A debate has erupted among researchers over the potential for the pandemic coronavirus, SARS-CoV-2, to spread through the air and—if it does so often enough—what to do about it.

Though talk of airborne transmission has been simmering since the beginning of the pandemic, it reached a boiling point this week following a letter penned by two researchers and addressed to “national and international bodies.” The letter, eventually signed by 239 researchers, urged those bodies to acknowledge the potential for airborne spread and to recommend control measures aimed at preventing it.

“Most public health organizations, including the World Health Organization, do not recognize airborne transmission except for aerosol-generating procedures [AGPs] performed in healthcare settings,” the letter stated.

The evidence on airborne transmission is “admittedly incomplete,” the letter went on, but “[f]ollowing the precautionary principle, we must address every potentially important pathway to slow the spread of COVID-19.”.

The letter was published Monday as a commentary piece titled “It is Time to Address Airborne Transmission of COVID-19” in the journal Clinical Infectious Diseases.

In the past months, many researchers and public health experts have criticized the WHO, saying it stumbled in the wake of fast-paced research relating to mask use and garbled messaging on the risks of virus transmission from people who show no symptoms of COVID-19, among other things.

These tiny droplets of respiratory secretions can be loaded with infectious virus, cozy in their moist bubbles.

When relating this to the spread of germs and disease, many epidemiologists try to keep things simple and categorize transmission as being either largely from the ballistic respiratory droplet route or the aerosol route.

The mumps virus is in the respiratory droplet category, for instance, which is spread from contact with saliva or close-range sneezes and coughs.

Not all large respiratory droplets fall within one or two meters, and not all aerosols travel far with infectious virus.

And we do not know how many SARS-CoV-2 virus particles a person has to inhale to get infected and come down with COVID-19.

The bulk of it points to close contact—being within two meters of an infected person where they could be exposed to respiratory droplets of any size, whether they’re plump ballistic ones or aerosolized.

With that combination of data, epidemiologists at the WHO and other health agencies placed SARS-CoV-2 in the respiratory droplet bin and acted accordingly.

They recommended that health care providers take precautions against infectious respiratory droplets.

In a study published in March in the New England Journal of Medicine, researchers at the US National Institutes of Health found that when they put SARS-CoV-2 into a three-jet Collison nebulizer, they could create SARS-CoV-2 aerosols that held viable virus aloft for up to three hours.

The study found that SARS-CoV-2 spread from one infected person to people at their table plus unrelated people at two nearby tables that happened to be in the stream of an air conditioning unit.

The authors of the study concluded that “aerosol transmission of SARS-CoV-2 due to poor ventilation may explain the community spread of COVID-19.”.

In a statement, infectious disease expert Babak Javid of Tsinghua University School of Medicine, Beijing, went further in knocking back the hypothesis of airborne spread in the Chinese restaurant.

This certainly supports transmission via the air conditioner airflow, be they small or large droplets.