In a classroom, 20, 25, or 30 students may spend several hours together in an enclosed space. Under these conditions, air quality can change rapidly throughout the day.
In recent years, CO₂ monitoring has become widespread in schools. It serves as a valuable indicator for assessing air exchange in an occupied room.
But does a satisfactory CO₂ level necessarily mean that the air in a classroom is of good quality?
Not always. Fine particulate matter, volatile organic compounds (VOCs), formaldehyde, pollen, allergens, and respirable aerosols may also be present in indoor air without being detected by a simple CO₂ sensor.
Understanding these various issues makes it possible to implement the most appropriate measures: limiting sources of pollution, ensuring effective air circulation and ventilation, and, when warranted, supplementing these measures with appropriate air treatment.
Why is air quality a particular concern in schools?
After home, school is one of the main places where children spend their time. The high occupancy rates in classrooms and the amount of time spent in enclosed spaces make the issue of air exchange and air quality particularly important.
There are many factors that can affect indoor air quality: occupants, furniture, building and finishing materials, cleaning products, activities carried out on the premises, as well as the outdoor environment.
And classrooms aren’t the only spaces affected. Cafeterias and dining halls, library/study centers, activity rooms, administrative offices, and gyms each have different usage patterns and potential sources of pollution.
Air quality is therefore an important issue for students, teachers, and all staff members working at the school.
What pollutants can be found in the air inside a school?
Discussing indoor air quality is not simply a matter of monitoring a single pollutant. Several categories of pollutants or indicators may be encountered.
CO₂: An Indicator of Air Exchange
Carbon dioxide (CO₂) is naturally exhaled during respiration. In an occupied room, its concentration increases when the air exchange rate is insufficient relative to the number of people present.
Measuring this parameter therefore makes it possible to identify a situation where the space is stuffy and to determine when it is necessary to ventilate more or to check that the ventilation system is working properly.
CO₂ also plays an important role in the regulatory framework for monitoring indoor air quality in schools.
Fine Particles
PM10, PM2.5, and even finer particles can come from indoor sources as well as from the outdoor environment.
Traffic, heating, construction, or episodes of air pollution can, in particular, affect concentrations near a facility and contribute to the transfer of particulate matter into buildings.
Indoors, certain activities and the resuspension of dust can also contribute to the measured concentrations.
VOCs and Formaldehyde
Volatile organic compounds (VOCs) include many substances that can be emitted by building and finishing materials, furniture, paints, adhesives, and certain cleaning products.
Among these, formaldehyde is one of the pollutants that are closely monitored in indoor environments.
Reducing emissions at the source, choosing appropriate materials and products, and ensuring adequate air exchange are the first steps to take.
Pollen and Allergens
Outdoor air is also a potential source of indoor pollution. Pollen and certain allergenic particles can enter buildings, particularly when the space is ventilated or through ventilation systems.
Their presence can become particularly problematic during periods of high pollen counts.
Respiratory Aerosols
Breathing, speaking, coughing, or sneezing produces respiratory particles and aerosols that can remain suspended in the air for varying lengths of time.
This phenomenon is not limited to COVID-19; more broadly, it is an issue during the seasonal circulation of respiratory viruses.
Air exchange helps dilute the particles and aerosols present in a room. In certain situations, appropriate filtration can complement this process.
Does a good CO₂ level mean that the air in a classroom is of good quality?
Not necessarily.
CO₂ is primarily an indicator of air exchange based on room occupancy. Measuring it is therefore particularly useful for determining whether a room needs more air circulation or ventilation.
However, a sensor that measures only CO₂ does not provide information on concentrations of fine particulate matter, VOCs, formaldehyde, or other air pollutants.
In other words:
CO₂ helps answer the question: “Are we ventilating the room enough?”
On its own, it does not provide an answer to the question: “What is in the air we breathe?”
That is why a comprehensive approach to air quality may require taking into account several factors, depending on the building's characteristics, its surroundings, and the activities carried out there.
Outdoor pollution, pollen, epidemics: when ventilating isn't enough to address all the challenges
Air out and ventilate are essential steps for renewing indoor air, removing CO₂, and reducing the concentration of many indoor pollutants.
However, certain situations may require a complementary approach.
In the event of outdoor pollution
Outdoor air is not always free of pollutants. Near a major road, during construction, or during certain episodes of air pollution, particles can enter the building.
The challenge, then, is to balance the need for fresh air with the need to limit exposure to outdoor pollutants.
During pollen season
Opening windows refreshes the air but can also allow pollen to enter when outdoor concentrations are high.
Once again, this situation illustrates why air quality and air exchange should not be confused.
During winter outbreaks
Air out and ventilate a room to refresh the air and dilute respiratory aerosols present in the room.
When air exchange alone is not sufficient to achieve the desired result, or in certain areas with particularly high occupancy, air filtration can serve as a supplementary measure.
