Office CO2 Monitor: What CO2 Level Means It Is Time to Ventilate?

An hour into a meeting with eight people, the room gets heavy. Attention drifts, someone yawns, and nobody thinks of opening a window. An office CO2 monitor makes that moment visible. It measures the carbon dioxide in the room in ppm (parts per million) and, on models with a traffic light, turns the number into green, yellow and red. The short answer to the question in the title: once the reading climbs past 1000 ppm, it is time to ventilate, and if it stays above 1500 ppm while people are in the room, the UK Health and Safety Executive treats that as a sign of poor ventilation that needs action.

Whether the display changes behavior or gets ignored after two weeks comes down to thresholds, placement and how the readings are used.

What is a good CO2 level in an office?

Everyone exhales carbon dioxide. Exhaled breath contains around 4 percent CO2, while outdoor air sits at roughly 420 ppm. In a closed room with people working, the level rises steadily until someone opens a window or the ventilation system brings in enough fresh air. A good office level is one that stays close to outdoor air: below about 1000 ppm throughout the working day, with short peaks that drop again after ventilating.

The 1000 ppm mark has a long history. In 1858 the Munich hygienist Max von Pettenkofer proposed 0.1 percent CO2 as the limit for good indoor air, and that figure, 1000 ppm, is still the most common yellow threshold on CO2 traffic lights. It also has a practical meaning: HSE notes that 1000 ppm corresponds to about 10 liters of fresh air per second per person, the ventilation rate many guides recommend for commercial buildings.

At office concentrations, CO2 itself is not toxic. It is a proxy. Where exhaled CO2 builds up, so do odors, humidity and airborne particles from breathing. That is why the number is useful, and also why it has limits: a CO2 reading says nothing about carbon monoxide, mold or fine dust, and a good value does not certify that the air is safe in every respect.

CO2 levels in ppm: which values mean fresh, stuffy or poor air?

There is no single official scale, but the ranges used by HSE and by German workplace and environmental guidance line up well. The table below combines them into one practical reading guide.

The point where opinions differ is red. A 1000/1500 ppm scheme turns red at the level where HSE asks for action, so people ventilate before the air is really bad. A 1000/2000 ppm scheme leaves the light yellow for a long stretch, and yellow is easy to ignore. For offices and meeting rooms, the earlier warning is usually the better choice.

What CO2 level does UK workplace guidance recommend?

The legal starting point in Great Britain is regulation 6 of the Workplace (Health, Safety and Welfare) Regulations 1992, which requires effective and suitable ventilation in enclosed workplaces. It does not set a CO2 limit. The practical numbers come from HSE guidance on CO2 monitors: readings consistently higher than 1500 ppm in a room in use indicate poor ventilation that you should improve.

HSE also gives a ventilation rate to aim for. The fresh-air supply should not normally fall below 5 to 8 liters per second per person, and 10 liters per second per person is recommended in many guides for commercial buildings. Since 1000 ppm corresponds to roughly that 10-liter rate, keeping an office below 1000 ppm and treating 1500 ppm as the action line fits the guidance well.

HSE is clear about the limits of the method. CO2 is only a broad guide to ventilation and does not demonstrate “safe levels”. Monitors are also unsuitable where processes produce CO2 or where air cleaners remove contaminants but not CO2. Do not confuse any of this with the workplace exposure limit for CO2 of 5000 ppm in EH40, which applies to work involving CO2 as a hazardous substance, such as dry ice or brewing, not to office air quality.

If your office is in Germany, the reference is the workplace rule ASR A3.6. Below 1000 ppm no further measures are needed, 1000 to 2000 ppm calls for a ventilation plan, and above 2000 ppm for further measures.

How does a CO2 monitor with a traffic light work?

Inside a proper unit sits an NDIR sensor. It sends infrared light through a small chamber and measures how much of it CO2 molecules absorb at a specific wavelength. That is a real CO2 measurement. Cheaper gadgets often have only a VOC sensor and calculate an estimated “eCO2” value from other gases, which can be wildly off when someone wipes the desk with cleaner or wears strong perfume. Check the spec sheet for NDIR before trusting a device in a workplace.

The traffic light is simply logic on top of the reading. You define two thresholds, and the device shows green below the first, yellow between them and red above the second. On the Milesight AM307, the three levels are called Excellent, Polluted and Bad, and you set the limits over NFC with the ToolBox smartphone app. The AM307 LED blinks green, yellow or red to match, and a buzzer sounds once the Bad threshold is exceeded; it can be switched off. Next to the LED, a 4.2-inch E-Ink screen shows the CO2 value with a trend chart, temperature and humidity.

One detail matters when you troubleshoot. The AM307 traffic light also reacts to its built-in TVOC sensor. If the LED turns yellow or red while the CO2 figure on the screen looks fine, the cause may be cleaning products, disinfectant or new furniture rather than stale air. And if the light stays red even after the windows have been wide open for ten minutes, check the configured thresholds and the mounting position before blaming the sensor.

How often should you ventilate an office?

A quick estimate shows why there is no single answer. An adult at a desk exhales roughly 20 liters of CO2 per hour. Two people in a 30-square-meter office with about 80 cubic meters of air raise the level by around 500 ppm per hour, so a freshly aired room at 450 ppm reaches 1000 ppm in a little over an hour. Eight people in a meeting room with 67 cubic meters get there in about 15 minutes.

German workplace guidance turns this into rule-of-thumb intervals for window ventilation: open up after about 60 minutes in an office and after about 20 minutes in a meeting room. The minimum duration depends on the temperature difference between inside and outside, roughly 3 minutes in winter, 5 minutes in spring and fall, and up to 10 minutes in summer.

