The hydrogen sulphide could be produced by bacterial activity in the system, and may be a small quantity compared with the total gas production in this case. For a bit more information see
this .
Some physiological effects of H2S
PHYSIOLOGICAL RESPONSE OF ADULT HUMANS TO HYDROGEN SULPHIDE **
Effect..................................Concentration
.......................................Mg (H2S)/1 Kg (Air)
Least Detectable Odour...............0.01-0.7
Offensive Odour........................3-5
Eye Irritation.............................10
Irritation Mucous Membranes
and Lungs...............................20
Irritation of Respiratory Tract......50-100
Olefactory Nerve Paralysis.........150
Headache, Dizziness................200
Nausea, Excitement,
Unconsciousness......................500-600
Rapidly Fatal...........................700-2000 **
Source Nordstron, G.A.: J.B. McQuilty: "Manure
Gases in the Animal Environment." University of
Alberta - 1976.
Note that H2S has the property of paraylising the nerves so that the smell is undetectable at the concentrations that cause dizziness (and it's toxic)