CO₂ vs PM2.5 vs TVOC: How to Read Your Air Quality Monitor

To read an air quality monitor, first identify the parameter and unit, then check the time period and what was happening in the room. CO₂, PM2.5 and TVOC describe different conditions. A normal reading for one does not establish that the others are low. An unusual reading at a particular moment does not necessarily indicate a persistent indoor air quality problem. Readings can be affected by various factors, including ventilation, cooking emissions, perfumes, and other chemical products. To interpret the results, review trends over time rather than relying solely on the current reading.

Air quality monitoring is about identifying causes and taking action—not chasing a perfect number.

Start with the label and unit

Read CO₂ alongside occupancy

Indoor CO₂ readings are primarily used to understand how occupancy and ventilation affect indoor air. CO₂ trends are easier to interpret when you also record how the room is used, how many people are present, and when ventilation conditions change.

A rise during a crowded meeting followed by a fall after occupants leave can help you understand the room’s behaviour. However, CO₂ is not a complete measure of ventilation effectiveness or overall indoor air quality. CDC notes that one concentration target may not be appropriate for every space and occupancy.↗

For the same reason, avoid treating 1,000 ppm as a universal boundary between safe and unsafe air. NIST has specifically addressed the limitations of applying that number as a general ventilation or air quality criterion. ↗

Read PM2.5 alongside particle-producing activities

Cooking, smoke, candles and outdoor pollution can influence indoor particle concentrations. ↗ If PM2.5 rises, record the activity and its timing before deciding what caused the change.

A practical example is to observe the room before cooking, during cooking and afterwards while using an externally vented range hood. Keep the monitor in the same appropriate position. The result may help you compare occasions, but it is not a controlled performance test of the hood.

If comparing a reading with a published guideline, check the required averaging period. A brief peak and a 24-hour average do not describe the same exposure.

Read TVOC as a sensor-dependent signal

Consumer VOC sensors do not respond equally to all compounds. Their readings and baseline behaviour depend on the sensor and processing method. ↗

When TVOC changes, ask whether a cleaning product, fragrance or new material was recently introduced. Avoid assuming that every increase has the same cause or health significance.

Compare trends from the same device under similar conditions. Two products may display different numbers because they use different reference gases, algorithms or output definitions.

Do not deliberately expose the sensor to solvents or spray products to test it unless the manufacturer provides a specific verification procedure.

Use an observation and response routine

1. Check the reading. Confirm the parameter, unit and whether it is a current or averaged value.

2. Check the context. Note occupancy, activities, outdoor conditions and recent device movement.

3. Check the setup. Confirm the sensor inlet is clear and the device has completed its specified start-up period.

4. Make a relevant change. For example, stop an avoidable source or adjust ventilation when appropriate for outdoor conditions.

5. Observe the trend. Allow for the device’s response time and record what changed.

Change one condition at a time where practical. This makes the result easier to interpret than moving the device, opening windows and switching equipment simultaneously.

What if a colour indicator disagrees with your expectations?

Look at the underlying values and the manufacturer’s explanation of its score. A single colour can hide which parameter triggered the change, and different brands can use different categories.

If readings remain unusual, check the manual and contact technical support with a record of the conditions. Persistent concerns about a specific pollutant may require a more targeted assessment.

For placement advice, read Where to Place an Air Quality Monitor in Your Home. For the meaning of each measurement, see What Does an Air Quality Monitor Measure?

Frequently asked questions

Why is CO₂ low while PM2.5 is high?

The measurements describe different pollutants. A particle source can be active even when the room’s CO₂ concentration is relatively low.

Does a green display mean the room is safe?

It means the device’s scoring rules place the measured conditions in that category. It does not evaluate every possible indoor hazard.

Should I compare a quick reading with a daily guideline?

Only after matching the averaging period and measurement basis. Use the applicable guideline’s own instructions.