WHO says the coronavirus isn't airborne, but 239 scientists disagree. Either way, your precautions should remain the same.

• Nearly 240 scientists signed an open letter asking the World Health Organization to acknowledge that the novel coronavirus could be transmitted through the air.
• A growing body of research suggests that coronavirus aerosols can travel more than 6 feet from an infected person — meaning social distancing isn't enough to stop transmission.
• But people who avoid large gatherings, practice social distancing, and wear masks can lower their risk of infection regardless.

An open letter from 239 scientists to the World Health Organization contains an alarming message. "We are concerned that the lack of recognition of the risk of airborne transmission of COVID-19 and the lack of clear recommendations on the control measures against the airborne virus will have significant consequences," they wrote.

The letter, published Monday, questions WHO's stance that the coronavirus spreads almost exclusively via respiratory droplets when a person coughs or sneezes. Respiratory droplets are relatively large — more than 5 microns in size — and heavy, meaning they fall to the ground quickly.

But talking, coughing, singing, or exhaling can also release smaller particles (less than 5 microns) called aerosols or microdroplets. For airborne transmission to occur, these particles have to stay in the air and remain infectious over time. The measles virus, for instance, can live for up to two hours in the air where an infected person coughed or sneezed.

WHO has said there's no evidence the coronavirus spreads this way, but a growing body of research suggests otherwise. Studies have shown that microdroplets carrying the coronavirus can linger in the air for several minutes, allowing them to travel more than 6 feet away from an infected person.

That's especially true in crowded, indoor areas without good ventilation. Some scientists have suggested that airborne transmission may have been responsible for coronavirus outbreaks at a choir practice in Washington state and a call center in South Korea.

"Airborne transmission appears to be the only plausible explanation for several superspreading events," the scientists wrote, adding, "people may think that they are fully protected by adhering to the current recommendations, but in fact, additional airborne interventions are needed for further reduction of infection risk."

The signatories called for three interventions designed to reduce transmission in offices, schools, hospitals, and nursing homes. The first two involve ventilation: circulating a steady supply of clean, outdoor air and using filtration systems or ultraviolet light to remove virus particles from indoor air. The third intervention involves reducing crowds in public buildings and transit systems.

Social distancing and face masks are still strong forms of protection

The current WHO recommendations for reducing coronavirus transmission focus mainly on handwashing and social distancing. In their open letter, the scientists said these two measures were "insufficient" to protect against airborne transmission.

The organization, however, also advises people to avoid crowded places, and it started recommending the use of face masks in public on June 5. So for people following the full list of WHO safety recommendations, there's little reason to panic.

Research shows that face masks play a significant role in lowering airborne transmission. An April study in the journal Nature found that surgical masks reduced the transmission of multiple human coronaviruses (not including this new one, SARS-CoV-2) emitted in the form of aerosols from patients in a hospital clinic.

The study also found that prolonged close contact was probably necessary for airborne transmission to occur. That's why avoiding large gatherings and social distancing when outside one's household can protect against this form of transmission.

Still, places where groups are unavoidable — like restaurants, offices, or schools — may want to consider new ways to improve their air quality. Scientists have found that UV light can destroy other coronaviruses like the ones that cause SARS and MERS — a sign it may also work on the novel virus. WHO warns against using UV light as a skin disinfectant but doesn't mention its efficacy as an air purifier.

WHO has been slow to recognize new science

WHO has yet to acknowledge that airborne particles can spread the coronavirus outside medical settings.

"Especially in the last couple of months, we have been stating several times that we consider airborne transmission as possible but certainly not supported by solid or even clear evidence," Dr. Benedetta Allegranzi, WHO's technical lead on infection control, told The New York Times on Saturday.

The agency's latest coronavirus update, on June 29, noted that scientists hadn't been able to culture the virus from air particles inside hospital rooms. This step, it added, "is critical to determining the infectiousness of viral particles."

But the new open letter argues that air particles pose a risk of exposure "beyond any reasonable doubt."

The debate represents yet another instance in which WHO has shied away from issuing public-health recommendations based on emerging science.

In early April, just days after the CDC began recommending face coverings in public, WHO said there was no need for healthy people to wear masks.

Then last month, WHO's technical lead for the coronavirus, Dr. Maria Van Kerkhove, suggested that asymptomatic spread of the coronavirus was "rare." The statement conflicted with a spate of recent research suggesting that asymptomatic spread played a major role in coronavirus transmission. WHO later walked back its remarks, saying there was a "misunderstanding."