An empty San Francisco International Airport on the eve of Memorial Day weekend in 2020 . Credit ... Jim Wilson / The New York Times
The idea may have been fueled by observations that people who were closest to a sick person seemed most at risk of infection . That led medical experts to recommend hand washing and social distancing as the best ways to contain a respiratory virus .
But scientists showed decades ago that large droplets may evaporate and shrink as they are expelled , becoming tiny aerosols that linger in the air . That is , a patient with the flu isn ’ t just expelling the virus in large droplets . According to Yuguo Li , an air quality expert at the University of Hong Kong , that patient may exhale , cough or sneeze droplets in any number of sizes .
The smallest will drift through the air and be inhaled directly into the lungs — a scenario that requires precautions far different than wiping down surfaces or washing hands .
To Dr . Li and other air quality experts , it was obvious from the start of the pandemic that the coronavirus was carried aloft . The SARS coronavirus , a close relative that emerged in Asia in 2002 , was airborne — why would the new one be any different ?
In January 2020 , Chinese researchers described a cluster of infections that included a 10‐year‐old child who had no symptoms but whose scans revealed “ ground‐glass lung opacities ,” a sign of infection with the new coronavirus .
Dr . Donald K . Milton of the University of Maryland , who has studied respiratory virus transmission for decades , knew what that meant : The coronavirus was being inhaled into the lungs .