A carbon dioxide monitor is a useful observation tool. Its reading becomes meaningful when the source of CO₂, the people present, the outdoor baseline and the air-exchange conditions are understood.
Literature summary: indoor CO₂ reflects generation, exchange with outdoor and adjacent air, mixing and time. Occupancy and activity change the source rate. A low reading in an empty room cannot establish ventilation performance for a crowded period.
NIST warns that a single 1000 ppm rule can misclassify ventilation. A comparison target needs a stated ventilation objective and appropriate assumptions. CO₂ is not a complete measure of particles, gases or all indoor-air hazards.
Decision example — A room rises during a meeting and falls after people leave. Keep the occupancy and fan-state record with the trend; do not label the rise a failed unit or the later fall a passed ventilation test without checking actual outdoor-air delivery.
Field method
What to record and compare
Record place, time, weather, occupancy, instrument and operating state. Keep observations separate from explanations.
01
GCT field method 1 — Identify the room, approximate occupancy count, activity, door/window state and HVAC mode. Record timestamps with a time zone.
02
GCT field method 2 — Check the instrument’s intended CO₂ measurement method, range, calibration instructions and placement. Keep it away from a person’s immediate breath and document the location.
03
GCT field method 3 — Record a representative outdoor baseline and an occupied indoor trend in ppm. Retain startup and changing-occupancy periods instead of silently treating them as steady conditions.
04
GCT field method 4 — Have the responsible provider compare the trend with the required ventilation objective and direct airflow or control evidence. State assumptions about mixing and other CO₂ sources.
05
GCT field method 5 — After an authorized correction, repeat under comparable occupancy. A lower reading under fewer occupants is not by itself proof of improved ventilation.
New source links and claims reviewed November 4, 2026. A source explains the stated mechanism; it does not diagnose a particular building or endorse ICY.
National Institute of Standards and Technology
NIST’s Indoor CO2 Tool Can Help Assess Ventilation and Indoor Air Quality
Occupancy, activity, outdoor concentration and room conditions affect CO2 interpretation; A universal 1000 ppm rule can misclassify ventilation
Quit Blaming ASHRAE Standard 62.1 for 1000 ppm CO2
CO2 requires contextual interpretation; CO2 is not a complete measure of indoor air quality; this 2020 paper is not a claim about every current local standard
Follow emergency procedures for alarms, acute symptoms or suspected hazardous releases. Do not use a reassuring CO₂ number to override those procedures or disable required ventilation.
