The origin of the six-foot distancing recommendation is something of a mystery. “It’s almost like it was pulled out of thin air,” said Linsey Marr, an expert on viral transmission at Virginia Tech University.

When the virus first emerged, many experts believed that it was transmitted primarily through large respiratory droplets, which are relatively heavy. Old scientific studies, some dating back more than a century, suggested that these droplets tend not to travel more than three to six feet. This observation, as well as an abundance of caution, may have spurred the C.D.C. to make its six foot suggestion, Dr. Marr said…

With aerosol transmission, safety generally increases with distance; the farther the aerosols travel, the more they diluted become. “It’s like being close to a smoker,” Dr. Marr said. “The closer you are, the more you’re going to breathe in.”

And distance aside, the more people there are in a room, the higher the odds that one of them will be infected with the coronavirus. A six-foot rule helps reduce that risk, said Donald Milton, an aerosol expert at the University of Maryland: “If people are six feet apart, you can’t pack them in. And so, it’s safer just because it’s less dense.”