Aerosols, the crucial missing link

This is really the most crucial question in the fight against the spread of the virus. The answer will have a huge impact on developments, not only in the coming months, but also in the years to come. Are aerosols playing an important role in the infection with the virus, or not?

WHO and RIVM say they do not, or hardly do. This means that the entire control policy is aimed at keeping people at a distance from each other (1.5 meters in the Netherlands). And if you (professionally) come close to an infected person, you wear optimal protection.

As I have indicated in many of my blogs (1, 2, 3, 4, 5), supported by many scientific papers since 2004, there is another important way in which people can become infected. That is through micro-drops (aerosols), which under certain circumstances continue to float in indoor spaces. People, who are present in these spaces, run an enormous risk of being infected by them.

I have given many examples of super spread events (SSEs) where a large proportion of those present have become infected. The choir rehearsal near Seattle and the event on March 5th in Kessel are well documented. Church services, choir rehearsals, après-ski, cruises and naval ships, nursing homes; everywhere where many people were infected in one fell swoop, this could not have happened because an infected person was so close to all those individuals present that the big outbreak started.

It is much more likely that this happened because of aerosols. I have written extensively about that. Discussion was whether the patterns found in the past for influenza also apply to COVID-19.

I have now found even more evidence that this is indeed the case.

It is important to realize that the recent findings of Homeland Security’s virus laboratory came from research into the COVID-19 virus and not from research into influenza. Those findings, however, resemble the conclusions from earlier influenza research:

– The virus can’t cope well with higher temperatures and higher humidity, and certainly not if they’re accompanied by sunlight.

– The virus thrives best indoors at low humidity.

I came across a study from 2013, in which experiments were carried out to determine which types of droplets are excreted when one coughs and how they spread indoors at different humidity levels.

This is that study: High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs

In this setup all kinds of measurements were taken at different levels of humidity.



The results of these experiments are abundantly clear and are best represented by the graph below. All experiments were done at 20 degrees Celsius. And again it is shown that the infectivity of the virus is smallest around 40% relative humidity (in fact 6.2 g/kg absolute humidity). And that the infectivity above that limit also remains low.


The research concludes that after 60 minutes in an indoor space with an air humidity below 23%, the chance of infection is 5 times higher than above 43%.

Results that are in line with what I have shown in many blogs over the past month and a half.

Now there are two possibilities:

– Or you stay on the line of the WHO and RIVM and you say the aerosols don’t play an important role in the spread of the virus. So you focus on just keeping social distance.

– Or you recognize that these aerosols do play a major role in explosive contamination in indoor spaces and you take adequate measures to ensure that the chance of being infected by aerosols in shops, offices, schools, care institutions, homes, ships, churches, halls, restaurants, cafés, clubs and party halls is kept to a minimum. And this can be done by ensuring an air humidity of more than 6 g/kg and good ventilation.

Now the good news is that meetings with several people have been forbidden since mid-March. So, for the time being, super spread events have been kept to a minimum (except  in healthcare institutions).

But now that we are slowly trying to start up more economic and social activities again, a policy to reduce the spread of the virus via aerosols will become very important.

RIVM, OMT and the government are making a serious mistake if they continue to ignore the spread via aerosols and thereby keep the chances of infection in indoor spaces unnecessarily high.

Think about the results of this research: At an indoor humidity of 43% (at 20 degrees), the chance of being infected by aerosols is 5 times smaller than at a humidity of 20%.

(Now the good news is that there is usually a lot of water in the outside air between May and September. And that has an effect on the humidity in indoor areas.

But that’s not always the case. And certainly not the rest of the year.

Plus, if you use air conditioning in the summer, the humidity indoors gets lower and that can be detrimental when there is less than 6g/kg in the air).

13 antwoorden
  1. catrien noorda
    catrien noorda zegt:

    Wat is jouw mening over vliegtuigen. Daar heerst een extreem lage luchtvochtigheid < 15%. Maar ik heb nog geen verhalen gehoord over super spread in een vliegtuig.

    • Maurice
      Maurice zegt:

      Ik denk dat er wel grotere risico’s in vliegtuigen zijn. Maar ik begrijp dat daar betere filtering is.
      Ik zou zelf als ik moet vliegen zeker mondbescherming gaan drage,

    • Robin
      Robin zegt:

      Volgens mij was er in de jaren 80 een geval waarbij een vliegtuig aan de grond moest blijven voor ongeveer 3 uur lang. 1 persoon in dat vliegtuig was besmet met influenza, na drie uur was 77% van de inzittenden besmet. Dit was wel zonder dat het filtersysteem aan stond.

      Filtersystemen zijn wel verbeterd, maar het zou voor de omzittenden nog wel een risico zijn

  2. Mark
    Mark zegt:

    Ik zag uw interview op Youtube bij Ad Verbrugge.
    In ziekenhuizen worden diverse maatregelen getroffen ten aanzien van de luchtbehandeling en de luchtkwaliteit. Op cleanroomniveau zijn deze instellingen sinds 2016 al op scherp gesteld vanuit het Toetsingskader luchtbeheersing operatieafdeling 2016. Hierbij zijn veel infectiepreventie en microbiologen betrokken en is er in de techniek, bij vakverenigingen en TNO veel ervaring op op gebied van klimaatventilatie. Er worden in de gezondheidszorg nu veel goed maatregelen omtrent deze installaties veel getroffen. In de stukken van REHVA en TVVL maak ik op de temperatuur en absolute vochtigheid geen invloed op het COVID virus heeft. Uit uw onderzoek blijkt dat dit klaarblijkelijk wel van invloed is, en waarbij mijn gevoel is dat dit ook aan de orde is ( ik ben techneut en geen wetenschapper). Vochtigheid is altijd van invloed geweest op de gezondheid en de verspreiding van deeltjes.

