ATLANTA (Reuters) — The Centers for Disease Control and Prevention posted updated guidance on its website Monday that said COVID-19 can spread through airborne transmission, a month after it took down a similar warning.

It said some people could get infected by exposure to the novel coronavirus in small droplets and particles, or aerosols, that can linger in the air for minutes to hours.

Aerosols lingering in the air could be a major source of COVID-19 transmission, a group of U.S. scientists warned in an unrelated open letter published in medical journal Science on Monday.

The focus must be on protecting against airborne transmission because individuals with COVID-19 release thousands of virus-laden aerosols and far fewer droplets while breathing and talking, the scientists said.

CDC’s Monday update acknowledges some earlier reports that showed limited, uncommon circumstances where people with COVID-19 infected others who were more than 6 feet away or shortly after a COVID-19-positive person left an area, the agency said.

In these instances, the CDC said transmission occurred in poorly ventilated and enclosed spaces that often involved activities that caused heavier breathing, like singing or exercise.

Last month, the CDC published – and then took down – its guidance warning possible airborne transmission of the novel coronavirus.