from the passenger to the driver. Comparing the spreading patterns of a passive scalar within the car cabin (Fig. 6), the general trend suggests a decreasing level of transmission as the number of open windows is increased, similar to the results found for the driver-to-passenger transmission.
The all-windows-closed configuration (Config. 1) shows the highest concentration level at the driver (~8%). This value, however, is lower than the 11% reported for the inverse transport, i.e., from the driver to the passenger (Fig. 5B), a difference that can be attributed to the fact that the air-conditioning creates a front-to-back mean flow.
As before, the lowest level of scalar transport corresponds to all-windows-open scenario (Config. 6), although we note that the concentration load here (about 2%) is noticeably higher than that for the driver-to-passenger transmission (about 0.2%). The streamline patterns for this configuration (fig. S5) show that the air enters through both the rear windows and exits through the respective front windows. There is, therefore, an average rear-to-front flow in both the left and right halves of the cabin, which enhances transmission from the passenger to the driver.
Among the remaining configurations (Configs. 2 to 5), Config. 3 shows a slightly elevated level of average concentration. The counterclockwise interior circulation pattern is at the heart of this transmission pattern. A substantial reduction in the average concentration can be achieved by additionally opening the rear window adjacent to the passenger (Config. 4).
This allows for much of the scalar released by the passenger to be immediately flushed out through the rear window, analogous to the way in which opening the driver-adjacent (FL) window helps to flush out the high-concentration contaminants from the driver before they can circulate to the passenger (Fig. 5C, Config. 5).
More information: John A. Lednicky et al. Isolation of SARS-CoV-2 from the air in a car driven by a COVID patient with mild illness, (2021). DOI: 10.1101/2021.01.12.21249603