Indoor Air Quality Monitoring: What CO₂, TVOC and PM Each Tell You — and How to Measure Them

First, correct the most widespread misconception

Many believe ASHRAE 62.1 caps indoor CO₂ at 1,000 ppm. ASHRAE's Position Document on Indoor Carbon Dioxide says otherwise: Standard 62.1 has never set an indoor CO₂ limit; misread text from earlier editions let the "1,000 ppm limit" circulate for decades. The real origin: 1,000 ppm corresponds to the level of body-odour acceptability at roughly 8 L/s of outdoor air per person — a ventilation-rate indicator, not a health threshold.

The European standard EN 16798-1 is clearer: it grades CO₂ above the outdoor baseline into four categories — Cat I 550 ppm, Cat II 800 ppm, Cat III/IV 1,350 ppm — each mapping to a different per-person outdoor-air expectation. Note these are differentials: outdoor baseline now commonly runs 420–500 ppm and must be factored in; comparing indoor absolute readings against an old table misjudges the result.

GB/T 18883-2022: the whole limit table

The Chinese national standard GB/T 18883-2022, effective February 2023, is the benchmark for indoor air assessment in residential and office buildings. The monitoring-relevant chemical and physical parameters are set out below.

• Construction acceptance is governed separately by GB 50325, whose Class I civil buildings (dwellings, hospitals, schools) carry a stricter formaldehyde limit of 0.07 mg/m³. For project-bound monitoring, confirm first which standard the client is being accepted against.
• GB/T 18883's reference methods are laboratory methods: TVOC by Tenax-tube sorption, thermal desorption and GC; formaldehyde by spectrophotometry. Online sensor readings do not replace a CMA report — they carry the continuous-trend and early-warning role, complementary to periodic lab testing.

The CO₂ layer: why NDIR is the only sound principle

The mainstream CO₂ principle is non-dispersive infrared (NDIR): CO₂ characteristically absorbs 4.26 µm infrared light, and the more there is, the more is absorbed. It is specific to CO₂, free of cross-interference and long-term stable — which is why decades have produced no replacement. Three things to check when specifying: range (0–5,000 ppm is ample for IAQ — avoid personal-safety 0–5% Vol tiers), automatic baseline calibration (ABC, which self-zeroes on periodic exposure to fresh outdoor air — be careful with ABC in rooms that are never opened up), and the temperature-drift spec.

Placement trips people up more than selection: CO₂ is not uniform in a room — occupied zones, ventilation dead corners and return grilles can differ by hundreds of ppm. Mount at breathing height in the densely occupied zone (about 1.1–1.2 m for seated work), away from doors, windows and supply-air jets; one sensor represents one ventilation zone, and open-plan floors need multiple points per zone.

CO₂-driven ventilation: the savings beyond safety

Demand-controlled ventilation (DCV) is where a CO₂ sensor earns its keep: a seated adult exhales roughly 18–19 litres of CO₂ per hour, and with adequate ventilation the room settles at "outdoor baseline + G/Q" (G the generation rate, Q the outdoor air per person). In spaces with swinging occupancy — meeting rooms, classrooms, dining halls, cinemas — fixed-rate ventilation sized for peak occupancy massively over-ventilates off-peak. Closing the loop from a CO₂ sensor onto dampers or fan speed typically returns 20–40% of ventilation energy in classrooms and meeting rooms while never under-delivering at peak.

A 2023 addendum to ASHRAE 62.1 recast the DCV control value as a maximum differential above ambient, converging with EN 16798-1's category approach — which in turn means DCV set-points must be anchored to the local outdoor baseline, not one national number.

The TVOC layer: realistic online practice

TVOC spans hundreds of compounds between n-hexane and n-hexadecane, emitted by materials, furniture, cleaners and printers — independent of headcount. So it measures sources, complementing CO₂'s measurement of ventilation. The reference method is laboratory GC (8-hour mean); online sensors take the equivalence route — electrochemical or MOS elements responding to the VOC mixture as a total reading for continuous trend and early warning.

Two realities at selection: first, an equivalence TVOC reading (in mg/m³ or ppm, calibrated on e.g. isobutylene) is not strictly comparable with the GC method's toluene-equivalent mg/m³ — compliance calls still need lab sampling; second, sensors respond weakly to high-boiling compounds, and formaldehyde must be measured with a dedicated element — a normal TVOC reading does not certify formaldehyde compliance.

For particulates, laser-scattering PM2.5/PM10 monitors are mature online; just separate cleanroom-grade from everyday IAQ-grade accuracy needs. For the CO layer (premises with combustion appliances), apply the occupational/indoor limits from our CO monitoring article. With that, every layer sits in place: CO₂ → ventilation, TVOC → sources, PM → particulates, plus dedicated CO and formaldehyde where applicable.

Frequently asked questions