Half a year into the pandemic, it's well-accepted that coronavirus can be spread when an infected person expels respiratory droplets by coughing or sneezing.
But can the virus be transmitted in microscopically small droplets that are released into the air by talking or just breathing? And if so, could you contract the virus from across a room, or after the infected person leaves the room?
As with so many aspects of the coronavirus, the answer is unclear, debated, and under study.
Still, evidence that these invisible "aerosols" can spread infection indoors more stealthily than thought prompted 239 scientists, including engineers and ventilation experts, to urge the World Health Organization to address the risk. "We are advocating for the use of preventive measures to mitigate this route of airborne transmission," said a letter published this week in the journal Clinical Infectious Diseases.
Charles Haas, a professor of environmental engineering at Drexel University and an endorser of the letter, ticked off those measures: "Universal masking. Avoid crowds. Avoid confined spaces. Keep a physical distance. And for indoor spaces, improve ventilation."
In other words, we should do all the difficult, sometimes impractical things we are doing now -- and more.
Here are some things to consider.
Some experts dismiss this distinction as a matter of semantics because the only difference between droplets and microscopic droplets is size. But size matters for several reasons.
Virus-laden droplets from a cough or sneeze are comparatively large and heavy, and quickly fall to the ground or a surface such as a table. It is possible -- though not conclusively shown -- that you can be infected by touching a contaminated surface, then touching your nose, mouth, or eyes. That's why public health experts urge frequent hand washing.
In contrast, microdroplets, also called aerosols, are so tiny (an average human hair is 10 times wider) that they can float in the air. The measles virus is highly contagious because it can survive in the air for a couple of hours, infecting people who walk by and inhale it. Before vaccination, each person with measles spread it to 12 to 18 others on average.