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Outdoors, a CO2 monitor should read close to 400 to 420 ppm, the background level of the atmosphere. In a closed, occupied room, the reading should rise above that baseline as people exhale; the rate of rise depends on how many people are present, the volume of the room, and the ventilation rate. A monitor that never moves from 400 ppm in a crowded room is either not measuring CO2 or is not sampling the room air. The number you act on is not a single target but a band: below about 800 ppm is well ventilated for most homes, 800 to 1,000 ppm suggests ventilation is marginal, and sustained readings above 1,200 to 1,500 ppm indicate the room needs more fresh air. These bands are general guidance, not a legal standard, and the listing for a given product may state different alarm thresholds. The sensor type matters as much as the number: NDIR sensors measure CO2 directly, while low-cost VOC sensors estimate it and can drift when other sources of volatile compounds are present.
The baseline: what a monitor reads in fresh air
Before you trust any indoor reading, check the monitor outdoors or in a room that has been ventilated with windows open for several minutes. The atmosphere currently averages around 420 ppm CO2, and outdoor air in most inhabited areas sits between roughly 400 and 450 ppm depending on local sources such as traffic and vegetation. A monitor that reads 600 ppm in fresh outdoor air is offset; one that reads 300 ppm is also suspect. Many NDIR sensors offer a calibration routine that sets the current reading to 400 ppm, and the listing may or may not mention it. If the manual does not describe calibration, treat the absolute number as indicative and the trend as the reliable part.
The baseline matters because every indoor decision is relative to it. A room reading 700 ppm when outdoor air is 420 ppm has added about 280 ppm from occupants and building sources. A room reading 700 ppm when the outdoor baseline is 500 ppm has added only 200 ppm. Without a baseline, the same number can mean different things.
The rise: occupancy, room volume and ventilation arithmetic
Human breath contains roughly 40,000 to 50,000 ppm CO2, and an average adult at rest exhales about 200 to 250 millilitres of CO2 per minute. In a sealed room, the concentration rises at a rate set by the number of people, their activity level, and the volume of the room. A rough rule for a typical room is that each adult adds several hundred ppm per hour in a space with no ventilation. That is why a bedroom with the door closed can climb from 500 ppm to over 2,000 ppm overnight, while the same room with a window cracked open may stay below 900 ppm.
The arithmetic is useful because it tells you what a monitor should show. If four people sit in a 30 cubic metre meeting room with no ventilation, the CO2 concentration will rise by roughly 1,000 ppm per hour, so a monitor should cross 1,500 ppm within an hour. If it does not, either the room is larger than you think, the ventilation is better than you think, or the sensor is not responding. A monitor that responds slowly will understate peaks; the question of response time is covered in how fast a CO2 monitor should respond.
The bands: what different concentrations mean for a room
The table below summarises the bands commonly used in indoor air quality guidance. They are not legal limits for homes, and a product listing may set its own alarm thresholds at different points. The values are general domain guidance, not figures taken from any product listing.
| Reading (ppm) | What it suggests | Typical action |
|---|---|---|
| 400 to 450 | Fresh outdoor air baseline | None; use as calibration reference |
| 450 to 800 | Well ventilated occupied room | None; ventilation is adequate |
| 800 to 1,000 | Ventilation marginal for comfort and cognition | Open a window or increase mechanical ventilation |
| 1,000 to 1,500 | Poor ventilation; complaints of stuffiness likely | Ventilate before the room is used again |
| 1,500 to 2,000 | Very poor; sustained levels affect comfort and focus | Increase ventilation urgently; check occupancy |
| Above 2,000 | Severely under-ventilated | Leave or ventilate immediately; investigate source |
These bands are for occupied spaces. A closed, unoccupied room can sit at 600 to 800 ppm simply because it has not been ventilated, and that is not a health concern in itself. The number becomes meaningful when people are present and the reading rises above the baseline.
The sensor: why two monitors can disagree
NDIR (non-dispersive infrared) sensors measure CO2 directly by absorbing infrared light at the wavelength CO2 absorbs. They are the reference technology for consumer monitors and are generally stable over months. A product such as the GOWENIC wall mount monitor states an NDIR sensor in its listing, which is the specification to look for if you want a number you can trust. Other monitors use a low-cost VOC sensor and estimate CO2 from the total volatile organic compound reading. That approach can respond to cooking, cleaning products, and even breath, but it does not measure CO2 specifically. A monitor of that type may read high in a room with a scented candle and low in a crowded room with no other VOC sources. The listing may not state the sensor type; if it does not, treat the CO2 figure as an estimate.
