“ If an asymptomatic kid can have that going on in his lungs , it ’ s going to be very difficult to contain , because asymptomatic people are going to be contagious ,” Dr . Milton recalled thinking when the Chinese report came out .
Donald K . Milton of the University of Maryland has studied respiratory virus transmission for decades . Credit ... Shuran Huang for The New York Times
Other evidence for airborne transmission quickly piled up . There was a New York man who became infected simply by driving his neighbor to the hospital , the 712 infected passengers and crew members of the Diamond Princess cruise ship , a choir practice in Seattle , and diners at a restaurant in China .
Still , the erroneous distinction between large and small droplets remained entrenched , despite attempts from physicists and air quality experts to set the record straight .
“ The resistance was thick . The walls of the silo were thick ,” said Richard Corsi , dean of the College of Engineering at the University of California , Davis . “ I ’ ll be blunt , it ’ s been frustrating as hell from the very start .”
Some scientists felt the health agencies were stonewalling because the consequences of indoor aerosol transmission — high‐quality masks , air filtration , building closures — would require a herculean response .
William Bahnfleth , an architectural engineering expert at Penn State University , said he was shocked by “ the paralysis of the public health community , the demand for more and more conclusive data .”
It took the C . D . C . until April 2020 to recommend masking and until October 2020 to acknowledge aerosol transmission of the coronavirus , and even then only obliquely . The World Health Organization was forced to review its stance in July 2020 , after 239 experts issued a statement demanding it .
A watershed moment came in the spring of 2021 , when three major medical journals published papers on airborne transmission of the coronavirus .
Still , the W . H . O . did not use the word “ airborne ” to describe the virus until December 2021 , and the C . D . C . has yet to do so .