Carbon dioxide (CO2)
Carbon dioxide
Indoor CO2, carbon dioxide, the gas everyone breathes out, is a marker of how well a room is ventilated. The reading tracks how much fresh outdoor air reaches a room relative to the people breathing in it.
What does Carbon dioxide (CO2) mean?
Carbon dioxide is the colorless, odorless gas every person exhales.
What the indoor number tells you is how well a room is ventilated. It rises when the people in a space exhale faster than outdoor air can replace the stale air, and it falls the moment the air exchanges. ASHRAE, the engineering body that sets ventilation standards, reads it that way, and so do the EPA and WHO.
It shows up in a home audit because by morning a closed bedroom in a tightly sealed modern build can climb well past the fresh-air baseline you would breathe on an outdoor walk. The fix is mechanical. You move more outdoor air through the room: open the door, crack a window when the outdoor air is clean, run a fan.
The building outpaced its ventilation, and the room is the part you control.
Sources cited for Carbon dioxide (CO2)
Every number on this Carbon dioxide (CO2) page traces to one of the 2 sources below: 1 government or regulatory.
Government & regulatory
Institutional & standards
Read the full Carbon dioxide (CO2) entry
Related air science
- PM2.5 (fine particulate matter) PM2.5 means particulate matter 2.5 micrometers across or smaller.
- PM10 (coarse particulate matter) PM10 is the mass of all airborne particles up to 10 micrometers in diameter, the fine PM2.5 fraction plus the coarse fraction between 2.5 and 10 micrometers that a PM2.5 reading does not capture.