When someone coughs, talks or even breathes, they send tiny respiratory droplets into the surrounding air. The smallest of these droplets can float for hours, and there is strong evidence that they can carry live coronavirus if the person is infected.
Until this week, however, the risk from these aerosols wasn’t incorporated into the World Health Organisation’s formal guidance for nations. The WHO instead suggested that the coronavirus was primarily transmitted by coughing or sneezing large droplets into someone’s face, rather than being a longer-term threat that can float in the air.
It took pressure from scientists to start to change that.
More than 200 scientists published an open letter to the WHO on July 6 warning about airborne transmission of Covid-19 via aerosols and urging the organisation to recognise the risks. The WHO responded Thursday afternoon with an update in which it acknowledged the growing evidence of airborne spread of the disease, but it did so with hesitation.
As professors who study fluid dynamics and aerosols, we believe it is important for people to understand the risks and what they can do to protect themselves.
What is an aerosol and how does it spread?
Aerosols are particles that are suspended in the air. When humans breathe, talk, sing, cough or sneeze, the emitted respiratory droplets mix in the surrounding air and form an aerosol. Because larger droplets quickly fall to the ground, respiratory aerosols are often described as being made up of smaller droplets that are less than 5 microns, or about one tenth the width of a human hair.
In general, droplets form as a sheet of liquid breaks apart. You’ve probably experienced this phenomenon by blowing soap bubbles. Sometimes the bubble doesn’t fully form, but instead breaks apart into many droplets.
Similarly, in humans, small sheets and strands of liquid — mucus — often stretch across portions of the airway. This most often occurs in locations where the airway opens and closes again and again. That happens deep within the lungs as the bronchioles and alveolar sacs expand and contract during breathing, within the larynx as the vocal folds vibrate during speech, or at the mouth, as the tongue and lips move while talking. The airflow produced by breathing, speaking and coughing breaks apart these sheets of mucus, just like blowing the soap bubble.