There are two primary groupings of gases that are generally monitored in industry: 1) combustible/organic compounds, and 2) toxic/inorganic compounds. And there is some considerable overlap between these broad categories. Organic (hydrocarbon) compounds are generally also toxic as well as being combustible. And a few inorganic (non-hydrocarbon) compounds may also be combustible as well as being toxic.
This article will focus on organic compounds that are usually combustible and may also be toxic. We will exclude inert compounds and, generally, inorganic compounds.