Electrochemical cells use a chemical reaction, electrolysis, to produce a small voltage through the movement of electrons within the cell.  It can be used to measure all sorts of different species in a very cost effective way.

The electrochemical cells are inherently affected by both temperature and humidity and as such the resolution can be limited, but by restricting the flow of gas to the cell, very low concentrations may be measured to look at trends at a much lower cost than by using traditional technology.

Learian StreetBox Low Cost AQM Monitor

ICOM Indoor CO Monitor