Support for the appeal of protective measures against the airborne spread of Covid-19

A publication by Lidia Morawska and Donald K. Milton in the journal "Clinical Infectious Diseases" (CID) published on Monday appeals for attention to the role of airborne microdroplets and aerosols in the spread of Covid-19.


239 scientists from 32 countries have signed the paper entitled "It is time to Adress Airborne Transmission of COVID-19" as supporters (1). One of these supporters of the appeal is Professor Hartmut Herrmann, head of the Atmospheric Chemistry Department (ACD) at the Leibniz Institute for Tropospheric Research (TROPOS).


The Morawska and Milton Declaration addresses the medical community and the responsible national and international authorities, not least the WHO. It clearly emphasizes that the protective measures recommended so far against the infection and spread of Covid-19 viruses should be supplemented everywhere by ventilation and air exchange measures, as otherwise these do not provide sufficient protection against transmission by floating microdroplets or droplet nuclei.


Depending on the particle size, one distinguishes droplets (larger than 5 µm) from smaller particles (droplet nuclei or infectious aerosols, smaller than 5 µm). The transformation is fluent; droplet nuclei can be generated from droplet size particles by drying out in the air (2).


Especially the WHO (Worlds Health Organisation) or also the American health authority CDC do not sufficiently warn of the danger of the long-lived infectious particles floating in the air, according to the author and the supporters.


The German Robert Koch Institute (RKI) confirms: "Longer time spent in small, poorly ventilated or unventilated rooms can increase the probability of infection by aerosols even over a distance of more than 2 m (...). An effective air exchange can reduce the aerosol concentration in a room". (2)


Also in Germany, where at present public life is gradually returning and many institutions are reopening, we must consider whether the recommendations take sufficient account of the aspect of airborne infectious particles and, if necessary, adjust measures accordingly. A possible renunciation of the wearing of masks should therefore be based on caution and carefulness and does not appear to be appropriate at the moment.


The statement by Lidia Morowska and Donald K. Milton includes concrete recommendations for implementation:


  • 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



Prof. Herrmann and the Director of TROPOS, Prof. Macke, and thus TROPOS as a whole, support the appeal that the aerosol aspect should not be ignored in the spread of the COVID-19 virus.



(1) Morawska L, Milton D K. It is time to Adress Airborne Transmission of COVID-19. Clinical Infectious Diseases 2020.





(2) Robert Koch Institut: SARS-CoV-2 Steckbrief zur Coronavirus-Krankheit-2019 (COVID-19), Stand: 26.6.2020




Prof. Hartmut Herrmann

Head of the Atmospheric Chemistry Department

Leibniz Institute for Tropospheric Research (TROPOS)

Phone +49 341 2717- 7024


Tilo Arnhold

TROPOS Public Relation

Phone  +49-341-2717-7189