CO₂ readings track occupancy because people are the source. Each seated adult adds about 18 liters of CO₂ per hour to the room, so doubling the number of people doubles the rate of rise, and the level the room settles at is set by people divided by fresh-air rate. A monitor’s history chart is effectively a record of who was in the room and how well the ventilation kept up with them.
The production side
A resting adult exhales around 0.3 liters of CO₂ per minute. Sitting and working, about 0.3 to 0.35. Walking around or presenting, 0.5 to 0.7. Children produce roughly half to two thirds of an adult’s output. A room of ten seated adults is adding about 3 liters of CO₂ per minute, or 180 liters an hour.
The removal side
Fresh air enters at about 420 ppm and leaves at the room’s concentration. The more air moves through, the more CO₂ it carries out. The room settles where inflow of CO₂ from people equals outflow in exhaust air. In steady state:
Indoor CO₂ ≈ Outdoor CO₂ + (CO₂ produced per second ÷ fresh-air flow per second)
With 0.005 liters per second per person and 10 liters per second per person of fresh air, that is 420 + 500 = about 920 ppm. Halve the fresh air and it becomes about 1,420 ppm. This is why ventilation standards, which specify fresh air per person, imply a CO₂ level; see what 1,000 ppm CO₂ means.
What the curve looks like
- Empty room: decays toward outdoor levels. The speed of the decay is a direct measure of the air-change rate.
- People enter: immediate small spike from breath near the sensor, then a steady ramp.
- Ramp slope: proportional to the number of people divided by room volume. Twice the people, twice the slope.
- Plateau: the steady state, set by people divided by fresh-air rate. A plateau above 1,000 ppm means ventilation is under-sized for the crowd.
- People leave: exponential decay back toward outdoors.
Room size changes the timing, not the destination
A large room takes longer to fill and longer to clear, but its steady-state level depends only on people and fresh-air rate. A big lecture hall with weak ventilation and a small office with weak ventilation end up at the same plateau if the people per unit of fresh air are the same; the hall just takes longer to get there. See why a monitor spikes when people enter for the short-term effects.
Using this to check a room
- Note the reading when the room has been empty for an hour; it should be near outdoors.
- Note the plateau after the room has been fully occupied for an hour or more.
- If the plateau is under 1,000 ppm, ventilation matches occupancy. If 1,000 to 1,500 ppm, it is marginal. Above 1,500 ppm, either cut occupancy or increase fresh air.
- Compare the decay after people leave against other rooms: slow decay means low air-change rate.
Building systems automate this as demand-controlled ventilation, opening dampers as CO₂ rises. In a home, the same logic says a monitor in the most crowded room tells you when to open a window. The office air quality monitors guide covers loggers with the history charts that make the curve visible.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
How much CO₂ does one person produce?
A seated adult produces about 0.3 liters per minute, or 18 liters per hour. Light activity roughly doubles it; children produce less.
How many people can a room hold before CO₂ exceeds 1,000 ppm?
It depends on the fresh-air rate. Roughly, each seated person needs about 8 to 10 liters per second of outdoor air to hold the room near 1,000 ppm.
Can a CO₂ monitor count people?
Approximately. In a room with steady ventilation, the rate of rise is proportional to the number of people. Building management systems use this for demand-controlled ventilation.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.