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Duct cleaning - air quality
How well is a room actually being ventilated? You cannot see fresh air, and measuring airflow directly is technical - but there is a simple proxy: the carbon dioxide level. Because people continuously exhale CO2 and only fresh-air ventilation dilutes it, the CO2 concentration in an occupied room is a readable indicator of how well it is ventilated. Here is how CO2 monitoring works as a proxy for ventilation quality.
The short answer
CO2 monitoring works as a proxy for ventilation quality because of a simple relationship: people continuously breathe out carbon dioxide, and the only thing that removes it from a room is fresh-air ventilation bringing in outside air (which is low in CO2, around 400 ppm). So in an occupied room, the CO2 concentration reflects the balance between the CO2 the people produce and the fresh air brought in to dilute it. If the room is well ventilated for its occupancy, the CO2 stays low - generally below around 800 to 1000 ppm indicates adequate ventilation. If it is poorly ventilated, the CO2 builds up - around 1500 ppm and above indicates poor ventilation that should be improved. So the CO2 level is a readable, measurable stand-in for something you otherwise cannot see: how well the space is being ventilated for the number of people in it. It does not measure the ventilation directly, but it tracks it closely enough to be a genuinely useful proxy - which is why CO2 monitors are widely used to check ventilation quality.
Why ventilation is hard to see
Ventilation quality - how well a room is being supplied with fresh air for the people in it - is important but hard to judge directly. You cannot see fresh air, or feel whether enough of it is coming in; a room can feel stuffy, but that is a vague impression, not a measurement. And measuring the ventilation directly - the actual rate of fresh air being supplied per person - is technical, requiring airflow measurement and knowledge of the occupancy, not something you can readily check. So there is a real difficulty: ventilation quality matters, but it is not easy to know whether a given room, at a given time, is being adequately ventilated.
This matters because inadequate ventilation has real effects - a stuffy, unpleasant environment, reduced concentration and cognitive performance for the people in it, and, as became widely understood, a higher risk of airborne infection spreading. So knowing whether a space is well ventilated is genuinely useful, but the difficulty of measuring it directly gets in the way. This is the problem that CO2 monitoring solves: it provides an easy, measurable proxy for ventilation quality, standing in for the direct measurement that is hard to make. Rather than trying to measure the fresh-air rate directly, you measure the CO2, which tracks it - turning an invisible, technical quantity into a simple reading. That is the value of the CO2 proxy: it makes ventilation quality visible and measurable when it otherwise is not.
The relationship that makes it work
The reason CO2 works as a proxy for ventilation is a simple, reliable relationship between people, CO2 and fresh air. People continuously breathe out carbon dioxide - it is a product of respiration, so wherever there are people, CO2 is being produced. And the main thing that removes CO2 from an indoor space is fresh-air ventilation: bringing in outside air, which is low in CO2 (the outdoor level is around 400 ppm), dilutes and carries away the CO2 the people produce. So in an occupied room, the CO2 concentration is set by the balance between the CO2 being produced by the people and the fresh air being brought in to dilute it.
This balance is what makes CO2 track ventilation. If plenty of fresh air is coming in relative to the number of people, the CO2 they produce is diluted away and the level stays low. If too little fresh air is coming in for the number of people, the CO2 builds up, because it is being produced faster than it is removed. So the CO2 level rises when ventilation is inadequate for the occupancy, and stays low when it is adequate - which is exactly the relationship needed for a proxy. The CO2 does not measure the ventilation directly, but because it responds to the balance of people and fresh air, its level reflects how well the space is ventilated for its occupancy. High CO2 means the fresh air is not keeping up with the people; low CO2 means it is. That is the physical basis of the proxy.
Reading the levels
Because CO2 tracks ventilation, the CO2 level can be read against rough thresholds to judge ventilation quality. The baseline is the outdoor level, around 400 ppm - a perfectly ventilated space would approach this, though occupied indoor spaces sit above it. Generally, an indoor CO2 concentration below around 800 to 1000 ppm indicates adequate ventilation - the fresh air is keeping up with the occupancy, and the space is reasonably ventilated. As the CO2 rises above this, it indicates the ventilation is becoming inadequate for the number of people. Around 1500 ppm and above indicates poor ventilation, which should be treated as a priority to improve - the fresh air is clearly not keeping up.
So the numbers give a readable scale: low CO2 (approaching the outdoor level, or well under 800-1000 ppm) means good ventilation; rising CO2 means declining ventilation for the occupancy; high CO2 (1500 ppm and above) means poor ventilation needing attention. This is not an exact measurement of the fresh-air rate, but a useful indication of where the ventilation stands - enough to tell whether a space is reasonably ventilated or not, and to flag when it is not. And because the CO2 responds to the actual balance of people and fresh air in real time, it reflects the real, current situation - a room that is fine when lightly occupied but poorly ventilated when full will show the CO2 rising as it fills, capturing the real-time ventilation quality. So reading the CO2 level against these rough thresholds gives a practical read on ventilation quality, which is the whole point of the proxy.
