The World Health Organization has acknowledged new evidence that the coronavirus spreads more widely in the air than it had previously suggested, as the Trump administration gave official notification of its withdrawal from the group.

A day after a group of scientists said the global body was underplaying the risk of airborne transmission between people, a senior WHO official said there was “evidence emerging” of airborne transmission of the coronavirus, but that it was not definitive.

Speaking at a media briefing in Geneva on Tuesday, Benedetta Allegranzi, the WHO’s technical lead for infection prevention and control, said: “...The possibility of airborne transmission in public settings – especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out.

“However, the evidence needs to be gathered and interpreted, and we continue to support this.”

The WHO has previously said the virus that causes the Covid-19 respiratory disease spreads primarily through small droplets expelled from the nose and mouth of an infected person that quickly sink to the ground.

But in an open letter to the Geneva-based agency, published on Monday in the Clinical Infectious Diseases journal, 239 scientists in 32 countries outlined evidence that they say shows floating virus particles can infect people who breathe them in. Because those smaller exhaled particles can linger in the air, the scientists in the group had been urging WHO to update its guidance.

“We wanted them to acknowledge the evidence,” said Jose Jimenez, a chemist at the University of Colorado who signed the paper.

“This is definitely not an attack on the WHO. It’s a scientific debate, but we felt we needed to go public because they were refusing to hear the evidence after many conversations with them,” he told Reuters.

Jimenez said historically, there has been a fierce opposition in the medical profession to the notion of aerosol transmission, and the bar for proof has been set very high. A key concern has been a fear of panic.

“If people hear airborne, healthcare workers will refuse to go to the hospital,” he said. Or people will buy up all the highly protective N95 respirator masks, “and there will be none left for developing countries.”