Measuring indoor air quality makes it possible to transform a subjective impression—stuffy air, humidity, discomfort—into concrete, actionable data.
In an office, school, daycare center, or any other occupied building, air quality changes throughout the day. It depends, in particular, on the number of occupants, air exchange, ventilation operation, the materials present, and the activities taking place on the premises.
What indicators should be monitored? How should CO₂ concentrations be interpreted? What is the difference between a one-time monitoring campaign and continuous monitoring? Here is the key information you need to know to implement effective indoor air quality monitoring.
What is indoor air quality?
Indoor air quality, often referred to by the acronym IAQ, refers to the quality of the air breathed inside buildings.
It can be influenced by several categories of pollutants or environmental factors:
- volatile organic compounds, or VOCs, emitted in particular by certain materials, paints, adhesives, or cleaning products
- fine particles
- carbon monoxide
- formaldehyde and benzene
- Radon in the affected areas
- biological pollutants such as mold or certain allergens
- the concentration of carbon dioxide
- temperature and relative humidity.
CO₂, temperature, and humidity alone are therefore not sufficient to identify all the pollutants present in a building. Monitoring these parameters, however, provides essential information on the airtightness of the premises, occupant comfort, and the operation of ventilation systems.
Why measure indoor air quality?
Measuring indoor air quality serves several purposes.
Check the air exchange rate
In an occupied room, CO₂ concentrations rise primarily as a result of human respiration. A high concentration may indicate that air exchange is insufficient relative to the number of occupants present.
CO₂ is therefore a particularly useful indicator for checking that the ventilation system is working properly and for identifying rooms that are regularly unventilated.
Improving occupant comfort
Temperatures that are too high, air that is too dry, or excessive humidity can reduce the level of comfort inside a building.
Simultaneous monitoring of CO₂, temperature, and humidity provides a more comprehensive view of indoor conditions and helps identify problematic periods or areas.
Identify HVAC system malfunctions
A CO₂ concentration that remains high despite the ventilation system being operational may indicate:
- insufficient fresh air flow
- inappropriate programming
- clogged or malfunctioning equipment
- a higher occupancy rate than initially assumed
- poor air distribution throughout the premises.
Continuous monitoring then helps the operator identify the cause of the problem rather than simply noting a one-time occurrence.
Manage the building more precisely
When measurement data is fed into a building management system, it can be used to trigger alerts, compare different zones, or adjust the operation of HVAC equipment.
Ventilation that is controlled based on actual occupancy conditions can also limit over-ventilation when the spaces are sparsely occupied. The goal is to balance air quality, comfort, and energy efficiency.
Which metrics should be measured?
The choice of parameters depends on the type of building, its use, and the problem being addressed.
CO₂
Expressed in parts per million (ppm), the CO₂ concentration primarily indicates the level of air exchange in an occupied space.
It allows you to verify whether the air exchange rate is appropriate for the actual occupancy. However, CO₂ should not be presented as a comprehensive measure of indoor air pollution: an appropriate concentration does not rule out the presence of particulate matter, VOCs, or other pollutants.
The temperature
Temperature directly affects occupants' thermal comfort. Monitoring it also makes it possible to analyze the operation of the heating and air conditioning systems, detect discrepancies between zones, and verify compliance with setpoints.
Relative humidity
Excessive humidity can lead to condensation and mold growth. Conversely, very dry air can make you feel uncomfortable.
Moisture measurements must be interpreted in conjunction with temperature and the building's characteristics.
VOCs, particulate matter, and specific pollutants
Depending on the context, additional measures may be necessary:
- PM2.5 and PM10 particles
- volatile organic compounds
- formaldehyde
- benzene
- carbon monoxide
- radon.
These pollutants require appropriate measurement technologies and protocols. A monitoring campaign conducted by a professional or an accredited organization may be essential when it is necessary to pinpoint a source of pollution or comply with a regulatory requirement.
How do you measure indoor air quality?
There are two approaches that can be used. They are complementary and do not address exactly the same need.
The one-time measurement campaign
A one-time campaign involves measuring certain pollutants over a specific period. It may include sampling, on-site measurements, and laboratory analyses.
This method is suitable for:
- make an initial diagnosis
- search for a specific pollutant
- check on the situation after construction work
- meet certain regulatory requirements
- determine the source of suspected pollution.
However, the result is still representative of the conditions observed during the campaign. A single measurement may not detect events that occur at other times of the day or year.
Continuous measurement with connected sensors
Continuous monitoring involves installing sensors in representative areas of the building. The readings are recorded at regular intervals and can be transmitted to a monitoring system or a web BMS.
This approach makes it possible to:
- view changes throughout the day
- identify recurring overspending
- compare rooms or buildings
- assess the impact of land use on CO₂
- detect a malfunction in the ventilation system
- measure the effects of a corrective action
- Keep a record of indoor conditions.
For commercial buildings, schools, and Public Access Building, continuous monitoring is particularly useful for moving from a simple assessment to operational management.
How do you properly install an air quality sensor?
The reliability of the measurement depends as much on the sensor as it does on its location.
The sensor must be installed in an area representative of the air breathed by the occupants. In particular, the following should be avoided:
- right next to a window or door
- an air inlet or outlet
- a radiator or another heat source
- direct exposure to the sun
- an area where there is little air circulation
- installation too close to an occupant's face.
In a building with multiple zones, it is best to equip spaces that differ in occupancy or use: meeting rooms, offices, classrooms, dining areas, or high-occupancy zones.
