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Dangers of COVID-19 aerosols are underestimated

July 9, 2020

A group of 239 researchers has called for an update of COVID-19 guidelines, as transmission via ultra-fine suspended matter in the air was not sufficiently taken into account. Now the WHO also sees the danger.

Man sneezing
Image: picture-alliance/dpa/PA/Jordan

Do I still need to wear a mask when shopping? When do the pubs and bars open again? What's with the distance rules in restaurants? When can I fly again without worries?

While many European countries are pressing for a return to normality as quickly as possible – at least that's the way it seems – after the corona crisis has been overcome, 239 researchers from various disciplines are sounding the alarm in the journal of "Clinical Infectious Diseases."

Pending danger in enclosed spaces

Aerosols are the smallest suspended particles and droplets in the air, which are smaller than five micrometers. When breathing out, speaking, laughing or singing, this fine mist spreads throughout the room. The larger droplets quickly fall to the ground, but the finest particles can remain suspended in the air for hours – especially in closed rooms.

If an infected person stays in such a closed room, he can infect many others in a very short time – without ever having direct contact with them.

COVID-19: All about aerosols

WHO reconsiders its position

In response to the public appeal, the World Health Organization is now also ready to change course. Until now, the WHO had mainly assumed droplet infection was the main culprit of the disease's spread.

Read more: WHO launches panel to review international COVID-19 response

The WHO now also recognizes "emerging evidence" for the airborne spread of the novel coronavirus, said Maria Van Kerkhove, technical manager for the COVID-19 pandemic at the WHO, in a press briefing.

On the basis of the new assessment, the next step would be to adapt the WHO corona guidelines accordingly. A month ago, the WHO had already had to revise its assessment of protective masks.

Aerosols as the most important transmission path

According to the researchers, investigations on influenza and also on the coronavirus MERS-CoV showed that the viruses spread mainly via aerosols. "We have every reason to believe that SARS-CoV-2 behaves in a similar way and that aerosols are a decisive transmission pathway," said the appeal, which was signed mainly by experts from the fields of chemistry, physics and engineering sciences, but less so by virologists and physicians.

Street scene during times of corona at the party mile in New York
Young people in New York celebrate the reopening of the party mile – at least outdoors.Image: Reuters/J. Moon

The current protective measures - washing hands, wearing a mask, keeping your distance - are primarily aimed at a possible droplet and smear infection. But these alone are not enough. The researchers argue that there has been insufficient evidence of aerosol transmission to date.

Regular airing helps

Therefore, the authors of the appeal call for regular and effective room ventilation with fresh air from outside. Air circulation indoors, such as using fans or air conditioning, should be avoided – especially in public buildings, schools, workplaces, hospitals, and retirement homes. Existing ventilation systems should be expanded to include extraction and air filtration systems and/or germicidal, ultraviolet light.

Above all, overcrowded rooms and crowds of people in closed rooms should be avoided, not only in bars or clubs, but also in public buildings and transport.

Indicators show need for reassessment

A sudden increase of corona cases after bar or restaurant visits, as well as after choir performances, show that the infections were probably transmitted by aerosols in the indoor air, according to Prof. Dr. Clemens Wendtner, head physician of the Infectiology and Tropical Medicine Department of the Munich Schwabing Clinic, who was not involved in the appeal.

Germany: Approximately 400 Corona infections in slaughterhouses in Rheda-Wiedenbrück
Did the "repeatedly circulated, unfiltered cold air " in slaughterhouses lead to frequent corona infections?Image: picture-alliance/dpa/B. Thissen

It is also possible that the ventilation systems that "repeatedly circulated, unfiltered cold air" in slaughterhouses led to the frequent occurrence of corona infections, according to Wendtner. The physician considers the ventilation measures proposed in the appeal to be sensible and had suggested a change of course from the WHO. 

"In view of the still increasing global infection figures and the simultaneous implementation of relaxation measures in some countries, a WHO call for protection against aerosols containing SARS-CoV-2 would be desirable and, from a scientific point of view, urgently required," said Prof. Dr. Wendtner.

Adapting behavior to new findings

At the beginning of the pandemic, transmission via surfaces was "probably somewhat overestimated," according to Dr. Isabella Eckerle, Professor of Infectious Diseases at the University of Geneva, who was not involved in the appeal.

On the other hand, the transmission via indoor air due to the proximity to sick people (family celebrations, choir rehearsal, and fitness studios, etc) has been somewhat underestimated. "I think the previous very strict distinction between either aerosol or droplet infection is not sufficient to cover all transmission scenarios.”

The coronavirus is not a classical aerosol-transmitted pathogen, such as measles or chickenpox, said Eckerle. "A similar scenario is not likely to occur with SARS-CoV-2."

Special masks for all are also not a solution

Although professional FFP3 filter masks could prevent aerosol infection, Eckerle believes that a recommendation for this type of mask in the general population is neither "reasonable" nor "feasible".

"I believe that we must be pragmatic and translate the knowledge we currently have about the pathogen into meaningful, actionable recommendations. Accordingly, superspreading events in enclosed spaces should be prevented,” says the professor of infectious diseases at the University of Geneva.