    Heeft u met partijen zoals REHVA, TNO, TVVL en Royal Haskoning etc contact hierover gehad? Hebben zij alle bronnen wel in beeld of is dit door deze partijen meegewogen in de conclusie?

  3. Hans
    Hans zegt:

    Airplanes – this is what Pieter Elbers wrote to KLM clients (on March 13, 2020)

    As a frequent traveler with KLM, you trust us to not only take you where you need to go, but to do so safely, comfortably and responsibly. Today, as the world comes to terms with an unprecedented health scare in COVID-19 (Corona virus), these values are more important to us than ever. Therefore, I’d like to take this opportunity to share some insight into the different steps that we are taking to guarantee that our flights continue to meet and exceed the highest standards of safety and hygiene.

    Safety & hygiene
    Before each departure, our planes are always thoroughly cleaned and disinfected. These procedures are designed to the highest standards and include both your personal space – tray tables, seat armrests and headrests, seatbelt buckles etc. – as well as communal areas such as the galleys, carpets and lavatories.

    Once in the air, our cabin crew works diligently to keep our planes clean. The air in the cabin is continuously refreshed with outside air and our hospital-grade HEPA filters are 99.99% effective in preventing the spread of airborne bacterial and viral organisms such as COVID-19. And in the event that a passenger does begin to show symptoms, all of our flights are equipped with special Corona-kits and our crew is trained in containment measures.

    This strong focus on hygiene extends to our entire organization, including catering services. Ingredients are fresh and sourced locally, and our catering crew is also trained to meet the strictest guidelines for hygiene and safety. This short video provides more information about hygiene at KLM.

    I fully understand the hesitation many people feel at the moment when deciding whether to book a flight or not. But you can feel secure in the knowledge that KLM has taken every precaution necessary to protect your health and the health of your loved ones. In this emotional time, you can be assured that we are guided and consistently act by fact-based, circumstance-driven decision making. On a daily basis, we work closely with the top experts at the World Health Organization and Rijksinstituut voor Volksgezondheid en Milieu (RIVM) in the Netherlands.

    No change fee
    I also understand that your concern may not be about your journey with KLM, but about the many unknowns about the situation on the ground at your destination. That’s why you can book your KLM flight with the confidence of being able to change your booking without a change fee. We are in constant touch with all the relevant authorities to adjust our network as required. This means that we can immediately respond when new travel restrictions are announced. For the latest information on flight status, please visit our travel advisory page or use the KLM app and let us keep you posted 24/7 with push messages.

    In 100 years of operations, KLM has faced many challenges and always emerged stronger. We were able to do so because we put you, our customer, at the heart of our operation. With that commitment in mind, I am confident that we will overcome this challenging time as well. As you know 33.000 KLM staff will go further to recognize your concerns and take care of you. Thank you for choosing KLM, and safe travels.


    Pieter Elbers

    President & CEO KLM Royal Dutch Airlines

  4. Ariane Greep
    Ariane Greep zegt:

    Maurice, sla de handen ineen met de tandartsen! Ik was net bij mijn tandarts en mondhygiëniste en hun hele beleid is er op gericht om aerosols tegen te gaan! Ramen open, andere apparatuur gebruiken waardoor aerosols niet zoveel gaan zweven, krabbers om tanden schoon te krijgen en zo kunnen ze verder werken. Protocol in heel Nederland!!! Onderzoek dat maar eens en dat kan jou helpen in jouw verhaal. RIVM moet ooit eens gaan luisteren.

  5. Rob
    Rob zegt:

    Naast aerosolen spelen waarschijnlijk stofdeeltjes (PM2.5) ook een rol in de Covid-19 crisis. Deze zijn ook aan elkaar gelinked.

    Met dank aan o.a. het RIVM die toch mooie datasets bij elkaar legt, waarmee hier een correlatie wordt gelegd tussen verhoging van PM2.5 (+20%) en Covid-19 gevallen (+100%):

  6. Piet
    Piet zegt:

    Gezien de verlichting van de huidige maatregelen, kan je opmaken dat de theorie over aerosols en luchtvochtigheid ivm temperatuur weldegelijk worden meegenomen in de versoepelingen; immers zwembaden wel open maar sportscholen niet. Mogelijk heeft je blog toch invloed!

  7. Hans Asscheman
    Hans Asscheman zegt:

    Beste Maurice,

    Geweldig wat je bijdraagt aan nieuwe inzichten.

    Heb jij enig zicht wat er gebeurd bij een repetitie van een blaasorkest?
    Ik kan nergens rapporten daarover vinden. Ook onze regering geeft nergens uitsluitsel. In Duitsland is het advies 5 meter tussen de blazers bij indoorrepetities.

    Ik ben erg benieuwd of jij advies hebt wat betreft de verspreiding van aerosolen bij een blaasorkest.

    Hartelijk dank
    Met een vr. Groet


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