Use of relay in a gas sensor (safety issue)
There are relays that are made for use in hazardous locations such as one where explosive gases are present.  The term for these relays is intrinsically safe and they either operate on such low energy that they will not ignite flammable or explosive materials, or they are sealed in such a manner that the gasses (or explosive dust, etc.) are protected from the live parts.  

There are intrinsically safe barrier relays which are interposing relays designed to operate at intrinsically safe levels on one side and normal levels on the other.  

It's a broad and complex topic way beyond this answer, but hopefully those terms will provide you with a start for further research.