Air out, ventilate, treat the air: complementary actions
There is no single solution that can address all air quality issues.
The answer depends primarily on the pollutant or the situation at hand.
|
Observed situation |
Priority Action |
Possible follow-up action |
|
High CO₂ levels |
Ventilate / Improve ventilation |
Identify the causes of insufficient renewal |
|
VOCs or formaldehyde |
Reduce sources + ventilate |
Treatment tailored to the situation |
|
Fine Particles |
Identify and limit sources |
Particulate Filtration |
|
Pollen / Particulate Allergens |
Limit Exposure |
Particulate Filtration |
|
Respiratory Aerosols |
Air out / ventilate |
Additional HEPA Filtration |
An air purifier does not reduce CO₂ levels. Therefore, it is not a substitute for either air circulation or ventilation.
On the other hand, when it is properly sized and equipped with an appropriate filtration system, it can remove certain airborne pollutants.
What role does an air purifier play in a school?
An air purifier works by circulating the air in the room through one or more filtration systems before returning it to the room.
HEPA filtration is particularly effective at trapping particles present in the air that passes through the filter. It can thus help reduce the concentration of fine particles, certain particulate allergens, and airborne aerosols.
However, its use should be viewed as a supplement to measures aimed at reducing sources, aeration, and ventilation, and not as a substitute for them.
In a school setting, several criteria are important when choosing a solution:
- the nature of the targeted pollutants;
- the volume of the room;
- the required airflow;
- noise levels, which are particularly important in a classroom;
- the performance and safety of the technology used;
- maintenance and tracking of supplies.
Proper sizing is essential: installing a unit without taking into account the room’s volume and the intended purpose will not ensure adequate filtration.
The EOLIS Air Manager professional air purifiers, designed by NatéoSanté, feature H13 HEPA filtration combined with various stages of air treatment and can be sized to meet the specific requirements of the spaces they are intended for.
Measure to understand before taking action
When the cause of an air quality problem is unknown, measuring CO₂ alone may provide only a partial picture of the situation.
A more comprehensive analysis can track several parameters simultaneously: CO₂, PM1, PM2.5, and PM10 particles, VOCs, formaldehyde, temperature, and humidity.
Observing how they change over several days, under the actual conditions in which the facilities are used, makes it possible, in particular, to identify:
- lockdown periods;
- the impact of room occupancy;
- particle variations;
- certain one-time broadcasts;
- the effect of aeration or changes in practices.
This approach makes it possible to move beyond mere impressions to objective data and to define the actions to be taken more precisely.
Would you like to know what students and staff are really thinking?
NatéoSanté offers an indoor air quality assessment that measures various parameters under your facility’s actual operating conditions and then analyzes the results to identify appropriate areas for improvement.
→ Request an air quality assessment
A Comprehensive Approach to Indoor Air Quality in Schools
Improving air quality in an elementary school, middle school, or high school is therefore not a matter of choosing between a CO₂ sensor, opening windows, or an air purifier.
The first step is to understand the sources and pollutants present, and then to take the appropriate actions: reduce emissions at the source, ventilate the space, monitor relevant parameters, and, when necessary, treat the air.
This approach makes it possible to tailor actions to the specific characteristics of each institution rather than applying a one-size-fits-all solution to every situation.
Frequently Asked Questions About Air Quality in Schools
Can CO₂ be used to measure the air quality in a classroom?
CO₂ is primarily used as an indicator of air exchange in an occupied room. A high concentration may indicate that air exchange or ventilation is insufficient given the number of occupants.
However, this measurement does not provide information on the concentration of other pollutants that may be present, such as fine particulate matter, VOCs, or formaldehyde.
What are the main pollutants found in a school?
Depending on the building, its surroundings, and the activities carried out there, indoor air may contain fine particles, VOCs, formaldehyde, pollen and allergens, and biological contaminants, among other things.
CO₂ is particularly useful as an indicator of air stagnation and air exchange.
Does an air purifier reduce CO₂ levels?
No. An air purifier that recirculates and filters the air in a room does not remove the CO₂ produced by its occupants.
To reduce its concentration, you must refresh the air through ventilation.
Is an air purifier useful during the winter flu season?
Appropriate HEPA filtration can help reduce the concentration of airborne particles and aerosols in the air passing through the filtration system.
It can therefore serve as a supplementary measure to air circulation, ventilation, and other preventive measures, although it does not guarantee that transmission will not occur.
Should classrooms be ventilated during a pollution spike?
It is still necessary to ventilate indoor air, particularly to remove CO₂ and pollutants generated indoors. However, certain outdoor pollutants can enter the building.
Ventilation procedures and schedules can therefore be adjusted based on local conditions and outdoor air quality. Monitoring both CO₂ and particulate matter can also help provide a better understanding of the conditions within the facility.
Last updated: September 1, 2026
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