Carbon dioxide (CO2)

Carbon dioxide

Indoor CO2 is a ventilation marker, not a poison. The reading tracks how much fresh outdoor air is reaching a room relative to the people breathing in it.

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, EPA, and WHO all treat it the same way, as a proxy for ventilation rather than a regulated toxin. 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 the bedroom air mover. The building outpaced its ventilation, and the room is the part you control.