What it does and does not tell you
Using CO2 as a ventilation proxy well means understanding what it does and does not tell you. What it tells you is how well the space is ventilated for its occupancy - whether enough fresh air is coming in for the number of people - which is genuinely useful and is what the proxy is for. A rising or high CO2 level is a reliable indication that the ventilation is not keeping up with the people, prompting a check of the ventilation or a reduction in occupancy. So as an indicator of ventilation adequacy for the people present, CO2 is a sound, practical proxy.
What it does not tell you is everything about air quality. CO2 is a proxy for ventilation - for fresh-air supply relative to occupancy - not a direct measure of every pollutant or hazard in the air. It does not detect, say, a specific contaminant, a grease-laden extract, or carbon monoxide (which is a different gas and a different hazard). So CO2 monitoring answers the question of ventilation adequacy for occupancy, not the question of whether the air is free of all hazards. Used within that scope - as a proxy for how well a space is ventilated for its people - it is very useful, and it is why CO2 monitors have become a common, practical tool for checking ventilation quality. Understanding it as a ventilation proxy, with that specific meaning, is what lets you use it well: a simple, readable stand-in for the fresh-air adequacy that is otherwise hard to see, not a complete measure of air quality. Within its scope, it turns an invisible quantity into a number you can act on.
The takeaway
CO2 monitoring works as a proxy for ventilation quality because of a simple, reliable relationship: people continuously exhale CO2, and the only thing that removes it from a room is fresh-air ventilation bringing in outside air (low in CO2, around 400 ppm). So the indoor CO2 concentration reflects the balance between the CO2 the people produce and the fresh air diluting it - low when the space is well ventilated for its occupancy, rising when the fresh air is not keeping up. Below around 800 to 1000 ppm generally indicates adequate ventilation; around 1500 ppm and above indicates poor ventilation to prioritise improving.
This makes CO2 a readable, measurable stand-in for something you otherwise cannot see or easily measure - how well a space is being ventilated for the people in it. It does not measure the ventilation directly, and it is a proxy for fresh-air adequacy rather than a complete measure of air quality - it will not detect a specific contaminant or carbon monoxide. But within that scope it is genuinely useful, responding to the real balance of people and fresh air in real time. That is why CO2 monitors are widely used to check ventilation: they turn the invisible question of fresh-air adequacy into a simple number you can read and act on. Understand it as a ventilation proxy, and it is a practical, reliable tool.
Questions
Because people continuously exhale CO2 and the only thing that removes it from a room is fresh-air ventilation. So the indoor CO2 level reflects the balance between the CO2 the people produce and the fresh air diluting it - low when the space is well ventilated for its occupancy, rising when the fresh air is not keeping up. That makes the CO2 level a readable proxy for how well a space is ventilated.
Because measuring the fresh-air rate directly is technical - it requires airflow measurement and knowledge of the occupancy, not something you can readily check. You cannot see or feel fresh air. CO2 provides an easy, measurable proxy that stands in for the direct measurement, turning an invisible, technical quantity into a simple reading you can act on.
The outdoor level is around 400 ppm. Indoors, below roughly 800 to 1000 ppm generally indicates adequate ventilation; as CO2 rises above this, the ventilation is becoming inadequate for the occupancy; around 1500 ppm and above indicates poor ventilation that should be a priority to improve. These are rough thresholds, but enough to tell whether a space is reasonably ventilated.
Because more people produce more CO2, and if the fresh-air ventilation does not increase to match, the CO2 builds up faster than it is removed. So a room that is fine when lightly occupied can show rising CO2 as it fills - which captures the real-time ventilation quality, reflecting the actual balance of people and fresh air at any moment. The CO2 responds to the real, current situation.
No - it is a proxy for ventilation (fresh-air supply relative to occupancy), not a direct measure of every pollutant. It does not detect a specific contaminant, a grease-laden extract, or carbon monoxide, which is a different gas and hazard. So it answers whether a space is well ventilated for its people, not whether the air is free of all hazards - a useful but specific question.
At the levels seen in poorly ventilated rooms, the CO2 itself mainly causes stuffiness, drowsiness, headaches and reduced concentration rather than acute danger - but it is a signal that the ventilation is inadequate, which matters for comfort, cognitive performance and the risk of airborne infection spreading. So a high reading is a prompt to improve the ventilation, both for the CO2 and for what its level indicates about the fresh air generally.
CO2 shows whether fresh air is keeping up with the people - but a ventilation system only performs while it is clean. Our duct cleaning keeps the ventilation working, so the fresh air it moves stays adequate.
