Coronavirus particles were found to linger in a vehicle for hours, in a study that showed the potential risk of car sharing or using a taxi during the pandemic.
The findings, by researchers at the University of Florida, adds to the weight of evidence that shows people with the virus release tiny infectious airborne particles that can be breathed in by others.
Researchers fixed a particle-collecting device, called a personal cascade impactor sampler, to the sun visor on the passenger side of the patient's car.
The patient, a woman in her 20s, who tested positive for Covid-19 but had only mild symptoms, then drove home from the clinic she initially attended after she fell ill. During the 15-minute trip, the windows were closed and the air conditioning was on.
Two hours later, a researcher wearing personal protective equipment turned off the sampler – which had been less than a metre from the woman with its intake pointed at the car roof – and removed the device.
When scientists analysed the filters, they detected coronavirus genetic material on several of them.
"Our data highlights the potential risk of Sars-CoV-2 transmission by minimally symptomatic persons in the closed space inside a car ... and suggest that a substantial component of that risk is via aerosolised virus," the researchers said.
Although coronavirus genetic material was detected on several of the sampler's filters, the researchers were able to grow coronavirus particles in the laboratory only from the filter that collected particles between 0.25 and 0.5 micrometres in diameter (between one 4,000th and one 2,000th of a millimetre). Particles this small can penetrate deep into the lungs if breathed in.
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It is well known that respiratory pathogens can spread by three routes: contact, droplet and airborne spread, and early on in the pandemic, public health messages tended to emphasise the dangers of contact or droplet infection.
Contact spread happens directly from person to person, or from a contaminated surface to a person, while droplets are larger particles released that rapidly land on another person or on a surface when a person coughs, sneezes or even simply talks.
Airborne spread involves much smaller particles – typically less than 5 micrometres (one 200th of a millimetre) in diameter – that remain suspended in the air for up to several hours.
As well as this latest piece of research, there have been several studies showing that suspended particles are a risk factor for spreading the virus, as is the case with some other respiratory viruses, such as measles and chickenpox.
Dr Davinder Pal Singh, a cardiologist at NMC Royal Hospital in Dubai, who recovered from Covid-19, said particles containing the virus could persist in the air for hours.
He said the duration of contact a person had with an infectious patient iwas a significant factor in the spread of the disease, with longer exposure times increasing the risk of spread and of more severe illness.
“Precautions a person should take are to wear a mask and maintain a two-metre distance as much as possible,” he said.
Studies in recent months have shown that face masks can protect against droplet or airborne spread of the virus, and health authorities also highlight the importance of maintaining good ventilation to flush out suspended particles.
Research on hamsters published in December indicated that infections caused by airborne spread might cause more severe illness than infections by other routes, because the animals infected this way contained greater numbers of virus particles and lost more weight.
The Florida study was published online by medRxiv, but has not yet been reviewed by other scientists.