CO vs CO2: Understanding the Critical Differences

Tuesday 7th October 2025

CO (carbon monoxide) is extremely dangerous even at low levels (50ppm+) and comes from incomplete combustion. CO2 (carbon dioxide) is naturally occurring and only dangerous at very high levels (800ppm+ indoors). Both are colourless and odourless. You need a CO detector for safety; CO2 monitors help with air quality. CO can kill quickly; CO2 mainly affects comfort and concentration at typical indoor levels.

Do you know the difference between carbon monoxide and carbon dioxide? These two gases might sound similar, but understanding what is the difference between them could literally save your life. Both are invisible threats lurking in our homes, offices and public spaces yet they pose completely different risks and require different detection methods.

Carbon monoxide vs carbon dioxide: what’s the difference?

[Keep existing chemical structure diagram]

CO and CO2 are both colourless, odourless gases. However, they’re chemically different: CO is one oxygen atom bound to one carbon atom, whereas CO2 is two oxygen atoms bound to one carbon atom. This difference in CO vs CO2 bond length and molecular structure makes all the difference when it comes to their effects on human health.

Carbon dioxide (CO2) vs carbon monoxide (CO) represents a crucial distinction every homeowner should understand. CO2 is a product of many natural processes in the human body and is safe at normal levels. Due to the similarity in their chemical structure, however, CO is a dangerous gas that’s toxic even at low levels.

Chemical diagram: CO (Carbon and Oxygen) vs CO2 (Carbon and two Oxygen) — Chemical structure of CO vs CO2

What is CO?

Carbon Monoxide is released during ‘incomplete combustion’. This usually happens when a fuel, such as coal, wood or oil, is burned without enough oxygen present. This deadly gas is dangerous even at very low levels. Because its molecules are very similar in structure to CO2, they bind to red blood cells in the body and become ‘stuck’. As a result, the amount of oxygen that can be transported to the body’s essential organs is reduced. This lack of oxygen can cause fatigue, breathlessness, headaches and eventually death.

Levels as low as 50ppm (parts per million) will cause harm, particularly with prolonged exposure, whereas 700ppm can quickly be fatal.

Any level of CO therefore warrants concern, which can only be detected with a working CO detector.

What is CO2?

Carbon Dioxide is released by many natural chemical processes, including combustion, respiration and decomposition. CO2 is used to give fizzy drinks their bubbles, and as a medium in some fire extinguishers. It’s also present in the air we breathe, where it’s safe at normal levels (under 800ppm). However, at extreme levels, CO2 can cause asphyxiation by reducing the level of oxygen available.

CO & CO2 can both cause headaches — Both CO and CO2 can cause headaches

CO vs CO2 symptoms

Both CO and CO2 can cause headaches, but the similarities end there. CO vs CO2 symptoms vary significantly:

CO symptoms include:

Headaches and dizziness
Nausea and vomiting
Fatigue and weakness
Chest pain
Confusion

CO2 symptoms at high levels include:

Drowsiness
Headaches
Increased heart rate
Mild narcosis

CO vs CO2 smell

Here’s a crucial point about CO vs CO2 smell: neither gas has any detectable odour. You cannot smell either carbon monoxide or carbon dioxide, which makes them particularly dangerous. This is why detection equipment is absolutely essential.

CO or CO2, which is more dangerous?

Carbon monoxide vs carbon dioxide: let’s compare their danger levels. CO is significantly more dangerous at lower concentrations. Carbon monoxide can be lethal at just 700ppm, whereas CO2 becomes dangerous only at much higher concentrations (around 40,000ppm for immediate danger).

CO2 and indoor air quality

Although carbon dioxide is naturally present in the air, high levels can have negative effects on human health. Poor ventilation in an enclosed environment can lead to raised CO2 levels. This often leads to headaches, fatigue and poor concentration. Safe indoor levels are below 800ppm; in classrooms, offices and other public venues, levels can easily reach over 1000ppm.You can improve indoor air quality by opening windows to improve ventilation, installing air purifiers, and reducing damp and humidity with dehumidifiers. Measuring CO2 levels with an indoor air quality monitor is the best way to track this and know when improvements must be made.

10 Year Life Digital Carbon Monoxide Alarm - UltraFire UBCO1D — UltraFire Digital Display CO detector

Do I need a carbon monoxide or dioxide detector?

This question arises frequently, and the answer depends on your situation. CO and CO2 detector needs vary:

CO detectors should be installed to detect the presence of CO. Without this, it’s impossible to detect the presence of this deadly gas, which is dangerous even at low levels. A CO alarm with a digital display allows users to monitor levels too low to trigger the alarm, encouraging them to improve ventilation.

CO and CO2 monitor systems have traditionally been used in commercial premises like breweries or laboratories, where fatal levels of CO2 might be released by chemical reactions. However, new technology has made these detectors more affordable and accessible. CO and CO2 sensor technology is now recommended in schools, offices and even homes to track and improve indoor air quality.

Will CO2 set off a CO detector?