To measure CO₂, it is recommended to use a sensor that employs NDIR technology, which stands for non-dispersive infrared optical measurement.
How do you interpret a CO₂ measurement?
Thresholds must always be interpreted in light of the building, its use, and the applicable standards.
As part of the regulatory monitoring of air quality in certain “ Public Access Building ” in France, Cerema presents, in particular, two action thresholds:
| CO₂ concentration | Interpretation within This Regulatory Framework |
|---|---|
| Less than 800 ppm | Air exchange rate considered satisfactory |
| Between 800 and 1,500 ppm | Situations to Monitor and Ventilation Measures to Consider |
| Greater than 1,500 ppm | Air exchange rate deemed insufficient |
These values serve as guidelines for evaluating containment measures in the relevant facilities. They should not be considered universal health standards applicable to all buildings.
A single measurement is also less informative than a complete curve. The historical data makes it possible to determine the duration of the exceedances, their frequency, and their correlation with the occupancy of the premises.
The Cerema guide on air quality monitoring provides details on the use of these thresholds in the relevant “ Public Access Building .”
What regulations govern indoor air quality?
Indoor Air Quality Monitoring in Selected Public Access Building s in France
Since the revision of the regulations took effect on January 1, 2023, certain public-access facilities are required to implement regulatory monitoring of indoor air quality.
The measure applies in particular to facilities that care for young children, after-school programs, educational institutions, and certain social, medical-social, and health care facilities.
This monitoring includes, in particular:
- an annual assessment of ventilation systems, including a direct-reading CO₂ measurement
- an indoor air quality self-assessment at least once every four years
- a campaign to measure regulated pollutants after certain key stages in the building's life cycle
- the development and updating of an action plan.
Regulatory campaigns that require sampling and analysis must be conducted by accredited organizations. A connected sensor designed for the operational monitoring of a building does not, therefore, automatically replace these regulatory services.
Details of the program are available in Decree No. 2022-1689 on indoor air quality monitoring.
The European EPBD Directive
European Directive 2024/1275 on the energy performance of buildings, also known as the EPBD, expands the concept of building performance to include indoor environmental quality.
In particular, it reinforces the role of monitoring, control, and automation systems in buildings. Temperature, humidity, ventilation, and indoor air quality thus become key data points for simultaneously optimizing energy efficiency and occupant comfort.
The EPBD does not define a universal CO₂ threshold applicable to all buildings. Its specific provisions depend on the national measures adopted for its implementation. The European Commission’s guidelines on the EPBD nevertheless confirm the growing role of continuous monitoring and smart control of technical systems.
How can air quality monitoring be integrated into a " BMS "?
For a measure to be truly useful, it must lead to action.
Integrating data into a web BMS , among other things, allows you to:
- View CO₂ concentrations by area
- generate an alert when a threshold is exceeded
- Compare the values with the occupancy periods
- Check the ventilation system's operating hours
- Identify facilities with recurring exceedances
- adjust the settings or flow rates when the system allows it
- Keep a record of the situation before and after a procedure.
In an existing building, the installation of wireless sensors generally eliminates the need to add communication wiring to each room. This approach makes it easier to install monitoring systems in a building as part of a renovation or expansion project BMS.
Discover Enless's solutions for air quality monitoring.
Measuring CO₂, Temperature, and Humidity with Enless
The Enless CO₂, temperature, and humidity sensor allows for continuous monitoring of a building’s key environmental parameters.
Its NDIR sensor measures CO₂ concentration over a range of 0 to 5,000 ppm. The sensor also transmits temperature and humidity readings.
It can operate on LoRaWAN or using the LoRa Easy Enless protocol. Data can thus be transmitted to the BMS through a reception architecture tailored to the building.
The sensor also features:
- a light indicator to inform the occupants
- configurable activation thresholds
- a replaceable battery power supply
- an optional USB-C power supply
- a tear detection system
- with a battery life of up to five years, depending on usage patterns.
This solution is particularly well-suited for offices, schools, daycare centers, hotels, museums, and other buildings where installing additional cables would be complicated or costly.
What should you do when air quality deteriorates?
The measure does not directly improve air quality; rather, it helps identify situations that require action.
Depending on the cause identified, various actions may be considered:
- Ventilate the premises from time to time
- inspect and maintain ventilation equipment
- check the fresh air flow rates
- adjust the operating hours of the ventilation system
- reduce or eliminate a source of indoor pollution
- revise the layout or use of a room
- adjust the HVAC system controls
- conduct a supplementary assessment when a specific pollutant is suspected.
The effectiveness of these actions can then be verified by comparing the data recorded before and after their implementation.
Frequently Asked Questions About Indoor Air Quality Measurement
Which technology should you choose for measuring CO₂?
NDIR technology is generally preferred. It is based on the infrared absorption of CO₂ and allows for selective measurement of its concentration.
Why connect air quality sensors to the BMS ?
Connecting to the BMS allows you to centralize data, generate alerts, analyze historical data, and—when the system allows it—adjust the ventilation system’s operation to the building’s actual conditions.
Implement an air quality monitoring system
An effective strategy relies on three elements: properly positioned sensors, reliable data transmission, and defined actions to be taken when thresholds are exceeded.
Enless wireless solutions make it possible to add CO₂, temperature, and humidity monitoring in new or existing buildings and then transmit this data to BMS.
Discover the Enless CO₂, temperature, and humidity sensor, or contact our team to determine the right system for your building.
