Public health officials have urged everyone to practice social distancing during the COVID-19 pandemic, which means leaving some space between yourself and people you don’t live with. This new study adds to other research that finds that these guidelines -- the CDC advises leaving at least 6 feet between yourself and other people -- may not go far enough.
In the latest study, researchers in Singapore estimated how droplets of various sizes might travel from a person coughing to a person standing either 1 meter or 2 meters away, or 3.2 feet and 6.5 feet away, respectively.
The study found that at a distance of about 3 feet, a person standing in front of a simulated cough really gets a direct hit of virus, becoming covered with about 65% of all the droplets generated by that cough. Most of the droplets that reach the bystander are the fat, wet kind that are heavy and fall to the floor and nearby surfaces after a sneeze or cough. The study authors write that those droplets carry a “tremendous” viral load.
When the cougher and the bystander are 2 meters, or 6.5 feet, apart, fewer of the larger droplets reach the other person, but a cough still delivers just enough virus to potentially infect the other person.
But in both cases, the droplets traveled down from the nose and mouth of the person coughing, so most fell on the lower body of the bystander. Because of this, the researchers think people probably don’t inhale these larger droplets directly, but may pick them up on their skin or clothing and then infect themselves when they pick that virus up on their hands and touch their face.
“Obviously, you don’t get sick from virus landing on your clothing. You would have to breathe it in or you would have to, you know, rub your hands all over your pants and pick up enough virus then touch your nose, eyes, or stick your finger in your mouth,” says Linsey Marr, PhD, a civil and environmental engineer at Virginia Tech, who has been studying aerosol transmission of the virus. “I don't think that's a big risk for transmission with this virus.”
Certainly, she says, sneezes and coughs can be dangerous, but two people would have to be really close together, almost face-to-face, to pass the virus this way.
“It has to be right in your face. You have to be really close, and those droplets fly like mini cannonballs and would have to land in your eyes, in your nostrils, which point down, so it seems less likely, or on your lips,” Marr says.
Viruses can become airborne two ways: In big, heavier droplets that are expelled from the nose or mouth, and through aerosols, droplets that are so small that they dry out quickly in air. Larger droplets contain more copies of the virus, but they are also heavy and fall to the floor and nearby surfaces quickly after a sneeze or cough. Smaller aerosols, which can be generated from the back of the throat when we talk, speak, or even sing, dry out before they reach the floor, and that allows them to continue to float in the air for minutes or even hours, and may still be present after a person leaves a room.
The researchers say that shorter people, like young children and teens, may be more at risk when a taller adult coughs, since their faces are more in line with the stream of these droplets. They say that during the pandemic, shorter people may want to stay more than 6 feet away from taller people for that reason.
The new findings support earlier work from researchers at the Massachusetts Institute of Technology, which suggested that particles from a cough, buoyed by the warm air in our breath, could travel much farther than 6 feet in a turbulent gas cloud.
And because the smaller aerosols -- which are generated just by talking or singing -- can travel over longer distances, everyone should leave as much space between themselves and others as feasible.
Guidelines like the CDC’s are best guesses, and researchers have been trying to figure out exactly how the virus moves from person to person through the air.
“It's a good guideline, but we shouldn't think of 6 feet as a magic number. The farther the better,” Marr says.
As researchers continue to unravel the mysteries of how people catch COVID-19, public health experts say it’s important to take a layered approach to safety. No one strategy can fully protect you on its own. That’s why it’s important to take all recommended precautions to keep yourself safe from the virus, including washing and sanitizing your hands, wearing a face mask, limiting travel and social interactions, and keeping space between yourself and others.