No, CO2 will not set off a CO detector. These devices use different sensing technologies. A CO vs CO2 detector comparison shows they’re designed for completely different gases. CO vs CO2 sensor technology works on different principles, so you need the right detector for the right gas.

Detecting CO vs CO2

CO and CO2 detector installation requires different approaches. Carbon monoxide detectors should be installed to detect the presence of CO, whereas CO vs CO2 sensor systems for carbon dioxide monitoring focus on air quality management.

CO detector technology typically uses electrochemical sensors, whereas CO and CO2 sensor systems for carbon dioxide often use infrared technology. Understanding these differences helps you choose the right protection for your home.

Glossary of Terms

Carbon monoxide (CO): A toxic gas with one carbon atom and one oxygen atom, produced by incomplete combustion

Carbon dioxide (CO2): A natural gas with one carbon atom and two oxygen atoms, produced by respiration and combustion

ppm: Parts per million – a measurement of gas concentration

Incomplete combustion: Burning fuel without sufficient oxygen, producing dangerous CO

Electrochemical sensor: Technology used in CO detectors to identify carbon monoxide

Infrared sensor: Technology commonly used in CO2 monitors

Asphyxiation: Suffocation caused by a lack of oxygen

Indoor Air Quality (IAQ): The air quality within buildings, affected by CO2 levels, among other factors

CO vs CO2: both are colourless, odourless gases, which pose different health risks. The only way to stay safe from these gases is to have the appropriate detector fitted. For additional support in selecting the best detector for your needs, contact our friendly customer support team on 0800 612 6537.

All information is correct at the time of posting.

Frequently Asked Questions

Do I need a carbon monoxide or dioxide detector for my home?

You definitely need a CO detector for safety, as carbon monoxide can be lethal even at low levels and there’s no way to detect it without proper equipment. Whether you need a CO and CO2 detector depends on your specific situation. Most homes require CO detection for safety, but a CO and CO2 monitor can also help track indoor air quality. If you have fuel-burning appliances like gas boilers, wood burners or oil heaters, a CO detector isn’t optional – it’s essential for protecting your family.

What is the difference between CO and CO2 in terms of danger levels?

CO or CO2, which is more dangerous? Carbon monoxide is far more dangerous at lower concentrations. CO becomes harmful at just 50ppm and can be fatal at 700ppm, whereas CO2 only becomes dangerous at much higher levels (around 40,000ppm for immediate danger). The key difference is that CO prevents your blood from carrying oxygen properly, whereas CO2 mainly displaces oxygen in the air. This makes carbon monoxide vs carbon dioxide: let’s compare, a matter of immediate lethality versus long-term air quality concerns.

Will CO2 set off a CO detector, and can I use one device for both gases?

No, CO2 will not set off a CO detector. These devices use completely different sensing technologies. A CO vs CO2 detector comparison shows they’re designed for specific gases – CO vs CO2 sensor technology works on different principles. CO detectors typically use electrochemical sensors, whereas CO2 monitors use infrared technology. You cannot rely on one device to detect both gases, so you need the right detector for each gas.

Carbon monoxide vs carbon dioxide: what’s the difference in terms of CO vs CO2 symptoms?

CO vs CO2 symptoms vary significantly. Carbon monoxide poisoning causes headaches, dizziness, nausea, fatigue, chest pain and confusion – symptoms that can quickly become life-threatening. CO2 at high levels causes drowsiness, headaches, increased heart rate and mild narcosis, but these symptoms typically occur at much higher concentrations. The crucial point about CO vs CO2 smell is that neither gas has any detectable odour, making symptoms your only warning sign without proper detection equipment.

How does CO vs CO2 bond length affect their toxicity?

The CO vs CO2 bond length and molecular structure explain why these gases affect us so differently. Carbon dioxide (CO2) vs carbon monoxide (CO) comes down to molecular makeup: CO has one oxygen atom bound to one carbon atom, whereas CO2 has two oxygen atoms bound to one carbon atom. This structural difference means CO molecules are similar enough to oxygen to bind with red blood cells but different enough to get ‘stuck’, preventing oxygen transport. CO2, being larger and different in structure, doesn’t have this binding problem.

Do you know the difference between when to use a CO and CO2 sensor?

A CO and CO2 sensor serves different purposes. Use a CO detector anywhere you have fuel-burning appliances, sleeping areas or enclosed spaces where incomplete combustion could occur. A CO and CO2 monitor for carbon dioxide is more about air quality management – useful in offices, schools, bedrooms or poorly ventilated spaces where CO2 buildup affects comfort and concentration. Many modern CO vs CO2 detector systems can monitor both, but remember that CO detection is about safety, whereas CO2 monitoring is about air quality.

When comparing CO and CO2, which situations require immediate action?

Any CO detection requires immediate action – evacuate the area, get fresh air and call the emergency services if anyone shows symptoms. CO and CO2 monitoring shows different urgency levels: CO levels above 50ppm need investigation, and anything above 200ppm requires immediate evacuation. For CO2, levels above 800ppm indoors suggest you need better ventilation, and levels above 5000ppm require immediate fresh air.

All information correct at time of posting.