Fighting Covid-19 in a passenger car
The social distance of 2 meters (about 6 feet) is designed to minimize the spread of COVID-19 in open spaces and large enclosed environments. However, when we need to travel by car, the program is difficult to maintain. According to OICA's estimates, by 2019, there will be more than 1 billion cars on the roads, while the number in Bangladesh is about 400,000.
In order to return to an open economy, we must take the car more frequently and ride with other people. Therefore, if proper preventive measures are not taken, diseases may be spread. The large droplets and aerosols that cause exhalation, sneezing, coughing, or talking into the passenger compartment depend on the internal ventilation system. Before deciding whether to start travelling, we should consider taking adequate safety measures.
So, what are the procedures that can protect us while travelling? For the airborne SARS-CoV-2 that causes COVID-19, this is indeed a thorny issue. Understanding the air flow pattern inside the car can be used as a tool to recommend a new method to achieve safe driving.
Under the condition of "using AC power with the windows closed", streamlines are generated inside the vehicle compartment from the AC power inlet. The cold air moves to the rear seats and circulates continuously in the cabin. Now, if a patient with corona is sitting on his back and coughing or sneezing, he will produce droplets and aerosols. Due to the action of gravity, larger droplets will be deposited on the front seat cover and floor mats, but the plume of aerosol will reduce the air convection to the low-speed advection transport, and will be transported to the rear seat again together with the streamline.
Therefore, all passengers in the cabin will be exposed to droplets in the air; the risk of spreading diseases is high. If the infected passengers use surgical masks, this situation can be dealt with to some extent. Although the diffusion of particles in the air in the forward direction during exhalation is negligible, surgical masks increase the risk of profound leakage of droplets and aerosols through the sides, back, above, and below.
However, when using N95 masks, the leakage around the edges is minimal. Therefore, we can conclude that under the condition of "using AC with the windows closed", driving in a taxi with an infected passenger wearing a mask is obviously at risk of infection to a susceptible host.
A car with windows open moves at a certain speed generates velocity streamlines from the windows of the front and rear doors. The streamlines sweep the passengers and exit the cabin through the windows in the back door. It has been reported that high temperature and humidity will reduce the spread of Covid-19.
In the case of "windows open and closed AC" and "speeds less than 30 km/h", due to the effect of gravity, large drops fall on the plastic or wooden interior of the car, while the aerosol moves along the flow line and retreat; therefore spread to the entire cabin. If the vehicle is travelling at a speed higher than 30 km/h, the droplets will not be able to go far beyond the driver's seat, but the aerosol flow will drift very far and eventually leave through the rear door window.
If the infected passengers use surgical masks, the cabin conditions will be significantly improved. When the mask is usually loosely mounted on the face, a large amount of radiation is emitted from either side of the mask, and droplets from the front are minimal. The aerosol moves to the front of the cabin and returns with the large amount of air outside the car, but the internal streamline forces the aerosol to be discharged from the cabin immediately.
If an infected person uses N95 masks, the situation may get improved. It can make passengers less susceptible to infection. When carefully evaluating the two situations of "use of AC with closed windows" and "windows-open and AC-off", we can conclude that "AC-off and windows-open" is against COVID-19 a better way, and strongly recommended at least until the pandemic is over.
In addition to CDC's recommendations of regularly cleaning highly-touch points (such as door handles, seats, seat belts, gear shift, steering wheels, radio knobs and other controls) exposing the car to sunlight for 30 minutes (3 inches windows to open) may be a better choice, which can minimize the possibility of virus transmission and thus protect passengers.
Professor Dr Mohammad Mainul Karim, Department of Applied Chemistry and Chemical Engineering, University of Dhaka