Our collective knowledge and understanding of the virus is constantly changing. 

The World Health Organization (WHO) acknowledged earlier this week that there is emerging evidence showing that COVID-19 can be spread through tiny particles in the air—not just through actual droplets or contaminated surfaces. 

On Monday, July 6, 2020, an open letter to the WHO, signed by 239 scientists in 32 countries, outlined the growing body of evidence that shows the potential for the airborne spread of COVID-19. Led by Donald Milton, professor of environmental health at the University of Maryland, the letter implores the WHO to recognize and promote guidance to address this issue. 

Milton’s life work is to study the transmission of viruses, and the other lead author of the paper, Lidia Morawska, is a professor of environmental engineering at the Queensland University of Technology in Brisbane, Australia, and a specialist in aerosol science. The two of them, along with a team of similar experts, have been looking into the airborne spread of the virus for several months.  

The research has shown that the virus is carried by airborne droplets, some of which are larger and will fall on surfaces. Other droplet particles are far smaller and will hang in the air where they can be inhaled or sucked into ventilation and air conditioning systems, potentially spreading quickly to other areas. In a crowded area, indoors, with poor ventilation, there is a dangerous potential for these aerosol particles to infect many people. 

The expert group felt it was important to speak up and address an area that they feel is not being given as much attention or importance as it should in the fight to understand the COVID-19 virus. 

“There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several meters, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.”

The letter goes on to say:

"Most public health organizations, including the World Health Organization, do not recognize airborne transmission except for aerosol-generating procedures performed in healthcare settings. Hand washing and social distancing are appropriate, but in our view, insufficient to provide protection from virus-carrying respiratory microdroplets released into the air by infected people."

The letter’s authors and those who signed it recommend that the WHO advise the following precautions to help mitigate and prevent the spread of the virus through airborne particles. 

  • Provide sufficient and effective ventilation (supply clean outdoor air, minimize recirculating air) particularly in public buildings, workplace environments, schools, hospitals, and aged care homes.
  • Supplement general ventilation with airborne infection controls such as local exhaust, high-efficiency air filtration, and germicidal ultraviolet lights.
  • Avoid overcrowding, particularly in public transport and public buildings.

The WHO acknowledges and confirms that the research presented in the letter—as well as other ongoing studies—shows that there is emerging evidence of COVID-19 spreading through airborne particles. 

“We acknowledge that there is emerging evidence in this field, as in all other fields regarding the Covid-19 virus and pandemic and therefore we believe that we have to be open to this evidence and understand its implications regarding the modes of transmission and also regarding the precautions that need to be taken," Dr. Benedetta Alleganzi, WHO Technical Lead for Infection Prevention and Control said in a briefing following the publication of the letter. 

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WHO scientists and doctors also pointed out that many of the experts who contributed to the letter are engineers, who understand how ventilation affects the distribution and spread of the virus. 

In the coming weeks, the WHO expects to release a new briefing detailing up-to-date information about the spread of the virus, as well as emphasize the need for more research. 

"So, these are fields of research that are really growing and for which there is some evidence emerging but is not definitive," Alleganzi said. "And therefore, the possibility of airborne transmission in public settings, especially in very specific conditions crowded, closed, poorly ventilated settings that have been described cannot be ruled out. However, the evidence needs to be gathered and interpreted."

As new information comes out about the virus, it is important to continue to take precautions such as wearing facemasks, observing socially distancing guidelines, washing hands often, and staying informed on the most recent news and research.