What did they say for methane?
Methane LEL 5%, UEL 15%. Any concentration above the UEL is considered too rich. The majority of the gas composition given for flatulence is hydrogen at 50%.
I work with electrical devices (enclosures, motors) and instrumentation in refineries and chemical plants.
Most applications where hydrocarbon vapors are present are considered as area classification Class 1 Group C and D.
Areas where hydrogen may be present are Class 1 Group B.
The most stringent is where there is acetylene or ethylene oxide, Class 1 Group A.
http://www.engineeringtoolbox.com/hazardous-areas-classification-d_345.html