Good Arduino module with an analog output,

and a digital pin, which is set HIGH when a threshold is exceeded.
The threshold can be adjusted with a small potentiometer on the back. Replacing the module
of course, no professional security technology, different gases are detected.

Who ordered the module from China, should keep in mind that the explosion protection grille in the typical
padded shipping envelopes can be "smashed". This affects not necessarily the function,
but looks ugly.