Analog gas and air quality sensors have quite negligent readings. They need special calibration to measure correctly.
This is why CCS811B digital air quality sensor by AMS is so interesting. It can measure indoor air quality and CO2 content with digital readings from 400 up to 29206 ppm.
MOD-ENV has CCS811B and BME280 digital precise temperature, humidity and air pressure sensor assembled together. Both operate through I2C and are low power with sleep modes which makes them perfect for enviroment monitoring and IoT.
ESP32-POE, ESP32-POE-ISO, ESP32-EVB has UEXT connectors and can directly connect to MOD-ENV.
The price of MOD-ENV is between EUR 14.36 and 15.95 depend on quantity.
olimex 
Jun 21, 2019 @ 23:24:07
Seems the CCS811B is really a sensor for Volatile Organic Compounds (VOC) and not CO2 directly, albeit it “supports intelligent algorithms to process raw sensor measurements to output equivalent total VOC (eTVOC) and equivalent CO2 (eCO2) values, where the main cause of VOCs is from humans.”
Raw CO2 vs VOC sensors:
https://www.co2meter.com/blogs/news/5185132-co2-sensors-vs-voc-sensors-for-iaq-whats-the-difference