A CO2 monitor lets you replace the clock with a measurement. When the display turns yellow, open the windows wide, ideally on opposite sides of the room for cross ventilation. A tilted window exchanges very little air while cooling the wall around it and wasting heating energy in winter. After a few minutes the reading should fall clearly, often to 500 to 600 ppm.

In buildings with mechanical ventilation, the monitor works more as a check. If a room regularly passes 1000 ppm, the system is set too low for how the room is actually used, runs too short, or recirculates too much air. That is a job for facilities.

Where should you place a CO2 monitor in an office?

The same device can show a reassuring 500 ppm beside an open window and well over 1500 ppm an arm’s length from someone talking. Neither is a sensor fault; both are poor locations. The goal is a spot that represents the air where people actually sit.

HSE recommends placing monitors at head height and more than 50 centimeters away from people, because exhaled breath can push the reading misleadingly high. Keep the device away from windows, doors and air supply openings, and out of direct sun or radiator heat. The opposite mistake is just as common: a monitor tucked into a shelf recess or behind a cabinet sits in still air and reacts slowly.

The display also has to be visible, or it will not change behavior. In a meeting room, the wall facing the table is often a good compromise; in a small office, a wall beside the desk rather than directly in front of it. The AM307 mounts on a wall with adhesive tape or screws.

Try two or three positions over a few ordinary working days before fixing the device permanently, and compare the curves. In larger spaces, HSE notes that more than one sampling location will usually be needed, since partitions, furniture and uneven airflow create zones with their own pattern.

Do classrooms need CO2 monitors?

Classrooms are where CO2 rises fastest, because so many people share so little air. A class of 25 to 30 pupils typically has about 200 cubic meters of air. Children exhale less CO2 than adults, but numbers make up for it: even at a conservative 15 liters per head per hour, the level climbs by almost 1900 ppm per hour, so a room starting at 450 ppm passes 1000 ppm after roughly 20 minutes without ventilation. A double lesson with closed windows almost inevitably ends far above 2000 ppm.

In England, the Department for Education began supplying CO2 monitors to state-funded schools in 2021, with guidance to act on ventilation where readings are consistently above 1500 ppm. School ventilation design guidance (BB101) sets a similar bar for naturally ventilated classrooms: a daily average below 1500 ppm during teaching, with peaks above 2000 ppm lasting no more than 20 consecutive minutes.

So the answer is yes: a monitor turns an abstract rule into something the whole class can see, and many teachers hand the job of opening windows to a rotating pupil. Mount it on a wall rather than the teacher’s desk so everyone can see it and nobody moves it. On the AM307 the power button can be locked through the app. And be honest about what it does: it shows how well a room is ventilated, it does not measure infection risk.

How do you log office CO2 levels over LoRaWAN?

A display in the room helps the people sitting in it. Whether a room is ventilated too poorly in general only shows up in weeks of data. That is where a connected monitor pays off, especially once you look after more than one or two rooms.

The Milesight AM307 sends its readings over LoRaWAN, every 10 minutes by default and adjustable from 1 minute to 18 hours. A LoRaWAN gateway in the building receives the packets and forwards them to a network server, from where they reach the platform or dashboard of your choice. If the connection drops, the sensor stores up to 18,000 records locally and can resend them later. When a value crosses the Bad threshold, it sends an alarm packet right away instead of waiting for the next interval. It runs on four replaceable lithium batteries, which the manufacturer rates at up to about three years at a 10-minute interval, or on USB-C power.

The value lies in the analysis. If one meeting room goes red every Tuesday morning, that is a planning problem, not individual carelessness. A single short spike, on the other hand, often has a trivial cause. Make sure your dashboard shows how old the latest reading is, since a value from three hours ago says nothing about the room right now, and set up a separate alert for a sensor that has stopped reporting.

Where can you buy a CO2 sensor for your office?

Start with a true NDIR sensor rather than an estimated eCO2 value, because only that measures carbon dioxide. The thresholds should be adjustable so you can choose between a 1000/1500 ppm and a 1000/2000 ppm scheme. A readable screen with the actual number complements the traffic light, because a color alone never settles a discussion. Check whether the device can be wall-mounted and how it is powered, since a cable otherwise decides the location for you. Find out how calibration works, automatically or manually outdoors. Finally, decide whether a display in the room is enough or whether you want to log readings over LoRaWAN for several rooms. With these points in mind, you will quickly narrow down the office CO2 monitor that fits your rooms and your ventilation routine. If you are looking for a suitable device, you can find our models in the CO2 sensors category.

CO2 Ventilation FAQ

How accurate is a CO2 traffic light?

It depends mainly on the sensor. NDIR devices typically reach an accuracy of a few dozen ppm; the Milesight AM307 is specified at ±(30 ppm + 3% of reading), which is about ±60 ppm at 1000 ppm. That is more than enough to decide when to ventilate, though a traffic light is not a lab instrument.

Do CO2 sensors need calibration?

NDIR sensors drift slightly over the years, so occasionally, yes. Many devices use automatic background calibration: the AM307 treats the lowest value over seven days as the fresh-air level of 400 ppm. That only works if the room is properly aired at least once in that period; otherwise, a manual calibration outdoors is the better option.

What CO2 level makes you tired?

There is no sharp threshold, and sensitivity varies from person to person. Many people perceive air above roughly 1000 to 1500 ppm as stuffy, and German workplace guidance notes that elevated CO2 tends to impair attention. Whether CO2 itself or the other components of stale air cause the tiredness is not fully settled.