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

In an open letter published Monday, 239 scientists in 32 countries outlined evidence 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.