Under normal conditions,
– We breathe 450 l of air per hour, or 10.8 m3 per 24 h, or 13 kg …
– We emit per person 18 l of CO2 per hour
– The concentration [CO2] of the outside air is 400 ppm (parts per million), or 0.04%.
– The air we breathe out has a [CO2] concentration of 40,000 ppm, or 4%
– We therefore breathe in air at min 400 ppm to reject it at 40,000 ppm
– There are several hundred potential pollutants in a building (paints, glues, furniture, etc.), and it is impossible to measure them all. On the other hand, we can ventilate, and use CO2 as a marker
– In the case of COVID, it is assumed that aerosols are a significant source of contamination. A contagious person will emit contaminating aerosols, but also CO2 when breathing. If we limit CO2, we therefore limit the risks.
According to the World Health Organization (WHO) and the medical profession in general, good ventilation of indoor spaces is one of the preventive measures that make it possible to limit the spread of COVID-19 by aerosols. Therefore, the municipality of Kraainem has been providing CO2 sensors (*) free of charge since summer 2020 for spaces accessible to the public. The use of these devices makes it possible to determine when additional ventilation with outside air is required, for example by opening windows and doors. At the beginning of March 2021, we had distributed in the Municipality more than 170 sensors in restaurants, cafes, municipal buildings, police offices, shops, doctors’ offices, physiotherapy rooms, hairdressers, etc.
(*) The device indicates the CO2 concentration (in ppm, “parts per million”, 1% = 10,000 ppm) and also gives basic indications by means of indicator lights. The lower the values, the better (the CO2 concentration outdoors is estimated to be around 400 ppm). The question is, of course, what is the “safe” level of CO2, and for how long? To this question there is no clear answer at the moment (**). The reason is that it is difficult to say today which dose (= concentration x duration of exposure, see graph below) of aerosols becomes dangerous, and therefore no one can give a limit (concentration or duration ) under which the aerosol situation is 100% certain.
For now, we must therefore limit ourselves to the ventilation RD for non-residential buildings [New rules for the quality of indoor air in work premises – Federal Public Service for Employment, Labor and Social Dialogue (belgium. be)], which is actually 900 ppm for most buildings. With an orange light from 800 ppm for our CO2 sensor, we are doing a little better, but it is certainly not a 0 risk guarantee.
Graph: Dose = concentration x duration of exposure [CO2 is used as a marker of a potential virus concentration]: one hour at 2000 ppm is equivalent in terms of “dose” to 8 hours at 600 ppm (assuming that the outdoor concentration is 400 ppm). Nobody knows today if 600 ppm for 8 hours is “safe”, there are indications that even 500 ppm would not be sufficient (***) … FYI, this graph was established following several exchanges that I had with former colleagues (CTSC and University of Ghent) with whom I spent several years on the new version of the AR Ventilation when I worked as Energy Advisor for the Confederation of Construction before becoming mayor from Kraainem.
(**) Things get complicated as soon as we try to define a limit to be respected in order to obtain a “safe” situation under current conditions. What are we talking about: what kind of variant, what exposure time, the mask worn or not, the type of activities, … All kinds of parameters which can obviously play an important role. Ideally, we would like to be able to offer the above type of graph to managers and managers of buildings accessible to the public, the level of the curve would depend on the sector of activity considered, as well as other parameters mentioned above.
(***) Professor Jimenez from the University of Colorado, with whom we are in regular contact, recommends staying below 700 ppm at first, but also says that even at 500 ppm, it cannot guarantee a risk 0. On the other hand, in schools, for example, it is better to aim for 700 ppm by opening the windows as often as possible, rather than the 3000 ppm that are conventionally found in many establishments …. We must therefore limit ourselves, at least for the moment, to note that, if we stay in the green zone of the sensor (which turns orange at 800 ppm), the ventilation is “better” than what the legislation prescribes (900 ppm) .
Following steps ?
As soon as we accept a residual risk, we could obviously consider the sustainable reopening of a whole series of activities as long as they are strictly supervised by means of a strict protocol. This is the subject of Professor Clumeck’s carte blanche (among others) published in Le Soir of 2021-04-12, and which advocates a “covid safe” label. Other labels of the same kind are proposed and also go in the direction of reopening certain activities and / or establishments under good safety conditions, rather than decision-making for entire sectors without any distinction between those who can. ensure the required level of security and others.
There probably won’t be a quick fix, and all contributions are welcome. Opening windows and doors doesn’t pose a particular problem in summer, but once the temperatures drop, it’s something else. It is then that filtration and / or air purification systems can make sense, as long as there is a medical green light [sine qua non in my opinion before being able to consider any installation at the level of a municipality].
The “covid safe” label is undoubtedly one of the elements of the puzzle, but will have to be well thought out in order to avoid chaotic situations, discussions and conflicts of all kinds.
In Professor Clumeck’s carte blanche, it is mentioned that the municipalities could be invested with the responsibility of advising, supporting and verifying the “covid safe” compliance of public places. This is fundamental because the best system will not have much effect if control and / or monitoring is not assured.