Measurements · Gases & VOCs
Carbon Dioxide (CO₂)
Also known as CO2 level, CO2 concentration
Carbon dioxide: an indicator of ventilation and indoor air quality, and an asphyxiant at high concentration.
What it is
What is CO₂ as an air quality parameter?
Carbon dioxide (CO₂) is a colorless, odorless gas that people exhale continuously. Outdoor air sits at roughly 420 ppm and rising. Indoors, CO₂ accumulates in proportion to how many people occupy a space and how little fresh air is supplied to dilute it. That makes indoor CO₂ the most practical proxy for ventilation adequacy. It does not measure pollutants directly, but it shows whether a room is getting enough outdoor air to carry pollutants, pathogens, and odors away.
Because the signal is cheap to measure and easy to interpret, CO₂ has become the headline number of the healthy-building and classroom-ventilation movements.
Health and operational effects
At the concentrations found in ordinary buildings CO₂ is not directly toxic, but rising levels track stuffiness, drowsiness, and measurable declines in concentration and decision-making; studies have shown cognitive scores falling as indoor CO₂ climbs through the 1,000–1,500 ppm range. High CO₂ also signals that whatever else occupants emit, from bioeffluents to airborne viruses, is being under-diluted, which is why ventilation guidance surged in importance after 2020.
Operationally, CO₂ is the control signal for demand-controlled ventilation: instead of running air handlers at a fixed rate, a building modulates fresh-air intake to actual occupancy, saving heating and cooling energy while protecting air quality.
Limits
Carbon Dioxide (CO₂) Limit Values
The published values, by averaging period; each links to the standard that sets it.
8-hour TWA (PEL)
A03 Ventilation Design, monitoring option
A06 Enhanced Ventilation Design (points)
Indoor concentration
95th percentile, continuous
Acceptable
High performance
Sustained, above ambient
- WELL v2 900 ppm: or no more than 500 ppm above outdoor; one of four ways to meet the precondition
- WELL v2 750 ppm: or no more than 350 ppm above outdoor
- UBA CO₂ 1000 ppm: < 1000 ppm: hygienically acceptable
- UBA CO₂ 2000 ppm: > 2000 ppm: hygienically unacceptable
- LEED v5 1000 ppm: 3 points at or below
- LEED v5 800 ppm: 4 points at or below
- RESET Air 1000 ppm: below
- RESET Air 600 ppm: below
- ISO 23875 400 ppm: cab CO₂ no more than 400 ppm above ambient (clause 4.1)
CO₂ is reported in parts per million (ppm). The widely used rule of thumb, reflected in the German UBA guideline, treats sustained readings below 1,000 ppm as hygienically adequate and above 2,000 ppm as unacceptable. The WELL Building Standard uses CO₂ as a ventilation indicator: sensor data showing 900 ppm or less (or 500 ppm above outdoor) meets its ventilation precondition, and 750 ppm (or 350 ppm above outdoor) earns enhanced-ventilation points. Interpret CO₂ as a trend against occupancy, not as an instantaneous pass/fail. A brief peak in a packed meeting room matters less than a level that never recovers between sessions.
How it is measured
How Carbon Dioxide (CO₂) Is Measured
The dominant technology is non-dispersive infrared (NDIR). CO₂ absorbs infrared light at a characteristic wavelength, so the sensor passes an IR beam through a sample chamber and measures how much is absorbed. NDIR is selective, stable, and long-lived. The main accuracy pitfall is calibration drift. Many sensors use an automatic baseline correction that assumes the space periodically returns to outdoor levels, which can misread in spaces occupied around the clock. It is worth checking for hospitals, nursing homes, and 24-hour operations.
Photoacoustic and chemical sensors also exist, but NDIR is the workhorse for building monitoring.
Built into Aethair IAQ1
Measured by the monitor's own sensors, calibrated before it ships: nothing to add.
Aethair PRO cartridges2
A pre-calibrated cartridge in one of Aethair PRO's sockets: Aethair CO₂ (dual NDIR) and Aethair CO₂ (self-calibrating NDIR).
An instrument on Thiamis3
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing CO₂ sensors
Mount CO₂ sensors at breathing height in the occupied zone, away from doors, operable windows, and supply diffusers where fresh air would give an artificially low reading, and away from anyone's direct exhalation. Wall-mounted units at 1.1–1.5 m are typical for offices and classrooms. One sensor represents one ventilation zone; open-plan spaces with uneven occupancy may need several.
