Reference · cited
CO₂ levels: what every ppm range actually means
Carbon dioxide indoors is mostly exhaled breath. Outdoors it sits near 420 ppm; in closed rooms it climbs fast, and with it come the complaints this site is named for. Here is the whole scale, band by band, with the evidence and its honest limits.
Outdoor baseline, 2026. The global average measured by NOAA has risen from ~315 ppm in 1958 to about 420–425 today — indoors, this is the best you can get.
The bands
| CO₂ (ppm) | What it is | What the evidence says |
|---|---|---|
| ~420 | Outdoor air (NOAA global average) | Your baseline. Indoor air can only approach it, never beat it. |
| 400–800 | Well-ventilated indoor space | Aim here. Below 800, air is a poor suspect for how you feel. |
| 800–1,000 | Ventilation slipping | The famous "1,000 ppm rule" is a ventilation rule of thumb often attributed to ASHRAE — ASHRAE itself notes its standard sets ventilation rates, not a CO₂ health limit. |
| 1,000–1,400 | Typical stuffy meeting room | Controlled studies measured duller thinking here: decision-making scores dropped at 1,000 ppm vs 600 (Satish 2012); cognitive scores averaged ~15% lower around ~945 ppm than in a high-ventilation condition (Allen 2016). |
| 1,400–2,000 | Crowded, closed, complained about | Allen 2016 measured roughly halved cognitive scores at ~1,400 ppm conditions; occupants report stuffiness and drowsiness. |
| 2,000–3,000 | Closed bedroom by morning | With door and window shut, researchers measured ~2,600 ppm average overnight; opening the air path (~835 ppm) improved rated sleep quality and next-day performance (Strøm-Tejsen 2016). |
| 5,000 | OSHA workplace limit | The US legal 8-hour average exposure limit (PEL) for CO₂. Ordinary buildings should never be near it. |
| 40,000 | Immediately dangerous | NIOSH's IDLH level — an industrial-accident number, not a building number. |
How fast do rooms actually climb?
Order-of-magnitude estimates from published occupant generation rates (Persily 2017), assuming zero ventilation — real rooms leak, so treat these as ceilings on speed, not predictions:
| Space | Volume | People | Est. climb |
|---|---|---|---|
| Car on recirculation | ~3 m³ | 2 | ~180 ppm/min |
| Small meeting room, door shut | ~25 m³ | 4 | ~2,600 ppm/h |
| Classroom | ~180 m³ | 25 | ~1,400 ppm/h |
| Bedroom, door & window shut | ~35 m³ | 2 | ~950 ppm/h |
| Open-plan office | large | many | depends on HVAC — measure at your desk |
The pattern: the smaller the box and the more lungs in it, the faster the climb — which is why the two worst everyday offenders are the two nobody suspects: the car and the bedroom.
The honest caveat
The low-level cognition findings have real pushback: a 2019 study of astronaut-like subjects found no consistent acute cognitive effects even at 15,000 ppm of pure CO₂ (Scully 2019). One reading: in real rooms, high CO₂ travels with everything else ventilation removes — bioeffluents, VOCs, humidity. So treat the ppm number as what it provably is: the best cheap proxy for how much stale air you're rebreathing. Either way the remedy is identical — ventilate — which is why the debate, interesting as it is, changes no advice on this site.
Questions people actually ask
Is CO₂ at these levels poisoning me?
No. Between 400 and 5,000 ppm this is a comfort-and-performance story, not toxicity. The danger gas in homes is carbon monoxide (CO) — different molecule, different detector, real emergency. If a headache clears when you leave the house, take that seriously: details here.
Will houseplants fix it?
Not measurably. A person exhales far more CO₂ per hour than a windowsill of plants absorbs. Plants are lovely; a cracked window is a machine.
Does opening a window actually work?
It's the fastest fix there is — with cross-ventilation, a room can drop hundreds of ppm in minutes. The catch is wildfire season, when outdoor particles make "just open a window" the wrong call: see the smoke guide.
Are children hit harder?
Classrooms are among the worst-measured everyday spaces, and two lines of evidence say it matters: across 100 US elementary schools, higher ventilation rates went with better math and reading scores (Haverinen-Shaughnessy 2011), and a Danish crossover trial found children worked faster on schoolwork when classroom air was improved (Petersen 2016). Full story: CO₂ in classrooms.
My car hit 3,000 ppm — is that dangerous?
Not toxic — but it's the drowsiness band, and you're operating a vehicle. Recirculation mode in a small cabin is the fastest CO₂ climb in ordinary life: the recirculation trap.
What should I buy to see my number?
One NDIR monitor, $60–250, no subscription. The monitor guide explains the one spec that matters and the fake-CO₂ trap to avoid.
Sources
- Satish U. et al. (2012). Is CO₂ an Indoor Pollutant? Direct Effects of Low-to-Moderate CO₂ Concentrations on Human Decision-Making Performance. Environmental Health Perspectives. doi:10.1289/ehp.1104789
- Allen J.G. et al. (2016). Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers. Environmental Health Perspectives. doi:10.1289/ehp.1510037
- Strøm-Tejsen P. et al. (2016). The effects of bedroom air quality on sleep and next-day performance. Indoor Air. doi:10.1111/ina.12254
- Scully R.R. et al. (2019). Effects of acute exposures to carbon dioxide on decision making and cognition in astronaut-like subjects. npj Microgravity. doi:10.1038/s41526-019-0071-6
- Persily A., de Jonge L. (2017). Carbon dioxide generation rates for building occupants. Indoor Air. doi:10.1111/ina.12383
- NOAA Global Monitoring Laboratory — Trends in atmospheric CO₂ (global averages; Keeling record).
- OSHA Annotated PELs, Table Z-1 (carbon dioxide, 5,000 ppm TWA); NIOSH Pocket Guide (IDLH 40,000 ppm).
- ASHRAE (2022). Standard 62.1, Ventilation for Acceptable Indoor Air Quality — and ASHRAE's position document on indoor CO₂ (the standard sets ventilation rates, not a CO₂ health limit).