What should schools do to prevent the spread of SARS-CoV-2 in classrooms – Part 2

In a previous post, I mentioned how important is to monitor the environmental conditions inside a classroom in order to minimize the spread of the virus. Temperature, relative humidity, particulate matter, and carbon dioxide values can serve as indicators, and thanks to them we can have an estimation of the possible propagation of the virus in a classroom.

It seems to me that most people and governments are convinced that the virus is in the air (finally), and although masks work (in most cases), we all know that it is difficult to demand from kids to wear them 8 hours a day without touching their face or each other. Definitely, the virus will affect the psychology of the kids in the classrooms, and most importantly their social skills.

In Madrid, Spain, the authorities have decided to install 6000 cameras in schools. I am totally against this decision. Are they going to fine a kid when he/she touches his/her face? Who is going to watch the footage from 6000 cameras in real-time to determine that a breach of the protocol has occurred? A complete waste of money as later they will have to remove them because of privacy concerns, mark my word on that.

We have to realize that is important to give “some freedom” to kids for their mental sake and for them to grow. Technologically, we can achieve that by offering them the best air possible. In my previous post, a colleague of mine told me that schools in Spain and schools in the UK are not the same because the weather conditions are not the same, and he is right. In south Spain, schools don’t invest in heating, and they could rely on window ventilation, but in the UK (and north Spain) because temperatures drop low earlier, schools need to invest in mechanical ventilation that will recover heat as well.

Either way and although I love an open window for fresh air, I recognize the need for mechanical ventilation systems that will introduce fresh and clean air to a classroom keeping particulate matter and CO2 low and at the same time temperature and relative humility at optimal levels for kids to study and teachers to work.

Hypothetical simulation: Fresh air comes from the back window and stale air exits from the front window, an asymptomatic student is sitting on the front row

The SARS-CoV-2 virus travels inside the tiny droplets we exhale while speaking, sneezing, or coughing. Those droplets aka aerosols have different sizes and can travel from a few centimeters to a few meters far. Most importantly, they can float and be suspended on the air for various minutes depending on their mass, increasing the chances of infection. The conditions inside a room play an important role.

Number and size distribution of the droplets exhaled by talking, sneezing, and coughing

Keep in mind, most of the time, we can’t see the exhaled aerosols below 50μm in diameter.

Currently, there is a lot of debate on which technology should be adopted by schools, medical centers, airports, etc in the area of the ventilation and air treatment as some of them offer some drawbacks.

You see, some of these technologies like pure UVC lamps, ion generators, or similar unregulated photocatalytic oxidation (PCO) technologies may produce a large concentration of unwanted gases like Ozone (O3), which is an irritant for the respiratory tract. CO2 is another by-product that may occur during the oxidation process. Many manufactures (but not all) regulate the concentrations of ozone their purifiers produce to safe levels.

Personally, I have a few important criteria to consider when it comes to which air purification/ventilation system a school should invest. First is the price, public schools can’t afford to install expensive HVAC systems. They need to reply on affordable solutions, and most likely systems that don’t require a lot of hassle to install and maintain (old building, poor infrastructure, no staff), the same applies to some private schools.

Secondly, the performance of the system (air volume m3/h) and the energy it uses (watts/h) must be optimal to keep the energy bill down and get the most out of it in each classroom.

Depending on how air ventilators/purifiers are designed and move the airflow in a room it may increase the chances of spreading the virus before removing it from the room. Simulations have demonstrated that downflow systems are more efficient than overhead systems. In simple words, we need to suck the air from the lower level in a room and introduce fresh and clean air from the top.

Hypothetical simulation: Fresh and clean air is introduced from the ceiling and the stale air is discarded from the vents below

There are so many options right now for someone to choose from. Standalone air purifiers, window ventilators with filtration systems, light fixtures that purify the air while they illuminate the space, central HVAC with quantum plasma that kills 99.9% of the viruses and up to 1000 m3 /hour C.A.D.R, or even special designed devices that can capture all type of particles in outdoor environments like a playground. I can’t tell you which one is the best because it depends on various parameters like the available budget, the infrastructure of the building, location, the size of the classrooms, and the number of students.

An expert is very important during the decision making to plan wisely and deliver the best air for kids. There are a lot of regulations regarding the air ventilation standards in buildings, and each country has its own. For example, in Spain, the UNE-EN 13779 states that a classroom of 45 m² with a height from floor to ceiling of 2.5 meters, occupied by 25 students and a teacher in primary school should renew the air 10.4 times in an hour.

  • 45 m³/h per person (IDA 2) x 26 people = 1,170 m³/h.
  • Classroom volume: 45 m² x 2.5 m = 112.5 m³.
  • Number of air changes in a classroom: (1,170 m³/h) / 112.5 m³ = 10.4 air changes in an hour

The same regulation states that the CO2 concentrations inside a classroom shouldn’t be above 500 ppm. Here come the real-time, low-cost indoor monitors that can measure constant fluctuations and warn teachers about the air quality in a room.


It will be very naive of us to think that only alcohol and masks (which not all of them are equally made) will protect our kids during the course of a day in a classroom. Most importantly, we have to think about their mental health too.

Below you will find a list of companies that have developed various systems for air ventilation and air treatment and each of them offer a different technology and experience.

Many thanks to all for the information you shared with me. Special kudos to caloryfrio.com

Risk of coronavirus transmission in different settings

2 thoughts on “What should schools do to prevent the spread of SARS-CoV-2 in classrooms – Part 2

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