18 Nov 2021 Episode 188 Ventilation and air claening: sure, but how?

Thu, 11/18/2021 - 11:17

Dear colleagues,

I will follow up on the growing literature on 3rd vaccination and related issues in the next episode. 

But first, my attention was caught Wednesday morning by a radio interview with Prof. Bert Blocken, an engineer in “building physics”  at the Technical University of Eindhoven and the KULeuven (https://www.tue.nl/en/research/researchers/bert-blocken/). 

Ep 188-1:  Interview with Blocken in Knack magazine  (Dutch) May 2021.  The central thesis is that you need a combination of vaccination and ventilation to efficiently lower aerosols.  He points out that  proper ventilation is more than just opening a window.  He also realizes that a mechanical ventilation with fresh air from outside is very costly (and that cost has increased since the rise of gas price). Therefore he pleads for good quality air cleaners/purifiers in class rooms, horeca, fitness centers etc. Still the price for such a device is estimated at 2000 Euro (!). (During the radio interview he added that the total cost for air cleaning schools in Flanders would be 60 million Euro)

A three step approach, to be implemented by the health authorities, is proposed by Blocken:

  • Ranking the  quality of air purifiers.
  • Distribute them in schools, horeca and fitness
  • Provide a quality label to those who have the installation functioning.

Blocken was hopeful in May since the Minister of Health showed interest and commitment, but in the meantime, progress has been limited.  According to Minister of Education Ben Weyts on Thursday morning, C02 meters are present in 90 % of the schools (but not in every classroom) and in 70 % of horeca.   Last night, after the “inter-governmental consultation committee”, several of our many ministers emphasized the importance of ventilation, but they only referred to “opening a window”…..  

Ep 188-2: The most recent scientific paper of Blocken in Building and Environment.  They measured the influence of ventilation and air cleaning in a gym room with 35 people exercising.  The conclusion is that neither ventilation nor air cleaning alone could prevent accumulation of aerosols over 30 minutes, while their calculations showed that a combination of both measures could reduce aerosols by 80-90 %.

Of course, I’m unable to critically evaluate the technical and mathematical aspects of this paper, but as a “naïve referee”, I would request the authors to perform an additional experiment to actually prove that combined ventilation + cleaning has indeed this favorable effect…. It is also unclear to me how you can extrapolate these results to a class- or meeting room or a horeca site, where individual people produce less aerosols (“resting” versus “exercise” state), but -sometimes- these places are also more crowded than a gym.  

Ep 188-3: Using the terms “COVID-19”, “classroom” and “ventilation”, I found only one paper in PubMed by a Basque group.  They clearly show that CO2 rapidly increases over 1000 ppm in a classroom of 91 m3 with 15 people inside and that opening a window also rapidly reduces CO2 (see Fig 5 p. 9).  From the discussion at the bottom of the same page, however, it seems that the relation between CO2 and virus spreading has not been consistent.  

Interestingly, particle concentration (and size) suspended in the air actually increased when opening windows (and doors) See Fig 6 p. 10), suggesting of course that they came from outside.  Clearly, this physical particle measurement is insufficient to conclude anything about infectious particles.  

Ep 188-4: Using the search terms ((COVID-19) AND (CO2 measurement)) AND (virus spreading), I found a more thorough and recent German study on the effect of various air purifiers in a classroom in the same journal.  They state that

  • Window ventilation reduces respiratory particle concentration and improves air quality in terms of humidity and CO2.
  • However, window ventilation is not a reliable measure to reduce the concentration of virus-laden particles to a defined degree.
  • Particle concentration can be strongly reduced by air purifiers with closed windows, but opening the window or ventilation with fresh air will bring in “innocent” particles (pollen, air pollution) that cannot be distinguished from respiratory particles.
  • They also recognize that various parameters such as: room size, duration of stay, and activity of the people in the room have a major influence on the potentially inhaled dose of virus-laden particles
  • A major drawback is that these air purifiers make noise, which is not favorable

Therefore, ideally both measures (ventilation and air purification) should be combined intermittently. 

Ep 188-5: A reassuring paper in PLOS One for those who “suffocate” because of wearing a facemask.  Among 50 adult volunteers (median age 33 years; 32% with a co-morbidity), there were no episodes of hypoxemia or hypercarbia in rest or during exercise, wearing either no mask, a cloth mask or a surgical mask.


Provisional conclusion: There is a growing awareness about the importance of “ventilation” and the contribution of “air cleaning” is certainly important in winter time, when people hesitate to open a window. 

However, the scientific literature is scarce and not really conclusive on what exactly should be done to minimize the risk. 

I tried to find the official information on air purifiers at the Ministry of Health.  This is what I found:  https://www.health.belgium.be/nl/lijst-van-gecontroleerde-en-toegestane-ventilatiezuiveringsproducten-tegen-covid-19:  a long list of over 250 purifiers, but without any evaluation….

Step one (ranking quality) still has to start…..

It sounds trivial, but, after almost two years into the COVID pandemic, clearly “more interdisciplinary and translational research on how to guarantee safe indoor air is needed”, since it remains very unclear to me how an MD, an epidemiologist, a school director, a café owner, a restaurateur, a fitness room manager,…. should translate the complicated engineers calculations and the vague instructions by our health authorities into useful and healthy practice….

If anybody has more concrete info, please let me know….

Best wishes,