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CO2 concentration can be a good indication of the ventilation in your home. More specifically, it can help you ascertain whether exhaled air is building up in your home, or being properly ventilated. Therefore, in certain circumstances, keeping indoor CO2 levels low can help mitigate the risk of airborne virus transmission. If this is your main purpose for monitoring indoor air quality, we highly recommend the Gain Express Wall Mount. First off, the readings on this meter stayed incredibly close to those we saw on our professionally calibrated CO2 meter throughout our testing, giving us a high degree of confidence in its accuracy. Secondly, the convenient wall mount and large display allow it to be placed in an easily readable position. This negates the need to repeatedly check an app on your phone or constantly saunter over to the meter to keep an eye on CO2 levels. Thirdly, if you need help remembering to monitor CO2 levels at critical times, you can set custom alarms that will sound when the concentration reaches a particular level. Finally, if you're more interested in CO2 concentration trends and averages, this unit can quickly display averages for both the last 15 minutes and the last 8 hours. If you want to go further, the unit can be connected to a PC via an RS232 connection for continuous data logging. 
A slight knock against the Gain Express Wall Mount is its lack of a battery, which makes it harder to move it around the house as needed. Additionally, it is CO2-focused, so it won't be helpful during the summer fire season. However, if you're concerned explicitly with CO2, this meter offers reliable and straightforward tracking with a display that can be read from a distance. 

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The Yvelines Air Quality Tester provides reliable air quality monitoring at a significantly lower cost than many of the most popular models. It covers all the bases, offering two different particulate pollution measurements, as well as volatile organic compounds, formaldehyde, and CO2 measurements. We were particularly impressed with its PM2.5 &#40;the pollutants associated with wildfire smoke and other smoke-based sources of air pollution&#41; accuracy, as throughout our tests it stayed almost in lockstep with our professional-grade air quality meter. Simple controls let you scroll through recent short-term averages for each measurement. These averages change based on the measurement; for example, the formaldehyde average is 60 minutes, while the CO2 and PM2.5 averages are 10 minutes. The color screen is easy to read regardless of lighting conditions. The built-in battery allows you to take measurements without power for up to 12 hours at a time. 
While we find the short-term averages provided by the Yvelines Air Quality Tester to be quite useful, they likely won't be sufficient for those interested in tracking long-term air quality trends in their home. Doing so would require labor-intensive manual tracking of the unit's readings. Additionally, this unit's CO2 readings were consistently more than 10% off from those of our professionally calibrated CO2 meter. However, its CO2 readings were consistently 15% to 20% higher than expected. Since it typically read higher than the calibrated pro meter in our testing, we still feel it could be a useful tool for those wanting to make sure the CO2 concentration in their home doesn't get too high. Overall, this meter is a great and relatively inexpensive option for those that want to make sure the air quality in their home stays within acceptable limits during wildfire season or the cold winter months when proper ventilation becomes difficult. 

Air Quality Monitor Comparison