Virus’ rate of decay depends on surface

Most infections start with water droplets, tiny globes of water 5 microns or less in size. That's much smaller than a human hair, which is 60 to 120 microns thick.

In all settings, viruses need water to survive. “Viruses can withstand a small amount of dehydration,” says Dr. Paul Meechan, a former director of safety at the CDC and president of the American Biological Safety Association.

The problem is knowing “how long it will take a virus to dry out and become non-infectious,” Meechan says. “Eventually, the virus deteriorates and becomes inactive.” The speed at which that happens depends on whether the virus is in the air or on a surface.

Let’s start with air. Researchers don’t know how many virus-laden particles people infected with COVID-19 expel in the average droplet. An average cough, however, can produce as many as 3,000 droplets and a single sneeze can make up to 40,000, according to multiple studies cited in a 2009 World Health Organization report.

A droplet’s size determines where it goes after being expelled. Gravity forces larger ones to the ground.

“Aerosols are different,” says Dr. Stanley Deresinski, clinical professor of medicine and infectious diseases at Stanford University. “Very small particles may be suspended in the air for a long time, sometimes for hours. They're suspended by air currents.”

Airborne droplets can stay suspended long enough for someone to walk through and inhale the virus. Outdoors, wind disperses the virus.

A virus that doesn’t reach the ground or floor can fall on shared surfaces — or be transferred there by those with the pathogen on their hands. Whatever the case, unsuspecting people can pick it up. How long a virus lives depends on the surface: