Keeping windows open best for preventing COVID-19 spread in cars

Keeping car windows open and wearing face mask can help protect passengers from the risk of contracting COVID-19, even though this may expose them to outdoor air pollution, a study finds.

The study, conducted by the Global Centre for Clean Air Research (GCARE) in Surrey, UK, used sensors to assess in-vehicle exposure to pollutants and aerosols — including particulate matter <2.5 μm (PM_2.5) and CO₂ emission — during three different ventilation settings: open window, air conditioning using fresh air (AC-FA), and air conditioning using air recirculation (AC-RC). The data were collected over 45 runs covering 325 km distance carried out across three timings a day in morning, afternoon, and evening. [Environ Int 2021;doi:10.1016/j.envint.2021.106814]

“Occupant exposure to CO₂ levels was explored as a proxy of COVID-19 transmission as a result of co-exhalation of pathogen-laden particles by an infected occupant,” explained the researchers.

While build-up of exhaled CO₂ was more than fourfold higher during AC-RC due to lack of fresh air compared with AC-FA and opening windows, the exposure to outdoor PM_2.5 was highest when the windows were kept open. Exposure to PM_2.5 was reduced by 82.4 percent and 43.9 percent during AC-RC and AC-FA, respectively, owing to effective filtration of the intake air.

The researchers also went on to explore the likelihood of COVID-19 transmission from an infected passenger during scenarios of (i) loudly speaking, (ii) speaking, (iii) and oral breathing, under the three ventilation settings.

Under AC-RC, the likelihood of COVID-19 transmission was increased by 28.5 percent, 5.1 percent and 1.1 percent per hour during loudly speaking, speaking, and oral breathing, respectively, when the passenger was wearing a moderately efficient (50 percent efficiency) face mask. 

Improving ventilation conditions significantly reduced the risk of COVID-19 spread, from a rate of 28.5 percent per hour during AC-RC to 5.1 percent per hour during AC-FA and 2.3 percent per hour when the windows were open, under a loudly speaking scenario.

Under normal speaking, the rate of transmission was reduced from 5.1 percent to 0.8 percent and 0.4 percent per hour during AC-RC, AC-FA, and opening windows, respectively.

However, these probabilities could rise by fourfold if no passengers had a facemask on compared with donning a moderately efficient facemask, warned the researchers.

Thus, motorists face a dilemma: while keeping the windows open to maintain continuous intake of fresh air, combined with mask wearing, is the best way to protect against COVID-19, this increases exposure to outdoor air pollutants. However, keeping the windows close increases their risk of getting COVID-19. 

“To reduce exposure, alternating ventilation settings or avoiding pollution hotspots along the route are highly recommended,” the researchers suggested.

To balance the risk of COVID-19 transmission and external air pollution, the optimal way could be running AC on ambient mode (ie, by drawing fresh air from outside).

 “Our research found that if your priority is to reduce the risk of contracting COVID-19, wearing a mask and keeping car windows open is the ideal approach,” said lead author Professor Prashant Kumar, Director of GCARE at the University of Surrey, UK.