Sketch for the BME280 and CCS811 for use with this breakout board available at Tindie:
The Bosch BME280 is a simple but high resolution pressure/humidity/temperature sensor, which can be used in its high resolution mode but with power consumption of 20 microAmp, or in a lower resolution mode with power consumption of only 1 microAmp. The choice will depend on the application.
The AMS CCS811 is an air quality sensor that provides equivalent CO2 and volatile organic measurements from direct I2C register reads as well as current and voltage (effective resistance of the sensing element). Gas sensors, including this MEMs gas sensor in the CCS811 measure resistance of a substrate that changes when exposed to inert gases and volatile organic compounds. Changed in concentration vary exponentially with the changes in resistance. The CCS811 has an embedded ASIC calibrated against most common indoor pollutants that returns a good estimate of equivalent CO2 concentration in parts per million (400 - 8192 range) and volatile organic componds in parts per billion (0 - 1187). The sensor is quite sensitive to breath and other human emissions.