CO2 & ventilation
Fresh air is hard to measure directly, so buildings measure carbon dioxide instead. Here is why CO2 works as a proxy, what the thresholds mean, and what a rising line is telling you.
You cannot see fresh air, and measuring it directly, litre by litre at every desk, is difficult and expensive. So the building industry uses a clever shortcut: it measures carbon dioxide instead. Because people breathe CO2 out constantly, its level in a room tracks how well fresh air is keeping up with the people in it. A cheap sensor on the wall becomes a proxy for something that would otherwise be very hard to know.
Outdoor air sits at roughly 420 parts per million of carbon dioxide today, and that is the floor any indoor space starts from. Put people in a room and the number climbs, because each occupant adds CO2 with every breath. If the ventilation is bringing in enough fresh air, that added CO2 is diluted and carried away, and the level stays low. If it is not, CO2 accumulates and the reading rises. The gas itself is not the main hazard at these concentrations; its value is as a signal. A rising CO2 level is really telling you that fresh-air supply is losing the race against occupancy, and that other occupant-generated pollutants are almost certainly building up alongside it.
Reading the numbers
A rough ladder has become widely accepted for interpreting readings. Below about 800 parts per million indicates good ventilation, with plenty of fresh air. Between 800 and 1000 is generally considered acceptable. Above 1000 suggests ventilation should be improved, and sustained readings from around 1500 upwards are treated as a clear red flag for inadequate fresh air. UK advice from the SAGE environmental group during the pandemic set 1500 parts per million as a general upper limit, with a stricter 800 for higher-risk settings such as gyms, choirs or crowded spaces where aerobic activity and loud speech raise the stakes. The often-quoted figure of 1000 has long served as a rule-of-thumb comfort target rather than a hard safety limit.
It is worth being precise about what these numbers are and are not. They are ventilation and comfort indicators, not toxicity limits. The occupational exposure limit for CO2 as a hazardous substance sits far higher, at 5000 parts per million averaged over a working day, a threshold about preventing acute effects in industrial settings, not about comfort or concentration in an office. Another common benchmark, from ASHRAE, frames good ventilation as keeping indoor CO2 no more than about 700 parts per million above outdoor levels. All of these describe the same underlying idea from different angles: how much occupant-generated air is being allowed to accumulate.
The measurement itself is now cheap and reliable. Most monitors use a non-dispersive infrared, or NDIR, sensor, which reads CO2 by how much infrared light the gas absorbs in a small chamber. These devices are affordable enough to place in meeting rooms, classrooms and offices, and give a live, honest read on ventilation that no amount of guesswork can match. The catch is placement and interpretation: a sensor by an open window or in an empty room tells you little, while one in the occupied zone, watched over a full day, tells you plenty.
What a rising line tells you
The point of monitoring is that it turns an invisible problem into a decision. When CO2 climbs past comfortable levels in a particular room at a particular time, that is a specific, located ventilation shortfall you can act on, by increasing fresh-air supply, reducing occupancy, or investigating why the system is underperforming. It is the same shift from symptom to diagnosis that lets an operator tell whether a building is getting the airflow it needs rather than simply feeling that a room is stuffy. A number you can trend is far more actionable than a complaint you cannot pin down.
When the reading blames the ducts
A CO2 monitor identifies a ventilation problem but does not, on its own, solve it. Persistently high readings despite a system that should cope often point to a system not delivering its design airflow, and fouled ductwork is a frequent, hidden reason: dust and grease accumulate inside ducts and choke the fresh air before it arrives. Restoring performance means cleaning the system, and it also protects the efficiency features built into modern ventilation, such as heat recovery in commercial ventilation, which only work when air moves freely. Monitoring and maintenance are two halves of the same job: one tells you the air is failing, the other puts it right.
Questions
Because people exhale carbon dioxide constantly, its indoor level tracks how well fresh air keeps pace with occupancy. Measuring CO2 with a cheap sensor is far easier than measuring airflow directly, and a rising level signals that ventilation is falling behind.
Below about 800 parts per million indicates good ventilation, and 800 to 1000 is generally acceptable. Above 1000 suggests ventilation should improve, and sustained readings around 1500 or higher are a clear sign of inadequate fresh air.
At typical indoor levels, CO2 is not directly harmful, it is a ventilation indicator, not a toxin. The occupational exposure limit is far higher, at 5000 parts per million over a working day. The concern at office levels is comfort, concentration and what else is accumulating.
The stricter 800 parts per million figure is advised for higher-risk, high-occupancy settings such as gyms, choirs or crowded rooms, where aerobic activity and loud speech increase airborne risk. For general spaces, 1000 to 1500 is the more common reference range.
Persistently high readings can mean the system is not delivering its design airflow. Fouled ductwork is a common hidden cause, as dust and grease choke fresh air before it arrives. Cleaning the ductwork restores airflow so the ventilation can bring levels back down.
Phoenix Duct Clean · by the numbers
Phoenix cleans commercial ductwork and extraction systems across the region, restoring the airflow that brings levels down. Talk to us about a survey.