How do you monitor indoor air quality in a commercial building?

Good monitoring is designed around a question. The measurements, the sensor locations and the response procedure are chosen together, once you know what you need to find out. Commercial buildings generally monitor indoor air quality (IAQ) for one of three reasons: to check that ventilation is working as designed, to track particular pollutants such as fine particles, or to investigate complaints. Each needs different measurements and a different response. Sensors installed without a decision about how their readings will be interpreted and acted on can create concern without improving the indoor environment at all.

Five measurements and their limits

Carbon dioxide needs particular care, because it is so often presented as "the" IAQ metric. ASHRAE's position document on indoor carbon dioxide (approved February 2025) states that indoor CO₂ concentrations are not overall indicators of IAQ, although they can be a useful tool when their limitations are understood. CO₂ mainly reflects emissions from occupants and the effectiveness of ventilation; it does not track pollutants from materials, outdoor air or other sources. ASHRAE also stresses that sensor performance, location and calibration are critical if the conclusions are to mean anything. A meeting room with acceptable CO₂ can still have a particle or chemical problem.

The US Environmental Protection Agency similarly notes that, despite advances, there are still gaps in understanding the data quality and performance of air sensors used indoors. Low-cost sensor readings are best treated as indicators that guide investigation, not as compliance measurements.

Designing a monitoring scheme that means something

What the monitoring is for differs by space. Offices, meeting rooms, hotel rooms, restaurants and areas next to car parks have different sources and risks, and for each it helps to write down what the sensor is meant to reveal. Sensor selection follows from that purpose. The manufacturer's stated accuracy, range, response time, drift and calibration method need checking against the intended use; a sensor driving demand-controlled ventilation needs different performance from one that shows occupants a trend.

Location matters as much as the instrument. Positions dominated by a supply diffuser, an open window, direct sun or a local source such as a printer will mislead, unless that source is what you are investigating. Sensors should be mounted at a height representative of the occupied zone, and each location recorded so that later comparisons stay meaningful. Because sensor accuracy degrades over time, calibration or replacement intervals need agreeing at the outset, along with how they are verified and who does the work.

The data also has to be integrated properly. Each reading should carry its space, the ventilation system serving that space, a timestamp and a quality flag, so that missing or stale data can always be told apart from a good reading.

From a reading to a response

Thresholds and rules should be set with the building-services engineer and, where health questions arise, an appropriately qualified indoor-environment or occupational-health specialist, with reference to the ventilation standards and regulations that apply to the building. A dashboard turning red is not a diagnosis.

Each rule also needs a response: who receives the alert and how quickly; what they check first, whether ventilation operation, damper position, filter condition, occupancy or outdoor conditions; when the problem is escalated to the designer or a specialist; how the action and its outcome are recorded; and what, if anything, occupants are told.

If readings are used to adjust ventilation automatically, the controls designer must confirm that minimum ventilation rates and equipment constraints are always respected. More outdoor air is not always the right answer, either. During an outdoor pollution episode, increasing ventilation without adequate filtration can bring particles in.

Showing the data to occupants

Displaying IAQ data to occupants can build confidence, but only if the building team can explain the numbers and answer questions about them. What is shown, how it is explained and what happens when a value looks poor all need deciding in advance. Publishing data you are not prepared to act on erodes trust.

Checking the supplier's claims

• What accuracy and drift do the sensors have for each parameter, and how was that established?
• How are sensors calibrated or replaced, and at what interval?
• How does the system flag stale, missing or implausible values?
• Can the data be exported with its location and timestamp to other systems?
• What does the dashboard claim, and is any summary "score" explained?