The risk for infection decreases with increasing distance from the source and increasing time after exhalation, said the US Centers for Disease Control and Prevention.
The principal mode by which people are infected with SARS-CoV-2, the virus that causes COVID-19, is through very fine aerosolised particles released during respiration, the US Centers for Disease Control and Prevention said in new guidelines on Friday, almost a month after the Lancet found the virus is transmitted through the air.
The top US medical body said the risk of transmission is greatest within three to six feet of an infectious source where the concentration of these very fine droplets and particles is greatest.
Modes of SARS-CoV-2 transmission are now categorized as inhalation of virus, deposition of virus on exposed mucous membranes, and touching mucous membranes with soiled hands contaminated with virus.
“People release respiratory fluids during exhalation (quiet breathing, speaking, singing, exercise, coughing, sneezing) in the form of droplets across a spectrum of sizes.1-9 These droplets carry virus and transmit infection,” said the top US medical body.
The smallest very fine droplets, and aerosol particles formed when these fine droplets rapidly dry, are small enough that they can remain suspended in the air for minutes to hours, it said.
“The risk for infection decreases with increasing distance from the source and increasing time after exhalation,” it said.
Although infections through inhalation at distances greater than six feet from an infectious source are less likely than at closer distances, the health body added.
“These transmission events have involved the presence of an infectious person exhaling virus indoors for an extended time (more than 15 minutes and in some cases hours) leading to virus concentrations in the air space sufficient to transmit infections to people more than 6 feet away, and in some cases to people who have passed through that space soon after the infectious person left,” it said.
The body said that existing recommendations like physical distancing, use of well-fitting masks, adequate ventilation and avoidance of crowded indoor spaces remain effective against the virus.
“These methods will reduce transmission both from inhalation of virus and deposition of virus on exposed mucous membranes. Transmission through soiled hands and surfaces can be prevented by practicing good hand hygiene and by environmental cleaning,” it cautioned.