Carbon dioxide
CO₂ is a colourless, odourless gas that people exhale with every breath. Indoors it builds up whenever a room holds more people than its ventilation can clear — which makes it the single most reliable signal of how fresh, or how stale, the air really is.
Stale air costs more than comfort.
CO₂ itself is not toxic at indoor levels — but it is an exact proxy for ventilation, and poor ventilation has measurable consequences.
Cognitive performance
Decision-making and concentration measurably decline as CO₂ climbs. Effects appear well before anyone notices the air feels “off”.
Comfort & symptoms
Drowsiness, headaches and that mid-afternoon slump in stuffy rooms track closely with rising CO₂ — and lift again when air is refreshed.
Ventilation proxy
High CO₂ means stale air is not being exchanged — so other pollutants and airborne pathogens accumulate alongside it.
Compliance
WELL, EN 16798 and most workplace ventilation guidelines reference CO₂ thresholds directly. Continuous data is your evidence.
From fresh air to a room that needs action.
Outdoor air sits near 420 ppm. The further a room climbs above it, the less fresh air each occupant is getting — and the sooner ventilation should respond.
Indoors, almost all of it comes from us.
Unlike particulates or VOCs, the dominant indoor CO₂ source is human respiration — so levels follow occupancy and ventilation hour by hour.
People in the room
Every occupant exhales CO₂ continuously. A full meeting room or classroom can double its CO₂ within an hour without enough fresh air.
Under-ventilation
Sealed, energy-efficient buildings trap exhaled air. When supply rates drop — or systems run on a schedule that ignores occupancy — CO₂ climbs.
Combustion appliances
Gas stoves, boilers and unflued heaters release CO₂ directly into the space, adding to the occupancy load.
Crowding & long sessions
Density and duration compound. Long meetings, packed classrooms and shift handovers are predictable peaks worth alerting on.