A good indoor CO₂ level is under 800 ppm. Between 800 and 1,000 ppm is acceptable. Above 1,000 ppm most ventilation guidance says increase fresh air, and above 1,500 ppm the room is clearly under-ventilated for the people in it. Outdoor air is about 420 ppm, so a reading is really a measure of how far the room has drifted from fresh.
Where the numbers come from
Ventilation standards for buildings specify a fresh-air rate per person. When that rate is delivered, the steady-state CO₂ in an occupied room lands at roughly 700 ppm above outdoors, which is where the familiar 1,000 to 1,100 ppm guideline comes from. Public-health guidance published during the pandemic pushed the target lower, commonly to 800 ppm, because the concern shifted from comfort to diluting respiratory aerosols. Neither figure is a health limit for CO₂ itself; occupational limits are several thousand ppm.
The bands
| CO₂ (ppm) | Rating | What to do |
|---|---|---|
| Under 600 | Excellent | Nothing; ventilation exceeds occupancy |
| 600 to 800 | Good | Nothing; watch the trend if people arrive |
| 800 to 1,000 | Acceptable | Open a window or raise the fresh-air setting if the room will stay occupied |
| 1,000 to 1,500 | Poor | Ventilate now; reduce occupancy if it keeps rising |
| 1,500 to 2,500 | Very poor | Ventilate immediately; check the ventilation system is running |
| Above 2,500 | Investigate | Sealed room, failed HVAC or a CO₂ source such as fermentation or a cylinder |
Room by room
Bedrooms
Two people in a closed bedroom will push CO₂ past 1,500 ppm within a few hours and often past 2,500 ppm by morning. Cracking the door or a window is usually enough to halve the morning reading. If the house has mechanical ventilation, a bedroom extract or trickle vent matters more than anywhere else.
Living rooms and kitchens
Larger volume and more door movement keep these lower, typically 600 to 1,000 ppm. A gas hob adds a noticeable spike; the cooker hood should remove it.
Home offices
One person in a small closed room reaches 1,000 ppm in under an hour. This is the room where a monitor pays for itself in afternoon concentration.
Offices and meeting rooms
Meeting rooms fill fastest. A reading above 800 ppm at the start of a meeting usually means it will exceed 1,500 ppm before the meeting ends. See air quality monitors for offices.
Classrooms
Thirty children in a closed classroom can reach 2,000 ppm in a lesson. Several countries now set 800 to 1,000 ppm targets for schools and recommend a monitor in every classroom.
Trend matters more than one reading
A single reading tells you where the room is; the slope tells you where it is going. A room rising 100 ppm every ten minutes will be at 1,500 ppm within the hour. Monitors with a history chart make the slope visible.
Before trusting a reading
- Make sure the monitor uses an NDIR sensor. Estimated “eCO₂” readings can be hundreds of ppm off.
- Keep it a meter from the nearest person; breath inflates the number.
- If it reads under 400 ppm or never drops when a window opens, calibrate it. See how to calibrate a CO₂ monitor.
Enter your reading in the CO₂ level checker for an interpretation and next step.
Frequently Asked Questions
Is 1,000 ppm CO₂ bad?
It is the point where most ventilation guidance says add fresh air. It is not dangerous, but a room that sits at 1,000 ppm all day is under-ventilated for the number of people in it.
What CO₂ level is normal in a bedroom overnight?
With the door and window closed, 1,500 to 2,500 ppm by morning is common for two people. Opening the door or cracking a window typically halves that.
Why does my monitor read below 400 ppm?
Outdoor air is about 420 ppm, so a lower reading means the sensor baseline has drifted. Calibrate it in fresh air following the manual.