Many listings on Amazon, AliExpress, eBay and in other places incorrectly state that the CCS811 is a Carbon Monoxide (CO) sensor. It is not. It reports eTVOC (equivalent Total Volatile Organic Compounds) and eCO2 (equivalent Carbon Dioxide). The CCS811 is not capable of measuring Carbon Monoxide and any listing or description of it that says it does is simply incorrect. It also doesn’t truly measure CO2, but that’s a story for another day.
I’ve been playing with the CCS811 in a project and have noticed that many places incorrectly call it a Carbon Monoxide - CO - sensor. It does not measure Carbon Monoxide and is not a Carbon Monoxide sensor. You can confirm this by reading its datasheet - which doesn’t mention carbon monoxide or CO even once.
It’s not even truly a carbon dioxide sensor, though it’s sold as one. Calling it a CO2 sensor is at least somewhat defensible (at least the datasheet describes it this way) but calling it a CO sensor is completely wrong. But that’s an issue for another time.
If you want to measure carbon monoxide, it’s important that you use something that actually measures real carbon monoxide, like an MQ-7. And if you find an interesting part in a product listing, always read its datasheet before ordering it or using it in a project, because product listings can be misleading to outright incorrect.