// the science, carbon dioxide
What does CO₂ do to you?
You breathe out carbon dioxide. In a closed room full of people, it builds up, and it's the single clearest sign that you're rebreathing your own stale air. At the levels found indoors CO₂ isn't poisoning you, but rising CO₂ tracks with that heavy, can't-think, need-a-window feeling, and with measurably worse focus and sleep.
ppm, today's outdoor baseline. Your indoor floor.
ppm, routine in meeting rooms, classrooms, and bedrooms overnight.
cognitive scores at 1,400 ppm vs ~550 ppm in a Harvard controlled study.
What the ppm number means
CO₂ is measured in parts per million. Roughly:
- 400–600 ppmFresh, essentially outdoor air
- 600–1000 ppmFine, a typical occupied room
- 1000–1400 ppmStuffy, ventilation is slipping
- 1400–2000 ppmFoggy, focus & comfort start to dip
- 2000–5000 ppmDrowsy, open a window
- 5000+ ppmOccupational limit (8-hour average)
Because CO₂ is produced by people and cleared by ventilation, it's the best proxy for how fresh a room's air is, and, during cold & flu season, for how much rebreathed air (and aerosols) you're sharing.
CO₂ and your brain
Satish et al., EHP, 2012
In a controlled chamber, decision-making performance dropped moderately at 1,000 ppm and sharply at 2,500 ppm, compared with a 600 ppm baseline, at levels people hit in ordinary rooms.
Allen et al. (COGfx), EHP, 2016
Harvard researchers found cognitive-test scores about 15% lower at 945 ppm and ~50% lower at 1,400 ppm versus ~550 ppm. Better-ventilated 'green' conditions scored far higher across all domains tested.
An honest caveat: later real-world studies have found weaker or mixed effects, and researchers still debate how much is CO₂ itself versus the other pollutants that build up alongside it. What isn't debated: high CO₂ means poor ventilation, and poor ventilation is bad for focus, comfort, and health.
Beyond focus
Sleep
A bedroom with the door and windows shut can climb past 2,500 ppm overnight. Better-ventilated, lower-CO₂ bedrooms have been linked to better sleep and next-day alertness.
That 3pm slump
Drowsiness, headaches, and the stuffy-room feeling often track with rising CO₂ in packed offices and classrooms.
Shared air
CO₂ doubles as a check on how much air you're rebreathing from others, handy when airborne bugs are going around.
An easy fix
Unlike most pollutants, the fix is usually free: crack a window, open a door, turn up the ventilation. You just have to know.
Sources
- Satish et al., CO₂ & decision-making performance, Environmental Health Perspectives (2012)
- Allen et al., COGfx green-building cognition study, Environmental Health Perspectives (2016)
- Strøm-Tejsen et al., Bedroom ventilation & sleep quality, Indoor Air (2016)
- NOAA Global Monitoring Lab, global CO₂ trend (~420 ppm)
- OSHA, carbon dioxide occupational exposure limit (5,000 ppm, 8-hour)
Your air follows you. So should the number.
CO₂ is invisible and rises silently, you only notice once you're already foggy. Partycle reads true NDIR CO₂ wherever you are, so you catch the stuffy room before it catches you, bedroom, office, or the back of a packed bus.