May 12, 2021

CDC: SARS-CoV-2 Transmitted via Inhalation of Very Fine Droplets, Aerosol Particles

A new scientific brief published by CDC on Friday emphasizes that SARS-CoV-2 is transmitted by exposure to infectious respiratory fluids, including inhalation of very fine respiratory droplets and aerosol particles that contain infectious virus. Previous CDC guidance acknowledged airborne transmission of SARS-CoV-2 but maintained that most infections were spread through close contact. According to CDC, very small fine droplets and aerosol particles can remain suspended in the air for minutes to hours. In addition to inhalation of the virus, the agency’s updated guidance states that SARS-CoV-2 can be transmitted via deposition of infectious respiratory droplets and particles on exposed mucous membranes in the mouth, nose, or eye as well as by touching mucous membranes with soiled hands contaminated with the virus.

CDC’s scientific brief also stresses that SARS-CoV-2 transmission can occur when individuals inhale virus in the air farther than six feet from an infectious source. According to the updated guidance, enclosed spaces with inadequate ventilation or air handling and increased exhalation of respiratory fluids—such as if an infectious person is engaged in physical exertion or raises their voice—can increase the risk of SARS-CoV-2 transmission indoors.

“With increasing distance from the source, the role of inhalation likewise increases,” CDC states. “Although infections through inhalation at distances greater than six feet from an infectious source are less likely than at closer distances, the phenomenon has been repeatedly documented under certain preventable circumstances.”

The World Health Organization recognizes close-range inhalation as a mode of transmission for SARS-CoV-2. According to WHO, “the virus spreads mainly between people who are in close contact with each other” and infection can occur when an individual inhales aerosols or droplets that contain the virus. The organization also notes the role of aerosols in the spread of SARS-CoV-2 in poorly ventilated or crowded indoor settings.

In February, 13 experts in aerosol science, occupational health, and infectious diseases called on the Biden administration to acknowledge inhalation as the key route of exposure to SARS-CoV-2 and take immediate action to limit aerosol transmission. A letter addressed to senior administration officials advocated for a range of measures, including updating CDC and OSHA guidelines to fully address transmission via small inhalable particles. A month later, a group of congressional leaders expressed similar concerns that CDC’s guidance did not reflect experts’ current understanding of transmission and therefore did not adequately protect workers from exposure.

CDC’s new scientific brief on SARS-CoV-2 transmission is available from the agency’s website.