September 25, 2020

the importance of ventilation


Over the past few weeks, the incidence of COVID-19 in Spain. Furthermore, recent scientific evidence shows that minors seem to be infected between a third and a half of adults and that if they are older than 10 years, they are just as contagious as adults. Faced with this situation, families and teachers have increased concern about planning to return to classrooms in the safest possible way.

Back to school in a pandemic: children suffer the consequences of the emotional overflow of COVID-19

Back to school in a pandemic: children suffer the consequences of the emotional overflow of COVID-19

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The issue affects all educational stages, from early childhood education to university. In the case of universities, their representative bodies, in many cases, approved plans during the month of July based mainly on occupational safety protocols for their workers.

The measures set by the communities

To return to classrooms safely, the main thing is that the incidence of the disease is low (for example, less than 25 cases per 100,000 population in the last 14 days).

Currently, all the autonomous communities far exceed these incidence values. Despite this, the authorities have decided to start the course and have agreed unified measures for the entire territory.

Among them are some necessary ones, based on scientific evidence, such as the mandatory use of the mask even if you keep a distance greater than 1.5 m for all students over 6 years of age and for teachers – even more so, because they will speak more frequently-. It is also recommended to wash your hands on a regular basis and ventilate the classrooms on a frequent basis. In addition, it is encouraged to take classes with the windows open.

However, it will also be mandatory to measure body temperature for students and teachers. This measure has been shown of doubtful efficacy, since a significant percentage of those infected, especially if they are minors, may not manifest fever and, in general, the disease is much milder.

In addition, some autonomous communities (Galicia, Catalonia and Andalusia, for example) propose to carry out PCR tests, despite the fact that the test may be negative because, for example, the viral load is low if the infection is recent.

Finally, it is also recommended to reduce the ratios per classroom, the blended presence and follow the isolation protocols once a case is detected.

The aerosol problem

The last evidences scientific show that in addition to being transmitted by drops or fomites (by contact with surfaces where there is an active virus), the virus can be transmitted using aerosols.

More of 200 experts asked to WHO to include aerosols among the main modes of transmission of COVID-19.

The institution included the new route of transmission as secondary, although it seems to be relevant in a healthcare environment or isolated, like a cruise.

Despite the fact that the possibilities of contagion in the courtyard, dining room, changing rooms or school entrance must not be neglected, students spend most of their time in classrooms. That is why it is very important to prevent these from becoming high-risk areas.

Aerosol transmission may be of little relevance in open spaces, unless the safety distance is not respected or there is physical contact. However, in closed spaces such as the classroom, even if the safety distance, this route can be an important part of the transmission of the disease if the space is not well ventilated or the use of masks is limited.

In short, in addition to the mandatory use of masks and the need for them to be well adjusted, the air quality and ventilation, the duration of classes and a low number of students per classroom, not only to reduce transmission but to facilitate traceability, must be factors to take into account when scheduling the next course.

Classes outdoors?

Everyone should wear a mask inside colleges and universities. But how can we lower the concentration of the virus in the air in case someone is infected?

A possible solution to reduce transmission would be to give as many classes as possible outdoors. With a mask, distance and relative silence, outdoors not only airborne transmission would be reduced, but also through contact with surfaces, since solar radiation reduces the time that the virus is active in a surface.

In New York, outdoor teaching was already chosen during successive pandemics of tuberculosis at the beginning of the 20th century and is being promoted from the mayor’s office for the next course. By the way, there are very few areas in Spain where the average temperature in autumn and winter is lower than in New York, so the cold should not be an impediment. In this sense, recently several municipalities, such as that of Barcelona, have offered those responsible for autonomous education facilities and parks.

Measuring air quality in classrooms

If teaching abroad is not feasible, different experts have created a spreadsheet to estimate the probability of transmission of COVID-19 by aerosols based on the local incidence of the epidemic, the number of infected people, the size of the classroom, the occupation, the duration of the class, the percentage of people with a mask and the type of mask, temperature, humidity (the virus remains active less time at low relative humidities) and ventilation and air purification, among other variables.

Regarding ventilation and air purification, we must remember that knowing said quality is not only important as an indicator of virus concentration, but also influences the concentration of students and teachers.

To estimate air quality we can measure CO₂ concentration simply and relatively cheaply. In fact, the Belgian government will facilitate the acquisition of CO₂ meters for all classrooms in the country. CO₂ concentration values ​​should be below 800 ppm. From the CO₂ concentration we can calculate ventilation of the space.

How to improve ventilation

Obviously, the first option is to use the Natural ventilation to reduce the concentration of viruses. It is highly recommended to have the windows open, as you will impose Belgium for the next course.

The ventilation rates of the different teaching spaces range from 0.1-8 ACH (air changes per hour, air changes per hour) for a primary school to between 2 and 12 ACH for a university classroom. The ACH variable expresses the fraction of initial air that remains in the classroom, a value of 1 means that after one hour 36% of the initial air remains.

If natural ventilation is not enough to improve air quality, which is the case in many older buildings, forced ventilation is necessary. Outside air should be used and not recirculated, since the latter seems to be the reason for some of the bulbs in restaurants. In addition, ventilation devices should have filters installed, at least of type MERV-13.

Finally, if the classroom cannot be naturally or mechanically ventilated, it is advisable to install air purifiers HEPA to minimize the risk of transmission, even though they have a high price.

In short, following what is indicated in this Harvard School of Public Health guide To ensure the ventilation of the classrooms, the ideal is to improve the ventilation systems, incorporate air purifiers (or, at least, humidifiers), perform the maximum of outdoor activities and open the doors and windows to, at least, achieve 5 ACH (air changes per hour) in 100 m² classrooms occupied by 25 students.

The foregoing does not exclude the mandatory use of masks, hand hygiene, regular cleaning of surfaces and material, and maintaining a safety distance to minimize the transmission of SARS-CoV-2.

A study recently published estimates that the rate of transmission of COVID-19 in 22 summer camps for children under 12 years of age, with follow-up of cases in groups bubble, constant hand hygiene and activity mainly outdoors (although also with indoor activity) was relatively low. From the initial 30 cases of COVID-19, which had contact with 253 children, only 12 secondary infections occurred. This infection rate is almost six times lower than that of the general population.

This article was originally published on The Conversation. You can read it here.

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