The Dienmern 5-in-1 listing states a Swiss-made CO2 sensor with a range of 400 to 5,000 ppm and accuracy of plus or minus 50 ppm. That is a plausible specification for an NDIR sensor and gives you a figure you can compare against the bands above. The listing for the 8-in-1 model from the same brand does not publish a sensor type or accuracy figure, so the two products should not be treated as equivalent even though they share a brand and a battery capacity.
The alarm: what a threshold does and does not tell you
Many monitors trigger an audible or visual alarm when CO2 exceeds a set threshold. The GOWENIC listing states that it triggers alarms when levels exceed safe thresholds, but does not publish the threshold values. The 16-in-1 monitor from HSCLPM uses a seven-level AQI indicator rather than a single CO2 alarm, and the listing does not state the CO2 threshold at which each colour changes. If the alarm point matters to you, look for a listing that publishes it, or check the manual before relying on the alarm.
An alarm is not a ventilation system. It tells you the concentration has crossed a line; it does not tell you why or what to do. In a bedroom, the useful response is to open a window or adjust mechanical ventilation, not to silence the alarm. A monitor that alarms at 1,000 ppm is more conservative than one that alarms at 1,500 ppm, but neither is a substitute for understanding the trend.
The limits: when a CO2 number is the wrong question
CO2 is a ventilation indicator, not a pollutant measurement. A room can have low CO2 and high particulate matter from cooking or outdoor pollution, and a CO2 monitor will not show it. If you are concerned about smoke, dust, or wildfire air, you need a PM2.5 monitor; the guide to choosing a PM2.5 monitor covers that. If you are concerned about carbon monoxide, a CO2 monitor will not detect it, and you need a dedicated CO alarm. A CO2 monitor also does not measure humidity or temperature unless the listing states those sensors, and even then the accuracy of the secondary sensors is often not published.
For logging and trend analysis, a monitor that stores data is more useful than one that only displays a live number. The question of how to get the data out is covered in how to export and read CO2 monitor data. For a review of a specific monitor with published sensor specifications, see the Qingping CO2 monitor review.
What to look for on the display
A CO2 monitor should read near 420 ppm outdoors, rise with occupancy, and fall when you ventilate. If it does not, check the sensor type and the calibration before you trust the number. The bands above are guidance, not law, and the product listing is the only place to find the alarm thresholds and accuracy figures for a specific model.
Frequently Asked Questions
What should a CO2 monitor read in a bedroom overnight?
In a closed bedroom with one or two people, the reading will typically rise from the outdoor baseline of about 420 ppm to somewhere between 1,000 and 2,500 ppm depending on room volume and ventilation. A reading that stays below 800 ppm suggests good ventilation; a reading above 2,000 ppm suggests the room needs a window open or mechanical ventilation.
Why does my CO2 monitor read 400 ppm in a crowded room?
Either the sensor is not measuring CO2 directly, or it is not sampling the room air, or it needs calibration. A monitor using a VOC sensor can read low in a crowded room if other VOC sources are absent. A monitor placed near a window or in a draught may read lower than the room average. Check the sensor type in the listing and try the calibration routine if one is described.
What is a dangerous CO2 level indoors?
CO2 is not usually dangerous at the levels found in homes and offices. Sustained readings above 2,000 ppm indicate poor ventilation and can cause drowsiness and poor concentration. Levels above 5,000 ppm are considered an occupational exposure concern, and the listing for the Dienmern 5-in-1 states a detection range up to 5,000 ppm. A CO2 monitor is not a substitute for a carbon monoxide alarm.
Do I need an NDIR sensor?
If you want a number you can compare against ventilation guidance, yes. NDIR sensors measure CO2 directly and are the technology used in the monitors that publish accuracy figures, such as the Dienmern 5-in-1 with its stated plus or minus 50 ppm. VOC-based sensors estimate CO2 and can be misled by other sources. The GOWENIC listing states an NDIR sensor; the HSCLPM and Dienmern 8-in-1 listings do not state the sensor type.
How often should I calibrate a CO2 monitor?
The listing does not publish a calibration interval for any of the products referenced here. NDIR sensors typically drift slowly, and many consumer monitors include an automatic baseline correction that assumes the lowest reading over a period is fresh air. If your monitor has a manual calibration routine, follow the manual. If it does not, compare it outdoors with a known reference before relying on absolute values.
Can a CO2 monitor tell me if my room is well ventilated?
Yes, within limits. CO2 is a good tracer for human-generated ventilation because people are the main indoor source. A reading below 800 ppm in an occupied room generally indicates adequate ventilation. The monitor cannot tell you about pollutants from other sources, such as particles from cooking or VOCs from furniture, so it is one indicator rather than a complete air quality assessment.