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

Series of Innovators: SPEC Sensors


SPEC Sensors was founded in 2012 in the USA (California) by Dr. Joseph Stetter and Ed Stetter. Their gas sensors, analog and digital, are one of a kind as the technology and hard work has allowed them to minimize the sensor size to only 20x20x3 mm in order for gas sensing to become a part of our everyday lives. The company has foreseen that sensors of all types are going to be integrated into smartphones and other wearable devices, becoming an integral part of our everyday lives and building the Internet of Things (IoT).

IoT low-cost sensors are in my DNA as I believe they will allow us to understand environmental issues otherwise, we would have dismissed or we would not be aware.

Let’s read together what they have to share with us and reach a conclusion at the end.


  • Tell us a bit about your motivation in developing air quality sensors?

We saw the need and opportunity for low cost electrochemical sensors with good performance – before SPEC, high performance sensors were too expensive and too big to enable new applications in wearables and distributed monitors. SPEC was born out of the research driven by Eco Sensors / KWJ Engineering, Inc. led by Dr. Joseph Stetter.  We were able to build SPEC Sensors using modern technology in a new high performance plastic package.Read More »

When is the best time to Exercise Outdoors in England?

Based on 2019 data I analyzed the concentration of Nitrogen Dioxide (NO2) and Particulate Matter (PM2.5), when available, as they are two of the most common pollutants in urban environments due to vehicle traffic and household heating.

In a recent study, scientists selected 140,072 adults and after following them for an average of 2 years, they found that physical activity, even in high PM2.5 levels, is an important high blood pressure (hypertension) prevention strategy as the markers remained stable.

This is a great piece of news. However, I prefer to avoid pollution in the first place and not playing the constant game of good and bad. In our lives, many other factors contribute to health issues, and by eliminating air pollution you make sure that the eternal battle of survivor leans towards your favor.

Let’s take some random locations from across England and see which day and time of the week is the best to exercise outdoors. All data are from the official monitoring network (AURN).


I randomly chose to analyze data from the London Marylebone Road (MY1), which is an urban traffic station.

London Marylebone Road (MY1)

Based on the NO2 data, from Tuesday to Friday I don’t recommend anyone to run near that area as the highest concentrations are registered those days especially between 07:00 to 21:00 throughout the year. Remember the annual safe mean according to WHO is set to 40 μg/m3. Weekends and late nights between 00:00 to 06:00 in the morning seem to be adequate to exercise as traffic seems to calm down a lot. January, February, March, and June are the worst months too.Read More »

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

Masks provided decent protection against the SARS-CoV-2 but if a carrier of the virus (40% of them are asymptotic) stays inside a home/business for a long time then the chances of transmitting the virus are higher because some particles may escape.

According to experts the best way to prevent the spread of the virus is to keep introducing fresh air inside a room.

Unfortunately, most rooms don’t meet the recommendation of 8 air changes an hour for a 10 square meter (107 square foot) room occupied by 4 people. A clear example is a classroom that doesn’t meet the recommended ventilation rates. This is a common mistake across all schools in the world.

Read More »

Building an IoT Air Quality Monitor that will survive the competition

Building a hardware startup is a hard business, we are well aware of that. Right now, there are a lot of indoor/outdoor Air Quality Monitors in the market that support low-cost sensors, unfortunately, even when the cost of the materials (aka BOM) is low, companies struggle to survive because of an initial bad business model.

If you want to enter into the game of the air quality or in general of the IoT, you have to ask yourself: How do I turn what I do into a compelling service?

When you sell a product you earn some money, typically three times more of what you have spent. However, your income relies only on that model and the problem with technology is that it evolves constantly, so tomorrow a new company will sell a product a bit better than yours because there will be a new sensor in the market and all future clients will shift their attention towards that device. Then you have to wait for a year or more in order to realize that you have to shut down your product and taking down with you a few hundred to thousand clients that have bought your AQM — More e-waste and frustration for the end-users.

For this reason, you have to invest in software development as well, and here comes my expertise. I have seen a lot of Air Quality Monitors, and I have collaborated with a fair number of companies in the field, but I always see the same mistake.

Some companies believe that they have to give customers free data support and services for life and if they turn their product into a paid service and they ask money based on a subscription model then they will lose costumers, but this is not necessarily correct. You are not going to ask someone to subscribe to your service to get just numeric values of a pollutant, you have to offer them something more than that. Something the competitors can’t offer because this feature is unique in your ecosystem. Maybe it could be restricted with a patent too.

Consumers really value good products especially when they can’t find something similar.

You are going to ask me, “What can I offer them?” You have two options, stay tuned for future articles or contact me in order to discuss how we can collaborate.


Review: BRISE Multi-Shield Mask

Another great mask to wear against air pollution and the COVID-19 disease. BRISE the company behind the air purifiers just released the BRISE Multi-Shield Mask and they come in three different colours, Dark Black, Peach Red, and Sky Blue.

BRISE Multi-Shield Masks have some key characteristics that separate them from the competition. A certified filter against PM2.5 pollutants, which can block oily and non-oily pollutants, keep in mind that the most common N95 masks are not resistant to oil). Double water-resistant washable layer that can withstand up to 200 times of hand-washing by maintaining excellent filtration efficiency and long-term effectiveness. Finally, the organic cotton inner layer offers a gentle and comfortable touch to the skin.

The mask offers 3 layers of protection.Read More »