Coronavirus can be spread by tiny particles suspended in the air, sometimes for hours, says the US Centres for Disease Control.
Its updated guidance says this airborne route of transmission is still uncommon – bigger droplets from coughs, sneezes and talking are still the main source.
People are at higher risk of catching it the longer and closer they are to someone who has the virus.
Poorly ventilated enclosed crowded spaces are are riskier than outdoors.
Last month, the CDC published – and then took down – a draft version of the guidance warning about possible airborne transmission, saying it had been posted in error.
At the time, the World Health Organization (WHO) said it knew of no new evidence to suggest this was how the virus was spreading, although it agreed that aerosol transmission was possible in some circumstances.