Commuters social distance while waiting for a train at the Fulton Street subway station and complex on Tuesday in lower Manhattan in New York City. Photo: AFP

Supercomputer-driven models simulated in Japan suggested that operating commuter trains with windows open and limiting the numbers of passengers may help reduce the risk of coronavirus infections, as scientists warn of the airborne spread of the virus.

In an open letter published on Monday, 239 scientists in 32 countries outlined evidence that they say shows floating virus particles can infect people who breathe them in.

The World Health Organization (WHO) acknowledged "evidence emerging" of airborne transmission, but said it was not definitive.

But questions remain about how many infections occur through that route. How concentrated the virus is in the air may also decide contagion risks, said Yuki Furuse, a professor at Kyoto University.

In the open letter, scientists urged improvements to ventilation and the avoidance of crowded and enclosed environments, recommendations that Shin-ichi Tanabe, one of the co-authors of the letter, says Japan broadly adopted months ago.

"In Japan, the committee for COVID-19 countermeasures insisted on the 3Cs at an early stage," said Tanabe, a professor at Waseda University in Tokyo, referring to Japan's public campaign to avoid 'Closed spaces, Crowded places and Close-contact settings.'

Japan has tamed the pandemic, with more than 19,000 confirmed cases and 977 deaths so far.

The recent study by Japanese research giant Riken using the world's fastest supercomputer, the Fugaku, to simulate how the virus travels in the air in various environments advised several ways to lower infection risks in public settings.

Its lead researcher, Makoto Tsubokura, said that opening windows on commuter trains can increase the ventilation by two to three times, lowering the concentration of ambient microbes.

Other findings advised the installation of partitions in offices and classrooms, while in hospitals, beds should be surrounded by curtains that touch the ceiling.