More than 200 scientists from around the world are challenging the official view of how the coronavirus spreads in an open letter to the World Health Organisation (WHO).
Experts warn there is growing evidence the virus can be transmitted through microscopic particles that can hang in the air for long periods and float several metres.
These droplets are of particular concern at indoor venues with poor ventilation and confined spaces such as public transport – even when people maintain a social distance.
The researchers have accused the United Nations agency of failing to issue appropriate warnings about the risk.
Currently, the WHO maintains you only have to worry about two types of transmission: inhaling respiratory droplets from an infected person in your immediate vicinity or – less common – touching a contaminated surface and then your eyes, nose or mouth.
But 239 researchers in an open letter contend that the guidance ignores growing evidence that a third pathway plays a significant role in contagion.
It’s believed this third mode of infection is behind a number of so-called “super-spreading” events including the infection of diners at a restaurant in China who sat at separate tables.
It also may explain the transmission among choir members in the US state of Washington who took precautions during a rehearsal.
Researchers say multiple studies demonstrate that particles known as aerosols – microscopic versions of standard respiratory droplets – can hang in the air for long periods and float for several metres, making poorly ventilated rooms, buses and other confined spaces dangerous even when people stay 1.8 metres from one another.
“We are 100 per cent sure about this,” said Lidia Morawska, a professor of atmospheric sciences and environmental engineering at Queensland University of Technology in Brisbane.
She makes the case in an open letter with experts from 32 countries which is set to be published next week in a scientific journal.
It comes as the WHO reported a record increase in global coronavirus cases, with the total rising by 212,326 in 24 hours.
The biggest increases were from the United States, Brazil and India.
Deaths remained steady on Saturday at about 5000 a day.
The scientific challenge raises the importance of wearing masks to help prevent the escape of exhaled aerosols as well as inhalation of the microscopic particles.
The use of masks has been another issue of contention in the pandemic, with experts offering varied advice on their effectiveness and authorities also flipping between advice.
The spread could also be reduced by improving ventilation and zapping indoor air with ultraviolet light in ceiling units.
WHO officials have acknowledged that the virus can be transmitted through aerosols but say it occurs only during medical procedures such as intubation that can spew large quantities of the microscopic particles.
Dr Benedetta Allegranzi, a top WHO expert on infection prevention and control, said in responses to questions from the Los Angeles Times that Morawska and her group presented theories based on laboratory experiments rather than evidence from the field.
“We value and respect their opinions and contributions to this debate,” Dr Allegranzi wrote in an email.
But in weekly teleconferences, a large majority of a group of more than 30 international experts advising the WHO has “not judged the existing evidence sufficiently convincing to consider airborne transmission as having an important role in COVID-19 spread”.
She added that such transmission “would have resulted in many more cases and even more rapid spread of the virus”.
Jose Jimenez, a University of Colorado chemist who signed the letter, said the idea of aerosol transmission should not frighten people.
“It’s not like the virus has changed,” he said.
“We think the virus has been transmitted this way all along, and knowing about it helps protect us.”