The WHO has long held that the coronavirus is spread primarily by large respiratory droplets that, once expelled by infected people in coughs and sneezes, fall quickly to the floor. But in an open letter to the WHO, scientists have outlined evidence showing that the virus can spread indoors through aerosols that linger in the air and can be infectious even in smaller quantities than previously thought.

Until recently, most public health guidelines have focused on social distancing measures, regular hand-washing and precautions to avoid droplets. But the signatories to the paper say the potential of the virus to spread via airborne transmission has not been fully appreciated even by public health institutions such as the WHO.

The paper was posted Monday on the journal Clinical Infectious Diseases.