June 9 (UPI) -- Airborne transmission of the coronavirus through human speech is the "primary contributor to its rapid spread," according to a review of existing research published Wednesday by the Journal of Internal Medicine.
This spread occurs when those who are infected, whether or not they have developed symptoms, speak without a mask or other covering over their nose and mouth, the researchers said.
This allows respiratory droplets, or tiny amounts of liquid emitted from the throat and lungs during speech or coughing, to spread to others.
If the speaker is not wearing a mask, these droplets, or aerosols, can pass through the air and into the nose and throat of others nearby.
If these aerosols reach the lower respiratory tract, including the lungs, they can cause COVID-19, the researchers said.
This highlights the "dual role of masks in both containing the spread of disease and mitigating its severity," they said.
"We've all seen some spit droplets flying when people talk, but there are thousands more too small to be seen by the naked eye," study co-author Adriaan Bax said in a press release.
"When the water evaporates from such speech-generated, potentially virus-rich droplets, they float in the air for minutes, like smoke, thus putting others at risk," said Bax, a researcher and biophysicist with the National Institute of Diabetes and Digestive and Kidney Diseases.
Early in the pandemic, the World Health Organization and the Centers for Disease Control and Prevention, among others, said the coronavirus was spread both through these airborne droplets, as well as contaminated surfaces.
However, as researchers have studied the virus and its transmission, respiratory droplets emitted from the nose and mouth that pass through the air to others have been seen as the primary drivers of spread, according to the CDC.
This is why agency officials and others have strongly urged the public to wear face coverings and maintain "social distancing" -- or 6 feet of space from others -- to limit the spread of COVID-19.
"We are in the midst of a renaissance of discovery and understanding as to how aerosols communicate airborne infectious disease," Harvard biomedical engineer David A. Edwards, who has researched COVID-19 spread, told UPI in an email.
"Speech, cough and breath droplets ... are surely involved in airborne disease transmission [but] we do not have sufficient data to date to assume that one predominates over the other," said Edwards, who was not part of the new study, though his research was included.
When a person infected with the coronavirus speaks or coughs, they emit tiny respiratory particles that are 95% to 99% liquid, according to Bax and his colleagues.
Once exposed to the air, the liquid in these droplets, which contain varying amounts of the virus, evaporate, causing virus cells to disperse and potentially spread to others.
Even droplets containing small amounts of virus can fuel transmission, particularly in confined and crowded indoor spaces, the researchers said.
Masks that block at least 50% of these aerosols, including many of the cloth face coverings that have become common during the COVID-19 pandemic, can help limit how many droplets escape when an infected person speaks and significantly reduce virus spread, according to the researchers.
Other masks that block up to 95% of emitted droplets, such as N95 and surgical masks, are even more effective, they said.