Carbon dioxide is the easiest indoor-air number to act on. People breathe it out, so when a meeting room, classroom or shop floor fills up and fresh air cannot keep pace, CO2 climbs. A CO2 reading in parts per million (ppm) is a practical gauge of whether a space is getting enough outdoor air, and a logged history shows you exactly when and where it is not.
Where indoor CO2 comes from
Outdoor air holds roughly 420 ppm of CO2. Indoors, the main source is us: every occupant adds CO2 with each breath, and ventilation is what carries it back out. Unvented fuel-burning appliances add more. That is why CO2 rises fastest in small, busy rooms with little fresh air, and why Health Canada describes indoor CO2 as a stand-in for how well a space is ventilated.
What the numbers mean
- About 400 to 450 ppm: outdoor air. A room at this level is getting plenty of fresh air, or it is empty.
- Up to 1,000 ppm: Health Canada's recommended long-term exposure limit for homes is 1,000 ppm averaged over 24 hours (2021 residential indoor air quality guideline for carbon dioxide). It is a useful yardstick for any occupied space.
- Above 1,000 ppm: the room is under-ventilated for the number of people in it. This is usually when people start calling a room stuffy.
- 2,000 ppm and up: ventilation is clearly not keeping up. Act now rather than at the end of the day.
Workplace exposure limits for CO2 are much higher (commonly 5,000 ppm over an 8-hour shift), but those are safety limits, not comfort or ventilation targets.
Why it matters
Health Canada based its guideline on the combined evidence from schools, offices and controlled studies, which linked higher CO2 with headaches, tiredness and trouble concentrating, and with lower scores on decision-making and school tests. It is careful to add that the studies cannot say whether CO2 itself or the other pollutants that build up in a poorly ventilated room cause those effects. Either way, the fix is the same: more fresh air when a space is busy.
Monitor it continuously, not once
A handheld meter tells you about one moment in one room. CO2 moves with occupancy: it climbs through a full meeting, drops at lunch and climbs again in the afternoon. A logger that records every few minutes shows you:
- which rooms run high, and at what times of day
- whether the ventilation brings in enough outdoor air at peak occupancy, or only once the building empties
- whether a change, such as a new ventilation schedule, actually brought the numbers down
What EDGE Air measures
EDGE Air logs CO2, an air-quality (VOC) level, temperature and humidity in one unit:
- CO2: 400 to 2,000 ppm, ±40 ppm + 5% of reading (at 25 °C (77 °F) and 50% RH), 1 ppm resolution
- Air quality (VOC): a five-level scale
- Temperature: -10 to +60 °C (14 to 140 °F); humidity: 0 to 99% RH, non-condensing
- Readings every 1 to 30 minutes, an on-device display and buzzer, and 4 AA batteries that last up to 2 years, with USB-C power as an option
Its range covers the levels where ventilation decisions are made. It is not a safety monitor for places where CO2 can build far past 2,000 ppm, such as rooms with compressed CO2 cylinders or dry ice.
Alarms that fit the problem
Set a CO2 limit, for example 1,000 ppm for a meeting room. When a room crosses it, MaxLinc calls, texts, emails and sends a push notification to everyone the logger is shared with, and repeats the alarm until the reading is back in range. EDGE Air also supports alarms for sudden CO2 spikes and for thresholds tied to long-term exposure. A persistence delay keeps a short peak, like a ten-minute stand-up in a small room, from setting off the alarm, so the alerts you get point to rooms that stay under-ventilated.
Bringing CO2 down
- Bring in more outdoor air: raise the ventilation rate or run it longer during occupied hours, and have the system checked by a qualified contractor.
- Open windows when the outdoor air is good. During wildfire smoke, keeping outdoor air out can matter more than getting CO2 under 1,000 ppm.
- Match people to rooms: move large meetings to larger rooms, or keep them shorter.
- Keep fuel-burning appliances properly vented and maintained.
Then check the graph: if next week's afternoon peak is lower, the change worked.
Get started
See the EDGE Air WiFi monitor (no monthly fee on WiFi), or EDGE Air Cellular for sites without WiFi. Property managers can see how CO2 fits with temperature, humidity and leak sensors on the property management page. For damp storage rather than stale air, read our guide to the Mold Index and humidity.