Indoor environment factor · measured in ppm (parts per million)
Carbon dioxide (CO₂)
Also: CO2
A colourless, odourless gas produced by breathing and combustion. Indoors it is the most reliable indicator of ventilation: rising CO₂ means the air is being breathed more times before it is replaced, and it rises together with every other pollutant that people emit.
Why it matters indoors
Carbon dioxide is the gas every person exhales at roughly 20 litres an hour at rest. In a sealed room it accumulates in proportion to the number of occupants and inversely to the fresh-air supply, which makes it a direct measure of how well a space is ventilated. It is not toxic at the concentrations found in buildings; its value is as a proxy — when CO₂ is high, so is everything else that people emit, including the respiratory aerosols that carry infections.
Where it comes from indoors
Occupants are the main source. Gas cooking, unflued heaters and candles add to it. Plants remove negligible amounts. Outdoor air, at about 420 ppm and rising slowly year on year, sets the floor a room can be ventilated down to.
Health and performance effects
Between 1000 and 2500 ppm, controlled studies report measurable declines in decision-making and concentration, more headaches and fatigue, and higher sick-leave rates in offices and schools. Above 5000 ppm — rare outside industrial settings — CO₂ itself causes headache and shortness of breath. The indirect effect matters most: poorly ventilated rooms transmit respiratory infections more readily.
Guideline values
The World Health Organization publishes no CO₂ guideline, because CO₂ is a ventilation indicator rather than a pollutant with a dose–response relationship. National and professional bodies do:
| Body | Value | Meaning | Source |
|---|---|---|---|
| ASHRAE 62.1 | about 700 ppm above outdoor (≈ 1100 ppm) | steady-state level corresponding to the standard’s ventilation rate for offices | ASHRAE 62.1 |
| German Environment Agency (UBA) | < 1000 ppm / 1000–2000 ppm / > 2000 ppm | hygienically harmless / elevated / unacceptable | UBA 2008 |
| UK Department for Education (BB101) | 1000 ppm (mechanical) · 1500 ppm (natural ventilation) | daily average during occupied hours in classrooms | BB101 2018 |
| Historical (Pettenkofer, 1858) | 1000 ppm | origin of the widely used threshold | — |
Slovak, UAE and Hong Kong values are being compiled in the indoor air quality limits table.
How it is measured
Non-dispersive infrared sensors shine infrared light through the air and measure how much is absorbed at the wavelength characteristic of CO₂. They are accurate to a few tens of ppm and stable for years when periodically re-referenced to outdoor air. Cheaper “eCO₂” sensors estimate CO₂ from VOC readings and should not be relied on for ventilation decisions.
What the studies report
12 of 175 Findings — those with the strongest study design.
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Mechanical ventilation | difference 491 | — · classroom | ventilation intervention study in classrooms: mechanical ventilation set to a carbon dioxide threshold in twelve classrooms and the measured concentrations compared with the baseline week | F-1651 A ventilation intervention study in classrooms to improve in… (2013) | machine |
| Mechanical ventilation | mean 841 | — · classroom | ventilation intervention study in classrooms: mechanical ventilation set to a carbon dioxide threshold in twelve classrooms and the measured concentrations compared with the baseline week | F-1652 A ventilation intervention study in classrooms to improve in… (2013) | machine |
| Cognitive performance | difference -13.57 | college students · test chamber | controlled chamber exposure experiment with college students in Chongqing: 4-hour exposures to 500 ppm and 2500 ppm pure CO2 with and without added negative air ions, cognitive test battery and symptom questionnaires | F-0399 Effects of increasing indoor negative air ions on cognitive … (2023) | machine |
| Cognitive performance | difference 11 | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0416 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Cognitive performance | difference 23 | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0417 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Cognitive performance | difference 44 | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0418 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Cognitive performance | difference 94 | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0419 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Cognitive performance | decision-making performance filed under cognitive-performance | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0420 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Cognitive performance | decision-making performance filed under cognitive-performance; the focused activity scale moved in the opposite direction | adult volunteers · exposure chamber (office-like) | blinded controlled exposure study with balanced crossover order: 22 participants in six groups exposed in an office-like chamber to CO2 at 600, 1,000 and 2,500 ppm in three 2.5-hour sessions on one day, ventilation rate and temperature constant, computer-based Strategic Management Simulation test of decision-making performance analysed with analysis of variance | F-0421 Is CO2 an Indoor Pollutant? Direct Effects of Low-to-Moderat… (2012) | machine |
