Six months after a pandemic that has killed more than half a million people, more than 200 scientists around the world are challenging the official view of how the coronavirus is spread.
The World Health Organization and the US Centers for Disease Control and Prevention contend that you need to worry about only two types of transmission: inhaling respiratory drops from an infected person in your immediate vicinity or, less commonly, touching a contaminated surface and then your eyes, nose or mouth.
But other experts maintain that the guide ignores the growing evidence that a third way also plays an important role in contagion.
They claim that multiple studies show that particles known as aerosols, microscopic versions of standard respiratory droplets, can remain airborne for long periods and float dozens of feet, making rooms, buses, and other confined spaces with poor ventilation are dangerous, even when people remain six feet from each other.
“We are 100% sure of this,” said Lidia Morawska, professor of atmospheric science and environmental engineering at Queensland University of Technology in Brisbane, Australia.
She presents the case in an open letter to WHO accusing the United Nations agency of failing to issue appropriate warnings about the risk. A total of 239 researchers from 32 countries signed the letter, which will be published next week in a scientific journal.
In interviews, experts said that the transmission of aerosols appears to be the only way to explain various “super-diffusion” events, including the infection of diners at a restaurant in China who sat at separate tables and of choir members at the Washington state that they took precautions during a trial.
WHO officials have acknowledged that the virus can be transmitted through aerosols, but they say it occurs only during medical procedures such as intubation that can shed large amounts of microscopic particles. CDC officials did not respond to multiple requests for comment.
Dr. Benedetta Allegranzi, a leading WHO expert on infection prevention and control, said in responses to questions from The Times that Morawska and her group presented theories based on laboratory experiments rather than evidence from the field.
“We value and respect your views and contributions to this debate,” Allegranzi wrote in an email. But in weekly teleconferences, a large majority of a group of more than 30 international experts advising the WHO “did not judge the existing evidence convincing enough to consider that airborne transmission plays a major role in the spread of COVID-19 “
He added that such transmission “would have resulted in many more cases and an even faster spread of the virus.”
Since the coronavirus was first detected in China in December, the understanding of how it is spread has evolved considerably, resulting in changing patterns regarding mask use.
At first, the WHO and CDC said the masks were excessive for ordinary people and should be kept for health workers. Later, the CDC recommended masks only for people with COVID-19 symptoms.
Then in April, after it became clear that people without symptoms could also transmit the virus, the CDC suggested masks for everyone when physical distancing was difficult, a position that the WHO eventually adopted.
Now, as outbreaks proliferate and governors order a new round of closings, nearly every state in the US has made or recommended face coatings primarily to prevent users from spreading the disease.
Proponents of aerosol transmission said that properly worn masks would help prevent the escape of exhaled aerosols, as well as the inhalation of microscopic particles. But they said the spread could also be reduced by improving ventilation and removing indoor air with UV light in the ceiling units.
José Jiménez, a chemist at the University of Colorado who signed the letter, said the idea of aerosol transmission should not scare people. “It is not that the virus has changed,” he said. “We believe that the virus has been transmitted this way all the time, and knowing that helps protect us.”
He and other scientists cited several studies that support the idea that aerosol transmission is a serious threat.
As early as mid-March, a study in the New England Journal of Medicine found that when the virus was suspended in the fog under laboratory conditions, it remained “viable and infectious” for three hours, which the researchers said was equivalent to half an hour in real world conditions.
It has already been established that some people, known as “super spreaders”, are especially good at exhaling fine material, producing 1,000 times more than others.
A recent study found coronavirus RNA in the hallways near the rooms of COVID-19 patients. Another raised concerns that aerosols laden with the virus were dumped by floor cleaning teams and by health workers taking off personal protective equipment.
Researchers in China found evidence of aerosols containing the coronavirus at two Wuhan hospitals.
It was the outbreak among the choir members in Mount Vernon, Washington, and a report on the incident in The Times that sparked the interest of several of the aerosol advocates. Of 61 singers in a March 10 rehearsal, all but eight fell ill, even though the members used hand sanitizer and avoided hugging or shaking hands. Two people died.
A team led by Shelly Miller, a professor of mechanical engineering at the University of Colorado, investigated church hall plans, furnace specifications, choir members locations, and hours of attendance. The researchers diagrammed the movements of the singer who was identified as the person who unknowingly brought the virus into practice.
Inhalation of aerosols “probably dominated the transmission of infections during this event,” the researchers wrote in a peer-reviewed article, concluding that the ill person, who had symptoms similar to a common cold, was unlikely to have a time within six feet of singers or having touched surfaces in common with them.
“We believe that shared air in the community room, combined with high emissions of respiratory aerosols from singing, are likely to be major contributing factors,” the newspaper said.
Finally, researchers from a wide spectrum of disciplines, including several who have studied the role of aerosols in the spread of influenza, SARS, and other infectious diseases, joined forces to campaign for greater recognition of aerosol transmission.
They said that the coronavirus is less contagious through the air than measles, but that the risk of transmission increases the longer it remains stagnant and the more people continue to breathe.
In interviews, they said WHO officials had unfairly set a higher bar to show aerosol spread than required for acceptance of the other two routes. “For them, the drops and the touch are so obvious that they are proven, but in the air it is so extravagant that it needs a very high level of evidence,” Jiménez said.
The test would require exposing large numbers of healthy people to aerosols emitted by patients with COVID-19, a study that scientists said would be unethical.
Donald Milton, a professor of environmental health at the University of Maryland and an aerosol expert who co-wrote the letter, said the average person breathes 10,000 liters of air each day.
“You only need one infectious dose of the coronavirus in 10,000 liters, and it can be very difficult to find it and show that it is there, which is one of the problems we have had,” he said.