Every device and option that measures Carbon Dioxide (CO₂) 5
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
Aethair IAQ built-in sensors | Built-in sensors | Aethair IAQ | |
 Aethair CO₂ (dual NDIR) 0d02 0d04 | Gas sensor cartridge | Aethair PRO | |
 Particles Plus · Particles Plus 5301, Particles Plus 5301-AQM1, Particles Plus 5301-AQM2… | Particulate & Dust Monitors | Thiamis | |
 RAE · MultiRAE Lite, MultiRAE, MultiRAE Pro… | Gas Detectors | Thiamis | |
 Trolex · Sentro X (TX6310) | Gas Detectors | Thiamis |
Where it is monitored
Solutions That Monitor Carbon Dioxide (CO₂)
Each one links to the solution page showing what that industry measures it for.
Healthcare Air Monitoring
Core parameterCO₂ is the clearest proxy for ventilation adequacy in a hospital's densely occupied, infection-sensitive spaces (waiting rooms, patient rooms, and exam rooms), where poor air exchange raises the risk of airborne transmission.
HVAC & Building Management
Core parameterIn HVAC and building-management applications, CO₂ is not just an air quality indicator but a control signal: demand-controlled ventilation modulates fresh-air intake to actual occupancy using CO₂ as the input, cutting the energy cost of conditioning air nobody needs while protecting indoor air quality.
Indoor Air Quality
Core parameterCO₂ is the anchor of any indoor air quality program because it is the clearest available signal of whether a space is being ventilated in step with how many people occupy it.
Landfill, Biogas & Odor
Core parameterLandfill and biogas sites generate carbon dioxide alongside methane as organic matter decomposes, and the CO₂ fraction is a useful indicator of the decomposition and gas-collection process.
School Air Monitoring
Core parameterCO₂ is the parameter schools lead with, because a full classroom drives it up quickly and elevated CO₂ is linked to reduced concentration and learning performance.
Cleanrooms, Labs & Manufacturing
Often addedParticle counting and controlled environments
Article Library
Reading on Carbon Dioxide (CO₂)
How Building Certifications Use Air Quality Monitoring
How WELL and LEED v5 incorporate air quality monitoring, why both frameworks are moving toward continuous performance, and what that means for building teams.
Workplace Air Quality Monitoring for Offices and Employers
How to monitor workplace air quality in offices, what to measure for employee health and focus, and how continuous connected monitoring documents the result.
Indoor Air Quality in Schools: Standards & Monitoring 2026
Indoor air quality in schools: which standards apply, what to monitor, and how CO2 and ventilation affect student attendance, health, and test scores.
WELL Building Standard Air Quality: CO2 & PM2.5 Limits 2026
WELL v2 air quality limits: PM2.5 at 15 µg/m³, formaldehyde at 50 µg/m³, CO2 at 900 ppm for ventilation monitoring, plus the monitoring WELL verifies.
OSHA Air Quality Standards 2026: PELs & 29 CFR 1910.1000
OSHA air quality standards for indoor and outdoor work: permissible exposure limits, Table Z-1, 29 CFR 1910.1000, and the heat and wildfire smoke triggers.
IAQ Monitoring Parameters: PM2.5, CO2, VOCs & Formaldehyde
Indoor air quality monitoring parameters explained: PM2.5, CO2, VOCs, and formaldehyde, plus the sensors, thresholds, and OSHA, ASHRAE 62.1, and WELL limits.
Related
More in Gases & VOCs
Questions
Carbon Dioxide (CO₂) FAQ
What CO₂ level indicates poor ventilation?
Sustained indoor CO₂ above about 1,000 ppm suggests ventilation is not keeping pace with occupancy; above 1,500–2,000 ppm indicates clearly inadequate fresh air. Because outdoor air is around 420 ppm, the excess over that baseline is what reflects human occupancy and dilution.
Does CO₂ measure air pollution directly?
No. CO₂ itself is not a pollutant at building concentrations; it is a proxy. It indicates whether a space is well ventilated, which in turn governs the buildup of pollutants, odors, and exhaled aerosols. For pollutants themselves, pair CO₂ with PM2.5 and VOC monitoring.
What is the best sensor technology for CO₂?
Non-dispersive infrared (NDIR) is the standard for building and IAQ monitoring: selective, stable, and durable. Check whether a given sensor relies on automatic baseline calibration, which can be inaccurate in continuously occupied spaces.
How can I monitor CO₂ in my building?
Aethair IAQ measures CO₂ with a built-in NDIR sensor (Aethair PRO takes a CO₂ cartridge). That makes it a single-device fit for most indoor air quality and ventilation programs. Use the configurator to add PM2.5, VOC, temperature, and humidity for a complete healthy-building configuration.
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