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A teacher had tested positive for Covid-19
17 December 2020

A teacher had tested positive for Covid-19

In the classroom, we accepted that our teacher was Covid-positive and examined what we should do to prevent it from infecting students. Ofcourse, we can open the window and reduce the risk, but this does not mean that the infected air is cleaned correctly and quickly. What if there are areas in the classroom where clean air cannot reach? Of course, the risk of contamination of indoor air will decrease due to clean air. But when we add winter conditions to this, the management of the situation becomes more difficult.

In the previous post, we mentioned that natural ventilation would be a romantic discourse in eliminating the risk of Covid (LinkedIn – CFD Covid-19 Ventilation Post). Because there is no control of the air entering the environment and it is possible for some areas to be insufficiently ventilated. We name insufficient ventilation as the aging of the air, especially in clean room projects. One of the most effective ways to eliminate this situation is to use machines that clean the indoor air. You will be trapped in the infected air caught by these hepa filter machines, which you can get for a few hundred dollars.

Nowadays, we complete analyzes for different purifiers, in different locations (office, clinic, houses, hotels, etc.). Our aim is to answer the question of whether the purifier can clean everywhere sufficiently by providing homogeneous air circulation in the indoor environment. When a “Covid-positive” person enters the environment, viruses that may spread as droplets will adhere to the surfaces and again increase the risk (Figure 1). For this reason, even if you already have a air purifier, the use of a mask does not lose its importance. However, the infected air that starts to spread from the mouth of the masked person is caught by the air purifer in the environment and kept in the filter.

A teacher had tested positive for Covid-19

Figure 1. Distribution of particles emitted into the inhaled air under different conditions (

The important criterion is how many times the machine passes the air through its own filter. The height of this class (Figure 2) is approximately 3m and its area is 60m2. The “Ashrae Standards” include international acceptances that specify the design criteria we use to ensure competence in indoor air quality (IAQ). It states that quality air in a standard class will be acceptable after changing 5 times per hour.

A teacher had tested positive for Covid-19

Figure 2. A class where window opening is not preferred due to cold weather

Changing the air of a volume of 3m x 60m2, ie 180m3, 5 times requires an air purifier that supplies 900m3 of clean air per hour. It is written in catalogs of machines to be used, how many cubic meters/hour capacity they can work with. By looking at this, you can make a very comfortable requirement calculation. The air change number chart for homes, offices or other spaces is below.

A teacher had tested positive for Covid-19

Figure 3. Air Change Rate table for different spaces

In this case, we see how important the function of purifiers is when providing clean air. In parallel with this, when you try to meet your need for fresh air by opening the window during winter periods, you cause the air you have heated inside to go out quickly, causing serious energy loss. When we think carefully, the issue has enormous effects, including climate change and using energy resources correctly.

Let’s go back to our classroom and admit that our teacher is Covid-positive. Some of the virus in the air coming out of his/her mouth will remain suspended on mask, a very small part in the form of droplets, and some in the air. Polluted and dangerous particles spread to the environment after long hours on a winter day when no purifier is used in our class. The weighted gas coming out of the human mouth will be CO2 and will move by settling towards the ground due to its molecular weight in the environment. In the simulations, we defined a lower molecular weight gas for the virus to show air suspension. We share this information from the literature in our project reports. This pollutant, which has a lower molecular weight, is small and tends to remain suspended in the environment, spreads around itself with the body temperature of the person, and at the end of a while, mixing with the fresh air entering through the open door is caused by natural convection movements caused by the students’ body temperature and breathing. It shows a risky distribution (Figure 4). If the 5000ppm contaminated gas was only CO2, it would lead to headache, low concentration, heart compression and sleepiness problems. CO2 ratio is the most used parameter to measure IAQ. For example, the CO2 PPM (particle per million) value should be 300ppm in the atmosphere (outdoor air) and should not exceed 1000ppm indoors (What are safe levels of CO and CO2 in rooms?).

A teacher had tested positive for Covid-19

Figure 4. Distribution of contaminated gas concentration in the classroom

Let’s watch what happens when the two air purifiers operate to avoid the situation in Figure 4.


As shown in the video, we wanted to clean the environment in a short time and by running two air purifiers for 20 minutes, we changed the air of the environment 6 times. An important issue is that the purifier’s vents that draw in contaminated air must be open. The air intake vent should not be obstructed. For example, if you examine the comparisons in Figure 6, you will see that the indoor air is aging in some regions and the LMA value is high. Because the air may not circulate homogeneously everywhere in the room. For this reason, the correct location of the purifiers to be used to suction the air can be determined by analysis. Based on the results of the current situation, the accumulation of contaminated gas in the sections parallel to the floor compared with the conditions in which the purifier is operated. In this case, it is not beneficial to have the door open. It is necessary to create a pressure or temperature difference for natural ventilation. The spread of the current situation is visualized in Figure 5. The density is much higher than the upper limit set in the legend. They can be compared with the values in the Figure 6 visual.

A teacher had tested positive for Covid-19

Figure 5. Current situation; only the door is open and the polluting gas dispersed from the teacher’s mouth for a long time (4 hours).

There were analyzes that we completed by placing purifiers at different points. Here, our aim is to show how the performance of the air purifier changes with the location where it is placed.

A teacher had tested positive for Covid-19

Figure 6. The images were taken from the neck level of the children parallel to the floor. In the visuals on the left, it indicates the ppm level of the polluted air in the cross section. On the right, the aging level of the air is indicated. The more homogeneous the air circulation is, the less it will age and the risk of contaminated gas will decrease.

The main purpose of the study is to increase awareness. Thanks to air purifiers, our indoor air can have better quality during or after the pandemic. We wish the pandemic is temporary. Especially for those who will build the future in schools, we should determine our priorities well for our children. We know that poor air quality negatively affects student performance (EPA – Student Healt and Academic Performance). In order to consume less energy in schools, the ventilation system is not operated at a sufficient level or in some countries, ventilation systems are not even available. Preventing this will start with gaining awareness. If you want to see this kind of analysis especially for your individual neighborhoods, send us the photos of your room and the documents of your machines to our email address. Let’s examine the results together!