| Ventilation | maximum level recorded as measure other; the value after is printed as a range and sits in extra | university students and staff (outbreak contacts) · university classroom | outbreak investigation with a contact cohort: 27 tuberculosis cases and 1665 contacts in underventilated university buildings in Taiwan, indoor CO2 measured before and after ventilation engineering, incidence of TB among contacts analysed with adjustment for contact investigation and latent TB infection treatment | F-0443 Effect of ventilation improvement during a tuberculosis outb… (2020) | machine |
| Tuberculosis | other 97 (50–99.9) | university students and staff (outbreak contacts) · university classroom | outbreak investigation with a contact cohort: 27 tuberculosis cases and 1665 contacts in underventilated university buildings in Taiwan, indoor CO2 measured before and after ventilation engineering, incidence of TB among contacts analysed with adjustment for contact investigation and latent TB infection treatment | F-0444 Effect of ventilation improvement during a tuberculosis outb… (2020) | machine |
| Tuberculosis | — | university students and staff (outbreak contacts) · university classroom | outbreak investigation with a contact cohort: 27 tuberculosis cases and 1665 contacts in underventilated university buildings in Taiwan, indoor CO2 measured before and after ventilation engineering, incidence of TB among contacts analysed with adjustment for contact investigation and latent TB infection treatment | F-0445 Effect of ventilation improvement during a tuberculosis outb… (2020) | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
How strong is the evidence
-
In classrooms, mechanical ventilation may lower indoor carbon dioxide: two independent measurement campaigns found lower concentrations where central mechanical ventilation was present than where it was not.
A Claim is Coridum reading the Findings together, not a quotation from one study — according to Coridum. The grade follows the study designs behind it: A is the strongest, D means a single study or a mechanism.
How Coridum measures it: Coridum devices use a non-dispersive infrared (NDIR) sensor — the same principle as laboratory analysers — with automatic baseline calibration against outdoor air.
Identifiers
- Molecular formula
- CO₂
- IUPAC name
- carbon dioxide
- CAS Registry Number
- 124-38-9 · PubChem · CTD
- InChIKey
- CURLTUGMZLYLDI-UHFFFAOYSA-N
- MeSH
- D002245
- SMILES
O=C=O
Frequently asked questions
What CO₂ level is too high indoors?
Outdoor air is around 420 ppm. Below 1000 ppm is generally considered good ventilation; 1000–2000 ppm indicates insufficient fresh air and is associated with drowsiness and reduced concentration; above 2000 ppm is widely classed as unacceptable for occupied rooms.
Is CO₂ itself dangerous at these levels?
Not directly — occupational limits are 5000 ppm for an eight-hour day. The concern at 1000–2000 ppm is what the number stands for: stale air carrying exhaled aerosols, odours and other pollutants, and the measurable drop in cognitive performance.
How fast does CO₂ rise in a closed room?
Quickly. Four people in a closed 20 m² bedroom can push CO₂ from 500 to over 2000 ppm within an hour or two. A classroom of 25 pupils with windows shut can pass 2000 ppm before the first break.
Sources
- ANSI/ASHRAE Standard 62.1: Ventilation and Acceptable Indoor Air Quality. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
- Building Bulletin 101: Guidelines on ventilation, thermal comfort and indoor air quality in schools. UK Department for Education, 2018.
- Umweltbundesamt (German Environment Agency): Gesundheitliche Bewertung von Kohlendioxid in der Innenraumluft, 2008.
- Effects of increasing indoor negative air ions on cognitive performance and health of… (2023). doi:10.1155/2023/8298339 CKB 0687
- O’Sullivan Review of carbon dioxide removal and recycling in space (2026). NPJ Microgravity. doi:10.1038/s41526-026-00608-x CKB 1236
- Zhu et al. LSTM-based IoT-enabled CO2 steady-state forecasting for indoor air quality monitoring…. Electronics. doi:10.3390/electronics12010107 CKB 0783
CKB = document number in the Coridum Knowledge Base, our curated library of peer-reviewed studies. Follow the DOI for the original paper.
Cite this page
Coridum Research. Carbon dioxide (CO₂). Coridum Knowledge G-0003, 16 September 2026. https://coridum.com/k/G-0003/