CO2 monitor purchasing guide

When we breathe out, we release carbon dioxide (CO₂) - a gas that’s naturally produced by our bodies. In outdoor spaces, fresh air helps keep CO₂ levels low. But when we’re indoors, especially in rooms without good airflow, CO₂ can build up because the air isn’t being replaced often enough with fresh air from outside. The more people in the room and the longer they stay, the more CO₂ accumulates.

This is important because high CO₂ levels can indicate poor ventilation. If there’s not enough fresh air circulating, we end up breathing in more of the air that other people have exhaled - this is called “rebreathed air”. Since exhaled air can also contain respiratory droplets that may carry viruses, good ventilation helps reduce the risk of spreading illnesses.

CO₂ itself is a colourless and odourless gas, so we can’t see or smell when levels are getting too high. That’s why a CO₂ monitor is useful - it’s a small device that measures the amount of carbon dioxide in the air and gives you a reading so you can tell if a space is well-ventilated or not.

There are different types of CO₂ monitors available, and they vary in terms of features, accuracy, and price. This guide will help you choose the right CO₂ monitor based on your specific needs, whether it’s for your home, school, workplace, or another indoor setting.

Read more about why CO₂ monitoring matters here.

What to look for

Sensor type
For the most accurate CO₂ readings, choose a monitor that uses a non-dispersive infrared (NDIR) or a photoacoustic sensor. These types of sensor provide the most reliable and accurate readings because they specifically measures CO₂ and isn’t easily affected by other substances in the air, or changes in heat and humidity.

Other sensor types can give less accurate results.

Real-time or frequent readings
It’s important to pick a monitor that takes measurements at regular intervals - or one where you can adjust how often it measures, depending on your needs.

Frequent readings help you track CO₂ levels throughout the day, so you can spot trends and make changes to improve ventilation when needed.

Display size
Choose a monitor with a display that’s easy to see and read from a distance. A clear, well-sized display makes it quicker and easier to check CO₂ levels at a glance.

Manual calibration
Choose a monitor that allows you to manually calibrate the unit. Manual calibration helps ensure your CO₂ monitor continues to take accurate readings over time, especially if it's used in spaces where outdoor air isn't regularly introduced. Some monitors can drift or misread levels if they're left to auto-calibrate. This feature isn’t available on all models, so is worth looking out for.

Data logging and apps
Some CO₂ monitors can store past readings or connect to apps that track data over time. This is especially helpful for places like workplaces, schools, or public spaces where it’s useful to monitor long-term ventilation trends and make ongoing improvements.

Power source: battery or mains power?
Decide whether you need a portable, battery-powered monitor or a mains-powered one.

Battery-powered models offer flexibility and can be easily moved around, but frequent measurements will drain the battery faster. Mains-powered devices are better for continuous monitoring in one location without worrying about battery life.

CO₂ range and accuracy
Look for a monitor that measures CO₂ levels in the range of 400–10,000 parts per million (ppm) and has a high level of accuracy - ideally within a small margin of error, like ±50 ppm or ±5%.

What to avoid

VOC-only monitors
Some air quality monitors focus on measuring Volatile Organic Compounds (VOCs) instead of CO₂, or they estimate CO₂ levels based on other substances in the air. These aren’t reliable for measuring ventilation - make sure the monitor specifically measures CO₂.

Unknown brands
Stick to well-known, trusted brands and check for independent reviews or third-party testing to ensure accuracy and quality.

Helpful tips

Positioning
Place the monitor at breathing height (like at table level), away from open windows, air vents, or fans - and avoid breathing directly on it, as that can give misleading readings.

Calibration
Follow the manufacturer’s instructions to regularly calibrate your device and keep it accurate.

Using your data
Use the CO₂ readings to guide decisions on improving ventilation - like opening windows, using portable air filters, like HEPA filters, or adjusting air conditioning and ventilation systems.

For more detailed information on CO₂ monitors, including reviews and more, check out the articles available at Breathe Safe Air.

With thanks to Ethan Brooke of Breathe Safe Air for his input into this article.