By Jill Pease
A new World Health Organization report is a step forward in acknowledging the role of inhalation of infectious disease particles in COVID-19 transmission, writes a group of experts in an editorial in the journal Aerosol Science and Technology, but WHO’s report falls short by not addressing how to minimize airborne transmission.
The editorial’s authors include John Lednicky, Ph.D., a research professor in the University of Florida College of Public Health and Health Professions department of environmental and global health, and colleagues from the U.S., U.K. and Sweden.
“The WHO report is a big step forwards in that it finally spells out that airborne transmission of pathogens can occur through ‘small droplets that contain pathogens.’ This is especially important for respiratory viruses,” said Lednicky, a member of UF’s Emerging Pathogens Institute. “Previously, the WHO stated that large droplets produced by coughing people were the major way that respiratory viruses were spread person to person. These droplets were said to fall one to two meters away from the person, and this is why there was the three-foot, then the six-foot, rule for safety when COVID-19 became a pandemic.”
At the inception of the COVID-19 pandemic, public health authorities advised that COVID-19 was spread person-to-person through contact with the virus SARS-CoV-2 in large droplets expelled in coughs and sneezes. However, a mixture of large, intermediate and small particles that contain SARS-CoV-2 are expelled by people when they cough and sneeze. Scientists later learned that whereas COVID-19 may be transmitted through large droplets, the primary mode of transmission is through smaller sized droplets emitted in coughs and sneezes and when talking, breathing and singing. Intermediate-sized particles can evaporate into smaller-sized droplets within micro-seconds. The smaller droplets are of concern as they can stay adrift in the air for relatively long periods of times in enclosed spaces, where they pose an inhalation risk, Lednicky said.
Using innovative sampling techniques, Lednicky and colleagues in the UF College of Engineering published results in summer 2020 that provided some of the first evidence to the international public health community that viable, or “live,” SARS-CoV-2 can be present in small droplets in the air of the rooms of COVID-19 patients, suggesting spread through aerosol transmission of particles of various sizes, not just respiratory droplets. News outlets such as The New York Times called this the “missing piece of evidence that infectious virus can be found in the air.”
WHO’s new report suggests the term “infectious respiratory particles” to refer to the pathogens in expirations. But the term “aerosol” in the context of infectious disease transmission is better understood and less restrictive, the editorial authors argue.
“If the term ‘aerosol transmission’ was used, there would be no dichotomy between ‘large’ and ‘small’ particles, and one would understand that viruses can be in small particles that stay adrift and move far away from the emitter … that there is indeed an inhalation risk in enclosed places,” Lednicky said.
While the WHO report focuses on transmission mechanisms, it has little to say about infection control measures, the editorial’s authors write, instead indicating the need for further meetings before updating guidance on mitigating infectious disease spread.
“Infection control folks need to know how to best break transmission chains,” Lednicky said. “The challenge now is for hospitals and clinics to be designed with proper air exchanges, clean-up of air in hospital rooms, and use of practices that minimize inhalation risks to airborne pathogens, such as wearing of N95 face masks and eye protection.”