TRANSMISSION MODES: Dangers of aerosols being underestimated

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.”

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. 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. 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.

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. 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.

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.

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. 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.