What CO2 level indicates poor ventilation?

As a guide, readings consistently above about 1,500 parts per million suggest ventilation is inadequate. Below about 800 ppm is generally good, and around 1,000 ppm is a commonly used threshold for acceptable.

Is high CO2 in the home dangerous?

At the levels found in homes, CO2 is not usually harmful in itself, but it is a useful sign that ventilation is poor and that moisture and other pollutants may be building up, which can affect comfort, sleep and the building.

What is a normal CO2 level in a bedroom?

A well ventilated bedroom typically stays below about 1,000 ppm, though it can spike overnight with the door closed. A room that sits high for long periods points to inadequate ventilation.

How do I lower CO2 at home?

In the short term open a window. Longer term, improve ventilation: open trickle vents, check extract fans, or make sure a mechanical system such as MVHR is running and properly commissioned.

Does MVHR protect against radon?

Not on its own. MVHR gives constant fresh air and, because a balanced system does not depressurise the home, it avoids actively drawing radon up from the ground, and airtight homes with MVHR tend to show lower radon. But in a radon affected area MVHR is not a substitute for proper radon protection, which means a radon barrier and a sump or ventilated subfloor. Treat MVHR as a help, not a fix.

How do I deal with radon in a new or airtight home?

Check whether your address is in a radon affected area using the UKHSA radon map, and if it is, follow the standard protection: a radon resistant membrane across the footprint plus provision for a radon sump or subfloor ventilation. Positive input ventilation can help at lower radon levels, but above roughly 500 becquerels per cubic metre a sump or subfloor system is usually needed. MVHR supports good air quality alongside these measures.

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CO2 levels and ventilation

Carbon dioxide, or CO2, is one of the most useful indicators of how well a home is ventilated. People breathe it out, so when it builds up indoors it is a sign that fresh air is not coming in fast enough. This guide explains what CO2 readings mean and what to do about high levels.

By VentRight Editorial · Last updated 2026-07-08 · Impartial · Sourced

Why is CO2 a good measure of ventilation?

People breathe out carbon dioxide, so indoor CO2 rises when a room is occupied and falls when fresh air comes in. That makes it a simple, direct proxy for how well a space is ventilated: if CO2 stays low, enough fresh air is arriving; if it climbs, ventilation is not keeping up. It is easy to measure with a cheap monitor.

CO2 itself is not usually harmful at the levels found in homes, but it tracks the other things you cannot see, such as moisture and indoor pollutants, that build up when ventilation is poor. That is why it is used as an indicator: a room with high CO2 is almost certainly under-ventilated in other ways too.

What is a good CO2 level indoors?

Outdoor air is around 400 parts per million. Indoors, below about 800 ppm is generally considered good, around 1,000 ppm is a commonly used threshold for acceptable, and readings consistently above about 1,500 ppm suggest ventilation is inadequate. These are widely used guide figures rather than a single legal limit, but they are a useful yardstick.

A well ventilated home, or one with MVHR, typically holds bedrooms and living rooms comfortably below 1,000 ppm even when occupied. Spikes are normal, for example in a bedroom overnight with the door closed, but a room that sits high for long periods is telling you fresh air is not reaching it.

What does a high CO2 reading mean?

A high CO2 reading means the fresh air supply is not keeping up with the people in the room. It does not mean the CO2 itself is dangerous at household levels, but it signals that moisture and other pollutants are likely building up too, and that the space feels stuffy. A bedroom that climbs high overnight is a common example.

Context matters: a brief spike when a room is full is normal, while a persistently high reading points to a ventilation problem. If a room is high even when lightly occupied, or does not recover when people leave, the ventilation is the thing to look at, whether that is trickle vents, extract fans or a mechanical system.

What should I do if CO2 levels are high?

In the short term, open a window or door to bring the reading down. As a longer-term fix, improve the ventilation: open trickle vents, check that extract fans work, or, in an airtight home, make sure the mechanical system is running and commissioned correctly. Persistent high readings in a modern home suggest the ventilation needs attention.

If you have MVHR or continuous extract and CO2 is still high, the system may be off, set too low, or poorly commissioned, and is worth checking. In an older home, better background ventilation or a mechanical system may be the answer. A monitor helps you see whether a change actually works.

Questions

What CO2 level indicates poor ventilation?: As a guide, readings consistently above about 1,500 parts per million suggest ventilation is inadequate. Below about 800 ppm is generally good, and around 1,000 ppm is a commonly used threshold for acceptable.
Is high CO2 in the home dangerous?: At the levels found in homes, CO2 is not usually harmful in itself, but it is a useful sign that ventilation is poor and that moisture and other pollutants may be building up, which can affect comfort, sleep and the building.
What is a normal CO2 level in a bedroom?: A well ventilated bedroom typically stays below about 1,000 ppm, though it can spike overnight with the door closed. A room that sits high for long periods points to inadequate ventilation.
How do I lower CO2 at home?: In the short term open a window. Longer term, improve ventilation: open trickle vents, check extract fans, or make sure a mechanical system such as MVHR is running and properly commissioned.
Does MVHR protect against radon?: Not on its own. MVHR gives constant fresh air and, because a balanced system does not depressurise the home, it avoids actively drawing radon up from the ground, and airtight homes with MVHR tend to show lower radon. But in a radon affected area MVHR is not a substitute for proper radon protection, which means a radon barrier and a sump or ventilated subfloor. Treat MVHR as a help, not a fix.
How do I deal with radon in a new or airtight home?: Check whether your address is in a radon affected area using the UKHSA radon map, and if it is, follow the standard protection: a radon resistant membrane across the footprint plus provision for a radon sump or subfloor ventilation. Positive input ventilation can help at lower radon levels, but above roughly 500 becquerels per cubic metre a sump or subfloor system is usually needed. MVHR supports good air quality alongside these measures.; Source: UKradon (UK Health Security Agency)