Ventilation in schools II: A tool for assessing its effectiveness

Now that autumn is coming to an end and the cold seems to be here to stay, ventilation in schools remains a priority. The guidelines and recommendations follow the same line: reduce social distance, mask and ventilation.

In our previous post on ventilation we analyzed what the regulations tell us and what is the current situation in classrooms. Today, we present you the estimation tool we mentioned to assess the CO₂ concentration of a space and the level of ventilation.

This is a very simple calculation tool, based on a tenminutal balance, where the CO₂ concentration of the air in a classroom is simulated, depending on a series of input parameters.

Below we explain, as a didactic guide, its use and application.

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Classroom dimensions

First of all, the physical dimensions of the spacemust be known : the usable area and the clear height.

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2. Alumni

Secondly, the number of people in the classroom is introduced. Based on the number of students and their age, the CO₂ production rate is determined based on a study (Persily and de Jonge, 2017. Indoor Air) used by the GUIDE FOR VENTILATION IN CLASSROOMS. The tool multiplies the number of students by the CO₂ production associated with the range of students in the drop-down menu, adding the presence of an adult in a teaching capacity.

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3. Timetables

Thirdly, the start and end times of classes are chosen, as well as the start and end times of the mid-morning break. Full occupancy is estimated during class time and empty classrooms both at break time and outside class time.

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4. Ventilation level

This is undoubtedly the most difficult section to estimate. Three sub-sections are set out in the tool:

• Air renewal due to building infiltration. The usual deficiencies in the joints and enclosures of the building generate a flow of natural and uncontrolled ventilation in the classroom. To determine this flow rate, three ages have been established for the building: before 2000, built between 2000 and 2013, and buildings constructed after 2013. Depending on the age, 0.1, 0.2 or 0.3 ACH are assigned. An ACH is the Air Change per Hour, i.e. if we consider 0.1 ACH we are saying that 10% of the air in the room is renewed every hour due to infiltrations.

• Mechanical ventilation. Older buildings usually do not have any mechanical ventilation system. If the building was constructed after 1998, compliance with RITE would require a ventilation system to ensure air renewal. In the tool, if "YES" is selected, the calculation assumes a renewal of 12.5 l/s per person.

• Window opening. Finally, the effect of opening windows in classrooms is simulated, which is undoubtedly the most complicated part to estimate. The flow of air exchanged with the outside through the opening of windows depends on many factors, from the type of window (tilt and turn, side...), its size, the height, the outside wind on the façade, the temperature difference between inside and outside, the possibility of cross ventilation, among others. This tool allows to play with the window opening schedule and the ventilation level estimated in ACH. In general, a slight opening of windows can ensure a ventilation level of 0-1 ACH, a medium opening level can reach 1-4 ACH and a wide opening can reach 5-10 ACH.

Thus, the tool allows us to play with all these parameters and visualize an estimated evolution of CO₂ in the classroom, which will allow us to identify whether we are ventilating effectively (and efficiently).

You can download and use this Excel tool at this link.

In general, levels of 800 ppm are usually established as ideal. RITE establishes 900 ppm as a reference for classrooms (IDA2, see our previous post) and non-recommended levels above 1500 ppm. However, the references consulted suggest a level below 700 ppm to reduce the danger of spreading viruses. This means reaching very high ventilation levels (5-6 ACH) which, on the other hand, are a source of great thermal discomfort at this time of year in winter.

As we said in our previous post, to go from theory (our estimation tool) to practice (what happens in classrooms these days of cold pandemic), we must measure. It is important to optimize the effectiveness and efficiency of air conditioning and air renewal systems, which are sized and regulated according to real needs. Because yes, 5 ACH of ventilation can guarantee 700 ppm of CO₂ continuously but what is the energy and economic cost of trying to maintain thermo-hygrometric comfort conditions with this level of natural ventilation? Has the time come to equip schools with extra blankets and coats, to be able to ensure clean and fresh air?.

We have already presented our MICA Lite device as a specific tool to identify and validate the ventilation protocol in educational centres. The data is converted into valid information to be able to act, both at the level of the LED light that indicates when it is necessary to ventilate, and through monitoring in the My inBiot cloud.

We agree that proper air quality is vital for a comfortable and healthy learning space. If you need more information or support in measuring air quality, inBiot can help you.