Understanding Indoor CO2 Levels
Carbon dioxide is a colourless, odourless gas that accumulates indoors whenever people breathe. Outdoor air sits at roughly 420 ppm today, but indoor concentrations can soar well beyond 2,000 ppm in stuffy rooms. Knowing the benchmarks helps you act before air quality deteriorates.
CO2 PPM Ranges at a Glance
- Below 600 ppm — Excellent. Close to fresh outdoor air; ventilation is outstanding.
- 600-800 ppm — Good. Typical of well-ventilated homes and offices. Cognitive function remains strong.
- 800-1,000 ppm — Fair. You may notice stuffiness or mild drowsiness. Ventilation could be improved.
- 1,000-1,500 ppm — Poor. Research shows measurable declines in decision-making and focus. Headaches and fatigue become common.
- Above 1,500 ppm — Unhealthy. Immediate action is needed: open windows, reduce occupancy, or activate mechanical ventilation.
What ASHRAE Recommends
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.1 targets indoor CO2 no more than 700 ppm above outdoor levels, which translates to roughly 1,100 ppm in most locations. Many IAQ experts now advocate a tighter target of 800 ppm for spaces where cognitive work or learning takes place.
Why It Matters
A landmark 2015 Harvard study found that cognitive scores dropped by 21 percent when CO2 rose from 600 to 1,000 ppm. Sleep quality also suffers when bedroom levels exceed 800 ppm — read more in our guide on how CO2 monitors improve sleep.
Practical Tips
- Measure first — you cannot manage what you do not measure. A quality NDIR-based monitor gives real-time readings.
- Ventilate strategically — even 10 minutes of cross-ventilation can cut CO2 in half. See our ventilation tips.
- Use our calculator — the CO2 calculator estimates how quickly levels rise based on room size and occupancy.
- Place your monitor correctly — our placement guide covers optimal positioning.
Keeping indoor CO2 below 800 ppm is the single best target for comfort, health, and productivity.