A driver for the Sensirion SGP40 MOx gas sensor. An example of this
chip from Adafruit is:

https://www.adafruit.com/product/4829

This is a moderately priced gas sensor that can detect volatile
organic compounds in the air. The driver uses the 3-clause BSD
licensed VOC algorithm provided by Sensirion to turn the raw sensor
metric into a VOC index which can indicate the quality of the air in a
particular indoor environment. All published functions of the chip
are supported and one unpublished feature.