Travelling safe during Covid pandemic: Transmission in taxi, bus and auto
A person is almost 300 times more likely to contract the coronavirus disease (Covid-19) sitting in an air-conditioned (AC) taxi with a passenger, who is Covid positive, than she is sitting in an autorickshaw, a recent study by two scientists at the Johns Hopkins University (JHU), Bloomberg School of Public Health, Department of Environmental Health and Engineering, found.
The probability of catching the infection is reduced by 250% in a taxi with the windows rolled down (non-AC) compared to one where the air conditioning is on. The researchers calculated that the risk in both types of taxis was reduced by 75% when the vehicles sped up from zero to 120km/hour.
The study titled “Risk analysis of different transport vehicles in India during COVID-19 pandemic” by Darpan Das and Gurumurthy Ramachandran was published in the peer-reviewed journal, Environmental Research, on May 11.
While Das is a postdoctoral fellow at JHU, Ramachandran is a professor in the environmental health and engineering department.
The researchers calculated the chances of contracting Covid-19 infection in four vehicles common to Indian metropolises —AC taxi, non-AC taxi, bus and auto rickshaw— using a range of ventilation rates (air exchanges per hour) taken from previous studies and found that autos were the safest of the four options.
All scenarios accounted for the presence of one index patient in the stationary vehicle and with everyone — five travellers in each of the vehicles except the bus, which seated 40 — wearing a mask (surgical or cloth).
Compared to the auto, the risk was 86 times higher in a non-AC taxi and 300 times more in an AC taxi (probability of infection: 0.000199, 0.0171, 0.061 respectively). A person was 72 times more likely to contract Covid-19 in a non-moving bus with its windows open and seating 40 people (probability of infection: 0.0143), compared to an auto surrounded by four others, including the index patient.
The researchers used the Wells-Riley model of airborne transmission of infectious diseases, which has been previously used for understanding transmission of tuberculosis and measles. This model, used to calculate the effect of ventilation on transmission, assumes that the air contains doses of the infectious virus, and the air is well-mixed. The model predicts that the concentration of doses of infectious virus will tend to be high in small, poorly ventilated rooms, and it would be lower in larger, better ventilated rooms.
“As an input to the model, we have considered different viral emission rates from the infected person—two to134 quanta/hr (amounts of airborne virus per hour) based on different activities such as singing, talking etc. Because of the small volume of the vehicles, it was assumed as a well-mixed room,” Das explained.
In the auto, five people (including the driver) sit beside each other in greater proximity, but on account of ventilation – the rate at which the air volume changes, or air exchange rate – the probability of infection is the least. In a bus, with 40 people seated, the probability of infection is higher on account of lesser ventilation. In the model used by the researchers, irrespective of where the traveller sits, everyone is exposed to the same air and thus, the same risk factor inside the vehicle (while it is stationary and moving).
In their study, Das and Ramachandran assume that infectious aerosols are evenly distributed throughout the vehicle’s interior volume of air, and are removed by ventilation, vehicle air filtration, gravitational deposition, and viral inactivation. The probability of infection is related to the number of airborne viruses inhaled.
“Due to the airborne nature of the virus, apart from six feet distance and masks, ventilation is also an important parameter to restrict the transmission of SARS-CoV-2,” Das said.
“This is not to say that auto rickshaws are completely safe. We’re saying that auto rickshaws are comparatively safer, if you have a mask on,” he said.
Using a risk estimator developed last year by US-based researchers studying a super-spreader event in the United States, Das and Ramachandran found that when the vehicle increased its speed from zero to 120 km/hr, this led to a decrease in the probability of infection by approximately 75% in both the AC and non-AC taxis, and the probability of infection reduced to 0.0043 for non-AC taxis and 0.0153 for AC taxis.
The researchers, however, did not calculate by how much the risk factor reduced if the bus is in motion but said over an email interview that it was likely to reduce. The paper also stated that the probability of risk is likely to remain the same in an autorickshaw whether it is stationary or in motion, because of its design.
The duo is now working on estimating risk of passengers in rail and air travel. “The ongoing pandemic has presented an opportunity to reconsider and redesign ventilation systems in public transit vehicles and also the role of mass transit to not only reduce risk of infection during this time, but also reduce exposure to PM 2.5 and other air pollutants for the future in a sustainable manner,” said Das.
The researchers also calculated the risk of a traveller in these four transportation modes from exposure to particulate matter (PM) 2.5, which are tiny particles of pollutants in the air that cause respiratory diseases and poor visibility. They found that the risk factor was the opposite of that for Covid-19. The risk of exposure to pollutants was lowest for AC taxis, followed by non-AC taxis, buses and auto rickshaws.
“In a non-Covid-19 scenario, when exposure to PM 2.5 is higher, it is ideal to travel in an AC taxi, compared to an auto. But in a Covid-19 scenario, which is far more dangerous, auto rickshaws are preferable,” said Das.
Das explained: “These are two competing risks. In case of Covid-19, the risk increases with a set of actions such as closing windows and reducing circulation but in case of exposure to PM 2.5, the risk reduces with these actions.”
“It is an interesting study, which is based on modelling, especially since we plan to open up the lockdown gradually in the future. We already know that ventilation can reduce the spread of the virus. From a regulatory point, we can ask people to roll down their windows while travelling in public transport. Of course, this becomes difficult in monsoon,” said city-based pulmonologist Lancelot Pinto, who was not involved with the study.
“Now that we are dealing with hypothesis that the virus is airborne, there is a chance that a previous passenger leaves a trail of virus in the taxi,” said Pinto adding that the study pointed to the fact that masking up, preferably in an N95 mask, remained essential to any mode of public transport.