Same experience with the CCS811 VoC sensor I had tested for many months.
The levels will drift higher and higher over time to the point you have no idea what is real or just drifted garbage readings.
The solution is to use this VoC sensor instead. It is advertised as a non drift sensor and after 5-6 months of testing it out I can say it certainly does not drift and stays stable for me.
If I remember correctly the default Adafruit library works out of the box on the Particle P1 & Photons. I have it working just fine.
You can see in the graph below that over a 30-day time frame the “Baseline” VOC & Co2 levels remain very stable. This is placed in a shop with a Laser Cutting Machine, where cleaning alcohol & glues are used regularly which cause the spikes in VOC levels you see.
Here is what a 12 hour reading looks like updated every min
