Air quality monitors can all detect a range of pollutants in the air, but not every model can pick up the same substances. Check the pollutants a monitor can detect before buying to make sure it can pick up those you’re most concerned about.

Some common pollutants include:

CO2 (carbon dioxide) and CO (carbon monoxide):

CO2 is naturally present in the air, but if it builds up due to a problem with your home heating system, it can become a danger. CO is a potentially deadly gas when found in high enough concentrations. It’s also colorless and odorless, so it can easily go undetected. An air quality monitor measures CO2 and/or CO in parts per million (PPM).

Particulate matter (PM), including dust, pollen, mold, and smoke:

Particulate matter is often found floating in the air and can trigger allergic reactions and other respiratory issues. PM refers to any particles that are between 2.5 and 10 micrometers in diameter, which are known as PM10. Those smaller than 2.5 micrometers are known as PM2.5. Not all air filters can remove PM2.5 particles, so you may have some in your home even if you have a filtration system.

Read more: The best air quality monitor