Indoor Air Quality in Schools

Helping students breathe clean, healthy and virus-free air

Following Covid19, the importance of maintaining clean and healthy indoor air quality can no longer be ignored, especially when school facilities are opening, and children of all ages are gearing up to return to the classroom. The question is no longer whether to ventilate, but how and what is the best way to do so.

Marko Petkovski, R&D Manager, Systemair, elaborates on the impact of different ventilation solutions in such critical spaces.

A shift in how society views ventilation

In the few months it took for Sars-Cov-2 to spread across the world, the science behind effective mitigation and protection from the virus became questionable, due to a lack of unified and straightforward communication from scientists and international healthcare bodies.

Social media, a leading source of information for a large part of the population, aggravated the issue of misinformation as even trusted bodies have issued contradictory statements, with shifting views and recommendations.

The connected nature of the modern world allowed the disease to spread faster than any virus in history. The resulting global lockdown had a significant economic, social and psychological impact as the virus turned society‘s daily life upside down.

All that opened a strong dialogue on ventilation. What was considered high-quality ventilation before Covid may not be regarded as sufficient in a time when there is a need for the highest level of indoor air quality (IAQ). Building owners and facility managers are eager for solutions to upgrade their ventilation systems without a need for significant investment.

Given the airborne nature of the virus, changes in ventilation norms are imperative to minimise the risk of infection as the world tries to resume normal activities and learn to live with Covid. This is especially critical for schools, as their doors open to receive children.

The solutions were found in decreasing or fully negating internal air leakage. If the whole system was designed with air mixing due to energy savings or maintaining indoor humidity, several HEPA or virucidal filter options came to the market. These filters lessen the chance of virus transmissions throughout the ventilation system. There are other options for removing viruses from airstream but, they were quickly found costly, especially in terms of the initial investment.

Why good indoor air quality is critical in schools

For schools, the first and most crucial step is to gauge indoor air quality in a classroom. Depending on the presence of people, their activity level, and the resulting emissions of bio effluents within a given space, CO2 is highly considered a reliable indicator of indoor air quality. Several studies have been conducted to find a correlation between CO2 concentration and cognitive student performance. More than a few have found that within one hour, CO2 concentration in a classroom can significantly affect students‘ concentration and decision-making performance, as well as cause dizziness and fatigue.

There are several types of particles in the air to which we as people react differently. When it comes to particles causing foul odours, it is easy to diffuse the concentration by simply opening the windows or the door to let fresh air in. This is a quick and easy solution to address odour as our own receptors can indicate when the odour is present. Other particles can be more problematic. Airborne viruses and bacteria, for example, can easily pass between people with no tangible signs until symptoms occur, putting students at greater risk. This is because, in the absence of tangible negative effects, people are less likely to open windows.

Also, during the cold season, many people opt to keep the windows and doors closed, preferring to breathe in stale air rather than risk cold drafts that may also cause colds.

A study took place in 150 classrooms in Graubünden, Switzerland. The study looked at the correlation between poor air quality and its influence on the number of Covid19 infections. While the study is still ongoing and cannot be generalised in short time frame, first results have shown that significantly more individuals were infected in poorly ventilated classrooms.

Mind your difference: Comparing the impact of no ventilation, natural and mechanical ventilation

To demonstrate the impact of different ventilation solutions in schools, a simple C02 calculation analysis has been done for a classroom with 165 m3. There are 8 classes per day, lasting 45 minutes each with a 5-minute break in between. Class has 30 students including teacher. Information about CO2 levels in human’s breath was taken from VDI standards.

Calculation simulates the impact of having ‚no ventilation’ and shows what happens with CO2 levels if students are kept inside the classroom for the whole day with windows kept closed. CO2 levels will rise consistently for about 2700 ppm each school hour. After just an hour, concentration of CO2 will be high enough to cause sleepiness among students. After two hours, there will be a high drop in students’ concentration and self-initiative.

Calculation simulates ‚natural ventilation’ and shows what happens if windows are open during each 5-minute break. The draft must be strong enough to achieve full air exchange within these 5 minutes. In this case, ppm levels will be averaging bellow 2000 ppm, which is already critical. Taking this into account, windows should be opened once more during school hour which would result in average ppm levels around 1000.

Calculation simulates the impact of ‚mechanical ventilation’ with the CO2 sensor set to 1000 ppm. Sensors adjust the unit’s operation depending on actual levels inside the room. (930 m3/h in our calculation). The unit can work at minimum operational cost while supplying students with enough fresh air to provide optimal indoor air quality. The draft will not be felt with careful design, while room conditions can be kept at optimal levels.

Using CO2 as an indicator and benchmark, it is clear that having no ventilation should no longer be an option for educational facilities. Although natural ventilation can keep CO2 levels relatively low, it still allows the infectious particles to stay inside for the whole hour before it is ventilated out. Should there be no possibility of installing mechanical ventilation, natural ventilation is still an option, should the school staff follow the guidelines.

Centralised vs decentralised mechanical ventilation systems

Most newly built schools are already expected to be designed and built with mechanical ventilation that follows the regulations and guidelines. The standards in place define optimal temperature, humidity and IAQ levels to ensure children have the best environment for learning. It is included in the planning stages, considering architecture and mechanical systems so that investment and operational price can be optimised.


This requires an understanding of common mechanical ventilation solutions in the market. The most relevant ones can be divided between centralised and decentralised mechanical ventilation systems. A centralised ventilation system refers to one stand-alone air handling unit, supplying air to several different classrooms. Decentralised ventilation systems refer to having one ventilation unit per classroom.

Most new schools will decide on centralised ventilation as it has lower investment costs. Air is distributed through ducts inside the school and regulated with a system of automatic dumpers or volume controllers. Ducts are mostly hidden into the false ceiling or with other »architectural tricks«.

A decentralised ventilation unit

Both options are valid and allow constant air exchange inside the classroom, removing the necessity of opening windows between breaks or during class time. Should a reader wish for more knowledge around centralised and decentralised ventilation, informative document has been published by another Systemair professional.

At the end of the day, it is important to remember that ensuring the best IAQ is choosing a solution that meets all critical criteria.

A centralised ventilation unit

Schools located in older buildings, however, are more problematic. In such situations, centralised ventilation would not be an ideal solution as classrooms usually have lower ceilings and investment in the building reconstruction and installation of duct systems would be too high. For such retrofit and refurbishment school projects, in most cases, it is more cost-effective to consider decentralised ventilation systems, where each classroom has its ventilation unit. The price of investment into units is higher, but the installation is easier as they only require two openings over the outdoor wall and electrical (sometimes water) connection for running the unit and reheating the air to room temperature.